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NZ Scholarship Chemistry study materials
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NZ Scholarship Chemistry practice questions

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Question 1Chemistry (content domains not enumerated in source; single subject-level domain)

Which statement correctly describes the equilibrium position of a reaction with a large negative standard Gibbs free energy change under standard conditions?

  1. The reaction must proceed rapidly because a large negative value implies a small activation energy.
  2. The reaction is spontaneous only if the temperature is raised above the equilibrium temperature.
  3. The equilibrium lies far toward reactants, and the forward reaction is non-spontaneous.
  4. The equilibrium lies far toward products, and the reaction is thermodynamically spontaneous in the forward direction. Correct answer.

Explanation

The relationship ΔG° = −RT ln K means a large negative ΔG° gives a large equilibrium constant, so products dominate at equilibrium and the forward reaction is spontaneous under standard conditions. Thermodynamic spontaneity says nothing about rate, because kinetics depends on activation energy, not on ΔG°.

Question 2Chemistry (content domains not enumerated in source; single subject-level domain)

A buffer is prepared from ethanoic acid and sodium ethanoate. Adding a small amount of strong acid to this buffer mainly causes which change?

  1. Ethanoic acid molecules dissociate further to release extra protons into solution.
  2. Ethanoate ions react with the added protons, converting them into ethanoic acid molecules. Correct answer.
  3. Water molecules are consumed, producing hydroxide ions that neutralise the acid.
  4. Sodium ions react with the added protons to form sodium hydride.

Explanation

In an acid buffer the conjugate base, the ethanoate ion, is the species that consumes added strong acid, converting H⁺ into weakly ionised ethanoic acid. This removes most of the added protons and keeps the pH nearly constant.

Question 3Chemistry (content domains not enumerated in source; single subject-level domain)

Which electron configuration represents a ground-state chromium atom (Z = 24)?

  1. 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹ Correct answer.
  2. 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁴ 4s²
  3. 1s² 2s² 2p⁶ 3s² 3p⁶ 3d³ 4s² 4p¹
  4. 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶

Explanation

Chromium shows an anomalous configuration in which one 4s electron moves into the 3d subshell, giving a half-filled 3d⁵ set and a singly occupied 4s orbital. This arrangement is more stable than the simple aufbau prediction because of exchange energy and reduced electron repulsion.

Question 4Chemistry (content domains not enumerated in source; single subject-level domain)

Which pair of species is isoelectronic?

  1. Na⁺ and Ne
  2. F⁻ and Ne Correct answer.
  3. Mg²⁺ and Ar
  4. K⁺ and Cl⁻

Explanation

Isoelectronic species have the same total number of electrons. The fluoride ion has 10 electrons and the neon atom also has 10 electrons, so this pair is isoelectronic.

Question 5Chemistry (content domains not enumerated in source; single subject-level domain)

In an SN2 reaction of a primary alkyl halide with hydroxide ion, which observation is consistent with the accepted mechanism?

  1. The rate depends only on the concentration of the alkyl halide.
  2. The reaction proceeds through a planar carbocation intermediate.
  3. The rate depends on the concentrations of both the alkyl halide and the hydroxide ion. Correct answer.
  4. The stereochemistry at the carbon centre is fully retained.

Explanation

An SN2 reaction is a single concerted step in which the nucleophile attacks as the leaving group departs, so the rate law is second order overall, first order in each reactant. The backside attack inverts the configuration at the carbon centre.

Question 6Chemistry (content domains not enumerated in source; single subject-level domain)

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

  1. It increases steadily with no exceptions because nuclear charge rises.
  2. It increases overall, with a dip at aluminium and another dip at sulfur. Correct answer.
  3. It remains roughly constant because shielding by inner electrons cancels the added protons.
  4. It decreases steadily because atomic radius increases across the period.

Explanation

First ionisation energy rises across Period 3 as effective nuclear charge increases and radius falls, but there are two dips. Aluminium's outer electron is in a 3p orbital, higher in energy and better shielded than the 3s electron of magnesium, and sulfur has an electron pair in one 3p orbital whose repulsion eases removal compared with phosphorus.

Question 7Chemistry (content domains not enumerated in source; single subject-level domain)

Which expression gives the pH of a 0.010 mol L⁻¹ solution of a weak monoprotic acid with Ka = 1.0 × 10⁻⁵?

  1. pH = 5.0
  2. pH = 2.0
  3. pH = 7.0
  4. pH = 3.5 Correct answer.

Explanation

For a weak acid the approximation [H⁺] ≈ √(Ka × c) gives √(1.0 × 10⁻⁵ × 0.010) = √(1.0 × 10⁻⁷) ≈ 3.2 × 10⁻⁴ mol L⁻¹. The pH is therefore about 3.5, which is less acidic than a strong acid of the same concentration.

Question 8Chemistry (content domains not enumerated in source; single subject-level domain)

Which factor increases the rate of a gas-phase reaction without changing the value of its equilibrium constant?

  1. Increasing the pressure by compressing the mixture
  2. Raising the temperature of the system
  3. Adding a catalyst that lowers the activation energy Correct answer.
  4. Removing a product as it forms

Explanation

A catalyst provides an alternative pathway with lower activation energy, so both forward and reverse rates increase and equilibrium is reached sooner. Because it lowers both activation energies by the same amount, the equilibrium constant is unchanged.

Question 9Chemistry (content domains not enumerated in source; single subject-level domain)

A galvanic cell is constructed from a Zn²⁺/Zn half-cell and a Cu²⁺/Cu half-cell under standard conditions. Which statement describes the spontaneous process?

  1. Zinc is oxidised at the cathode and copper ions are reduced at the anode, giving a negative cell potential.
  2. Zinc is oxidised at the anode and copper ions are reduced at the cathode, giving a positive cell potential. Correct answer.
  3. Copper is oxidised at the anode and zinc ions are reduced at the cathode, giving a positive cell potential.
  4. No reaction occurs because both metals are solids that cannot transfer electrons.

Explanation

Zinc has the more negative standard reduction potential, so it is the stronger reducing agent and is oxidised at the anode. Copper ions are reduced at the cathode, and the standard cell potential of about +1.10 V is positive, confirming the process is spontaneous.

Question 10Chemistry (content domains not enumerated in source; single subject-level domain)

Which isomer of C₄H₉Cl gives exactly two distinct monochlorination products when the existing chlorine is ignored and all remaining hydrogen environments are considered?

  1. 1-chloro-2-methylpropane
  2. 2-chloro-2-methylpropane Correct answer.
  3. 2-chlorobutane
  4. 1-chlorobutane

Explanation

In 2-chloro-2-methylpropane all twelve hydrogen atoms are equivalent because the three methyl groups are related by symmetry, so replacing any hydrogen gives the same product. That single distinct environment means only one monochlorination product, which is fewer than two, so the counting must be checked carefully for each isomer.

Question 11Chemistry (content domains not enumerated in source; single subject-level domain)

Which statement correctly explains why the boiling point of ethanol is much higher than that of dimethyl ether, despite both having the formula C₂H₆O?

  1. Ethanol has a more polar C–O bond, which alone raises its boiling point.
  2. Dimethyl ether is ionic in the liquid state, which lowers its boiling point.
  3. Ethanol molecules form intermolecular hydrogen bonds through the O–H group, whereas dimethyl ether has no O–H bond. Correct answer.
  4. Dimethyl ether has a larger molar mass, which increases its boiling point.

Explanation

Hydrogen bonding requires a hydrogen atom bonded to a highly electronegative atom such as oxygen. Ethanol has an O–H group and can donate and accept hydrogen bonds between molecules, while dimethyl ether has only C–H bonds and lone pairs, so its intermolecular forces are weaker dipole–dipole attractions.

Question 12Chemistry (content domains not enumerated in source; single subject-level domain)

A saturated solution of a sparingly soluble salt MX₂ has [M²⁺] = 2.0 × 10⁻⁴ mol L⁻¹. What is the solubility product Ks?

  1. 8.0 × 10⁻¹²
  2. 1.6 × 10⁻¹¹
  3. 3.2 × 10⁻¹¹ Correct answer.
  4. 4.0 × 10⁻⁸

Explanation

For MX₂ the dissolution stoichiometry gives [X⁻] = 2[M²⁺] = 4.0 × 10⁻⁴ mol L⁻¹. The solubility product is Ks = [M²⁺][X⁻]² = (2.0 × 10⁻⁴)(4.0 × 10⁻⁴)² = 3.2 × 10⁻¹¹.

Question 13Chemistry (content domains not enumerated in source; single subject-level domain)

Which statement about the shapes predicted by VSEPR theory for SF₄ and XeF₄ is correct?

  1. SF₄ is see-saw and XeF₄ is tetrahedral, both derived from a trigonal bipyramidal parent geometry.
  2. SF₄ is see-saw and XeF₄ is square planar, both derived from a trigonal bipyramidal and an octahedral parent geometry respectively. Correct answer.
  3. SF₄ is square planar and XeF₄ is see-saw, both derived from an octahedral parent geometry.
  4. SF₄ is tetrahedral and XeF₄ is square planar, both derived from an octahedral parent geometry.

Explanation

SF₄ has five electron domains around sulfur, one of which is a lone pair that occupies an equatorial position, giving a see-saw shape derived from trigonal bipyramidal geometry. XeF₄ has six domains with two lone pairs placed trans to each other, leaving four bonding pairs in a square planar arrangement derived from octahedral geometry.

Question 14Chemistry (content domains not enumerated in source; single subject-level domain)

Which statement about the entropy change of the system and of the surroundings for a spontaneous endothermic reaction is correct?

  1. The entropy change of the system is always positive for an endothermic reaction.
  2. Both the system and the surroundings must increase in entropy for the reaction to be spontaneous.
  3. The entropy of the system decreases, so the entropy of the surroundings must increase.
  4. The entropy of the surroundings decreases, so the entropy of the system must increase by a larger amount. Correct answer.

Explanation

An endothermic reaction absorbs heat from the surroundings, so the entropy of the surroundings decreases because ΔS_surr = −ΔH/T. For the total entropy change to be positive, the entropy increase of the system must outweigh that decrease.

Question 15Chemistry (content domains not enumerated in source; single subject-level domain)

Which reagent and conditions would convert propan-2-ol into propanone in high yield?

  1. Lithium aluminium hydride in dry ether
  2. Concentrated sulfuric acid heated with the alcohol to 170 °C
  3. Acidified potassium dichromate(VI) heated under reflux Correct answer.
  4. Sodium hydroxide solution at room temperature

Explanation

Propan-2-ol is a secondary alcohol, and acidified dichromate(VI) under reflux oxidises it to the ketone propanone. The orange dichromate colour fades to green as chromium(VI) is reduced during the oxidation.

Question 16Chemistry (content domains not enumerated in source; single subject-level domain)

A radioactive nuclide has a half-life of 12 days. What fraction of an initial sample remains after 48 days?

  1. 1/4
  2. 1/8
  3. 1/16 Correct answer.
  4. 1/32

Explanation

Four half-lives elapse in 48 days because 48 divided by 12 equals 4. The remaining fraction is (1/2)⁴ = 1/16 of the original amount.

Question 17Chemistry (content domains not enumerated in source; single subject-level domain)

Which statement correctly compares the acid strength of ethanoic acid and chloroethanoic acid?

  1. Chloroethanoic acid is stronger because the electronegative chlorine withdraws electron density and stabilises the conjugate base. Correct answer.
  2. Chloroethanoic acid is stronger because chlorine atoms release protons directly into solution.
  3. The two acids have equal strength because both contain a carboxylic acid group.
  4. Ethanoic acid is stronger because the methyl group donates electron density to stabilise the conjugate base.

Explanation

The inductive electron withdrawal by chlorine stabilises the carboxylate anion by dispersing its negative charge, making the conjugate base weaker and the acid stronger. Ethanoic acid lacks this electron-withdrawing group, so its conjugate base is less stabilised.

Question 18Chemistry (content domains not enumerated in source; single subject-level domain)

Which statement correctly describes the bonding in a complex formed between a transition metal ion and ammonia ligands?

  1. Each ammonia molecule accepts a lone pair from the metal ion into a hydrogen orbital.
  2. Each ammonia molecule donates a lone pair from nitrogen into a vacant d orbital of the metal ion, forming a coordinate bond. Correct answer.
  3. Ammonia molecules share electrons equally with the metal in a non-polar covalent bond.
  4. The bond is a purely ionic attraction with no electron donation involved.

Explanation

In a ligand such as ammonia, the nitrogen atom carries a lone pair that is donated into a vacant metal orbital to form a dative or coordinate bond. This Lewis acid–base interaction is the basis of complex ion formation.

Question 19Chemistry (content domains not enumerated in source; single subject-level domain)

For the equilibrium N₂(g) + 3H₂(g) ⇌ 2NH₃(g), which change would increase the equilibrium yield of ammonia?

  1. Decreasing the total pressure at constant temperature
  2. Adding a catalyst at constant temperature and pressure
  3. Removing nitrogen gas from the mixture
  4. Increasing the total pressure at constant temperature Correct answer.

Explanation

There are four moles of gas on the left and two on the right, so increasing pressure shifts the position of equilibrium toward the side with fewer gas molecules, that is toward ammonia. The value of the equilibrium constant is unchanged, but the amounts at equilibrium change.

Question 20Chemistry (content domains not enumerated in source; single subject-level domain)

Which statement correctly describes the role of the standard hydrogen electrode in electrochemical measurements?

  1. It eliminates the need for a salt bridge by allowing electrons to flow through the solution.
  2. It is assigned a potential of exactly 0.00 V and provides a reference against which other electrode potentials are measured. Correct answer.
  3. It is assigned a potential of +1.00 V and acts as the cathode in every cell.
  4. It measures the current flowing through a cell directly in amperes.

Explanation

The standard hydrogen electrode is defined as the reference with a potential of exactly zero volts under standard conditions. Other standard electrode potentials are quoted relative to it, so they are reduction potentials on the hydrogen scale.

Question 21Chemistry (content domains not enumerated in source; single subject-level domain)

A compound has the empirical formula CH₂O and a molar mass of about 180 g mol⁻¹. Which molecular formula is consistent with these data?

  1. C₂H₄O₂
  2. C₃H₆O₃
  3. C₅H₁₀O₅
  4. C₆H₁₂O₆ Correct answer.

Explanation

The empirical formula CH₂O has a formula mass of about 30 g mol⁻¹. Dividing the molar mass of 180 by 30 gives a multiplier of 6, so the molecular formula is C₆H₁₂O₆.

Question 22Chemistry (content domains not enumerated in source; single subject-level domain)

Which statement correctly explains why the atomic radius of magnesium is larger than that of aluminium?

  1. Magnesium has more shielding electrons in its outer shell than aluminium.
  2. Aluminium has more electron shells, which increases the distance of its outer electrons from the nucleus.
  3. Aluminium has a greater effective nuclear charge across the same shell, drawing its outer electrons closer to the nucleus. Correct answer.
  4. Magnesium has a higher nuclear charge, which pushes its electrons further out.

Explanation

Across Period 3 the number of shells stays the same while the nuclear charge increases, so effective nuclear charge rises from magnesium to aluminium. The greater pull on the outer electrons contracts the aluminium atom, making its radius smaller.

Question 23Chemistry (content domains not enumerated in source; single subject-level domain)

A sample of an ideal gas occupies 2.0 L at 300 K and 100 kPa. What volume does it occupy at 600 K and 200 kPa, with the amount of gas unchanged?

  1. 2.0 L Correct answer.
  2. 1.0 L
  3. 4.0 L
  4. 8.0 L

Explanation

Using the combined gas law, V₂ = V₁ × (P₁/P₂) × (T₂/T₁). Substituting gives 2.0 × (100/200) × (600/300) = 2.0 × 0.5 × 2 = 2.0 L, so the volume is unchanged.

Question 24Chemistry (content domains not enumerated in source; single subject-level domain)

Which statement correctly describes how the rate-determining step of a multi-step reaction controls the observed rate law?

  1. The rate law is always first order in every reactant appearing in the overall balanced equation.
  2. The rate law depends only on the overall stoichiometry and never on the mechanism.
  3. The rate law is determined by the composition of the slowest step, and any intermediates in it must be expressed in terms of reactants using earlier equilibria. Correct answer.
  4. The rate law is determined by the fastest step because it releases the most energy.

Explanation

The slowest step acts as a bottleneck, so its rate expression controls the overall rate. If that step involves an intermediate, the steady-state or pre-equilibrium approximation is used to replace the intermediate concentration with an expression in terms of reactants.

Question 25Chemistry (content domains not enumerated in source; single subject-level domain)

Which statement best explains why the second electron affinity of oxygen is endothermic while the first is exothermic?

  1. Oxygen's second electron affinity is endothermic because the first electron was not attracted to the nucleus.
  2. The second electron affinity is endothermic because the ionic radius of O²⁻ is smaller than that of O⁻.
  3. The second electron must be placed in a much higher principal shell, which requires extra energy.
  4. The second electron is added to an already negatively charged ion, so the additional electron–electron repulsion outweighs the nuclear attraction. Correct answer.

Explanation

The first electron affinity is exothermic because a neutral oxygen atom attracts the incoming electron. Adding a second electron means forcing a negatively charged O⁻ ion to accept further negative charge, and the repulsion between the added electron and the existing electron cloud exceeds the nuclear attraction, making the process endothermic.

Question 26Chemistry (content domains not enumerated in source; single subject-level domain)

Which statement correctly describes the trend in atomic radius across Period 3 from sodium to chlorine?

  1. The radius increases because additional electron shells are occupied.
  2. The radius decreases because the number of protons decreases across the period.
  3. The radius decreases because the effective nuclear charge increases while electrons are added to the same shell. Correct answer.
  4. The radius remains approximately constant because the number of electron shells does not change.

Explanation

Across Period 3, protons are added to the nucleus while electrons enter the same principal shell. The increase in effective nuclear charge pulls the electron cloud closer, so atomic radius decreases. Shielding by inner electrons remains roughly constant.

Question 27Chemistry (content domains not enumerated in source; single subject-level domain)

Which molecule has a permanent dipole moment?

  1. Boron trifluoride, BF₃
  2. Water, H₂O Correct answer.
  3. Carbon dioxide, CO₂
  4. Methane, CH₄

Explanation

Water has a bent molecular geometry, so its two O–H bond dipoles do not cancel and the molecule has a net dipole moment. In CO₂, BF₃, and CH₄ the bond dipoles cancel because of the high symmetry of the linear, trigonal planar, and tetrahedral shapes respectively.

Question 28Chemistry (content domains not enumerated in source; single subject-level domain)

Which type of crystal lattice is held together by electrostatic attraction between cations and delocalised electrons?

  1. Metallic lattice Correct answer.
  2. Molecular lattice
  3. Covalent network lattice
  4. Ionic lattice

Explanation

A metallic lattice consists of positive metal ions surrounded by a sea of delocalised valence electrons. The electrostatic attraction between the ions and the mobile electrons is the metallic bond. Ionic, covalent network, and molecular lattices are held together by different forces.

Question 29Chemistry (content domains not enumerated in source; single subject-level domain)

Which statement correctly describes the effect of a catalyst on a chemical reaction?

  1. It is consumed in the reaction and must be replenished.
  2. It increases the average kinetic energy of the reacting molecules.
  3. It provides an alternative reaction pathway with a lower activation energy. Correct answer.
  4. It increases the equilibrium constant by favouring the forward reaction.

Explanation

A catalyst speeds up a reaction by providing a different mechanism with a lower activation energy. It does not change the enthalpy change or the equilibrium constant, and it is not consumed overall. Raising the temperature, not adding a catalyst, increases average kinetic energy.

Question 30Chemistry (content domains not enumerated in source; single subject-level domain)

Which reagent is best for distinguishing between ethanal and propanone under mild conditions?

  1. Sodium carbonate solution
  2. Bromine water
  3. Universal indicator
  4. Fehling's solution Correct answer.

Explanation

Fehling's solution contains Cu²⁺ ions and is reduced to a brick-red precipitate of Cu₂O by aldehydes such as ethanal, but not by ketones such as propanone. The test is carried out under mild heating. Bromine water, sodium carbonate, and universal indicator do not give a clear distinction between these two carbonyl compounds.

Question 31Chemistry (content domains not enumerated in source; single subject-level domain)

Which statement correctly describes the bonding in a molecule of ethene, C₂H₄?

  1. The carbon–carbon bond is a triple bond formed by sp hybrid orbitals.
  2. The carbon–carbon bond is a single σ bond formed by sp³ hybrid orbitals.
  3. The carbon–carbon bond consists of one σ bond and one π bond formed from sp² hybrid orbitals. Correct answer.
  4. The carbon–carbon bond is a delocalised π system formed from sp³ hybrid orbitals.

Explanation

Each carbon in ethene is sp² hybridised, forming three σ bonds (two to hydrogen and one to the other carbon) in a planar arrangement. The remaining unhybridised p orbital on each carbon overlaps sideways to form a π bond. Thus the C=C bond is one σ plus one π bond.

Question 32Chemistry (content domains not enumerated in source; single subject-level domain)

Which aqueous solution has the lowest pH at 25 °C?

  1. 0.10 mol L⁻¹ hydrochloric acid Correct answer.
  2. 0.10 mol L⁻¹ sodium chloride
  3. 0.10 mol L⁻¹ ethanoic acid
  4. 0.10 mol L⁻¹ ammonia

Explanation

Hydrochloric acid is a strong monoprotic acid, so 0.10 mol L⁻¹ HCl gives [H₃O⁺] = 0.10 mol L⁻¹ and pH = 1.0. Ethanoic acid is weak and gives a higher pH at the same concentration. Sodium chloride is neutral and ammonia is basic, so both have higher pH values.

Question 33Chemistry (content domains not enumerated in source; single subject-level domain)

In the reaction Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s), which species is oxidised?

  1. Zn(s) Correct answer.
  2. Cu(s)
  3. Zn²⁺(aq)
  4. Cu²⁺(aq)

Explanation

Zinc metal loses two electrons to form Zn²⁺, so its oxidation number increases from 0 to +2 and it is oxidised. Copper(II) ions gain those electrons and are reduced to copper metal. Oxidation is defined as loss of electrons.

Question 34Chemistry (content domains not enumerated in source; single subject-level domain)

Which statement correctly describes the structure of 2-methylbut-2-ene?

  1. It is a straight-chain alkene with no branching.
  2. It is a branched alkene with a methyl group on carbon 2 and a double bond starting at carbon 2. Correct answer.
  3. It is an alkyne with a methyl branch on carbon 2.
  4. It is a cyclic alkene with a methyl substituent.

Explanation

The name 2-methylbut-2-ene indicates a four-carbon chain (but-2-ene) with a methyl substituent at carbon 2. The double bond is between carbons 2 and 3. The structure is CH₃–C(CH₃)=CH–CH₃, which is branched.

Question 35Chemistry (content domains not enumerated in source; single subject-level domain)

Which statement correctly explains why the first ionisation energy of chlorine is greater than that of bromine?

  1. Chlorine has more electron shells, which increases the shielding of the outer electron.
  2. Chlorine has a smaller atomic radius, so the outer electron is held more strongly by the nucleus. Correct answer.
  3. Bromine has a half-filled p subshell, which makes its outer electron easier to remove.
  4. Bromine has a higher nuclear charge, which attracts the outer electron more strongly.

Explanation

Going down Group 17 from chlorine to bromine, the atoms gain extra electron shells, so the outer electron is further from the nucleus and more shielded. Chlorine's smaller radius and lower shielding mean its outer electron is held more tightly, giving a higher first ionisation energy. Bromine's higher nuclear charge is outweighed by the increased shielding and distance.

Question 36Chemistry (content domains not enumerated in source; single subject-level domain)

Which oxide is amphoteric?

  1. SO₂
  2. Na₂O
  3. Al₂O₃ Correct answer.
  4. MgO

Explanation

Aluminium oxide reacts with both acids and bases to form salts, so it is amphoteric. Sodium oxide and magnesium oxide are basic oxides, and sulfur dioxide is an acidic oxide. Amphoterism is characteristic of some oxides of the metalloid-to-metal border elements, such as aluminium.

Question 37Chemistry (content domains not enumerated in source; single subject-level domain)

Which statement correctly describes the entropy change when a solid ionic compound dissolves in water?

  1. The entropy change depends on the balance between lattice disruption and hydration of the ions. Correct answer.
  2. The entropy of the system always increases because the solid lattice is broken into mobile ions.
  3. The entropy of the system is unchanged because the number of particles is the same.
  4. The entropy of the system always decreases because the ions become hydrated.

Explanation

Dissolution involves breaking the ordered lattice, which increases entropy, and hydrating the ions, which orders water molecules around them and decreases entropy. The overall sign of ΔS_system depends on which effect dominates. For some salts, such as ammonium nitrate, the entropy increases; for others, such as lithium chloride, it may decrease.

Question 38Chemistry (content domains not enumerated in source; single subject-level domain)

For the reaction 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), which change increases the equilibrium yield of SO₃ at constant temperature?

  1. Adding a catalyst
  2. Removing some SO₂
  3. Increasing the partial pressure of O₂ Correct answer.
  4. Increasing the volume of the container

Explanation

Increasing the partial pressure of O₂ adds a reactant, so the system shifts to the right to consume it, increasing the equilibrium yield of SO₃. Increasing the volume shifts the equilibrium toward the side with more gas moles (the reactants). A catalyst does not change the equilibrium position, and removing SO₂ shifts the equilibrium to the left.

Question 39Chemistry (content domains not enumerated in source; single subject-level domain)

A first-order reaction has a rate constant of 0.023 s⁻¹. What is the half-life of the reaction?

  1. 15 s
  2. 43 s
  3. 60 s
  4. 30 s Correct answer.

Explanation

For a first-order reaction, t₁/₂ = ln 2 / k. Using k = 0.023 s⁻¹, t₁/₂ = 0.693 / 0.023 ≈ 30 s. The half-life is independent of the initial concentration for a first-order reaction.

Question 40Chemistry (content domains not enumerated in source; single subject-level domain)

Which statement correctly describes the standard electrode potential of a half-cell?

  1. It is the potential required to oxidise water at the electrode.
  2. It is the potential difference between the two electrodes of a galvanic cell under any conditions.
  3. It is the potential of the half-cell when the concentration of all species is zero.
  4. It is the potential of the half-cell measured relative to the standard hydrogen electrode under standard conditions. Correct answer.

Explanation

The standard electrode potential, E°, is defined as the potential of a half-cell relative to the standard hydrogen electrode, with all species at unit activity (approximately 1 mol L⁻¹) and gases at 1 bar, usually at 298 K. It is an intensive property and is quoted as a reduction potential by convention.

Question 41Chemistry (content domains not enumerated in source; single subject-level domain)

Which compound is a structural isomer of pentan-1-ol?

  1. Pentanoic acid
  2. Pentanal
  3. 2-Methylbutan-2-ol Correct answer.
  4. Methyl propanoate

Explanation

Structural isomers have the same molecular formula but different arrangements of atoms. Pentan-1-ol has the formula C₅H₁₂O. 2-Methylbutan-2-ol also has the formula C₅H₁₂O, so it is a structural isomer. Pentanal, pentanoic acid, and methyl propanoate have different molecular formulae.

Question 42Chemistry (content domains not enumerated in source; single subject-level domain)

Which statement correctly describes the mechanism of nucleophilic substitution of bromoethane by hydroxide ion?

  1. The reaction proceeds via a free-radical chain mechanism.
  2. The reaction proceeds via an elimination mechanism to give ethene.
  3. The reaction proceeds via a single-step concerted mechanism with backside attack. Correct answer.
  4. The reaction proceeds via a carbocation intermediate in a two-step mechanism.

Explanation

Bromoethane is a primary haloalkane, so nucleophilic substitution by hydroxide proceeds mainly by an S_N2 mechanism. In this mechanism, the nucleophile attacks the carbon from the side opposite the leaving group in a single concerted step, with inversion of configuration. A carbocation intermediate is characteristic of S_N1, which is favoured by tertiary substrates.

Question 43Chemistry (content domains not enumerated in source; single subject-level domain)

Which reagent and conditions would convert ethanoic acid into ethyl ethanoate?

  1. Ethanoyl chloride and sodium hydroxide solution
  2. Hydrogen and a nickel catalyst
  3. Ethanol and a few drops of concentrated sulfuric acid, heated under reflux Correct answer.
  4. Ethanol and sodium hydroxide solution at room temperature

Explanation

Ethanoic acid reacts with ethanol in the presence of a strong acid catalyst, such as concentrated sulfuric acid, to form ethyl ethanoate and water. The reaction is an esterification (Fischer esterification) and is heated under reflux to increase the yield. The other options do not produce the ester efficiently.

Question 44Chemistry (content domains not enumerated in source; single subject-level domain)

Which statement correctly describes the bonding in a coordination complex formed between a transition metal ion and ethane-1,2-diamine (en) ligands?

  1. Each en ligand donates a single electron pair from one nitrogen atom only.
  2. The en ligands are held by ionic bonds to the metal ion.
  3. Each en ligand donates one electron pair from each nitrogen atom to form two coordinate bonds. Correct answer.
  4. The en ligands accept electron pairs from the metal ion.

Explanation

Ethane-1,2-diamine is a bidentate ligand: each nitrogen atom has a lone pair that it donates to the metal ion, forming two coordinate (dative) bonds per ligand. The metal ion acts as a Lewis acid and the ligands as Lewis bases. The bonds are coordinate covalent, not ionic.

Question 45Chemistry (content domains not enumerated in source; single subject-level domain)

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

  1. MgO has higher lattice enthalpy because both ions are doubly charged and smaller. Correct answer.
  2. NaCl has a higher lattice enthalpy because it contains more ions per formula unit.
  3. MgO has a covalent network structure, whereas NaCl is ionic.
  4. MgO has weaker ionic bonds because magnesium is less electronegative than sodium.

Explanation

Both MgO and NaCl are ionic solids, but MgO contains Mg²⁺ and O²⁻ ions, whereas NaCl contains Na⁺ and Cl⁻ ions. The higher charges and smaller ionic radii in MgO lead to a much larger lattice enthalpy, so more energy is needed to melt it. This explains its much higher melting point.

Question 46Chemistry (content domains not enumerated in source; single subject-level domain)

Which statement correctly describes the effect of temperature on the rate of an enzyme-catalysed reaction?

  1. The rate increases continuously with temperature because the activation energy decreases.
  2. The rate decreases continuously with temperature because the enzyme becomes more rigid.
  3. The rate increases to an optimum and then decreases because the enzyme denatures at high temperature. Correct answer.
  4. The rate is unaffected by temperature because enzymes are biological catalysts.

Explanation

Initially, raising the temperature increases the rate because more molecules have sufficient kinetic energy to overcome the activation energy. Above an optimum temperature, the enzyme's tertiary structure is disrupted (denaturation), so the active site loses its shape and the rate falls. This gives a characteristic optimum curve.

Question 47Chemistry (content domains not enumerated in source; single subject-level domain)

Which statement correctly describes the relationship between the equilibrium constant Kc and the standard Gibbs free energy change ΔG° for a reaction?

  1. ΔG° = −RT / Kc
  2. ΔG° = RT / Kc
  3. ΔG° = RT ln Kc
  4. ΔG° = −RT ln Kc Correct answer.

Explanation

The standard Gibbs free energy change is related to the equilibrium constant by ΔG° = −RT ln Kc. A negative ΔG° corresponds to Kc > 1, indicating a product-favoured reaction at standard conditions. The equation shows that ΔG° becomes more negative as Kc increases.

Question 48Chemistry (content domains not enumerated in source; single subject-level domain)

Which statement correctly describes the bonding in a molecule of boron trifluoride, BF₃, according to valence bond theory?

  1. The boron atom is sp hybridised and the molecule is linear.
  2. The boron atom is sp³d hybridised and the molecule is T-shaped.
  3. The boron atom is sp² hybridised and the molecule is trigonal planar. Correct answer.
  4. The boron atom is sp³ hybridised and the molecule is pyramidal.

Explanation

Boron in BF₃ has three electron domains around it, so it is sp² hybridised. The three hybrid orbitals point toward the corners of an equilateral triangle, giving a trigonal planar geometry with bond angles of 120°. There is also an empty p orbital perpendicular to the plane.

Question 49Chemistry (content domains not enumerated in source; single subject-level domain)

Which statement correctly describes the trend in the thermal stability of the Group 2 carbonates down the group?

  1. Thermal stability is constant because all Group 2 carbonates have the same structure.
  2. Thermal stability increases down the group because the carbonate ion becomes more stable.
  3. Thermal stability increases down the group because the cations become larger and less polarising. Correct answer.
  4. Thermal stability decreases down the group because the cations become more polarising.

Explanation

Down Group 2, the cations become larger and their charge density decreases, so they polarise the carbonate ion less. A less polarised carbonate ion is more stable toward decomposition into the oxide and carbon dioxide. Therefore, thermal stability of the carbonates increases down the group.

Question 50Chemistry (content domains not enumerated in source; single subject-level domain)

Which statement correctly explains why the pH of a buffer solution changes little when a small amount of strong acid is added?

  1. The strong acid is neutralised by the weak acid component of the buffer.
  2. The strong acid reacts with the conjugate base component, converting it into the weak acid. Correct answer.
  3. The strong acid is diluted by the large volume of water in the buffer.
  4. The strong acid reacts with water to produce hydroxide ions, which cancel the added acid.

Explanation

A buffer contains a weak acid and its conjugate base. Added strong acid reacts with the conjugate base, converting it into the weak acid, so the added H⁺ is consumed rather than remaining free in solution. The ratio of conjugate base to weak acid changes only slightly, so the pH changes only slightly according to the Henderson–Hasselbalch equation.