Reaction rates, Equilibrium, Acids/Bases, Redox Reactions

Measure of disorder or randomness in a system

Natural tendency for system to increase entropy (more random)

EXAMPLE – Diffusion◦As molecules are dispersed, entropy increases

◦Continued dispersal leads to a uniform solution

Remember, things tend towards an increase in entropy

Spontaneous reaction favors the products (exothermic) and releases free energy

C + O2 CO2

◦Exothermic◦Solid gas increases entropy

Gibbs free energy – max amt of E that can be used in another process

Entropy never decreases in a system and instead will increase over time

UNLESS you change the surroundings◦Spraying air freshener◦Spray it into a collapsible box

Study of reaction rates (rate at which a chemical reaction takes place)

Measured by:◦Rate of formation of products◦Rate of disappearance of reactants

◦Changes in concentration of reactants or products

ConcentrationPressureTemperatureSurface AreaAll of the above have a DIRECT relationship

When reactants collideNormally, molecules bounce off each other b/c of electron clouds repulsion

BUT, if those molecules have a LARGE amount of energy, they can overcome the repulsion and react

Molecules also must collide in the right orientation

Energy required to start a chemical reaction◦A nudge, a spark◦Potential E

Activated complex – “speed bump” of the reaction – point at which it could go either way

H2O + CO2 H2CO3 H+ + HCO3-

Another factor that affects reaction rate Speeds the reaction by lowering the

activation energy Not used up by reaction

Two basic categories for reactions1. Completion reactions – 1-way

(combustion, decomp, rusting)2. Reversible reactions – products

can re-form original reactants Reversible reactions often use 2

arrows b/c reactions occur at the same time

Chemical equilibrium is DYNAMIC, not STATIC

Chemical equilibrium – reactions in which the forward and reverse reaction rates are equal

Every reaction has a condition of equilibrium at a given temperature

That means that 2 reactants will react to form products until a state is reached where the amounts of products and reactants no longer change◦CO2 in a half-filled, sealed soda bottle

Things will stay that way until the system is somehow altered

Equilibrium constant, Keq – a number that expresses the necessary concentrations of reactants and products for the reaction to be at equilibrium

aA + bB cC + dD Keq = [C]c [D]d

[A]a [B]b

If Keq >1, the reaction favors the products If Keq <1, the reaction favors the reactants

Calculate the Keq of the following equation CO2 (g) + H2 (g) CO (g) + H2O (g)

If the [CO2] = 1.5 M, [ H2 ] = 1.5 M,

[ CO ] = 0.6 M, [ H2O] = 0.6 M Keq= [CO]1 [H2O]1 = [0.6] [0.6] = 0.16

[CO2]1 [H2]1 [1.5] [1.5] So this reaction favors the….

When a system at equilibrium is disturbed, the system adjusts in a way to reduce the change.

Chemical equilibria responds to 3 kinds of stress or change

1. Change in concentration2. Change in temperature3. Change in pressure

Increasing concentration of reactant will make the rate of the forward reaction faster than the reverse◦Called a shift right◦Continues until new equilibrium

H3O+ + HCO3 2H2O + CO2 Increasing concentration of product leads to shift left

Remember that endothermic & exothermic are opposites

Increasing the temp adds E so the endothermic will go faster to use it

If it is exothermic forward, increasing the temp favors the reactants

If it is endothermic forward, increasing the temp favors the products

Only affects gases Imagine volume has been decreased, increasing the pressure

Immediate effect is increase in concentration of both product & reactant

According to principle, system will adjust to decrease the pressure

A pressure increase favors the reaction that produces fewer molecules (stoichiometry)

2NOCl 2 NO + Cl2H2O + CO H2 + CO2

Acids – sour taste, conduct electricity well, react with many metals, generate hydronium ions (H3O+), turn litmus paper red

Bases – bitter taste, slippery feel, varying solubility, generate hydroxide ions (OH-), turn litmus paper blue

Strong acids & bases COMPLETELY dissociate or ionize in water (one way reaction)◦HNO3 + H2O H3O+ + NO3

-

◦NaOH Na+ + OH-

Weak acids & bases only partially dissociate (reversible reaction)◦HOCl + H2O H3O+ + ClO-

◦NH3 + H2O NH4+ + OH-

Acid – ionizes to form an H3O+ ion when added to water

Base – generate OH- when dissolved in water

Acid – donates a proton (H+) to another substance

Base – accepts a proton (H+) NH3 + H2O NH4

+ + OH-

H2O is the Bronsted-Lowry acid & NH3 is the Bronsted-Lowry base

Always reactants

Conjugate Acid – Formed when a base gains a proton (H+)

Conjugate Base – Formed when an acid loses a proton (H+)

NH3 + H2O NH4+ + OH-

NH4+ is the conjugate acid & OH- is

the conjugate baseAlways products

Can act as an acid or a base depending on what it is combined with

Can act as a Bronsted-Lowry acid or base

H2O + H2O H3O+ + OH-

Called the self-ionization of waterResults in equal concentrations of H3O+ and OH- in pure water

[H3O+] = [OH-] = 1.00 x 10-7 M

[H3O+] x [OH-] =

1.00 x 10-7 x 1.00 x 10-7 = 1.00 x 10-14

Found to be true for other aqueous solutions at equilibrium

[H3O+] x [OH-] = 1.00 x 10-14

Also abbreviated as Kw

Have proportional amounts of H3O+ & OH-

[H3O+] x [OH-] = 1.00 x 10-14

H3O+ H3O+ H3O+

OH-OH-OH-

ACID NEUTRAL BASE

[H3O+] x [OH-] = 1.00 x 10-14

If [H3O+] = 1.00 x 10-2, what is [OH-]?

[OH-] = 1.00 x 10-12

If [H3O+] = 1.00 x 10-5, what is [OH-]?

[OH-] = 1.00 x 10-9

1909 – Soren Sorenson – negative exponents are annoying…

So let’s just look at the exponents!Logarithm – power to which 10 must be raised to equal that number

log 100 = 2 because 100 = 102

log 0.001 = -3 because 0.001 = 10-

3

log 10,000 =log 0.01 = log 10 = log 0.000001 = log 1 =

Represents the “power” of “Hydrogen”

pH = - log [H3O+]What is the pH of a 0.00010 M solution of HNO3?

pH = - log [1.0 x 10-4] = -(-4) = 4

What is the pH of a 0.2 M solution of a strong acid?

pH = - log [.2]pH = 0.70

[H3O+] x [OH-] = 1.00 x 10-14

pH + pOH = 14You can calculate [H3O+] by 1.00 x 10-14 / [OH-]

Then you can calculate pH

What is the pH of a 0.0136 M solution of KOH, a strong base?

[H3O+] = 1.00 x 10-14 / 0.0136[H3O+] = 7.35 x 10-13

pH = -log [H3O+] pH = - log [7.35 x 10-13]pH = 12.13

Lemonade has a hydronium ion concentration of 0.0050 moles/L. What is it’s pH?

pH = -log [H3O+] pH = 2.3What is it’s pOH?

Reaction of H3O+ & OH- to form water molecules and often a salt

H3O+ & OH- 2H2O

◦Neutral means [H3O+] = [OH-]HCl + NaOH H2O + NaClCommon way to deal with acid & base spills

Baking soda = NaHCO3,Ammonia = NH3

Change color at a certain pH level Red cabbage juice – changes to blue between 3 & 4 and to green at 8/9

Litmus paper – red or blue Phenolphthalein – turns bright pink in the presence of a base

Used to determine the unknown concentration of a known reactant

Uses an indicator to show the equivalence point

For strong acid/strong base… Equivalence point is where [H3O+] = [OH-] or where moles of acid = moles of base

Often uses phenolphthalein

Remember that electronegativity is a measure of how tightly atoms hold on to their electrons

Atoms with large electronegativity differences form ionic bonds by electron transfers

2Na + Cl2 2NaClCan be written as 2Na + Cl2 2Na+Cl-

Oxidation = Loss of electrons◦Na Na+

Reduction = Gain of electrons◦Cl2 2 Cl-

These 2 reactions happen together

Oxidation-Reduction or REDOXOIL RIG

Use “oxidation” numbersThe number of electrons that must be added or removed to convert the atom to elemental or neutral form

In other words, it’s the charge the atom would have if it were an ion

1. Look at the equation2. Assign known oxidation numbers3. Calculate unknowns & verify- Sum of all atoms in a molecule is

zero- Sum of all atoms in a polyatomic

is equal to the charge on that ion

Uncombined = 0 O2

Monatomic ion = ion charge Zn 2+

Flourine = -1 (most electronegative) Group 1 = +1 K Group 2 = +2 Ca Binary compounds – most electronegative element = ion charge

CaCl2

Hydrogen usually = +1 ◦If combo with metal, H = -1

Oxygen usually = -2◦ If combo with Flourine, O = +2◦ Can also be -1 in peroxides like H2O2

Transition metals have multiple oxidation states so save them for last

S2O72-

Oxygen = -2 so O7 = -14Entire molecule must = 2-So S2 + (-14) = 2-S2 = +12S = +6

Ca (OH)2

Ca = +2The entire molecule must = 0So (+2) + (OH)2 = 0(OH)2 = -2O = -2 so O2 = -4So -4 + H2 = -2H2 = +2 so H = +1

From the given, balanced formulas, assign oxidation numbers

2H3O+ + Zn H2 + 2H2O + Zn2+

Since Zn changes from 0 to +2 and some of the H changes from +1 to 0, it is a redox reaction

If the oxidation number goes UP during a reaction, it is oxidized

2H3O+ + Zn H2 + 2H2O + Zn2+

If the oxidation number goes DOWN during a reaction, it is a reduction

1s2 2s2 2p3 – 5 valence electrons, -3 oxidation number

1s2 2s2 2p6 3s1 – 1 valence electron, +1 oxidation number

1s2 2s2 2p5

1s2 2s2 2p6 3s2 3p1