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Chemistry 9

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Chemistry 9
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Atomic number and mass number Individual atoms are far too small to be weighed, but their masses can be compared using a mass spectrometer Measurements using modified mass spectrometers can also be used to determine ionization energies, which have provided evidence for the electronic structures of atom Atoms of different elements will definitely have different numbers of proton. This means that the number of protons in an atom decides which element it is. Proton number = atomic number = Z Mass number = A Proton no not necessarily equal to neutron no. Protons and neutrons are sometimes called nucleons. Nucleon number = mass number Molecular formula: C3H7I Empirical formula vs general formula vs molecular formula Define the term relative atomic mass. Average mass (X weight) of an atom (X element) relative to 1/12 mass of an atom of carbon 12 Mass spectrometry An accurate method of comparing the relative masses of atoms and molecules The relative masses of atoms are called relative atomic masses and the relative masses of molecules are called relative molecular masses. One the relative atomic mass scale, the masses of atoms and molecules are measured relative to one twelfth the mass of a carbon-12 atom A mass spectrometer separates atoms and molecuels according to their mass, and also shows the relative numbers of the different atoms and molecules present Before atoms can be separated and detected in a mass spectrometer, they must be converted to positive ions in the gaseous or vapor state Which functional group has an infrared spectrum with a broad absorption due to hydrogen bonding? Alcohol (X aldehyde) 5 Main stage Vaporization – Inject some of the chemical to be tested into the instrument. The high vacuum causes the sample to vaporize if it is not already a gas
Transcript
Page 1: Chemistry 9

Atomic number and mass numberIndividual atoms are far too small to be weighed, but their masses can be compared using a mass spectrometerMeasurements using modified mass spectrometers can also be used to determine ionization energies, which have provided evidence for the electronic structures of atomAtoms of different elements will definitely have different numbers of proton. This means that the number of protons in an atom decides which element it is.Proton number = atomic number = ZMass number = AProton no not necessarily equal to neutron no.Protons and neutrons are sometimes called nucleons. Nucleon number = mass numberMolecular formula: C3H7IEmpirical formula vs general formula vs molecular formula

Define the term relative atomic mass.

Average mass (X weight) of an atom (X element) relative to 1/12 mass of an atom of carbon 12

Mass spectrometryAn accurate method of comparing the relative masses of atoms and moleculesThe relative masses of atoms are called relative atomic masses and the relative masses of molecules are called relative molecular masses. One the relative atomic mass scale, the masses of atoms and molecules are measured relative to one twelfth the mass of a carbon-12 atomA mass spectrometer separates atoms and molecuels according to their mass, and also shows the relative numbers of the different atoms and molecules presentBefore atoms can be separated and detected in a mass spectrometer, they must be converted to positive ions in the gaseous or vapor state

Which functional group has an infrared spectrum with a broad absorption due to hydrogen bonding?

Alcohol (X aldehyde)

5 Main stage

Vaporization – Inject some of the chemical to be tested into the instrument. The high vacuum causes the sample to vaporize if it is not already a gas

Ionization – bombard the vaporized sample with a beam of high-energy electrons. These knock electrons off atoms or molecules in the sample, forming positive ions

e- + X X+ + e

Acceleration – accelerate the positive ions into the instrument by an electric field

Deflection – Deflect the moving positive ions with a magnetic field. Lighter ions are deflected more than heavier ions with the same charge

Detection – detect the charged ions as a tiny current and feed the signal to a computer. By first using a reference compound with a known structure and relative molecular mass, the computer can print a scale on the mass spectrum

The element bromine has two stable isotopes,. How many peaks corresponding to Br2+ ions would be seen in the mass spectrum of bromine? Justify your answer.

Page 2: Chemistry 9

Three peaks caused by Br2+ ionsBecause 79Br-79Br, 81Br-79BR, 81Br-81Br

Suggest another application of mass spectrometry, other than to determine the relative atomic mass of an element

Analysis of material from spaceDrug testing in sportIdentify molecules from sample with potential biological activities

Explain why there are two peaks in the spectrum of rubidium

Because it has two isotopes

Isotopes and relative isotopic massesWhen atoms of elements is pass through mass spectrometer, different peaks are produced. This shows that the atoms from which the ions formed must have different masses. These relative masses are best described as relative isotopic masses because they give the relative mass of particular isotopes, relative to atoms of carbon-12. The relative masses of pure isotopes are relative isotopic masses, whereas the relative masses of the atoms in an element ( containing a mixture of isotopes) are called relative atomic massesIsotopes have same number of protons, electrons, atomic numbers and chemical properties. However, isotopes have different numbers of neutrons, mass number and physical properties

Relative isotopic mass is the mass of on atom of an isotope relative to the mass of one atom of carbon-12, for which the relative mass is defined as exactly 12(3X35) + (1X37) /4 = 35.5Mass spectrometers can also be used to study molecules. After injecting a sample into the instrument and vaporizing it, bombarding electrons ionize the molecules and also break them into fragments. Because of the high vacuum inside the mass spectrometer, it is possible to study these molecular fragments and ions which do not normally existThe peak with the highest mass in the spectrum = relative molecular mass of the compoundFrom interpreting the data from mass spectrometers, chemists can reduce the isotopic composition of elements, the relative atomic masses of elements, the relative molecular masses of compound

Evidence for the electronic structure of atomsIn a mass spectrometer, by varying the intensity of the beam of electrons bombard the sample, it is possible to measure the minimum amount of energy needed to remove electrons from the atoms of an element. predict the electron structures of atomsEm1 = 1st ionization energy the product is one mole of gaseous ions with one positive charge.Ionization energies are always endothermicThe shells of electrons at fixed or specific levels are sometimes called quantum shells. The word quantum is used to describe something related to a fixed amount or a fixed levelWhen an energy level is filled, the electrons are paired up and in each of these pairs the electrons are spinning in opposite directions. The paired electrons can only be stable when they spin in opposite directions so that the magnetic attraction resulting from their opposite spins can counteract the electrical repulstion from their negative charge

Page 3: Chemistry 9

Understanding the pattern in ionization energiesElectron shell structure = 28,8,1Electron sub-shell structure 1s2, 2s2 ……Electron sub-shells = energy sub-levels

An orbital is a region is space around the nucleus of an atom in which there is a 95% chance of finding an electron, or a pair of electrons with opposite spinsS sub-shells – contain one orbitals – spherical ringP sub-shells – three orbitals – dumb-bell shaped and arranged at right angles to each otherEach box represents an orbitalsWhen there are two or more orbitals at the same energy level, electrons occupy these orbitals singly before they pair up

Why rare gas is inert ( e.g. why helium is stable)

Its first shell nearest the nucleus is full as it contains 2 electrons

If all the shells of an atom are full, the atom is very stable and it has a highly endothermic first ionization energy

Why neon is inert

Neon has a filled first shell with 2 electrons and a filled second shell with 8 electrons.

Therefore, neon has a higher first ionization energy than its neighbors in the periodic table

Electron structures and the periodic tableS block, p block , d block and f block is divided based on the electron structures of the elements. Therefore, the periodic table reflects the underlying electronic structures of the atomsS block – the last electron added goes into an s orbital in the outer shellP block – the last electron added goes into p orbital in the outer shellD block elements are all metals. These metals are much less reactive than the s block metals in group 1 and 2F- block are all metals. They are often called the lanthanides and actinides because they are the 14 elements immediately following lanthanum and actinium. Another name is inner transition elementsElectrical conductivity and 1st ionization energies are physical properties

Why elements in each group in periodic table have similar chemical properties?

The chemical properties of elements are determined by electronic structures e.g. number of electrons in the outer shell and how strongly they are held by the nuclear charge

Because they have similar electron structure.

E.g. Group 1 has one s electron in its outer shell

Why reactivity of group 1 metals increase down the group

Page 4: Chemistry 9

As the atomic numbers of the alkali metals increase, the outer electron is further from the positive nucleus. The outer electron is held less strongly by the nucleus, so the electron is lost more readily. (P.58)

(P.147) Down the group, the charge on the nucleus increases and the number of filled inner shells also increases. The shielding effect of the inner electrons means that the effective nuclear charge attracting the outer electrons is 1+.the outer electrons get further and further away from the same effective nuclear charge and so they are held less strongly

Melting pointLondon force, dipole-dipole attraction

Is this statement correct?

The trend in the m.p. of successive elements across period 2 is similar to that in period 3 correct

Metal

Why melting point rise from group 1 to group 2 to group 3?

The m.p. depends on strength of ________

Group 1 to Group 3, the bonding between atoms is metallic bond which is strong

The more electrons each atom contributes from its outermost shell to the shared delocalized electrons

The stronger the bonding and the higher the melting temperature

Why carbon and silicon have extraordinary melting point?

They have giant covalent structures. The bonds in these structures are highly directional, so most of the bonds must break before the solid melts. Therefore, the melting temperatures of group 4 elements are very high

Why non-metal elements in group 5,6,7,0 has low melting point

The non-metal elements in groups 5,6,7,0 form simple molecules. The intermolecular forces between these simple molecules are weak, so these element have low melting point

Explain why ionic compounds have relatively high melting temperature

Strong electrostatic attractions between ionsIons held in giant lattice and there are many ionic attractions So large amount of energy (X high temperature) needed to break apart ions (1M)

1 st Ionization energy High Ionization energy = unreactive

The ionization energy of an atom is determined by three atomic properties

The distance of the outermost electron from the nucleus As the distance increases, the attraction o the positive nucleus for the negative electron decreases and this tends to reduce the ionization energy

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The size of the positive nuclear charge As the positive nuclear charge increases, its attraction for outermost electrons increase and this tends to increase the ionization energy

The shielding effect of electrons Electrons in inner shells exert a repelling effect on electrons in the outer shells of an atom. This reduces the pull of the nucleus on the electrons in the outer shell. Therefore, the effective nuclear charge attracting electrons in the outer shell is much less than the full positive charge of the nucleus. Shielding effect increases as the number of inner shells increase

Why 1 st ionization energy increase across any period ( left to right)

Moving from left to right across any period, the nuclear charge increases as number of proton increase.Since the electrons added to the same outer shell and the shielding effect of full inner shells is constant, the increasing nuclear charge tends to pull the outer electrons closer to the nucleus.The increased nuclear charge and the reduced distance of the outer electrons from the nucleus makes the outer electrons more difficult to remove and the 1st ionization increases

Why 1 st and 2 nd ionization energies decrease down the group 2?

The shielding effect of the inner electrons means that the effective nuclear charge attracting the outer electron is 2+Down the group, the outer s electrons get further and further away from the same effective nuclear charge, and so they are held less strongly and the ionization energies decrease.

Why the 3 rd ionization energy is much higher than the 2 nd ionization energy in group 2

The removal of a third electron takes much more energy because the third electron has to be removed against the attraction of a much larger effective nuclear charge.

Why the rising trend in ionization energies across a period is not smooth but in a 2-3-3 pattern

This reflects the way in which electrons feed into s and p orbitals.Extra stability associated with filled sub-shells and with half-filled p sub-shells in which charge is distributed evenly into three orbitals

From above concept, why 1 st ionization of beryllium is higher than boron although moving across the period

Extra stability associated with filled sub-shellsBeryllium’s 1s2 2s2 electron structure in which all sub-shells are filled is more stable than that of boron which is 1s2 2s2 2p1

From above concept, why 1 st ionization energy of nitrogen higher than oxygen

Extra stability associated with filled sub-shellsThe electron structure of nitrogen is 1s2 2s2 2p3 and that of oxygen is 1s2 is2 2p4The half-filled 2p subshell in nitrogen, with on electron in each of the three 2p orbitals and its evenly distributed charge, is more stable than the 2p sub-shell in oxygen which contains four electrons.

For sodium, suggest why there is a big jumps in Em1 value between 1 st and 2 nd ionization energies

This suggests that sodium atoms have only one electron in an outer shell or energy level furthest from the nucleus.

Therefore, sodium must be in group 1

Page 6: Chemistry 9

Why the 1 st ionization energy is the smaller than the 2 nd ionization energy in case of sodium

The outer electron is easily removed because it is shielded from the full attraction of the positive nucleus by 10 inner electrons.Below this outer single electrons, sodium atoms have 8 e- in 2nd shell which are closer to the nucleus than the single outer electron

Why the last two ionization energy is the biggest in all elements

These two electrons feel the full attraction of the positive nucleus

Bonding and StructuresStructure has two types: Giant structures and simple molecular structuresMaterials with giant structures form crystals in which all the atoms or ions are linked by a network of strong bonds extending throughout the crystal. These strong bonds result in giant structures with high melting and boiling temperatures. E.g. Metals, ionic solids, polymers, ceramics (pottery) and glassSubstances with simple molecular structures consist of small groups of atoms. The weak intermolecular forces allow the molecules to be separated easily. So molecular substances have low melting and boiling tem.Tetrachloromethane is not polar

Metallic bonding and structures- Most metals are packed tgt as close as possible ( close-packing)_- Each atom in the middle of the layer of a metal crystal touches six other atoms in the same layer- This close packing allows atoms in one layer to get as close as possible to those in the next layer, forming a

giant lattice of closely packed atom in regular pattern.- A metallic structure is held together by attractions between metal ions (X atoms) and delocalized electrons

What is delocalized electron

Delocalized electrons are bonding electrons which are not fixed between two atoms in a bond.In this giant lattice of metal, electrons in the outer shell of each metal atoms are free to drift through the whole structure. These electrons do not have fixed positions and shared by several or many more atom – they are described as delocalized

Compare sigma bond and pi bond

Sigma bond is overlap of any0shaped orbitals along the line between the nuclei.Rotation is allowed around a sigma bondElectron density alone the line is stronger along the line between the two nuclei

However, pi bond is two dump-bell shaped p orbitals and by side by side overlapping No rotation about a pi bondElectron density is higher above and below the line between the two nuclei

By considering the strength and structure of the pi bond, explain why alkenes are more reactive than alkanes

Pi bond weaker than a sigma bond less energy needed to break pi bondPi bonds is more accessible to electrophilic attackHigher electron density in pi bond, so alkenes more susceptible to electrophilic attack

Page 7: Chemistry 9

What is metallic bond?

The strong electrostatic attractions between the positive metal ions and the sea of delocalized electrons result in strong forces between the metal atoms

Why metals have high melting and boiling temperature

Metal atoms are closely packed with strong forces of attraction between the positive ions and delocalized electrons( melting point) It takes a lot of energy to move the positive ions away from their position in the giant lattice and move around each other, allowing the metal to melt( boiling point) It takes even more energy to separate individual atoms in the metal

Why metals are good conductors of heat

When a metal is heated, energy is transferred to the electrons. The delocalized electrons move around faster and conduct the energy rapidly to other parts of the metal

Why metals are malleable --- can be bent or hammered into different shape

The bonds between metal atoms are strong but not directional because the delocalized electrons can drift throughout the lattice and attract any of the positive ionsWhen a force is applied to a metal, lines or layers of atoms can slide over each otherAfter slipping, the atoms settle into closely packed position again

What is meant by lattice

Used to describe a regular arrangement of atoms of ions in a crystal

Ionic StructureThe greater the difference between electronegativity, the greater the ionic character

Ionic Radii

- Radius of cation is smaller than its atomic radius because it loses electrons from its outer shell when turning into an ion

- Radius of anion is larger than its atomic radius because electrons are added to the outer shell

Explain why solutions of ions are able to conduct electricity

Because the ions (X electrons or particle) are free to move when a potential difference is applied

Suggest why aqueous solutions of calcium chloride and barium chloride of the same molar concentration, have different electrical conductivities.

Barium ions and calcium ions are of different sizes (1M)

Page 8: Chemistry 9

Why calcium ion is smaller than calcium atom?

because it loses electrons from its outer shell when turning into an ionthus It has one less sub shell of electronsThe ratio of protons: electrons has increased, more proton than electronsGreater effective nuclear charge (X greater nuclear chage)thus greater net force on remaining electrons so remainder of electrons held more closely.

Energy changes of forming ion

Sodium starts as a giant structure of atomEnergy is required to separate the sodium atoms ( enthalpy change of atomization)Energy is required to remove electrons to form sodium ions (1st ionization energy)Chlorine consists of moleculesEnergy is required to separate the chlorine atom (enthalpy change of atomization)Energy is involved in adding one electron to each chlorine atom to form chloride ion (electron affinity)

Born-Haber cycles are thermochemical cycles for investigating the stability and bonding in ionic compoundsAll the terms in the cycle can be measured experimentally except the lattice energyLattice energies are negative as usually defined. This means that the description larger and smaller can be ambiguous in comparing lattice energies. It is better to describe one lattice energy as more or less exothermic

Strength of ionic bonds are measured as lattice energies. The energy released is greater and the force of attraction between the ions is stronger if the charges on the ions are large and the ions are small allowing them to get closer to each other

Suggest why the melting temperature of magnesium oxide is higher than that of magnesium chloride, even though both are almost 100% ionic

Oxide ions have one more negative charge than chloride ion, thus greater charge density Moreover, oxide ion is smaller than chloride ionSo the ionic attraction between ions is stronger in MgOMore energy is required to break the ionic bonds and separate the ions in MgOGet 0 marks if mention intermolecular forces

Distinguish standard enthalpy change of formation and lattice energy

The lattice energy of an ionic solid is the standard enthalpy change when one mole of the compounds forms from free gaseous ionsStandard enthalpy change of formation is enthalpy change when one mole of compounds is formed from its element which is at most stable form

What is meant by isoelectronic?

Isoelectronic ions have different ionic radii.

Where the energy come from to turn sodium metal and chlorine molecules into NaCl

Ionic crystals are stable because of the large release of energy (lattice energy) when oppositely charged ions come tgt forming a crystal lattice

Page 9: Chemistry 9

What is meant by lattice energy?

The lattice energy of an ionic solid is the standard enthalpy change when one mole of the compounds forms from free gaseous ions

What is meant by first electron affinity?

The first electron affinity of an element is the energy change when each atom in one mole of gaseous atoms gains one electron to form one mole of gaseous ions with a single negative charge

The gain of the first electron is exothermic, but adding the second electron to a negatively charged ion is endothermic

Example: Which of these equations represents the electron affinity of chlorine?

A. Cl2 + 2E- eCl-B. Cl2 – 2e- eCl-C. 0.5Cl2 + e- Cl-D. Cl(g) + e- Cl- (g)

(All in gaseous state) Ans: D

How to test whether NaCl is entirely ionic?

The experimental lattice energy calculated from a Born-Haber cycle can be compared with the theoretical value calculated using the laws of electrostatics and assuming that the only bonding in the crystal is ionic test the ionic model and check whether the bonding in a compound is truly ionicThe difference between theoretical and experimental values of lattice energies is less than 3%Shows that ionic bonding can account almost entirely for the bonding is NaCl

Why MgCl is not purely ionic?

The experimental values of lattice energy are much more exothermic than the theoretical values which assume the only bonding is ionic. It can be concluded that the bonding is significantly stronger than that predicted by a pure ionic modelThis suggests that there is some covalent bonding as well as ionic bonding in MgCl

Why MgCl is partly ionic and partly covalent?

Positive metal ions attract the outermost electrons of negative ions, pulling electrons into the space between the ions. This distortion of the electron clouds around anion by positively charged cation is called polarizationPolarisation of negative anions by positive metal ions gives rise to some electron sharing.

Polarizing power v.s. polarizability

The polarizing power of cation depends on its charge and its radiusE.g. Polarizing power : Ca2+ > K+> Cs+

Page 10: Chemistry 9

The polarisability of anion depends on its size

Why fluoride ions is less polarizable than iodide ions

The larger the negative ion and the large its charge , the more polarizable it becomes.

Simple molecular structureE.g. O2Usually gases, liquids or soft solids at room temperature because of weak intermolecular forceRelatively low mp and bpDo not conduct electricity as solids, lquids or gases because they contain neither ions nor free electrons to carry the electric chargeMore soluble in non-polar solvents

Use your knowledge of intermolecular forces to predict the compound with the highest boiling point:

HF or H2O or NH3 orCH4 Ans: H2O

Giant covalent structuresDiamond, graphite and silicon, silicon dioxide and boron nitrideThe covalent bonds in diamond are strong and point in a definite direction, so diamonds are very hard and have very high melting temperaturesDiamond does not conduct electricity because the electrons in its covalent bonds are localized between pairs of atom

What is covalent bond

Formed when atoms share electrons.The atoms are held together by the electrostatic attraction between the positive charges on their nuclei and the negative charge on the shared electrons

What is the character of C-C double bond?

The two ends of a molecule cannot rotate relative to teach other, about the axis of the double bondThey have a higher electron density than a single bond

What is dative covalent bond

Is a bond in which two atoms share a pair of electrons, both the electrons being donated by one atom

Group 1, the alkali metalsThe metals are soft and easily cut with a knifeThe only oxidation state is +1 because only have 1 single s electron from the outer shellThe alkali metlas are powerful reducing agents because they react by giving up electrons to form M+ ions

Page 11: Chemistry 9

Water and chloride

All the metals react with water to form hydroxides and hydrogen.The rate of violence of the reaction increase down the group – lithium reacts steadily with cold water, caesium reacts explosively2Li(s) + 2H2O(l) 2LiOH(aq) + H2(g)All the metals react vigorously with chlorine to form colorless ionic chlorides. The chlorides are soluble in water, forming colorless solution. 2K(s) + Cl2(g) 2KCl(s)

Hydroxides Carbonates NitratesAll white solids All white White crystalline solidsSoluble in water, forming alkaline solutionsSolubility increases down the group( because lattice energy decrease)

Unusual metal carbonates in that they dissolve in water and form alkaline solution because carbonate ion is a baseCO32- + H2O HCO3- + OH-

Very soluble in water

Unusual feature of group 1 carbonates:Most of them do not decompose on heating. The exception is lithium carbonate, which breaks down to the oxide and carbon dioxide when hot

They crystals of sodium and potassium nitrates are much harder to decompose on heating than most other metal nitrates. On heating, these nitrates first melt and then on stronger heating start to decompose giving off oxygen. They only decompose as far as the nitrite2KNO3(s) 2KNO2(s) + O2(g)

Why compounds of sodium and potassium are widely used as chemical reagent?

The ions of alkali metals are unreactive, so they act as spectator ions which take no part in reactions when the reagents are usedMost sodium and potassium compounds are soluble in water, including their hydroxides and carbonates. In contrast, most other metal hydroxides and carbonates are insoluble so not available in aqueous solutionThe ions of alkali metals are colorless in aqueous solution so they do not hide or interfere with color changes. Sodium or potassium compounds are colored only if the negative ion is colored.

Why group 1 metals are kept in oil ?

To protect them from airGroup 1 metals are shiny when freshly cut but quickly become dull in air as they react with moisture and oxygen

Group 2, the alkaline earth metalsMany of the compounds of these elements occur as minerals in rocksAll elements exist in the +2 oxidation state in all their compoundsGroup 2 metals are reducing agents

Group 2 compounds are insoluble Harder and denser than group 1 metals and they have higher melting point In air, the surface of the metals is covered with a layer of oxide

Page 12: Chemistry 9

Oxygen

Apart from beryllium, the group 2 metals burn brightly in oxygen on heating to form white, ionic oxidesMagnesium burns very brightly in air with an intense white flame forming the white solid magnesium oxideCalcium burns brightly in air but with a red flame forming the white solid calcium oxide. Strontium reacts in a similar wayBarium burns in excess air or oxygen with a green flame, to form a peroxide BaO2, which contains the peroxide ion, O22-

Water

The metals Mg to Ba in group 2 react with waterThe reaction are not as vigorous as the reactions of the group 1 metalsThe rate of reaction increases down the groupMagnesium reacts very slowly with cold water but much more rapidly on heating in steamMg(s) + H2O(g) MgO (s) + H2 (g)Calcium reacts with cold water to produce hydrogen and a white precipitate of calcium hydroxideCa(s) + 2H2O (l) Ca(OH)2 (s) + H2(g)Barium reacts even faster with cold water and its hydroxide is more soluble

Oxides HydroxidesApart from beryllium oxide, oxides are basic, reacting with acids to form saltCaO (s) + 2HNO3 Ca(NO3)2 + H2O

MgO is a white solid made by heating magnesium carbonate MgO turns to Mg(OH)s in waterMgO has high mpCaO is white solid made by heating CaCO3CaO reacts very vigorously with water, product is calcium hydroxide

To some degree soluble in water forming alkaline solutionsSolubility increases down the groupMg(OH)2 only slightly soluble in waterCa(OH)2 is only sparingly soluble in water forming an alkaline solutionBa(OH)2 is the most soluble of the hydroxides.It has advantage over sodium and potassium hydroxides in that it cannot be contaminated by its carbonate because barium carbonate is insoluble in water

Carbonates NitratesAll insoluble in waterReact with dilute acidsDecompose on heating to give oxide and carbon dioxideBecome more difficult to decompose down the group ( more thermally stable)

Are colorless crystalline solidsVery soluble in waterDecompose to the oxide on heating2Mg(NO3)2 2MgO (s) + 4NO2(g) + O2(g)Become more difficult to decompose down the group

SulfatesAll colorless solidsBecome less soluble down the group ( unusual)Epsom salts consist of hydrated magnesium sulfate, MgSO4.7H2O which is a laxative

Why ionic radii of group 2 elements is smaller than that of group 1?

Why reactivity increases down the group?

Down the group, the charge on the nucleus increases and the number of filled inner shells also increases. The increasing number of filled inner shells means that atomic radii increase down the group. The tendency to react and form ions increase down the group (P.151)

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The shielding effect of the inner electrons means that the effective nuclear charge attracting the outer electron is 2+Down the group, the outer s electrons get further and further away from the same effective nuclear charge, and so they are held less strongly

Analytical Chemistry

Which liquid would react with phosphorus (V) chloride to give a gas which fumes in moist air?

Alcohol

Compound X is a white solid. On heating this compound, a colorless, acidic gas is the only gaseous product. A flame test is carried out on the solid residue and a reddish flame is observed. Compound X is?

CaCO3

Flame test

Particularly useful in identifying group 1 metal ions, which are otherwise very similarUse nichrome wire and concentrated hydrochloric acid

What is the principle of flame test?

Ionic compounds do not burn during a flame testThe energy from the flame excites electrons in the sodium atomsRaising them to higher energy levelsThe atoms then emit the characteristic yellow light as the electrons drop back to lower energy levels

Describe how you would carry out a flame test

Use a nichrome (platinum) wire to dip some Concentrated HCl.Salt mixed with wire and acidPlace in a non-luminous flame

Why HCl is used is used in flame test

Clean nichrome wireChlorides evaporate more easily and so color flames more strongly than less volatile compounds.Conc HCl converts involatile compounds such as carbonates to chlorides

In an experiment, 1-chloro-2-2-2-fluro-ethanol was heated in a test tube with aqueous silver nitrate and ethanol, using a water bath. Suggest why ethanol was used in this experiment

Solvent for both reactants To dissolve and mix the reactants(X acidify silver nitrate)

Halogenoalkane P forms the yellow precipitate faster than halogenoalkane Q. What is halogenoalkane P?

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Tertiary iodide or secondary iodide

What is the product formed by the reaction of halgoenoalkane with hot aqueous silver nitrate

Silver iodide and alcohol

Principle of Limewater test

A solution of Ca(OH)2 reacts with CO2 forming a white, insoluble ppt of CaCO3

Identify by name or formula, the compound that is dissolved in water to make limewater

Calcium hydroxide

On prolonged strong heating, compound A forms a white solid and a gas which turns limewater milky. Suggest the formula for the anion in Compound A. Justify your answer

CO32-Because carbon dioxide gas given off when this carbonate is heated and decomposed

Suggest a test for the hydroxide anion. Give the result of your test

Universal indicator turns blueTurn red litmus paper blueWarm with ammonium ions alkaline gas given up and turn red litmus paper blue

Green ChemistryCFCs and nitrogen monoxide, NO, are involved in the depletion of the ozone layer (X CFCs and NO are decomposed by UV radiation X)

Describe how SO2 and NO2 come to be present in the atmosphere and explain how they can damage a limestone building

SO2 and NO2 form when fossil fuels are burnt,,When petrol or diesel burn in vehicle engineVolcanoesThey react with water to form sulfurous acid and nitric acid which are acid ranThey reacts with limestone to form soluble compounds which dissolved building by neutralization

Why using a polystyrene cup has less environmental impact than using a paper cup

Raw material for paper cup requires cutting down treesPolystyrene cup uses less energy produce less CO2Polystyrene cup releases less sulfur based compounds into air so less chance of forming acid rain less chance of damaging buildingsPolystyrene cup releases no chlorine compounds which damages ozone layer

Information regarding the life cycle that would make any assessment of the environmental impact more reliable

Ease of recyclability e.g. whether easy to reuse

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Space taken up in landfillType and amount of gases formed if incineratedBiodegradabilityDurability

The use of polyethene packaging has been criticized mainly because?

Large amounts of oil are consumed in producing the monomer, ethane

(X polyethene degrades to form toxic products)

(X the complete combustion of polyethene produces dangerous fumes )

What does the term CFC stand for?

Chlorofluorocarbon

Suggest one use for CFCs, e.g. CCl2F2

RefrigerantFire retardantDegreasing solvent

Explain why many scientist consider the effect of CFCs on ozone to be harmful

O + O3 2 O2In the stratosphere, CFCs are broken down by absorption of UV radiation to form chlorine free radicalChlorine free radical acts as catalystBecause the chlorine will not be used up and is regenerated after catalyzing the reaction, the chlorine free radical persists in the atmosphere and continues to attack and start a new chain reaction.As a result, ozone is depleted and cause holes in ozone layer.UV reaching Earth’s surface increases as less UV absorbedCause skin cancer or mutation occurs

The compound of formula Ch2F2 has replaced several CFCs, e.g. CCl2F2, for commercial use. If molecules of CH2F2 reach the stratosphere, they do not break down to produce fluorine free radical. Suggest why C-F bonds are not broken in the stratosphere

Because the C-F bond is very strong (Xstable) (X reject any mention of electronegativity)And C-F bond is much harder to break than the C-Cl bond and UV does not have enough energy to break it

The compound CH2F2 acts as a greenhouse gas when it absorbs Infrared radiation. Explain why a molecule of CH2F2 is able to absorb this radiation

The molecule is polar, IR causes bonds to vibrate and stretching and bending increase

Magnesium chloride may be prepared from magnesium by reaction with chorine or with HCl. Compare these two preparations in terms of the atom economies of the reactions. No calculation is required.

Magnesium reacts with chorine to form only one product which is MgClMagnesium reacts with HCl to form hydrogen as well as MgCl, it forms more than one product.H2 is waste product

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Write chemical equationThe reaction of chlorine has atom economy which is higher(Ignore any comments about yield)

UV radiation causes polypropene to degrade. Suggest one advantage and one disadvantage of this

Polypropene will decompose naturally (X biodegradable)Polypropene will not require landfill as it can decompose in sunlight No need to incinerate (X no pollution or good for environment)Polypropene cannot be used when exposed to bright sunlightCannot be recycled or reused

Enthalpy Change

Define the term standard enthalpy change of formation of a compound.

Enthalpy change when one mole of a substance is formed from its element in their most stable states under standard conditions of 298K and 1atm pressure (100kPa)

Why the theoretical value of enthalpy change is different from the experimental value?

Heat absorbed by apparatus / heat lost to surroundingsThe substances are not completely burnt or incomplete combustionCannot ensure all products are at standard conditions at end of reaction, e.g. water may be produced as a gas

List three ways in which the design of the experiment causes the results to be so different from the data book value

Heat loss from the beaker because beaker not insulated/ heat loss as no lid on beaker/ no stirringIncomplete combustion of the alcohol or formation of sootNot all the energy from the flame is used to heat the beaker or the waterToo large a distance between flame and beakerHeat capacity of the beaker is neglected, and beaker absorbs heatEvaporation of the hot alcoholEvaporation of water which acts as solvent

Bond angleExplain why HCH angle in methanol is 109.5. Justify it

Because this is the minimum repulsion between four bond pairs of electrons

(Reject: four bond pairs give tetrahedral shape)

Explain why COH angle in methanol is 104-105

There is two lone pairs, therefore repel more than bonding pairs

EquilibriumDraw cycle even if they don’t ask you to do so

Page 17: Chemistry 9

Explain why the equilibrium shift to the right (favour forward reaction)

If pressure increase Shifts to the direction of fewer moles of gas molecules (Xatom)

If temperature decrease exothermic in forward direction (X equilibrium shifts in the exothermic direction)

The main reason that reaction rates increase with temperature is that:

A larger proportion of molecules have high energies

Organic chemistry

1-iodobutane can be made by reacting butan-1-ol with phosphorus (III) iodide, PI3, formed by reacting moist red phosphorus with iodide. Write the equation

PI3 + 3C4H9OH 3C4H9I + H3PO3

Give the structural formula for the organic product of the reaction between 1-iodobutane and ammonia

Ch3CH2CH2CH2NH2

What is the difference in the mass spectra of two structural isomer

Same m/e of the molecular ion but fragmentation pattern is different

What is essential for a species to act as a nucleophile

A lone pair of electrons

( XA negative charge, an unpaired electron or strongly polar bond)

Name a raw material from which liquid paraffin can be obtained

Crude oil

The role of the UV radiation in the reaction is to:

Break the Cl-Cl bond forming Cl free radical

Describe what is meant by cracking

Use of high temperature and heat in the absence of air. Catalysts may be used (1M)To break large molecules to form smaller molecules producing alkenes and alkanes

Explain why the propene could form two products

Bromide could add to either primary or secondary carbocation because propene is unsymmetrical

Hydrogenation:

Reagent = Hydrogen gasCatalyst: Nickel/ palladium/ platinum

Explain why the major product Is formed in the electrophilic addition reaction of propene

Page 18: Chemistry 9

Secondary carbocation is formed More stable than primary (Markownikoff’s rule)

Which of the following is true?

All the bond angles in buckminsterfullerene are 120 falseThe melting temperature of buckminsterfullerene is higher than that of graphite falseThere are delocalized electrons in buckminsterfullerene trueOn complete combustion, buckminsterfullerene forms carbon dioxide and water False

The best method of converting ethanol into iodoethane is ?

Heat red phosphorus, ethanol and iodine under reflux

(X heat potassium iodide and ethanol with concentrated sulfuric acid)

(X react ethanol and potassium iodide in the presence of dilute aicds)

(X heat iodine and ethanol under reflux)

Why tertiary alcohol is resistant to oxidation when heated with acidified potassium dichromate (VI)

Because it is tertiary alcohol and no C-H bond to break

Which isomer is a secondary alcohol? Justify your answer

Because the C atom bearing the OH group is attached to two other C atom and 1 H atom

Phosphorus pentachloride ( Phosphorus (V) chloride) is used to test for the presence of an OH group. Which isomers can react with PCl5?

Only primary alcohol

ROH + PCl5 RCL + POCl3 + HCl

Suggest why phosphoric (V) acid is used in the preparation of hydrogen iodide gas rather than concentrated sulfuric acid

Sulfuric acid oxidizes hydrogen iodide to iodinePhosphoric (V) acid does not oxidize hydrogen iodide to iodine

UncategorizedCO2 + H2O H2CO3H2O + CO2 H+ + HCO3-

When conc sulfuric acid is added to solid sodium bromide, bromine is produced. When concentrated sulfuric acid is added to solid sodium chloride, no chlorine is produced. The reason for this difference is ?

The chloride ion is a weaker reducing agent than the bromide ion

Explain why the gases from the engine are not cooled before entering the converter.

Page 19: Chemistry 9

Activation energy for the reaction is too high. If cooled molecule would not have enough energy to reactFewer molecules have the required activation energyCooling the gases would make the rate too slowTherefore, more molecules have energy equal or larger than activation energy at higher temperature

Most compounds of lead are insoluble, an exception being Lead(II) nitrate. Therefore a good method of preparing Lead(II) sulfate is?

Adding dilute sulfuric acid to Lead(II) nitrate solution

(X adding concentrated or dilute sulfuric acid to lead metal or lead oxide)

A hazard that is particularly associated with alkane is?

They are flammable (X toxic or corrosive)

If two impure samples containing carbonate react with acid, the amount of gases released are not consistent . Suggest a reason for this other than errors in the procedure, measurements or calculations

Different samples may have different amount of carbonate .

Describe what you would see if a test tube of hydrogen iodide gas was inverted in a beaker of water

Water rises in the test tube because HI is soluble (X water displace the gas)

State what you would observe if methanol reacts with sodium

Sodium dissolve to form a clear colorless solutionEffervescence forming

What should be used to crush tablet

Pestle and mortar

Practical skills --- Titration

Describe how you would make up exactly 100 cm3 of potassium iodate(V) solution in the volumetric flask

Transfer solution and rinsing the containerPour the washing to the volumetric flaskMake up to the markShake and invert

Explain why the mean titre should be based only on titration 2,3,4

Titres 2,3,4 are concordant and consistent, difference within __ cm3

Titre 1 is a trial and acts as a rangefinder, it may be out shot. Therefore, titre 1 is an outlier and anomalous

The most significant hazard when using conc Sulfuric acid

Page 20: Chemistry 9

Sulfuric acid is corrosive and can burns skin

Suggest, with a reason, how the student’s method of preparing the diluted solution could be improved

Use a pipette to measure acid, because pipette is more accurate than a measuring cylinderShake and invert the volumetric flask thoroughly, to ensure a uniform concentrationRinse out containers and transfer washing to the volumetric flask to ensure all the acid is transferred to the volumetric flaskUse a teat pipette to make up to the mark in volumetric flask to ensure volume of solution accurately measured

Suggest one reason why a student may obtain volumes outside the uncertainty of the burette when performing a titration

OvershootsWater left in burette’s air lock below tapBurette not verticalAlkali not at stated concentrationLeaking tapNot reading meniscus at eye levelFunnel left in top of buretteNot reading level against a white backgroundWashing pipette between titresWashing the flask with the solution that will go in itNot swirling flask

Practical skills --- Uncategorized

Suggest one reason why a water bath was used rather than heating the test tube containing the reaction mixture directly over a Bunsen flame if ethanol is used

Ethanol is flammable (X low boiling point =/= flammable)Another use of water bath:greater control of heating control rate of reaction to prevent the reaction getting too vigorous and out of controlTo allow constant heating and even heating


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