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Cambridge IAL Chemistry practice questions

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Question 1Physical chemistry

Which statement correctly describes the trend in first ionisation energy across Period 3 from sodium to argon?

  1. It generally increases, with dips at aluminium and sulfur. Correct answer.
  2. It remains approximately constant because nuclear charge and shielding increase together.
  3. It increases steadily with no exceptions from sodium to argon.
  4. It generally decreases, with peaks at aluminium and sulfur.

Explanation

Across Period 3 the nuclear charge rises while the outer electrons remain in the same shell, so the general trend is an increase in first ionisation energy. The dips occur at aluminium, where a 3p electron is removed in preference to the filled 3s subshell, and at sulfur, where electron pairing in a 3p orbital increases repulsion.

Question 2Physical chemistry

A gaseous ion X²⁻ has the electron configuration 1s² 2s² 2p⁶. Which element is X?

  1. Magnesium
  2. Fluorine
  3. Oxygen Correct answer.
  4. Neon

Explanation

The ion contains 10 electrons, and since it carries a 2− charge the neutral atom must contain 8 electrons, which identifies oxygen. Neon has 10 electrons, fluorine 9 and magnesium 12, so none of these gives a 2− ion with 10 electrons.

Question 3Physical chemistry

Which pair of species are isotopes of the same element?

  1. ²³Na⁺ and ²³Na
  2. ¹⁶O²⁻ and ¹⁸O
  3. ¹⁴N and ¹⁵N Correct answer.
  4. ¹⁴C and ¹⁴N

Explanation

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, which is exactly the case for ¹⁴N and ¹⁵N. The other pairs differ in proton number or in charge rather than being neutral atoms of one element with different mass numbers.

Question 4Physical chemistry

In the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g), the pressure is increased at constant temperature. What is the effect on the equilibrium position and on the value of Kp?

  1. Equilibrium shifts to the right and Kp is unchanged. Correct answer.
  2. Equilibrium shifts to the right and Kp increases.
  3. Equilibrium shifts to the left and Kp is unchanged.
  4. Equilibrium is unchanged and Kp decreases.

Explanation

Increasing pressure shifts the equilibrium towards the side with fewer moles of gas, which is the product side here (2 mol versus 4 mol). The equilibrium constant Kp depends only on temperature, so it is unaffected by a pressure change at constant temperature.

Question 5Physical chemistry

Which species has the electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶?

  1. S²⁻
  2. K⁺
  3. Ar⁺
  4. Cl⁻ Correct answer.

Explanation

The configuration shown contains 18 electrons, which corresponds to a species with 18 electrons. Chlorine has 17 electrons, so the chloride ion Cl⁻ has 18; the other options do not have 18 electrons.

Question 6Physical chemistry

Which quantity is defined as the enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions?

  1. Standard enthalpy of combustion
  2. Standard enthalpy of neutralisation
  3. Standard enthalpy of formation Correct answer.
  4. Standard enthalpy of atomisation

Explanation

The standard enthalpy of formation refers specifically to the formation of one mole of a compound from its constituent elements in their standard states. Combustion, atomisation and neutralisation describe different processes and are not defined in this way.

Question 7Physical chemistry

Which statement about a catalyst is correct?

  1. It increases the enthalpy change of the reaction so more product forms.
  2. It increases the activation energy so that only fast collisions succeed.
  3. It shifts the equilibrium position further towards the products.
  4. It provides an alternative route with lower activation energy without being consumed overall. Correct answer.

Explanation

A catalyst lowers the activation energy by offering an alternative reaction pathway, and it is regenerated at the end so it is not used up. It does not alter the enthalpy change or the position of equilibrium, only the rate at which equilibrium is reached.

Question 8Physical chemistry

Which expression gives the units of the rate constant for a reaction that is second order overall?

  1. mol⁻² dm⁶ s⁻¹
  2. s⁻¹
  3. mol dm⁻³ s⁻¹
  4. mol⁻¹ dm³ s⁻¹ Correct answer.

Explanation

For an overall second-order rate law, rate = k[A][B], the units of k are obtained by dividing mol dm⁻³ s⁻¹ by (mol dm⁻³)², giving mol⁻¹ dm³ s⁻¹. The other options correspond to first order, zero order and third order respectively.

Question 9Physical chemistry

In an electrochemical cell, which change occurs at the anode during discharge?

  1. Oxidation, with electrons accepted from the external circuit
  2. Reduction, with electrons released to the external circuit
  3. Reduction, with electrons accepted from the external circuit
  4. Oxidation, with electrons released to the external circuit Correct answer.

Explanation

By definition the anode is the electrode where oxidation occurs, and in a discharging cell oxidation releases electrons that flow through the external circuit. Reduction occurs at the cathode, where electrons are accepted.

Question 10Physical chemistry

Which statement explains why the first electron affinity of chlorine is negative (exothermic)?

  1. The incoming electron is repelled by the nucleus, releasing energy.
  2. The chlorine atom loses an electron, which releases energy.
  3. The incoming electron is attracted to the nucleus and the energy released exceeds the repulsion from existing electrons. Correct answer.
  4. Electron affinity is always negative because energy is required to add an electron.

Explanation

When an electron is added to a gaseous chlorine atom it experiences attraction from the nucleus and is stabilised, releasing energy, so the enthalpy change is negative. There is some repulsion from existing electrons, but the net process is exothermic for chlorine.

Question 11Physical chemistry

Which statement describes the shape and bond angle of a molecule of sulfur dioxide, SO₂?

  1. Trigonal planar with a bond angle of 120°
  2. Bent with a bond angle of about 104.5°
  3. Linear with a bond angle of 180°
  4. Bent with a bond angle of about 119° Correct answer.

Explanation

The sulfur atom in SO₂ has three electron domains (two bonding regions and one lone pair), giving a trigonal planar electron-pair geometry and a bent molecular shape. The lone pair compresses the O–S–O angle slightly below 120°, to about 119°.

Question 12Physical chemistry

A sample of a hydrocarbon contains 0.200 mol of carbon and 0.400 mol of hydrogen. Its relative molecular mass is 42. What is its molecular formula?

  1. C₃H₆ Correct answer.
  2. C₃H₈
  3. C₂H₄
  4. CH₂

Explanation

The empirical formula from the mole ratio is CH₂, with an empirical formula mass of 14. Dividing the relative molecular mass of 42 by 14 gives 3, so the molecular formula is C₃H₆.

Question 13Physical chemistry

Which statement correctly explains why the melting point of magnesium is higher than that of sodium?

  1. Magnesium has a larger atomic radius, so its bonding is stronger.
  2. Sodium has a higher charge and therefore stronger metallic bonding.
  3. Magnesium forms covalent bonds, whereas sodium forms metallic bonds.
  4. Magnesium has more delocalised electrons and a higher charge, so the metallic bonding is stronger. Correct answer.

Explanation

Both metals have a giant metallic lattice, but Mg²⁺ ions with two delocalised electrons per atom give stronger electrostatic attraction than Na⁺ with one. This stronger metallic bonding requires more energy to overcome, so the melting point is higher.

Question 14Physical chemistry

For the reaction 2NO(g) + O₂(g) → 2NO₂(g), the rate equation is rate = k[NO]²[O₂]. What is the effect on the initial rate of doubling the concentration of NO while keeping [O₂] constant?

  1. The rate increases by a factor of eight.
  2. The rate increases by a factor of two.
  3. The rate remains unchanged.
  4. The rate increases by a factor of four. Correct answer.

Explanation

The rate is proportional to [NO]², so doubling [NO] multiplies the rate by 2² = 4. The concentration of O₂ is unchanged, so its term contributes no additional factor.

Question 15Physical chemistry

Which statement about the Boltzmann distribution is correct?

  1. The area under the curve increases as temperature increases.
  2. Increasing temperature increases the fraction of molecules with energy greater than the activation energy. Correct answer.
  3. Increasing temperature raises the activation energy of the reaction.
  4. At higher temperature the most probable energy of the molecules increases while the total number of molecules increases.

Explanation

Raising the temperature broadens the distribution and shifts it to higher energies, so a greater fraction of molecules has energy at least equal to the activation energy. The activation energy itself is a property of the reaction pathway and does not change with temperature.

Question 16Physical chemistry

Which statement about a buffer solution made from ethanoic acid and sodium ethanoate is correct?

  1. It works only when the concentrations of acid and salt are very different.
  2. It maintains a pH of exactly 7 when small amounts of acid or alkali are added.
  3. It resists pH change on adding acid because the ethanoic acid removes added H⁺.
  4. It resists pH change on adding small amounts of acid because the ethanoate ion removes added H⁺. Correct answer.

Explanation

In an acidic buffer, the conjugate base (ethanoate) neutralises added acid, while the weak acid neutralises added alkali, so the pH is stabilised. A buffer is most effective when the acid and salt concentrations are comparable, and its pH is not necessarily 7.

Question 17Physical chemistry

Which equation represents the lattice enthalpy of potassium chloride?

  1. K⁺(g) + Cl⁻(g) → KCl(s)
  2. K(g) + Cl(g) → KCl(s)
  3. KCl(s) → K⁺(aq) + Cl⁻(aq)
  4. KCl(s) → K⁺(g) + Cl⁻(g) Correct answer.

Explanation

Lattice enthalpy is defined as the enthalpy change when one mole of an ionic compound is separated into its gaseous ions, which corresponds to KCl(s) → K⁺(g) + Cl⁻(g). The reverse process is the exothermic formation of the lattice from gaseous ions.

Question 18Physical chemistry

Which statement explains why the entropy of a system increases when a solid dissolves in water?

  1. The temperature of the solution always falls during dissolving.
  2. The ions become more ordered as they are surrounded by water molecules.
  3. The number of particles decreases when the solid dissolves.
  4. The ordered lattice is broken up and the ions become more disordered in solution. Correct answer.

Explanation

Dissolving breaks down the ordered ionic lattice and disperses the ions among water molecules, giving a greater number of arrangements and hence higher entropy. Hydration does order water molecules locally, but this is outweighed by the increased disorder of the ions.

Question 19Physical chemistry

Which statement about the standard hydrogen electrode is correct?

  1. It is assigned an electrode potential of +1.00 V by convention.
  2. It consists of hydrogen gas at 1 atm bubbled over platinum in 1 mol dm⁻³ H⁺ at 298 K. Correct answer.
  3. It must be connected to the negative terminal of the voltmeter in every cell.
  4. It uses a silver wire in 1 mol dm⁻³ hydrochloric acid at 298 K.

Explanation

The standard hydrogen electrode uses hydrogen at 1 atm bubbling over a platinum electrode in an acidic solution of unit activity at 298 K, and it is assigned a potential of exactly 0.00 V. It can act as either anode or cathode depending on the other half-cell.

Question 20Physical chemistry

A reaction has ΔH = +58 kJ mol⁻¹ and ΔS = +170 J K⁻¹ mol⁻¹. At which temperature does the reaction first become feasible?

  1. About 341 K Correct answer.
  2. About 3410 K
  3. About 2.9 K
  4. About 0.34 K

Explanation

Feasibility requires ΔG = ΔH − TΔS ≤ 0, so T = ΔH/ΔS = 58 000 J mol⁻¹ ÷ 170 J K⁻¹ mol⁻¹ ≈ 341 K. Above this temperature the positive entropy term outweighs the positive enthalpy term.

Question 21Physical chemistry

Which statement about the equilibrium constant Kc is correct?

  1. Its value changes with temperature but is unaffected by changes in concentration or pressure. Correct answer.
  2. Its value increases when the concentration of a reactant is increased.
  3. Its value always equals the rate constant of the forward reaction.
  4. Its value changes when a catalyst is added.

Explanation

Kc is a thermodynamic quantity that depends only on temperature for a given reaction. Changing concentrations or pressure shifts the position of equilibrium but leaves Kc unchanged, and a catalyst affects only the rate of approach to equilibrium.

Question 22Physical chemistry

Which statement correctly compares the lattice enthalpy of magnesium oxide with that of sodium chloride?

  1. NaCl has a more exothermic lattice enthalpy because it contains more ions per formula unit.
  2. The two compounds have equal lattice enthalpies because both are ionic.
  3. MgO has a much more exothermic lattice enthalpy because the ions are doubly charged and smaller. Correct answer.
  4. MgO has a less exothermic lattice enthalpy because magnesium is a metal.

Explanation

Lattice enthalpy increases in magnitude with higher ionic charges and smaller ionic radii, since the electrostatic attraction is proportional to the product of charges divided by the distance between ions. Mg²⁺ and O²⁻ are both doubly charged and smaller than Na⁺ and Cl⁻, so MgO has a much more exothermic lattice enthalpy.

Question 23Physical chemistry

For a first-order reaction, the half-life is 20 minutes. What fraction of the reactant remains after 60 minutes?

  1. One sixth
  2. One eighth Correct answer.
  3. One third
  4. One half

Explanation

Sixty minutes corresponds to three half-lives, since 60 ÷ 20 = 3. After each half-life the amount halves, so the fraction remaining is (1/2)³ = 1/8.

Question 24Physical chemistry

Which statement explains why the ionic radius of Na⁺ is smaller than that of Na?

  1. The sodium ion has an extra electron shell compared with the atom.
  2. The sodium atom gains an electron, increasing the electron–electron repulsion.
  3. The sodium ion has a greater nuclear charge than the sodium atom.
  4. The sodium atom loses its outer 3s electron, so the remaining electrons are in a shell closer to the nucleus with the same nuclear charge. Correct answer.

Explanation

Forming Na⁺ removes the single 3s electron, leaving the 2s and 2p electrons in a shell closer to the nucleus while the nuclear charge is unchanged. The reduced shielding and smaller number of shells give a smaller ionic radius than the atom.

Question 25Physical chemistry

In a reaction mechanism, the rate-determining step is the slow step. Which observation would support this for a reaction whose rate equation is rate = k[CH₃COCH₃][H⁺]?

  1. The slow step involves iodine and CH₃COCH₃ but not H⁺.
  2. The slow step involves all three reactants equally.
  3. The slow step involves CH₃COCH₃ and H⁺ but not iodine. Correct answer.
  4. The slow step involves only iodine.

Explanation

The species appearing in the rate equation are those involved in the rate-determining step up to the point of the transition state, so the slow step must involve propanone and hydrogen ions. Iodine does not appear in the rate law, so it must enter in a fast step after the slow one.

Question 26Physical chemistry

Which statement describes the trend in first ionisation energy across Period 3 from sodium to argon?

  1. It decreases steadily because the atoms become larger across the period.
  2. It remains constant because each element has the same number of electron shells.
  3. It increases overall, but decreases between magnesium and aluminium and between phosphorus and sulfur. Correct answer.
  4. It increases steadily with no irregularities because the nuclear charge increases.

Explanation

Nuclear charge rises and shielding stays roughly constant across Period 3, so first ionisation energy generally increases. Two dips occur: Al's outer electron is in a 3p orbital, higher in energy than Mg's 3s, and in sulfur the 3p electrons are paired, giving extra electron–electron repulsion compared with phosphorus.

Question 27Physical chemistry

Which pair of species are structural isomers of each other?

  1. CH₃CH₂CHO and CH₃COCH₃
  2. CH₃CH=CH₂ and CH₃CH₂CH₃
  3. CH₃CH₂CH₃ and CH₃CH₂CH₂CH₃
  4. CH₃CH₂CH₂OH and CH₃CH(OH)CH₃ Correct answer.

Explanation

Structural isomers share a molecular formula but differ in how atoms are joined. Propan-1-ol and propan-2-ol both have the formula C₃H₈O but the hydroxyl group sits on a different carbon, so they are positional isomers.

Question 28Physical chemistry

Which statement about the trend in melting point down Group 2 from magnesium to barium is correct?

  1. Melting point increases because the ionic charge on each metal ion increases down the group.
  2. Melting point increases because the number of delocalised electrons per atom increases down the group.
  3. Melting point stays approximately constant because all Group 2 metals have the same packing arrangement.
  4. Melting point decreases because the atoms become larger and the delocalised electrons are further from the nuclei, weakening the metallic bond. Correct answer.

Explanation

Group 2 metals are held together by metallic bonding in which each atom contributes two delocalised electrons. Down the group the atomic radius increases, so the attraction between the cations and the delocalised electrons weakens and the melting point falls from magnesium to barium.

Question 29Physical chemistry

Which statement about the structure of a molecule of ammonia, NH₃, is correct?

  1. It is trigonal planar with a bond angle of 120°, because there are three bonding pairs.
  2. It is pyramidal with a bond angle of about 107°, because the lone pair repels the bonding pairs more strongly. Correct answer.
  3. It is T-shaped with a bond angle of 90°, because the lone pair forces the bonds together.
  4. It is linear with a bond angle of 180°, because the nitrogen atom is sp hybridised.

Explanation

Nitrogen in ammonia has three bonding pairs and one lone pair, giving four electron domains arranged tetrahedrally. The lone pair repels the bonding pairs more than they repel each other, compressing the H–N–H angle to about 107° and giving a pyramidal shape.

Question 30Physical chemistry

Which statement about the effect of increasing temperature on the rate of a reaction is correct?

  1. The activation energy of the reaction decreases, so more collisions are successful.
  2. The enthalpy change of the reaction becomes more negative, so the reaction proceeds faster.
  3. The frequency of collisions between reactant molecules decreases, but each collision is more energetic.
  4. A greater fraction of collisions has energy greater than the activation energy, so the rate increases. Correct answer.

Explanation

Raising the temperature shifts the Maxwell–Boltzmann distribution to higher energies, so a larger proportion of molecules has energy at or above the activation energy. This increases the rate constant and hence the rate; the activation energy itself is essentially unchanged.

Question 31Physical chemistry

Which statement about the oxidation state of manganese in the manganate(VII) ion, MnO₄⁻, is correct?

  1. It is +6, because the four oxide ions contribute −8 and the ion carries a single negative charge.
  2. It is +2, because manganese is a transition metal with variable oxidation states.
  3. It is +4, because manganese commonly forms MnO₂.
  4. It is +7, because each oxygen is −2 and the overall charge is −1. Correct answer.

Explanation

Each oxygen atom is assigned an oxidation state of −2, giving a total of −8 from the four oxygens. The ion has an overall charge of −1, so the manganese must be +7 to make the sum equal −1.

Question 32Physical chemistry

Which statement about the bonding in a crystal of sodium chloride is correct?

  1. It consists of a giant lattice of Na⁺ and Cl⁻ ions held by electrostatic attraction in a 6:6 coordination. Correct answer.
  2. It consists of Na⁺ and Cl⁻ ions held by London dispersion forces only.
  3. It consists of a giant covalent lattice of Na and Cl atoms sharing electrons.
  4. It consists of discrete NaCl molecules held together by covalent bonds.

Explanation

Sodium chloride is an ionic solid in which each Na⁺ ion is surrounded by six Cl⁻ ions and vice versa. The lattice is held together by strong electrostatic attractions between oppositely charged ions, which is why the melting point is high.

Question 33Physical chemistry

A gas occupies 250 cm³ at 300 K and 100 kPa. What volume would it occupy at 400 K and the same pressure, assuming ideal behaviour?

  1. 167 cm³
  2. 188 cm³
  3. 333 cm³ Correct answer.
  4. 400 cm³

Explanation

At constant pressure, Charles's law gives V₁/T₁ = V₂/T₂, so V₂ = V₁ × T₂/T₁ = 250 × 400/300 = 333 cm³. The volume increases in proportion to the absolute temperature.

Question 34Physical chemistry

Which statement about the reaction of chlorine with cold dilute aqueous sodium hydroxide is correct?

  1. Chlorine is oxidised to chlorate(V) ions, with the oxidation state changing from 0 to +5.
  2. Chlorine is reduced to chloride ions only, and no oxidation occurs.
  3. Chlorine disproportionates to give chloride ions and chlorate(I) ions, with the oxidation state of chlorine changing from 0 to −1 and +1. Correct answer.
  4. Chlorine reacts as an acid to neutralise the sodium hydroxide without a change in oxidation state.

Explanation

In cold dilute alkali, chlorine undergoes disproportionation: one chlorine atom is reduced from 0 to −1 (chloride) and another is oxidised from 0 to +1 (chlorate(I), ClO⁻). The overall equation is Cl₂ + 2NaOH → NaCl + NaClO + H₂O.

Question 35Physical chemistry

Which expression gives the pH of a solution of a weak acid, HA, of concentration c and acid dissociation constant Ka?

  1. pH = ½(pKa + log c)
  2. pH = pKa − log c
  3. pH = pKa + log c
  4. pH = ½(pKa − log c) Correct answer.

Explanation

For a weak acid, Ka = [H⁺]²/c if the dissociation is small, so [H⁺] = √(Ka·c). Taking negative logarithms gives pH = ½pKa − ½log c, which is the same as pH = ½(pKa − log c).

Question 36Physical chemistry

Which statement about the standard electrode potential of a half-cell is correct?

  1. It is measured relative to the standard hydrogen electrode under standard conditions of 298 K, 100 kPa and 1 mol dm⁻³ solutions. Correct answer.
  2. It is measured with the half-cell under any conditions, provided the cell is at equilibrium.
  3. It is the e.m.f. of a cell in which both electrodes are identical.
  4. It is the potential difference produced when the half-cell is connected to a silver electrode.

Explanation

Standard electrode potentials are defined relative to the standard hydrogen electrode, which is assigned 0.00 V. All solutions must be 1 mol dm⁻³, gases at 100 kPa and the temperature 298 K, so the value is reproducible.

Question 37Physical chemistry

Which statement explains why the entropy change of the surroundings, ΔS_surroundings, is negative for an endothermic reaction?

  1. Heat is absorbed from the surroundings, so the surroundings become more ordered and their entropy decreases. Correct answer.
  2. The reaction is spontaneous, so the surroundings must lose entropy to compensate.
  3. The activation energy of the reaction is high, so the surroundings lose entropy.
  4. The reaction increases the number of gas molecules, which decreases the entropy of the surroundings.

Explanation

The entropy change of the surroundings is given by ΔS_surroundings = −ΔH/T. For an endothermic reaction ΔH is positive, so heat flows from the surroundings into the system, lowering the thermal energy and entropy of the surroundings.

Question 38Physical chemistry

Which statement about the reaction of an alkene with hydrogen bromide in the presence of organic peroxides is correct?

  1. The reaction proceeds by nucleophilic substitution and gives an alcohol.
  2. The reaction proceeds by electrophilic addition and gives a dibromoalkane.
  3. The reaction proceeds by electrophilic addition and gives the Markovnikov product.
  4. The reaction proceeds by a free-radical mechanism and gives the anti-Markovnikov product. Correct answer.

Explanation

Peroxides generate radicals that add to the alkene to form the more stable carbon radical, so the bromine atom ends up on the less substituted carbon. This free-radical pathway gives the anti-Markovnikov product, the opposite of the normal electrophilic addition.

Question 39Physical chemistry

Which statement about the shape of the ethanoate ion, CH₃COO⁻, is correct?

  1. The carboxylate group is pyramidal with bond angles of about 109° because the carbon has a lone pair.
  2. The carboxylate group is linear with a bond angle of 180° because the carbon is sp hybridised.
  3. The carboxylate group is tetrahedral with bond angles of 109.5° because there are four single bonds.
  4. The carboxylate group is planar with bond angles of about 120° because the carbon is sp² hybridised with three electron domains. Correct answer.

Explanation

The carboxylate carbon is bonded to three atoms and has no lone pair, giving three electron domains and sp² hybridisation. The delocalised π system makes the group planar with angles close to 120°, and the two C–O bonds are identical in length.

Question 40Physical chemistry

A 25.0 cm³ sample of hydrochloric acid is titrated with 0.100 mol dm⁻³ sodium hydroxide. The average titre is 20.0 cm³. What is the concentration of the acid?

  1. 0.100 mol dm⁻³
  2. 0.125 mol dm⁻³
  3. 0.0500 mol dm⁻³
  4. 0.0800 mol dm⁻³ Correct answer.

Explanation

Moles of NaOH = 0.100 × 20.0/1000 = 2.00 × 10⁻³ mol. The acid and alkali react in a 1:1 ratio, so moles of HCl = 2.00 × 10⁻³ mol in 25.0 cm³, giving a concentration of 2.00 × 10⁻³/0.0250 = 0.0800 mol dm⁻³.

Question 41Physical chemistry

Which statement about the reaction of a halogenoalkane with aqueous potassium hydroxide is correct?

  1. A primary halogenoalkane reacts mainly by SN2, in which hydroxide attacks the carbon bearing the halogen from the opposite side to the leaving group. Correct answer.
  2. A primary halogenoalkane reacts mainly by SN1, in which a carbocation intermediate forms first.
  3. The reaction produces an alkene by elimination as the major pathway in aqueous conditions.
  4. The reaction produces an ether because the hydroxide ion acts as an electrophile.

Explanation

With a primary halogenoalkane, steric hindrance is small, so the SN2 pathway dominates: hydroxide attacks the δ+ carbon from the side opposite the halogen and the C–X bond breaks as the C–O bond forms. This gives inversion of configuration at the carbon.

Question 42Physical chemistry

Which statement about the equilibrium 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), ΔH = −197 kJ mol⁻¹, is correct?

  1. Increasing the temperature increases the yield of SO₃ because the forward reaction is exothermic.
  2. Increasing the pressure increases the yield of SO₃ because the equilibrium shifts to the side with fewer gas molecules. Correct answer.
  3. Adding a catalyst increases the equilibrium yield of SO₃.
  4. Removing SO₃ as it forms decreases the value of the equilibrium constant.

Explanation

There are three moles of gas on the left and two on the right, so increasing the pressure shifts the equilibrium to the right to reduce the number of gas molecules, increasing the SO₃ yield. This is the basis of the high-pressure conditions used in the Contact process.

Question 43Physical chemistry

Which statement about the lattice enthalpy of an ionic compound is correct?

  1. It becomes more exothermic as the charges on the ions increase and as the ionic radii decrease. Correct answer.
  2. It depends only on the ionic radii and not on the ionic charges.
  3. It becomes more endothermic as the charges on the ions increase.
  4. It is always less exothermic than the hydration enthalpy of the same ions.

Explanation

Lattice enthalpy depends on the electrostatic attraction between ions, which increases with higher ionic charges and shorter interionic distances. Therefore compounds such as MgO, with 2+ and 2− ions and small radii, have much more exothermic lattice enthalpies than NaCl.

Question 44Physical chemistry

Which statement about the reaction of bromine with aqueous iodide ions is correct?

  1. Bromine is reduced to bromide and iodide is oxidised to iodine, because bromine is the stronger oxidising agent. Correct answer.
  2. Iodide is reduced to iodine and bromine is oxidised to bromide, because iodine is the stronger oxidising agent.
  3. Iodine is displaced as a gas and bromine remains in solution as Br⁻.
  4. No reaction occurs because both bromine and iodine are halogens with similar reactivities.

Explanation

Bromine is above iodine in the halogen group, so it is the stronger oxidising agent. It takes electrons from iodide ions, being reduced to bromide, while iodide is oxidised to iodine, which is seen as a brown colour in solution.

Question 45Physical chemistry

Which statement about the ideal gas equation pV = nRT is correct?

  1. The temperature must be in degrees Celsius for the equation to be valid.
  2. R has the value 8.31 J K⁻¹ mol⁻¹ when p is in kPa and V is in cm³.
  3. R has the value 8.31 J K⁻¹ mol⁻¹ when p is in Pa and V is in m³. Correct answer.
  4. The equation applies exactly to all gases at all pressures and temperatures.

Explanation

The gas constant R = 8.31 J K⁻¹ mol⁻¹ is consistent with SI units, so pressure must be in pascals and volume in cubic metres. Temperature must be converted to kelvin by adding 273 to the Celsius value.

Question 46Physical chemistry

Which statement about the trend in thermal stability of the Group 2 carbonates down the group is correct?

  1. Thermal stability decreases down the group because the cation becomes more polarising.
  2. Thermal stability increases down the group because the larger cation polarises the carbonate ion less. Correct answer.
  3. Thermal stability is unchanged down the group because all the carbonates contain the same anion.
  4. Thermal stability increases down the group because the lattice enthalpy becomes more exothermic.

Explanation

A small, highly charged cation such as Mg²⁺ polarises the large carbonate ion strongly, weakening a C–O bond and lowering the decomposition temperature. Down the group the cation becomes larger and less polarising, so the carbonate is more thermally stable and barium carbonate requires a higher temperature to decompose.

Question 47Physical chemistry

In a fuel cell, hydrogen and oxygen react to produce water. Which statement about the cell is correct?

  1. Oxygen is oxidised at the anode, and the cell reaction is non-spontaneous.
  2. Hydrogen is oxidised at the anode, and the overall reaction has a positive E°cell because the cell reaction is spontaneous. Correct answer.
  3. Hydrogen is reduced at the cathode, and the cell requires an external supply of electrical energy to drive the reaction.
  4. Both hydrogen and oxygen are oxidised, and the cell produces no electrical work.

Explanation

In a hydrogen–oxygen fuel cell, hydrogen is oxidised at the anode and oxygen is reduced at the cathode. The overall cell reaction is spontaneous, so E°cell is positive and chemical energy is converted into electrical energy.

Question 48Physical chemistry

A reaction has a rate equation rate = k[A][B]². When the concentration of A is doubled and the concentration of B is halved, by what factor does the rate change?

  1. It is quartered.
  2. It is unchanged.
  3. It is halved. Correct answer.
  4. It is doubled.

Explanation

The rate is proportional to [A][B]². Doubling [A] multiplies the rate by 2, and halving [B] multiplies it by (½)² = ¼, so the overall factor is 2 × ¼ = ½. The rate is therefore halved.

Question 49Physical chemistry

The lattice enthalpy of silver chloride cannot be measured directly by Born–Haber cycle experiments and must be found indirectly. Which statement explains why the value obtained from a Born–Haber cycle may differ from the value calculated from electrostatic theory?

  1. The experimental value ignores the ionisation energy of silver, so it is always too small.
  2. The experimental value includes some covalent character in the Ag–Cl bond, so the lattice is more stable than a purely ionic model predicts. Correct answer.
  3. The theoretical model assumes that the ions are polarisable, which makes the calculated value too exothermic.
  4. The experimental value applies only at 0 K, whereas the theoretical value applies at 298 K.

Explanation

A purely ionic model treats the ions as point charges, but silver chloride has significant covalent character because Ag⁺ is strongly polarising. The extra covalent interaction makes the real lattice more stable than the simple electrostatic calculation predicts, so the Born–Haber value is more exothermic.

Question 50Physical chemistry

For the equilibrium N₂O₄(g) ⇌ 2NO₂(g), ΔH = +57 kJ mol⁻¹, a sample is allowed to reach equilibrium in a closed vessel. Which change increases both the value of Kp and the mole fraction of NO₂ at the new equilibrium?

  1. Increasing the temperature Correct answer.
  2. Adding a catalyst
  3. Decreasing the temperature
  4. Increasing the total pressure at constant temperature

Explanation

Because the forward reaction is endothermic, raising the temperature shifts the equilibrium to the right and increases Kp, so the mole fraction of NO₂ rises. Changes in pressure alter the position of equilibrium but leave Kp unchanged at constant temperature.