A2 Unit 4 Kinetics, Equilibria and Organic Chemistry
Mr Lund 03 March 2015
1
AQA A2-LEVEL
Student Guide to Unit 4
Kinetics, Equilibria and Organic Chemistry
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Shared Areas Chemistry Read Chemistry Mr Lunds Classes A2 Chemistry Unit 4 All programs
A2 Unit 4 Kinetics, Equilibria and Organic Chemistry
Mr Lund 03 March 2015
2
Equilibria
dynamic (not static - do you know the difference!) equilibrium is achievable in a closed
system (e.g. solutions in a test tube if there are no gaseous reactants/products).
rates of the forward and reverse reactions at equilibrium are identical
concentrations are unlikely to be 50 50
Le Chateliers Principle and Qualitative Aspects
can you unambiguously write down what LCP states (see 148 of the AS textbook)?
remember this is a predictive tool used to determine the effect on the position of equilibria when a change in concentration, temperature or pressure is made
it is NOT an explanation of WHY it happens so avoid statements such as because of LCP, LCP causes and learn to state LCP predicts that ..
LCP is not suggesting that a system completely reverses a temperature change when establishing a new equilibrium as the new equilibrium will be that for the changed
temperature
it implies that the shift in the position (in terms of reactants and products) of equilibria is in the direction that seems to minimize the effect of that change
a new position of equilibria in which the relative rates of the forward and backward reaction are once again in balance under the new set of conditions is eventually arrived at
the position of equilibria is changed by:
concentration (which can be easily understood using rates/collision theory)
temperature (EA will be larger for the endothermic process so it will be relatively more
favoured by a rise in temperature)
pressure (only applicable where there is an imbalance between the number of moles of
gaseous particles on either side of the equation)
catalysts - do not affect position of equilibrium, just the time to achieve it
Monitoring Equilibria
remote sensing this is non intrusive (e.g. level of absorbance of a given wavelength of light by a coloured solution) so will not effect the position of equilibrium
titrimetric analysis this will effect the position of equilibria (since the concentration of one of the reactants or products will be changed) so is only applicable to a system with a slow
response to a change in conditions or where quenching (dilution or cooling) is used to slow
down the rate of reaction and thus the effect of the investigative technique
A2 Chemistry (Nelson Thornes) AQA 20 and Chapter 9 in the AS Book
A2 Chemistry (Heinemann) AQA 16 17 Q 1 - 3 on page 16 17 Q 1 on page 32
Chemguide Chemical equilibrium
How Science
Works: A, B
A2 Unit 4 Kinetics, Equilibria and Organic Chemistry
Mr Lund 03 March 2015
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Equilibrium Law (i.e. equation)
aA + bB cC + dD Kc =
eqm
b[B]
eqm
a[A]
eqm
d[D]
eqm
c[C]
only applies to systems at equilibrium so dont use initial concentrations
Kc is calculated from concentrations at equilibrium (note that the concentration of a solid is constant and so solids do not appear in the equilibrium expression)
equilibrium constant (Kc) is related to reaction stoichiometry (a, b, c etc) and is a constant at constant temperature
the value of Kc is an indicator of the position of equilibrium (reverse reaction = inverse value)
the value of Kc is not indicative of how fast the reaction proceeds
you must be able to calculate the numerical value of Kc (possibly using data from an experiment you will carry out yourself)
note that concentrations are used i.e. moles/volume not moles although quite often V will cancel down or cancel out completely (when the powers is the same on the top as the bottom) but show ALL working in exams
determination of the units of Kc you must show workings in the exam but .. check using (moldm-3)top powers - bottom powers
Summary Questions Page 21 1 - 3
Exam Style Questions Page 28 1
determination of the concentrations of reactants/products present at equilibrium, given appropriate data including the numerical value of Kc
this may involve the determination of an unknown value of x to solve an equation (but not a quadratic one on this syllabus)
you may be required to realise that the equation can be simplified by taking the square root of both the top and the bottom terms and the value of Kc(see the example on page 22 23)
you might also be asked to determine the amount of given reactant required in order to produce a given amount of product (see the example on page 23 24)
Summary Questions Page 24 1
Exam Style Questions Page 28 2
A2 Chemistry (Nelson Thornes) AQA 20 24
A2 Chemistry (Heinemann) AQA 18 20, 22 - 27 Q 4 6, 8 - 13 on pages 18 27 Q 3 on page 33
Chemguide Equilibrium equation
This equation can vary and
simply illustrates the principle
behind its construction
A2 Unit 4 Kinetics, Equilibria and Organic Chemistry
Mr Lund 03 March 2015
4
Changing Conditions and Equilibria
Pressure or Concentration Change
the value of Kc does not change with variations in concentration or pressure
to improve yield wrt a more expensive reactant a cheaper reactant could be added.
removing the product as it is formed (assuming that the response of the system is fast enough) would also improve yield whist also allowing reactants to be recycled (see Haber)
pressure only has an effect IF gaseous particles are involved and in addition the stoichiometric ratio of gaseous particles is unequal on either side of the equation
in all cases a pressure increase will increase the rate of reaction involving gaseous reactants
Kc and Temperature Change
value of Kc increases as temperature increases for endothermic reactions i.e. the equilibrium shifts to the RHS i.e. more products
value of Kc decreases as temperature increases for exothermic reactions i.e. the equilibrium shifts to the LHS i.e. less products
Temperature Exothermic reaction Endothermic reaction
increases Kc decreases Kc increases
decreases Kc increases Kc decreases
Kc and Catalysts
value of Kc DOES NOT CHANGE when a catalyst is used
therefore catalysts do not change the position of equilibria i.e. change the yield
HOWEVER, the rate of the reaction will be faster (for both the forward and backward reaction) hence equilibrium will be achieved sooner
adding a catalyst to a system already at equilibrium will not change its position
How science works Page 26 Development of the Haber Process
Summary Questions Page 27 1 - 3
Exam Style Questions Page 28-29 3 5
A2 Chemistry (Nelson Thornes) AQA 25 - 27
A2 Chemistry (Heinemann) AQA 18 27 Q 4 13 on pages 18 27 Q 3, 5 on page 33
Chemguide Equilibrium equation
How Science
Works: E, F
A2 Unit 4 Kinetics, Equilibria and Organic Chemistry
Mr Lund 03 March 2015
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Kinetics
review basic ideas of collision theory and activation energy from AS
Rate Equations
the rate of reaction is the rate of change of either the reactants or products in a chemical reaction
the rate can be determined from concentration versus time graphs
the mathematical expression for the graph on page 4 of reaction A + 2B C:
Rate = dt
Bd
dt
Ad
dt
Cd ][
2
1][][
there are different ways to express rate
(i) average rate
(ii) instantaneous rate (this is what the equations above depict) which is the gradient of
the tangent of the curve at selected values of concentration or time
(iii) initial rate is the instantaneous rate right at the start of a reaction
initial rate (t0) the gradient is easiest to determine with confidence since it goes through zero
Measuring Reaction Rates
see AS Module 2 guide for general practical techniques for monitoring progress in a given reaction
generally the main methods comprise two different strategies
1 following a single reaction measuring colour change, change in conduction/pH of one product or reactant
then plotting a concentration time graph and determining the rates at given concentrations
2 clock techniques measuring the time to an observable event from known different initial conditions e.g. the thiosulphate cross (the one you did at GCSE) or the iodine clock this must be relatively early in the reaction so that the concentration of other reagents
(other than the one being varied) can be deemed unchanged.
initial rate 1
t
How science works Page 5 Damn fast reactions indeed Summary Questions Page 6 1 - 5
Dont panic !! this can be explained
easily in words ask your teacher
A2 Unit 4 Kinetics, Equilibria and Organic Chemistry
Mr Lund 03 March 2015
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Rate Equations and Order of Reaction
each reactant may or may not affect reaction rate (so its not quite as clear cut as it seemed at GCSE/AS)
ALSO it is not necessarily directly proportional when it does we shall see why later
for a given reactant we can state
Rate [reactant]n
for two reactants A and B the general rate expression is:
Rate [A]m[B]n moldm-3s-1
m and n are the order of reaction for each reagent (values are limited to 0, 1 or 2 at A-Level)
overall order = individual orders)
unlike with equilibria the orders of reaction and thus the overall rate equation cannot be determined from reaction stoichiometry they can only by determined by experiments
e.g. 2H2(g) + 2NO(g) 2H2O(g) + N2(g)
doubling [NO] quadruples the rate i.e. double2 = 4
doubling [H2] doubles the rate
i.e. Rate [H2][NO]2
NOT as suggested by the ratio of reactants rate [H2]2[NO]2
k is the rate constant (units depend upon overall order)
rate = k[H2][NO]2 moldm-3s-1
units of k will vary depending upon the overall order of the reaction
determination of the units of k for 0, 1, 2 and 3rd order reactions overall should be attempted as they are likely in an exam and here is
another cheat for checking your answer:
units of k = (moldm-3)1 - overall order (s-1)
note that if the order is 0 this term will not appear in the rate equation e.g. A2BC0 = A2B x 1 = A2B
Maths tip:
Any number raised
to the power zero = 1
Note that rate always has these same
units.
[NO]2
The effect on
rate of the
change
here
..is raised
to this
power i.e.
its second
order wrt
to [NO]
A2 Unit 4 Kinetics, Equilibria and Organic Chemistry
Mr Lund 03 March 2015
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Catalysts and Rate Equations
catalysts will be involved in the rate expression (this might only be a modification of the value of k itself for example a solid in heterogeneous catalysis)
e.g. CH3COCH3(aq) + I2(aq) CH2ICOCH3(aq) + H+(aq) + I-(aq)
by experimentation it was found that the reaction is:
first order wrt [H+] and [CH3CO CH3] zero order wrt [I2] i.e. second order overall
rate [CH3COCH3]1[ H+]1[I2]0
rate = k[CH3COCH3][ H+] moldm-3s-1
what are the units of k?
Rate Determining Step
most reactions occur in several steps (this is exemplified by organic reaction mechanisms)
each step will take place at a different rate
the slowest step will determine the overall rate of the reaction and is known as the rate determining step
the order of the reaction regarding each reagent can provide information regarding its involvement in the rate determining step
obviously a reactant with 0 order will not be involved in the rate determining step
study of reaction kinetics can yield important information regarding the mechanism of a multi-step reaction
in the reaction above iodine would not be involved in the rate determining (slow) step
at this point you might ask your teacher to explain why there are variations in the mechanism for the hydrolysis of a haloalkanes as discussed on pages 15 16 (or perhaps research SN1 and SN2 yourself)
Summary Questions Page 9 1 4 Page 16 1
A2 Chemistry (Nelson Thornes) AQA 4 9, 14 - 16
A2 Chemistry (Heinemann) AQA 4 6 Q1, 2, 3 on page 5-6
Chemguide Order of reaction, rate expression, SN2
H+
A2 Unit 4 Kinetics, Equilibria and Organic Chemistry
Mr Lund 03 March 2015
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Concentration/Time and Rate/Concentration Graphs
rate/concentration graphs can show the order with respect to a given reagent
it is glossed over at A-Level that the other reagents will have to be present in xs so that their concentration can be deemed to be unchanged during the course of a reaction where a series
of measurements of the concentration of a given chemical are measured.
or in a clock technique the event measured in relatively early in the reaction so better reflects the known initial concentration of each reagent
zero order e.g. decomposition of ammonia (tungsten catalyst), rate is independent of concentration i.e. graphically it is a flat horizontal line
2NH3(g) 3H2(g) + N2(g)
first order e.g. thermal decomposition of dinitrogen monoxide to nitrogen and oxygen (gold catalyst), rate is directly proportional to concentration and will be a straight line gradient = k
(any points not on the straight line will be anomalies and require identification and
explanation (e.g. temperature variations))
second order e.g. thermal decomposition of ethanal to methane and carbon monoxide will produce a graph that curves upwards (rate against concentration2 is a straight line)
Initial Rates Method
initial reaction rates are determined by plotting the tangent to the time/concentration graph for different initial reagent concentrations at the start of the reaction (t = 0) when reaction
concentrations are accurately known (and at a fixed temperature/catalyst)
the gradient of this line is the initial rate
orders of reaction can be determined from initial rates data by inspection
the value of k can also be determined from this data
the best way to grasp this idea is to try examples
Summary Questions Page 13 1
Exam Style Questions Page 17 - 19 1, 3, 5
A2 Chemistry (Nelson Thornes) AQA 10 - 12
A2 Chemistry (Heinemann) AQA 7 11 Q4 on page 8
Q1 on page 14
Chemguide rate
W Do you have an idea as to why it might be zero
order?
A2 Unit 4 Kinetics, Equilibria and Organic Chemistry
Mr Lund 03 March 2015
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Variation of k with Temperature
Maxwell-Boltzmann Distribution Curve
distribution of energies amongst particles at different temperatures give rise to the Maxwell-Boltzmann distribution curves based on the Arrhenius expression
RTE A
Aek
A = Arrhenius constant (determined by collision frequency and orientation factor)
R = gas constant (8.31 JK-1mol-1)
T = absolute temperature (K)
the main significance of this equation (which you dont need to know unless you are trying to understand the subject) is that a small rise in temperature has an exponential (i.e. big)
effect on rate
ln(k) = ln(A)
ln(k) = - + ln(A)
y = m x + c
Plot ln(k) against 1/T gives gradient = - and intercept = ln(A)
Exam Style Questions Page 17 - 18 2,4
Page 158 1
A2 Chemistry (Nelson Thornes) AQA 10 - 12
A2 Chemistry (Heinemann) AQA 12 13 Q2-4 on page 14 - 15
Q1 on page 150
Chemguide Arrhenius expression
AE
R T
R
EA
R
EA
T
1
Extra info for
those who do
Maths how can we get
values for EA
and A
A2 Unit 4 Kinetics, Equilibria and Organic Chemistry
Mr Lund 03 March 2015
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Acid-Base Equilibria
Arrhenius definition of an acid - hydrogen ions and oxonium ions (H3O+(aq))
soluble base = alkali
alkaline solutions have relatively high [OH-(aq)]
Brnsted-Lowry Theory of Acids and Bases
Arrhenius limited to aqueous solutions but acid-base concept is broader e.g.:
NH3(g) + HCl(g) NH4Cl(s)
acid-base equilibria involve proton transfer
acids are proton donors, bases are proton acceptors
water is amphoteric
it behaves as an acid with NH3
and as a base HCl
conjugate acid-base pairs undergo proton exchange
competition occurs for protons between bases on either side of the equilibria
relative strength of the base determines the equilibria bias
a relatively strong base has a relatively weak conjugate acid and visa-versa
note that in the protonation of nitric acid in the nitration of benzene nitric acid acts as a base
Ionic Product of Water (Kw)
water undergoes slight auto-ionisation (dissociation of the water molecule)
ionic product of water again the equation defines the concept and dont forget units
Kw = [H+(aq)][OH-(aq)] = 1 x 10-14 mol2dm-6 at 298K
variation of Kw its value increases as temperature increases - LCP - (hence [H+(aq)] increases and therefore the pH of a neutral solution decreases!)
Summary Questions Page 31 1 - 3
A2 Chemistry (Nelson Thornes) AQA 30 - 31
A2 Chemistry (Heinemann) AQA 34 35, 36, 42 - 43 Q1 on page 35
Q6 on page 37
Chemguide Brnsted, ionic product
pH Calculations
You must make the
effort to learn the
definition of a Brnsted-
Lowry acid and base
A2 Unit 4 Kinetics, Equilibria and Organic Chemistry
Mr Lund 03 March 2015
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define pH using an equation NOT in words as it is more certain to get full marks.
pH = -log10[H+(aq)] learn to use your calculator !
pH of monoprotic (release a single proton into aqueous solution) acids e.g. HCl (note: pHs lower than 1, including ve values, are possible)
you should be able to calculate pH after a strong acid of known volume and concentration (or pH) is diluted by a known volume of water
pH of diprotic acids NOTE: [H+(aq)] = 2 x [H2SO4(aq)]
calculating [H+(aq)] from pH values
[H+(aq)] = 10- pH learn to use your calculator !!!!!!
calculating the pH of alkalis using Kw to calculate [H+(aq)]
note for Ca(OH)2 that [OH-(aq)] = 2 x [Ca(OH)2(aq)]
but why might it give a weaker acid than sodium hydroxide?
[OH-(aq)] = ][H
K
(aq)
w
How science works Page 33 Measuring pH
Summary Questions Page 31 Questions 1 - 5
Exam Style Questions Page 50 Questions 3
A2 Chemistry (Nelson Thornes) AQA 32 35
A2 Chemistry (Heinemann) AQA 38 40, 42 - 45 Q7 - 10 on pages 39 - 40
Q13, 14 on pages 37 38 Q 3, 4 on page 55
Q 7 on page 56
Chemguide Strong acids pH
A2 Unit 4 Kinetics, Equilibria and Organic Chemistry
Mr Lund 03 March 2015
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[HA(aq)]
(aq)](aq)][A[H-
]O
O
(l)2
-
3
[HA(aq)][H
(aq)](aq)][A[H
Weak Acids and Ka
weak acids are only partially ionised
HA(aq) + H2O(l) H3O+(aq) + A-(aq)
Kc =
[H2O] ~ constant
[A-(aq)] = [H3O+(aq)] = [H+(aq)]
given that it is a weak acid we can assume the degree of dissociation is minimal hence:
[acid]equilibrium ~ [acid]initial
Ka = Kc[H2O] =
Ka =
pKa = -log10Ka
Ka = 10- pKa
relatively higher Ka / relatively lower pKa = stronger acid (given the same concentration)
value is independent of concentration and therefore more useful
you should be able to calculate the pH of a weak acid of known concentration using Ka and
calculate Ka from the pH of a weak acid of known concentration
Summary Questions Page 38 1 - 3
Exam Style Questions Page 51 6
A2 Chemistry (Nelson Thornes) AQA 32 35
A2 Chemistry (Heinemann) AQA 35 36, 40 42 Q11 - 12 on pages 41 - 42
Q 1 on page 55
Chemguide Weak acid
[HA(aq)]
(aq)][H2
A2 Unit 4 Kinetics, Equilibria and Organic Chemistry
Mr Lund 03 March 2015
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Titration Curves
as an alkali is added to an acid the pH increases and visa-versa
you should also be able to calculate the pH at any point in the addition of sodium hydroxide to a monoprotic acid (and visa-versa) including weak acids
typical errors to avoid are not converting to moles and getting the stoichiometric ratio wrong
most likely you will forget to use the total volume of the solution created and thus get the concentration wrong and therefore the pH
another likely error with a weak acid where xs alkali has not been added is assuming that the remaining acid is fully dissociated i.e. forgetting to use Ka to determine [H+(aq)]
equivalence point is where two solutions have reacted in stoichiometrically the correct molar ratio this will be the vertical point on the titration curve where the pH changes markedly
pHs at equivalence point and appropriate curves for:
http://www.avogadro.co.uk/chemeqm/acidbase/titration/phcurves.htm
pay particular attention to the position of the initial pH for strong and weak acids and look carefully at how it changes at the start
also carefully note the position of the equivalence point the mid point of the vertical section
finally ensure that a sensible final pH is shown to reflect the use of a strong or a weak base.
you should be able to calculate concentrations of an unknown acid or alkali from the results of a titration pretty much as was the case for AS level but there will be more likely hood of diprotic acids cropping up (e.g. sulphuric acid)
Summary Questions Page 41 1 - 2
A2 Chemistry (Nelson Thornes) AQA 39 41
A2 Chemistry (Heinemann) AQA 44 48 Q15 on page 45
Q 16 on page 48
Q 2, 8 on pages 55 - 56
Chemguide Titration curves
SASB WASB SAWB WAWB
A2 Unit 4 Kinetics, Equilibria and Organic Chemistry
Mr Lund 03 March 2015
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Ka of Weak Acids from Titration Curves
at the half equivalence point (half neutralisation point as stated in your text book) given that:
[HA] = [A]
Ka = [H+(aq)]
pKa = pH
the half equivalence point can be determined practically by determining the pH at half the equivalence point (half the volume) from a plotted titration curve
note that it is not half the pH value itself at the equivalence point that is used!
alternatively a titration can be repeated with half the volume of the already determined equivalence point and the pH then measured using a pH meter
End Point of an Indicator
indicators can only be used for a titration curves with a vertical section of >2 pH units
suitable indicators for acid-base titrations will have an end point and range that lie within that vertical section i.e. will thus exhibit a sharply defined colour change
pH meters can be used on coloured solutions
indicators are weak acids
HIn(aq) H+(aq) + In-(aq)
Ka (or Kin) = ][HIn
]][In[H
(a q )
(a q )-
(a q )
at end point [In-(aq)] = [HIn(aq)] Kin = [H+(aq)]
pKa = pKin = pH
Summary Questions Page 41 1 - 2
Exam Style Questions Page 50 4
A2 Chemistry (Nelson Thornes) AQA 39 - 41
A2 Chemistry (Heinemann) AQA 49 51 Q17 - 18 on pages 50 51 Q 5 on page 56
Chemguide Indicators
A2 Unit 4 Kinetics, Equilibria and Organic Chemistry
Mr Lund 03 March 2015
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Buffer Solutions
the effect of pH changes e.g. lemon juice and the proteins in milk
buffers are designed to maintain pH stability
you must understand buffer solution in terms of the response of an equilibrium system to the addition of hydrogen or hydroxide ions
Acidic Buffer
HA H+ + A- reservoir of HA provided by acid
NaA Na+ + A- reservoir of A- provided by the sodium salt of the acid
Basic Buffer
NH3 + H+ NH4+ reservoir of NH3 provided by weak base
NH4Cl NH4+ + Cl- reservoir of NH4+ provided by the ammonium salt
Added H+(aq) is removed by A-(aq) provided by
the salt as the equilibrium shifts to the left
Added OH-(aq) is removed by
reacting with H+(aq) provided
by the acid as the equilibrium
shifts to the right
Added H+(aq) is removed by NH3 as the
equilibrium shifts to the right
Added OH-(aq) is removed by
reacting with NH4+(aq)
provided by the salt and the
equilibrium shifts to the left
An acidic buffer (for pH 7) consists of a
weak alkali and its soluble salt
A2 Unit 4 Kinetics, Equilibria and Organic Chemistry
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Important Buffer Solutions
Carbonic acid-Bicarbonate Buffer in the Blood is by far the most important process for maintaining the acid-base balance (in our bodies there are also phosphate and protein
buffers).
a pH change of over 0.5 can be fatal
buffer solutions are also used in:
the food industry (acidity regulators) fabric dyeing where AZO dyes are used hair care products which are kept slightly acidic pH 5.5 as alkaline conditions
make hair look rough as microscopic scales on the surface of the hair are made to
stand up.
calibrating pH meters biochemical research (enzymes are denatured by pH extremes) and there are numerous other areas
Preparation of Buffer Solutions
a buffer solution can be made in two different ways:
1 adding a suitable soluble salt to an acid
2 partially neutralising a weak acid with a strong base up to the required pH
when [HA] = [A-] the buffer solution is equally able to deal with the addition of acid and base by equal sized reservoirs
for this reason a weak acid with a pKa relatively close to desired pH is selected for more effective buffering (see the calculations in the next section)
Added H+(aq) is removed as the
equilibrium shifts to the right
Added OH-(aq) is removed by reacting with
H+(aq) shitting the equilibrium to the left
A2 Unit 4 Kinetics, Equilibria and Organic Chemistry
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[HA(aq)]
(aq)](aq)][A[H-
[ACID)]
](aq)][SALT[H
[SALT]
[ACID]
Calculating the pH of a Buffer Solution
HA H+ + A-
Ka =
but [HA] ~ [ACID]i and [A-] = [salt]
Ka =
[H+(aq)] = Ka
it is assumed that the volume of the weak acid solution is unchanged by the addition of a small quantity of its solid salt
since all particles are present in the same total volume of solvent we can make life easier by appreciating that the acid to salt:
moles ratio = concentration ratio
the best buffer will be that obtained at half the equivalence point:
[ACID] = [SALT]
and so [H+(aq)] = Ka
pH = pKa
equimolar amounts of acid and salt produces a buffer solution with a pH of the same numerical value as pKa
you should be able to determine the required combination of acid and salt to produce a buffer solution of a given pH
you should also be able to demonstrate by calculation that adding acid or alkali to a buffered solution changes the pH by less than for an un-buffered solution
Summary Questions Page 48 1
Exam Style Questions Page 49 1, 2
Page 51 5
Page 160 5
A2 Chemistry (Nelson Thornes) AQA 45 - 48
A2 Chemistry (Heinemann) AQA 52 54 Q17 - 20 on pages 52 54 Q6, 9 on page 56
Chemguide Buffer
Will the dilution
of a buffer
solution change
the pH?
A2 Unit 4 Kinetics, Equilibria and Organic Chemistry
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18
Organic Nomenclature
displayed formula every bond and every atom should be shown in examinations!!!
structural formula e.g. CH3CH2CH2OH
skeletal formula may be useful for prospective medical etc students
it is important that you understand the difference between 2-D displayed formula and the actual 3-D molecular shape as this will prove particularly relevant later
functional group one or more reactive sites on a hydrocarbon skeleton
homologous series same general formula + same functional group (hence same chemistry as the increase in chain length has little effect on their chemical reactivity)
nomenclature is based on four criteria:
root longest unbranched hydrocarbon chain (including main functional group)
suffix determines principal functional group (a pecking order exists)
prefix other changes to root molecule (e.g. side chains, functional group)
locant position of branch or substituent (e.g. double bond) on the main chain
look for the longest chain NOT the longest straight chain of carbons
start numbering at the end of the chain that results in the lowest numbers in the name or for the primary functional group position
alphabetical order is used where more than one type of functional group or branch is required in the prefix
mono, di, tri and tetra indicate multiple functional groups or branches of a given type (dont change alphabetical order)
commas and dashes are important - so learn how to do this correctly !!!!
An e is dropped if the next letter is a vowel: propan-2-ol, propane-1,2-diol
An a is added if inclusion of di, tri, etc., would put two consonants consecutively: buta-1,3-diene, not but-1,3-diene propanenitrile, not propannitrile or propanitrile.)
you will need to aware of nomenclature examples of the following (including cyclic variations):
alkanesalkeneshaloalkanes alcohols aldehydes and ketones
carboxylic acids esters acyl chlorides amines amides
amino acids benzene and its derivatives
Summary Questions Page 56 1 - 3
Exam Style Questions Page 64 1
A2 Chemistry (Nelson Thornes) AQA 52 - 56
Chemguide nomenclature
How Science
Works: H
How Science
Works: H How Science
Works: H
A2 Unit 4 Kinetics, Equilibria and Organic Chemistry
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UNIT 2
Isomerism
isomers have the same molecular formula but different chemical and/or physical properties
remember that there are a number of ways in which isomerism exists that you have already met:
UNIT 1
isomers
structural isomers stereoisomers
position chain functional geometrical optical
group
Structural Isomerism
structural isomers have the same molecular formula but different structural formula
one form of structural isomerism is called chain isomerism unbranched chain and branched chain i.e. different hydrocarbon skeleton
e.g. How many versions of C4H8 can you find that represent structural isomers?
functional groups that are present at different positions are called positional isomers (there will be different numbers in the name)
e.g. propan-2-ol and propan-1-ol
functional group isomerism exists where the molecular formula is the same but different functional groups (and therefore chemical properties exist)
e.g. propane-l-ol and methoxy ethane, but-2-ene and cyclobutane
UNIT 4
A2 Unit 4 Kinetics, Equilibria and Organic Chemistry
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20
How Science
Works: H
Stereoisomerism
Stereoisomers have the same molecular and structural formula but differ in the spatial
arrangement of their atoms.
Geometrical isomerism
this is consequential of the non-rotation of a double bond (unlike in alkanes) which is what you will state as the fundamental requirement in the exam
this lack of free rotation is consequential of the bond present in the alkene
a single carbon-carbon bond cannot give rise to this type of isomerism as (unless a chain is present) there is unrestricted rotation
Geometrical isomers have the same molecular formula, same structural formula but a different
spatial arrangement of the atoms due to the non rotation of the carbon-carbon double bond
whilst the minimum requirement is the presence of restricted rotation consideration should also be given to the substituents on each carbon
the two molecules on the left above are identical even though there is a carbon-carbon double bond as simply flipping vertically makes them super imposable
across a carbon-carbon double bond each carbon in turn must have different substituents
it doesn't matter whether the two groups are the same e.g. in the example on the right no amount of flipping or rotating makes them super imposable.
For geometrical isomerism to be possible both carbon atoms on the double bond must have
different atoms/groups attached to themselves, however, the carbon atoms can still both be
identical in that respect.
E and Z are used to distinguish between the two isomers (its quite EZy to do will
a little practice)
E isomers have the main grouping diagonally across the double bond
Z isomers have the main grouping on the same side of the double bond
cis and trans were previously used and in most cases E corresponds to trans and Z to cis BUT NOT ALWAYS
How science works Find out about the food industry and trans fats and the
hydrogenation of vegetable oils.
How Science
Works: A
How Science
Works: L
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the trick is to identify what the main groupings are:
where the two atoms directly bonded to the carbons of the double bond with the largest
atomic numbers (highest priority) are diagonally opposite then it is deemed an E isomer
where the two atoms directly bonded to the carbons of the double bond with the largest
atomic numbers (highest priority) on the same side then it is deemed an Z isomer
if on one of the carbons the atoms directly bonded are identical then to establish the
highest priority grouping a tie break situation arises in which you look at the next
highest priority atom attached to each of them e.g -CH2Br beats CH2Cl and so on
take care here when using older text books as some molecules deemed trans in the old system would actually be Z in the new system i.e. across the double bond in one system does not directly yield across the double bond in the other e.g. 3-bromobut-2-ene
Example: but-2-ene
Step 1: split the alkene Step 2: assign the relative priorities. The two attached atoms are C and H, so since the atomic numbers C > H then the -CH3 group is higher priority.
Step 3: look at the relative positions of the higher priority groups : same side = Z, hence (Z)-but-2-ene.
The two attached atoms are C and H, so since the atomic numbers C > H then the -CH3 group is higher priority. Therefore the two high priority groups are on the opposite side, then this is (E)-but-2-ene.
E-Z transformations are possible given an energy source e.g. photochemistry and eyesight:
Optical Isomerism
How many functional
groups can you see
here?
How can you test to
provide evidence for
each one?
CH3
CH3
CH3
CH3
CH3
O
nerve impulse to the brain
O
C H 3 C H 3
C H 3
C H 3
C H 3
lightto eye
CH3
CH3
CH3
CH3
CH3
O
CH3
CH3
CH3
CH3
CH3
O
nerve impulse to the brain
O
C H 3 C H 3
C H 3
C H 3
C H 3
lightto eye
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Optical isomerism exists where there is an asymmetric (i.e. chiral) carbon with four different
groups attached.
Optical isomers are non-super imposable molecules (enantiomers) which are mirror images of
one another.
dont just put mirror images as they can sometimes be superimposable where a plane of symmetry exists
if two groups are the same i.e. there is a plane of symmetry then a simple rotation yields the same spatial arrangement so they are not enantiomers
many chemicals synthesised in the lab produce equal amounts of both enantiomers (see lactic acid later) which is called a racemic mix
this is because the reagents are not stereo specific (rather like a left handed screwdriver)
however, living organisms tend to manufacture one enantiomer in preference to the other as determined by the reactive sites of the optically active enzyme used to construct it
enzymes are stereo specific reagents
the way that these enantiomeric molecules interact with biological systems can be different, for example:
carvone one enantiomer tastes of spearmint the other caraway
limonene one smells of lemons the other oranges (see if you can spot the chiral centre in
these enantiomers of limonene)
Can you give the
systematic name for this
molecule?
A2 Unit 4 Kinetics, Equilibria and Organic Chemistry
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Enantiomers rotate plane polarised light in opposite directions (clockwise and
anticlockwise).
this physical property is the means by which they can be distinguished
one enantiomer rotates the polarised light clockwise (to the right) and is the (+) enantiomer;
the other rotates the polarised light anticlockwise (to the left) and is called the () enantiomer.
a racemic mix consists of a 50:50 mix of both isomers and this will therefore NOT rotate plane polarised light as the two enantiomers cancel one another out
How science works Page 62 The thalidomide tragedy
Summary Questions Page 59 1 - 4
Exam Style Questions Page 64 2, 4, 5
A2 Chemistry (Nelson Thornes) AQA 57 - 59
Chemguide isomerism
How Science
Works: L
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Aldehydes and Ketones
Physical properties
lower Mr aldehydes and ketones are miscible with water due to polar C=O bonds ability to hydrogen bond with water molecules
higher Mr molecules have greater VdW with one another due to increasing size of hydrocarbon tail hence miscibility is reduced for energetic reasons
Preparation
primary alcohol aldehyde heat with K2Cr2O7(aq)/H2SO4(aq), distil off aldehyde (boiling point lower than alcohol)
secondary alcohol ketone reflux under heat with K2Cr2O7(aq)/H2SO4(aq)
orange dichromate(VI) ions (Cr2O72-aq)) are reduced to green chromium(III) ions (Cr3+(aq))
Distinguishing Between Aldehydes and Ketones
ACIDIFIED POTASSIUM DICHROMATE SOLUTION
tests are based on the fact that aldehydes can be easily oxidised to a carboxylic acid while ketones cannot be.
Aldehyde + [O] Carboxylic Acid
reflux with K2Cr2O7(aq)/H2SO4(aq) orange dichromate(VI) ions (Cr2O72-(aq)) are reduced to
green chromium(III) ions (Cr3+(aq))
Care: The above test is only applicable if it is clear that the unknown sample is not a primary
or secondary alcohol.
A2 Unit 4 Kinetics, Equilibria and Organic Chemistry
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NOTE: PRIMARY AND SECONDARY ALCOHOLS CANNOT BE OXIDISED BY
FEHLINGS OR TOLLENS HENCE DO NOT GIVE POSITIVE RESULTS WITH THE TESTS BELOW.
writing balanced redox equations under alkaline conditions is a little more involved than acidic conditions but you will find an excellent strategy on CHEMGUIDE see the link below
FEHLINGS TEST
warm with Fehlings solution blue complexed Cu2+(aq) is reduced to brick (an alkaline solution of a complexed red Cu2O(s)
copper ion) (this is the basis of the test for reducing sugars)
RCHO(aq) + 2Cu2+
(aq) + 4OH-(aq) RCOOH(aq) + Cu2O(s) + 2H2O(l)
TOLLENS REAGENT
warm with Tollens reagent complex ion [Ag(NH3)2]+(aq) is reduced to Ag(s), (aqueous silver nitrate in xs ammonia) hence the silver mirror effect
RCHO(aq) + 2[Ag(NH3)2]+(aq) + 2OH-(l) RCOOH(aq) + 2Ag(s) + 4NH3(aq) + H2O
strictly speaking you will get the carboxylate anion RCOO-(aq) under alkaline conditions rather than the carboxylic acid itself
additionally its worth knowing that methanoic acid (which has a hydrogen present HCOOH) can be oxidised to carbon dioxide via carbonic acid H2CO3 which then easily breaks down
into CO2 and H2O (see if you can work out the equations)
you should be aware of changes in the IR spectra during the oxidation reactions of compounds containing one or more oxygen atoms (and also the reduction reactions of said
molecules)
Summary Questions Page 67 1 - 4
A2 Chemistry (Nelson Thornes) AQA 66 67, 69
A2 Chemistry (Heinemann) AQA 65 67 Q 1 on page 66
Chemguide Ionic alkaline, Tollens, Fehlings
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Nucleophilic Addition
Reduction of Aldehydes and Ketones
polar nature of the carbonyl group - you would be wise to revise electro negativity and the nature of nucleophiles
aldehyde + 2[H] primary alcohol sodium tetrahydridoborate(III) (NaBH4) in water (+ ethanol as a universal solvent
ketone + 2[H] secondary alcohol for higher (longer chain) members)
BH4- provides the hydride ion, H- which acts as a nucleophile
mechanism is required (nucleophilic attack by hydride ion)
intermediate ion then gains H+ from the water present in the aqueous solvent just show H+ in the mechanism itself
Reaction with Hydrogen Cyanide
ethanal + HCN 2-hydroxypropanenitrile
HCN is made in situ using acidified sodium or potassium cyanide
HCN is a toxic gas so there are health and safety issues in its use
cyanide ions are toxic so this will not be done in the lab
note: an extra carbon is introduced into the chain so this is an important synthesis step
mechanism for reaction = nucleophilic addition
trigonal planar carbonyl group can be attacked from either side
hence products from aldehydes other than methanal exhibit optical isomerism i.e. there is an asymmetric (i.e. chiral) carbon with four different groups attached hence two non-
super imposable molecules (enantiomers) exist which are mirror images of one another.
a racemic mix is produced i.e. one that is 50:50 of both isomers and this will therefore NOT rotate plane polarised light in this case as the two enantiomers cancel one another out
symmetrical ketones do not yield enantiomers as no chiral centre is present
hydroxynitriles can be converted to a carboxylic acid by undergoing acid hydrolysis
this involves refluxing with a dilute acid (below I have shown the organic as named for clarity - although you MUST give its formula - as the other particles will always be the same
irrespective of the number of carbons in the chain)
the optical properties are preserved in the carboxylic acid
2-hydroxypropanenitrile + 2H2O + H+ 2-hydroxypropanoic acid + NH4+(aq) (lactic acid)
NA
NA
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lactic acid synthesised this way will exist as racemates BUT that produced biologically will not be since enzymes are stereo specific yielding only one optical isomer hence this DOES
rotate plane polarised light.
Summary Questions Page 63 1 3 Page 70 1 - 4
Exam Style Questions Page 83 2
A2 Chemistry (Nelson Thornes) AQA 60 63, 68, 70
A2 Chemistry (Heinemann) AQA 66 70, 57 62 Q 2 3 on pages 68 Q 4 -6 on page 70
Q 1, 3, 5 on page 82
Q 4 on page 149
Chemguide Aldehydes, Isomerism
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Carboxylic Acids and Esters
Nomenclature
be aware of different ways to write the acid functional groups RCOOH, RCO2H etc
benzenecarboxylic acid C6H5CO2H
general structural formula of esters: RCO2R` e.g. CH3CO2CH2 CH2CH3
you should be able to work out the acid/alcohol used to make an ester and visa versa
esters and carboxylic acids are functional group isomers (easily distinguished by IR or nmr see later - or a simple chemical test using sodium carbonate and testing for evolved CO2)
Summary Questions Page 73 1 4
Physical properties
lower members of the carboxylic acids and esters are miscible with water due to hydrogen bonding with water
higher members are less miscible with water as the extent of VdW with themselves becomes prevalent (they are more soluble in sodium hydroxide solution do you know why?)
most carboxylic acids are crystalline solids (hydrogen bonding) whilst esters are typically oils and fats (no hydrogen bonding) compared to similar sized hydrocarbons
melting points can be used to identify and determine the purity (to some extent) of organic solids
A2 Chemistry (Nelson Thornes) AQA 71 - 73
A2 Chemistry (Heinemann) AQA 71 75 Q 7 10 on pages 71 - 74
Chemguide Carboxylic acids, Esters
Synthesis of carboxylic acids
primary alcohol or aldehyde carboxylic acid reflux under heat with xs K2Cr2O7(aq) and H2SO4(aq)
orange dichromate(VI) ions (Cr2O72-aq)) are reduced to green chromium(III) ions (Cr3+(aq))
A word about REAGENTS: When the examiner asks for a reagent then it is the name on the bottle
NOT the active particle introduced e.g. H+(aq) is not a reagent but H2SO4(aq) is.
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Reactions of Carboxylic Acids and Esters
carboxylic acids are weak acids understanding required in terms of equilibria
chlorine-substituted ethanoic acids are more acidic due to negative inductive (I) effect of chlorine atoms due to their relatively high electronegativity
ethanoic acid + sodium hydroxide sodium ethanoate + water
ACID + BASE SALT + WATER
ethanoic acid + sodium carbonate sodium ethanoate + water + carbon dioxide
the latter is a useful test for the presence of COOH for which you will OBSERVE effervescence and that the gas evolved turns limewater (Ca(OH)2(aq)) cloudy and thus
INFER that CO2(g) was produced suggesting a carboxylic acid.
Ca(OH)2(aq) + CO2(g) CaCO3(s) + H2O(l)
acid catalysed esterification with an alcohol is relatively slow and gives a poor yield
ethanol + ethanoic acid ethyl ethanoate + water reflux under heat with cH2SO4
catalyst
acid + alcohol ester + water
Summary Questions Page 77 1 4
Hydrolysis of Esters
ethyl ethanoate + water ethanoic acid + ethanol sulphuric acid catalyst
initiated by nucleophilic attack by the water molecule on the C+ of the carbonyl group
acid catalysed hydrolysis not complete due to an equilibrium being established
alkali (hot NaOH) catalysed (saponification) hydrolysis is quicker and goes to completion
sodium salt of the carboxylic acid is produced since the acid produced reacts with the sodium
hydroxide which will drive the equilibrium RHS as acid is removed from the system
adding xs sulphuric acid protonates the alkanoate anion carboxylic acid
A2 Chemistry (Nelson Thornes) AQA 74 - 76
A2 Chemistry (Heinemann) AQA 71 74 Q 11 on page 74
Chemguide Carboxylic acids, esters hydrolysis
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Uses of Esters
solvents e.g. ethyl ethanoate for nail varnish
they evaporate relatively easily since there is no hydrogen bonding
esters have pleasant smells (unlike carboxylic acids which typically have unpleasant smells rancid fats ask to smell some butanoic acid youll get the idea) so are used in perfumes and in the food industry as flavourings:
butyl butanoate ethyl pentanoate 3-methylbutyl ethanoate
The cost of synthesising esters is often far less expensive than extracting them from natural sources
2-methoxyphenol is a waste product from the paper industry and can be used to make methyl vanillin (4-hydroxy-3-methoxybenzaldehyde) an artificial vanilla by the end of module 4 you might be able to draw its structure and suggest a possible synthesis strategy (although the
industrial process is more complex)
plasticizers added to plastics (e.g. PVC) to make them softer and more flexible as they weaken the IMF between polymer strands allowing them to slide over each other more
readily (loss over time makes the plastic brittle)
some phthalate based plasticizers have a possible association with birth defects although this is still a subject of some disagreement
Fats and Oils
animal fats + vegetable oils are triesters of propane-1,2,3-triol (glycerol) and fatty acids (long chained carboxylic acids)
they are triglycerides - three alcohol groups esterified by up to three different carboxylic acids (take care with the hydrolysis equation regarding the actual products e.g. under alkaline
conditions the anion of each acid group will be formed and each of these could be different)
oils have a higher degree of unsaturation cf fats (research E-Z isomerism and trans fats)
a greater number of double bonds reduces the relative flexibility (due to restricted rotation) of the molecule which in turn reduces the overlap efficiency of intermolecular forces
if a triglyceride undergoes alkaline hydrolysis (NaOH(aq))(saponification) then the salt of a fatty acid is produced e.g. sodium octadecanoate which is also known as sodium stearate and
better known as a soap
being ionic, soaps are soluble
the carboxylate anion RCOO- released is miscible with water (due to the hydrophilic carboxylate
group) and miscible with grease (due to its
hydrophobic tail) hence can solvate grease into
water allowing its removal
soap is precipitated out of solution by adding xs common salt (salting out) which can be understood from an equilibria point of view
How Science
Works: I
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Glycerol (glycerine)
the co-product of soap making, propane-1,2,3-triol (glycerol) has numerous uses:
it is a useful solvent e.g. in medicines, food colouring
its extensive hydrogen bonding makes it good at retaining water thus preventing drying out
(e.g. facial creams)
and can be used to make nitro-glycerine (which mixed with finely divided silicon(iv) oxide =
dynamite)
Biodiesel
a renewable fuel made from oils obtained from vegetable matter (e.g. rape seed) which generally consist of a combination of any three of five common carbon chains linked by a
glycerol structure
it can also be made from animal fat and waste oil
small industrial plants tend to be batch processes but larger plants can use continuous flow methods which are more economical
methyl esters are produced by reacting these oils with methanol and a strong alkali at around 60oC
this is called base-catalysed transesterification
the methyl groups replace the glycerol structure on each of the fatty acids
Note: there may be three different methyl esters produced but the general formula of each is:
CH3OOCCxHy where x and y depend on chain length and degree of unsaturation
(this can be written the other way around CxHyCOOCH3)
the ester produced does not readily mix with the propane-1,2,3-triol co product so can be separated using a separating tank or a centrifuge
any remaining glycerol can be extracted using water (hydrogen bonding)
there may be some soap bi-product so further processing will be necessary to achieve a level of purity acceptable for a biofuel
rape seed (the yellow stuff you see in fields) produces rape methyl ester (RME) which is very similar to the diesel obtained from crude oil
this can be used directly or as a small % of filling station diesel
Exam Style Questions Page 83 4
A2 Chemistry (Nelson Thornes) AQA 76 - 77
A2 Chemistry (Heinemann) AQA 75
Chemguide Fats and oils, soap, glycerol
How Science
Works: K
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Acylation using Acyl Chlorides or Acid Anhydrides
nomenclature named after the parent carboxylic acid hence deemed acid derivatives (as are esters and amides)
Acyl chlorides: -oic acid replaced with: -oyl chloride
X = Cl e.g. ethanoyl chloride = an acyl chloride
Acid anhydrides e.g. ethanoic anhydride
X = OR
both are readily attacked by nucleophiles (book error on 79) due to very polar carbonyl group the polarity of which is increased by the electron withdrawing effect of X
thus this group of compounds are more useful than carboxylic acids in synthesis due to their high reactivity due to the enhanced + of the carbonyl carbon and since -X is a good leaving group cf OH
used to join an acyl group to the oxygen of water, alcohol or phenol or the nitrogen of ammonia or an amine
(ethanoylation is specifically when R = CH3)
H+ is eliminated in each case in the final step
Hydrolysis
ethanoyl chloride + water ethanoic acid + hydrogen chloride
very exothermic reaction, steamy fumes of hydrogen chloride are produced even when exposed to air (due to the reaction between HCl and water vapour), hence anhydrous
conditions essential with acyl chlorides and must be stated in exams
reaction is faster than with haloalkanes due to additional polarising effect of C=O
hydrogen chloride fumes can be tested for using:
conc. ammonia a drop at the end of a glass rod will create a white
smoke of ammonium chloride
silver nitrate solution a drop at the end of a glass rod will go cloudy as white
silver chloride is precipitated
mechanism is nucleophilic addition-elimination (condensation)
NOTE: H is not abstracted by Cl- (think why HCl is a strong acid!)
Ethanoic anhydride + water ethanoic acid + ethanoic acid
NAE
O
R C
:Nu
O
R C
X
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Reaction with Alcohols and Phenols
ethanoyl chloride + ethanol ethyl ethanoate + HCl
yield of ester better than with carboxylic acid since reaction goes to completion
NOTE: acyl chlorides form esters with the phenol group unlike carboxylic acids. The
lone pair of the O in phenol is less readily available since these electrons are
delocalised into the ring system hence reducing the electron density and the thus
the effectiveness of phenol as a nucleophile (higher activation energy)
mechanism for reaction with an alcohol - ester formation is very similar to that with water
treat the alcohol as RO-H cf water as HO-H so RO- is added to the carbon rather than HO-
NOTE: H is not abstracted by Cl-
ethanoic anhydride + ethanol ethyl ethanoate + ethanoic acid
Reactions with Ammonia and Amines
ethanoyl chloride + ammonia ethanamide (a primary amide) + hydrogen chloride
violent reaction with aqueous ammonia at room temperature
ethanamide is the only product as further substitution does not occur due to strong electron withdrawing effect of C=O which makes the lone pair of the nitrogen less readily available
than with amines (cf haloalkanes + ammonia)
ethanoyl chloride + ethylamine N-ethylethanamide + hydrogen chloride (primary amine) (secondary amide)
ethanoyl chloride + phenylamine N-phenylethanamide + hydrogen chloride
(i) the product (an acyl derivative) is a white crystalline solid with a sharp melting point - can be recrystallised and used in the identification of the original amine
(ii) suggest reagents and mechanism for the synthesis of paracetamol N-(4-hydroxyphenyl)ethanamide
ethanoic anhydride + ammonia ethanamide + ethanoic acid
Summary Questions Page 82 1 4 Exam Style Questions Page 83 1, 3
A2 Chemistry (Nelson Thornes) AQA 78 - 81
A2 Chemistry (Heinemann) AQA 76 83 Q 12 - 15 on pages 77 81 Q 1 8 pages 82 - 83
Chemguide Acyl
NAE
NAE
NAE
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The Synthesis of Aspirin
(aspirin is 2-ethanoyloxybenzoic acid dont panic you wont be asked for this on the exam)
the benefits of willow bark, which contains salicylic acid (2-hydroxybenzoic acid) a similar compound to aspirin, have been known for millennia e.g. Hippocrates (~460 B.C - 377 B.C.),
African Hottentots and North American Indians
it acts as an analgesic (pain killer) and has an anti-pyretic effect (body temperature)
salicylic acid was first isolated around 1829
the next step was to find a way to synthesise it rather than rely on extraction from a natural source as this can be problematic:
the source might be rare, or seasonal, or have a low concentration, or have harmful
contaminants
in 1860 it was synthesised from phenol (a by product of the production of town gas from coal) using the Kolbe process (NaOH and high pressure CO2)
but the problem was that it was tough on stomachs so alternatives with a similar structure (hence retaining the benefits) were searched for
aspirin itself had been synthetically produced in 1853 by a French chemist named Charles Frederic Gerhardt but he didnt realise its potential and took it no further
in 1898, a German chemist named Felix Hoffmann rediscovered Gerhardt's formula
he gave it to his father who was suffering from the pain of arthritis and with good results (he had tried other formulations before that!!) so convinced the German pharmaceutical company
Bayer to patent it in 1900 (the patent was ignored by the allies during WW1 and thereafter
along with the patent they held for heroin!)
its sales increased dramatically during the Spanish Flu epidemic of 1918
aspirin can be synthesised from salicylic acid using ethanoyl chloride or ethanoic anhydride
ethanoyl chloride + 2-hydroxybenzoic acid aspirin + HCl (salicylic acid)
Ethanoic anhydride + 2-hydroxybenzoic acid aspirin + ethanoic acid (salicylic acid)
both are more readily attacked by nucleophiles than the corresponding acid
acid anhydrides offer certain advantages over acyl chlorides, despite being less reactive, in that they are:
cheaper, less corrosive (no HCl liberated), less readily hydrolysed
How science works Page 82 Aspirin
Exam Style Questions Page 83 1
A2 Chemistry (Nelson Thornes) AQA 82
A2 Chemistry (Heinemann) AQA 81
Q 14 on page 81
Chemguide Aspirin
How Science
Works: I, J
NAE
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Aromatic Chemistry involves compounds containing a benzene ring (aka arenes look out for C6H5-)
empirical formula CH, Mr =78, molecular formula C6H6 Structure and Stability of Benzene
hydrogenation with 3 moles of H2 suggests the equivalence of 3 carbon-carbon double bonds
How science works Page 85 Kekules dream
Kekule did propose a cyclic structure but it could not account for some major aspects of the chemistry of Benzene:
1. no electrophilic addition reactions (e.g. with Br2(aq) in the dark) unlike alkenes 2. you dont get two isomeric (1,2) disubstituted compounds 3. X-ray diffraction studies found intermediate between double and single and equal C-C
bond length i.e. a symmetrical structure
4. enthalpy of hydrogenation (208 kjmol-1) is less than 3 x cyclohexene (360 kjmol-1)
initially a resonance hybrid structure was suggested
the true structure and shape of benzene can be explained in terms of
the delocalisation of the electrons of 6 x 2p orbitals (whilst its not on the syllabus, knowledge of orbital
hybridisation would be helpful have a look on Chemguide or in an older A-
level book)
delocalised electrons increase
relative stability (less electron - electron repulsion)
Summary Questions Page 86 1 3 Exam Style Questions Page 93 6
A2 Chemistry (Nelson Thornes) AQA 84 - 86
A2 Chemistry (Heinemann) AQA 84 - 87
Q 1 2 on page 87
Chemguide Bonding benzene
How Science
Works: A
Why is it drawn like this?
How Science
Works: F
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Physical properties
non-polar colourless liquid and does not mix with water (no hydrogen bonding)
boiling point similar to 6 carbon aliphatic hydrocarbons but melting point is higher a planar structure allows better packing therefore more effective VdW.
Nomenclature
C6H5- phenyl group and simple monosubstituted aromatic compounds (arenes)
normally named as derivatives of benzene so benzene often forms the root of the name mono-substituted arenes are generally of formula C6H5X e.g. benzaldehyde C6H5CHO and
yield a peak of 77 on mass spectra due to the fragment C6H5+ (see later)
methylbenzene (toluene) chlorobenzene nitrobenzene benzaldehyde
benzenecarboxylic acid (benzoic acid) ethylbenzene (chloromethyl)benzene
some names also use phenyl or variations of it when the benzene is regarded as a side chain
phenol (instead of hydroxybenzene) phenylamine (aniline) (instead of
aminobenzene)
4-hydroxyphenyl ethanoate phenylethene (instead of ethenylbenzene)
aromatic compounds with more than one substituent
1 lowest numbering possible 2 alphabetical order for substituents (ignore di, tri etc)
2-hydroxybenzoic acid 2,4,6-trinitrotoluene 2,4,6-trinitrophenol (picric acid)
benzene-1,4-dicarboxylic acid (Terephthalic acid)
take heart naming aromatic compounds is complex but you will only have to deal with simple examples as it is far more important that you understand the chemistry!
Summary Questions Page 88 1 4
A2 Chemistry (Nelson Thornes) AQA 87 - 88
Chemguide Naming aromatic
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Dont use H2 for cHCL/Sn
Reactions of aromatic compounds
Electrophilic Substitution
high electron density in the ring attracts electrophiles
Br2(aq) is not decolourised (in the dark) electrophile must be powerful (+ve not just +)
why benzene resists attack by poor electrophiles stability of benzene compared to alkenes
electrophilic substitution rather than addition (c.f. alkenes) since this retains the relatively stable benzene ring structure hence is energetically more favourable
Nitration of Benzene and Methylbenzene
benzene nitrobenzene (a yellow oil) cH2SO4/cHNO3 refluxed at 50oC (nitrating mixture)
mechanism of formation of NO2+ the nitronium (old name = nitryl) cation (sulphuric acid acts as a homogeneous catalyst)
methylbenzene 2(and 4)-nitromethylbenzene cH2SO4/cHNO3 refluxed at 50oC
the methyl group is 2, 4, and 6 directing and activates the ring towards electrophilic substitution (hence faster rate) since it donates electron density into the ring thus making it
relatively less stable and more susceptible to attack by electrophiles.
further substitution requires more vigorous conditions (higher acid conc. and temperature) as the nitro group deactivates the ring towards electrophilic substitution by withdrawing electron
density from the ring (i.e. increasing the extent of delocalisation thus further stabilising the
ring) TNT is an explosive
nitrobenzene (a yellow oil) phenylamine cHCl/Sn or H2/Ni
C6H5NO2 + 6[H] C6H5NH2 + 2H2O
the above reaction is much simplified (whilst this equation is acceptable do you see why the initial product would not be phenylamine? a possible A* question perhaps?)
phenylamine is important for the production of AZO dyes since the end of 19th century (these replaced the old technique of mordant dying why not read about this it is interesting)
How Science Works Page 90 TNT
Exam Style Questions Page 92 1, 3
A2 Chemistry (Nelson Thornes) AQA 89 - 90
A2 Chemistry (Heinemann) AQA 88 - 89
Q 3, 4 on page 89
Chemguide Electrophilic substitution, nitration
ES
ES
ES
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Friedel-Crafts Acylation
role of the AlCl3 - halogen carrier why is it a lewis acid
anhydrous conditions - AlCl3 readily hydrolysed by water before it does its job
electrophilic substitution mechanism proceeds via an acylium ion intermediate
you should be able to write a full balanced equation for the above
benzene + ethanoyl chloride phenylethanone warm with AlCl3 catalyst anhydrous conditions
Summary Questions Page 91 1 - 4
Exam Style Questions Pages 92 93 2 - 5
A2 Chemistry (Nelson Thornes) AQA 91
A2 Chemistry (Heinemann) AQA 91, 184
Q 7, 8 on page 91
Q 1 8 on pages 94 - 95
Chemguide Acylation
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Amines
nomenclature of primary, secondary and tertiary amines (by the way amine has ONE m in it!!!)
note how this differs from alcohols and haloalkanes
amino can sometimes be used (see amino acids later on)
Physical properties
boiling points are elevated by the ability to hydrogen bond but are lower than similar sized alcohols due to the relative electronegativity of O and N compared (ASK if you dont understand the significance of this)
lower members are gases
liquid amines smell like rotting (fishy) flesh adding acid removes this smell WHY?
smaller primary amines are water soluble (hydrogen bonding) producing alkaline solutions
solubility decreases with chain length (as with alcohols) due to increased mutual VdW
phenylamine is not very soluble in water as the VdW between the rings is significant compared to hydrogen bonding between the amine group and water
produce alkaline solutions in water when they dissolve
Summary Questions Page 95 1 - 4
Basic properties
lone pair on N can be a nucleophile, base or ligand depending on the context
how good it is depends on the availability of that lone pair i.e. what the N is bonded to
Brnsted-Lowry bases proton acceptors, Lewis base = lone pair donor
ethylamine is more basic than ammonia due to +I inductive effect of the alkyl group (its also a better nucleophile than ammonia - see alkyl halides)
this explains the relatively more basic nature of small secondary amines but this does not hold true for tertiary amines where reduced solubility is a factor
phenylamine is less basic than ammonia since the lone pair on the nitrogen is less available due to delocalisation in the ring structure diagram
(phenylmethyl)amine is as basic as primary amines as the N is NOT bonded directly to the ring (note brackets in name why ?)
acid amides are relatively poor bases/nucleophiles/ligands as the strongly electronegative oxygen causes the lone pair on the nitrogen to be more withdrawn
pKa value of conjugate acid increases (i.e. is poorer) with increased basicity of the conjugate amine
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amines react with acids to form salts in a similar manner to ammonia
ethylamine + hydrochloric acid ethylammonium chloride + water
solvation of insoluble phenylamine achieved by the addition of HCl to form a soluble salt phenylammonium chloride (reversed by adding NaOH)
phenylamine + hydrochloric acid phenylammonium chloride + water
Summary Questions Page 97 1 - 3
Exam Style Questions Pages 108 9 1
A2 Chemistry (Nelson Thornes) AQA 94 - 97
A2 Chemistry (Heinemann) AQA 96 97, 99 - 100 Q 1, 2 on pages 96 97 Q 5, 6 on pages 99 - 100
Q 4 on page 104
Chemguide Amine name, amine base
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Preparation
From haloalkanes
bromoethane ethylamine alcoholic solution of NH3 under pressure (lots of by-products so not a good method)
mechanism NH3 acts as nucleophile
excess ammonia is used to improve the yield of the primary amine
if xs bromoethane is used, since the ethylamine produced is also a nucleophile (stronger than ammonia due to the +I inductive effect of the alkyl group) it can react with the xs
bromoethane to give diethylamine
further substitution can then occur to produce: triethylamine and tetraethylammonium bromide (a quaternary ammonium salt cf ammonium ions)
note that acyl chlorides only yield primary amide (mechanism reminder) lone pair withdrawn by strong d+ on C due to the polarity of C=O caused by the electronegativity
of the O
From nitriles
Ethanenitrile ethylamine reduction by H2/Ni
RCN + 4[H] RCH2NH2
(note: at AS you were also told that acid hydrolysis of nitrile yields a carboxylic acid)
LiAlH4 but not NaBH4 (not a powerful enough reducing agent) can also be used (dont put H2 in
balanced equation in this case!!)
Aromatic Amines
benzene nitrobenzene (a yellow oil) cH2SO4/cHNO3 refluxed at 50oC (nitrating mixture)
nitrobenzene (a yellow oil) phenylamine reduction cHCl/Sn (or H2/Ni)
phenylammonium chloride* is initially produced, and addition of NaOH yields phenylamine (via deprotonation) which is then obtained by steam distillation
* strictly speaking its actually phenylammonium hexachlorostannate(IV)
NS
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Uses of Amines
quaternary ammonium salts are used as cationic surfactants (long carbon chains help) in fabric
conditioners and hair conditioning products
the positive charges present will repel and add body to the hair/fabric whilst the tail section associates with the fabric
if you are particularly interested in laundry see: http://www.scienceinthebox.com/en_UK/glossary/
surfactants_en.html
http://www.chemistryquestion.com/English/Questions/ChemistryInDailyLife/27c_nonionic_s
urfactant.html
aryl amines are used in synthetic dyes
aryl amines are used to make certain drugs e.g. paracetamol
amines are used to make polymers e.g. polyurethanes (cavity wall insulation) and polyamides (nylons)
How Science Works Pages 100 - 1 Sulfa drugs
Summary Questions Page 101 1, 2
Exam Style Questions Pages 108 2 - 4
A2 Chemistry (Nelson Thornes) AQA 99 - 101
A2 Chemistry (Heinemann) AQA 97 98, 101 - 105 Q 7 on page 103
Q 13, 5-8 on pages 104 - 105
Chemguide Amine preparation, amine uses
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C
N H
C OOH
H
CH
2
CH 2
CH 2
Amino acids
there are 20 important naturally occurring amino acids (amine is on the C next to the acid
group CO2H)
you should recognise that there is a CHIRAL centre hence amino acids exhibit optical isomerism (name the exception)
some amino acids have been identified in space (those of you who are interested in science might read the next link)
http://www.newscientist.com/article/dn7895-space-radiation-may-select-amino-acids-for-life.html
the amino acid proline is a 2o amine all the others are primary amines
Extra Info for potential medical/biochemistry/pharmacy students:
amino acids in proteins are all L-isomers
this does not necessarily mean plane polarised light is rotated the same way
if L(+) then D(-) for a given amino acid and L(-)/D(+) dont exist in our natural system
CORN law I have provided some info about this on my site it is not on the exam but it may be helpful at your interview for medical school given the biochemical significance
Summary Questions Page 103 1, 2
Page 107 1
R can vary
R = H glycine
R = CH3 alanine
R = CO(OH)CH2
aspartic acid
etc
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Zwitterions
the amino group and the carboxyl group of each amino acid are both ionisable
the acidic carboxyl group (with a pKa of about 3 its a weak acid) is deprotonated
the basic amino group (with a pKa of around 9 its a weak base) is protonated
amino acids thus exist as zwitterions with both a positive and negative charge present
Zwitterions are amphoteric i.e. they exhibit both acidic and basic properties in solution because of the two functional groups
they thus form salts with both acids and bases (note all similar groups ionised as appropriate)
amino acids can thus act as buffers (hence regulate pH)
charge on the zwitterion ion depends on pH, -ve at high pH and +ve at low pH
at a given pH, the isoelectric point of the amino acid there will be no overall charge
as the + and cancel
this pH varies from amino acid to amino acid and provides a means of separating
them using a technique called
electrophoresis.
Extra Info for potential medical/biochemistry/pharmacy students:
you might be interested to find out how electrophoresis works by looking at this link
http://www.saburchill.com/IBbiology/chapters01/003.html
Amino acids exist as zwitterions in the solid state and thus have strongly ionic character
this explains their high solubility in polar solvents e.g. water
it also explains the high MPt (white crystalline solid when pure)
MPt too high to be accounted for by hydrogen bonding alone - supporting existence of Zwitterions and ionic nature of amino acids in the solid state
Exam Style Questions Pages 108 5, 8, 11
Adding OH-
pH increasing
Adding H+
pH decreasing
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A2 Unit 4 Kinetics, Equilibria and Organic Chemistry
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Condensation Polymerisation and hydrolysis
peptide (amide) links are formed between 2 amino acids by condensation reactions (where a small molecule such as water is eliminated) to form a dipeptide
Polypeptide (~50 amino acids) are formed by condensation polymerisation and catalysed by
enzymes
sequence of amino acids in a protein is called its primary protein structure
hydrogen bonding between C=O and N-H
controls shape - secondary protein structure - -
helix (coiled) and -pleated sheet (folded)
tertiary structure involves further bending and twisting (a good analogy is a knotted,
multicoloured telephone coil)
the stretching of wool is dependent on hydrogen bonding the length of which can be
reversibly increased up to a limit (very hot water
can break these ruining the fluffyness)
hydrolysis of proteins is achieved by refluxing with acid, base or enzyme catalyst (cf hydrolysis of an amide) in effect reversing the process shown in the diagram above
the liberated amino acids can then be separated by paper chromatography (developed by treating with ninhydrin which colours amino acids violet
some enzymes are selective and only partially hydrolyse certain proteins enabling amino acid sequences to be identified
enzymes are themselves proteins and are very specific in what they catalyse (substrate) due to their shape
their shape is dependent on hydrogen bonding hence their activity is sensitive to elevated temperatures where they are denatured
How Science Works Pages 107 Robots in the lab
Summary Questions Page 107 1, 2
Exam Style Questions Pages 109 6, 8 - 10
A2 Chemistry (Nelson Thornes) AQA 102 - 107
A2 Chemistry (Heinemann) AQA 111 114, 57 62 Q 8 - 12 on pages 111 113 Q 2 4, 5 - 6 on pages 115 - 116
Chemguide Zwitterions, peptide
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Polymerisation
Addition Polymerisation
addition across the carbon-carbon double bond of a single unsaturated monomer
occurs by a free radical mechanism (details not required) started by an initiator (e.g. a peroxide) which is incorporated at the start/end of the polymer chain
you should be able to work out monomer from polymer and visa-versa (tip use the >C=C< form of the monomer i.e. rewrite it)
if asked for a single repeating unit dont show brackets otherwise use [C-C]-n format
ethene poly(ethene)
propene poly(propene)
chloroethane poly(chloroethene) (PVC)
phenylethene poly(phenylethene) (polystyrene)
tetrafluoroethene poly(tetrafluoroethene) (PTFE or Teflon)
methyl 2-methylpropenoate perspex
Summary Questions Page 114 1-4
Exam Style Questions Pages 122 4, 5
A2 Chemistry (Nelson Thornes) AQA 112 - 114
A2 Chemistry (Heinemann) AQA 106 -108
Q 1-3 on page 106
Q 7, 8 on page 116
Chemguide Addition polymerisation
heat, pressure, catalyst
in all cases
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Condensation Polymerisation
Polyesters
two different types of monomers, one a diol, the other a dicarboxylic acid
this polymer is connected by ester linkages
you should be able to draw repeating units and determine the monomers used to make a given
polymer colour coding diagrams helps here
condition usually involve heat + catalyst
look for CO2- in structural formula
balanced equation (dont forget 2n H2O !!!!)
ethane-1,2-diol + benzene-1,4-dicarboxylic acid PET (ethylene glycol) (terephthalic acid)
the chain repeat unit of PET is:
commonly just called polyester PET, poly(ethylene terephthalate) was initially used as a
fibre (e.g. Terylene and Dacron)
it is now used extensively in plastic containers e.g. for fizzy drink bottles it does not smash on impact
you can read about the invention of polyester at:
http://inventors.about.com/library/inventors/blpolyester.htm
those of you doing textiles might also like to visit the polyester story at
http://schwartz.eng.auburn.edu/polyester/polyester.home.html
poly(2-hydroxypropanoic acid) used in surgery as its broken down by enzymes/body fluids over a number of days (why is this good?)
how would you synthesise it from ethene?
you can find out more about plastics from renewable raw materials and biologically degradable plastics at this site (its also a good example of (or idea for) an EPQ project!):
http://www.rsc.org/education/teachers/learnnet/green/docs/plastics.doc
You will find it useful to look at the reaction
pathways template that I have put on line
How Science
Works: I
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Polyamides
two different types of monomers, one a diamine, the other a dicarboxylic acid
this polymer is connected by amide linkages
an diacyl chloride can be used instead of the acid but what would be the pros and cons of this
nylon-6,6 is so called as both monomers have 6 carbons
nylon is a polyamide (just like proteins)
1,6-diaminohexane + hexanedioic acid nylon-6,6
you might be interested in the history of nylon its an ideal stocking filler for Xmas see: http://www.cha4mot.com/p_jc_dph.html and http://inventors.about.com/od/nstartinventions/a/nylon.htm
polyamides have extensive hydrogen bonding between
parallel strands (cf protein
structure)
when nylon is spun into fibres, amide groups on
adjacent chains form
hydrogen bonds making
nylon yarn strong.
kevlar is an example of an aromatic polyamide and is made from the monomers benzene-1,4-dicarboxylic acid and benzene-1,4-diamine
the molecule is flat because of the aromatic groups
the uses of Kevlar are related to its strength (its several times stronger than steel)
this is related to the packing together of sheets of molecules held together by hydrogen bonds formed between N H groups and C = O groups on adjacent molecules
here is a scuba diving site that has an excellent overview of natural and synthetic polymers have a look it is very good: http://njscuba.net/artifacts/matl_polymers.html
How Science
Works: I
Medieval chainmail similarity?
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Environmental Issues
polyalkenes are saturated, have no polar bonds and have strong C-C and C-H bonds hence they are relatively unreactive e.g. with acids, alkali or oxidants
this in turn makes them difficult to dispose of
non biodegradability means they last a long time but you can use landfill not ideal though
combustion yields toxic and greenhouse gases e.g CO and carbon particulates, NO2 and HCN from polyurethane in older upholstery or HCl and dioxins released through combustion
of halogenated plastics such as PVC and of course there will always be CO2 produced
recycling is expensive as the plastics must be identified and separated from other waste
energy production is an option but as this involves combustion there will be CO2 produced
using as chemical feedstock after cracking
use biodegradable/photodegradable polymers instead is an option
polyesters and polyamides both have polar bonds hence they are potentially biodegradable
they are broken down by (catalytic) hydrolysis in acid/alkaline solution or with enzymes although this can take a long time
recycling these materials e.g. terylene will save on natural resources and energy in their initial production (as well as reducing the need for landfill), however, as they are biodegradable
their structural integrity will diminish after repetitive usage
as with polyalkanes the cost of collection, separation and transportation (require energy and are labour intensive) must be taken into account
How Science Works Pages 120 - 1 Hermann Staudinger + Q 1,2
Summary Questions Page 119 1, 2
Page 121 1-4
Exam Style Questions Pages 121 - 123 1, 2, 3, 6, 7
A2 Chemistry (Nelson Thornes) AQA 115 - 120
A2 Chemistry (Heinemann) AQA 108 -110
Q 4 - 8 on page 109 - 110
Q 1, 5 on pages 115 - 116
Chemguide Condensation polymerisation
How Science
Works: J
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Aliphatic synthesis You should be able to write full chemical equations, and identify the type for all the reactions listed:
Oxidation, Reduction, Addition, Elimination, Addition-Elimination (Condensation),
Substitution, Hydration, Dehydration, Hydrogenation, Dehydrogenation, Hydrolysis
Mechanisms: Nucleophilic Addition NA, Nucleophilic Substitution NS, Electrophilic Addition EA,
Nucleophilic Addition-Elimination NAE, Free Radical Substitution FRS or Elimination E.
Alkanes haloalkane, alkene
methane + Cl2 chloromethane FRS
alkanes can be used to produce alkenes by thermal cracking
Alkenes haloalkane, alkane, alcohol, alkoxyalkane
ethene + HBr bromoethane EA
ethene + Br2 1,2-dibromoethane EA
ethene + H2 ethane Ni catalyst, ~200oC (catalytic hydrogenation)
ethene + H2O ethanol EA
Method 1 cH3PO4 on a silica support with high temperature and pressure,
Method 2 first react with cold cH2SO4, then warm with water
Alkyl Halides amine, alcohol, nitrile, alkene
bromoethane + NH3 ethylamine + HBr NS heat with xs ammonia under pressure to minimise further substitution
bromoethane + OH-(aq) ethanol + Br- NS reflux with dilute NaOH dissolved in water
bromoethane + CN- propanenitrile + Br- NS boil under reflux with alcoholic NaCN or KCN (NOT HCN)
propanenitrile + 4[H] propylamine reduction by H2/Ni
bromoethane + OH- ethene + H2O + Br- E reflux with NaOH dissolved in ethanol
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Alcohols carbonyl (aldehyde and ketone), carboxylic acid, ester, haloalkane, alkene
ethanol + [O] ethanal + H2O mild conditions - K2Cr2O7(aq)/H2SO4(aq) dist