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NCEA L3 Chemistry study materials
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NCEA L3 Chemistry practice questions

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Question 1Chemistry (NCEA Level 3 content unspecified by source)

Which statement correctly describes the equilibrium constant expression for the reaction N2(g) + 3H2(g) ⇌ 2NH3(g)?

  1. Kc = [NH3]^2 / ([N2][H2]^3) Correct answer.
  2. Kc = ([N2][H2]^3) / [NH3]^2
  3. Kc = [NH3]^2 / ([N2]^2[H2]^3)
  4. Kc = [NH3] / ([N2][H2])

Explanation

For a general equilibrium aA + bB ⇌ cC + dD, the equilibrium constant is the product of product concentrations raised to their stoichiometric coefficients divided by the product of reactant concentrations raised to theirs. Here NH3 has coefficient 2, N2 has 1 and H2 has 3, giving Kc = [NH3]^2 / ([N2][H2]^3). Pure solids and liquids are excluded, but all species here are gases.

Question 2Chemistry (NCEA Level 3 content unspecified by source)

A solution has a hydrogen ion concentration of 1.0 × 10^-3 mol L^-1 at 25 °C. What is its pH?

  1. 14.00
  2. 1.0 × 10^-3
  3. 3.00 Correct answer.
  4. 11.00

Explanation

pH is defined as the negative base-10 logarithm of the hydrogen ion concentration. Taking -log10(1.0 × 10^-3) gives exactly 3.00. The value is dimensionless and reported to the same number of significant figures as the concentration.

Question 3Chemistry (NCEA Level 3 content unspecified by source)

Which particle is emitted when a nucleus undergoes beta-minus decay?

  1. A proton
  2. A helium nucleus
  3. An electron Correct answer.
  4. A neutron

Explanation

In beta-minus decay a neutron in the nucleus converts into a proton, emitting an electron (the beta particle) and an antineutrino. The electron is created at the moment of decay and ejected from the nucleus. The mass number stays the same while the atomic number increases by one.

Question 4Chemistry (NCEA Level 3 content unspecified by source)

Which type of bonding is present in solid sodium chloride?

  1. Metallic bonding
  2. Hydrogen bonding
  3. Ionic bonding Correct answer.
  4. Covalent network bonding

Explanation

Sodium chloride is formed from a metal cation (Na+) and a non-metal anion (Cl-), held together by strong electrostatic attraction in a three-dimensional lattice. This electron-transfer-based attraction is ionic bonding. The lattice explains its high melting point and its conductivity only when molten or dissolved.

Question 5Chemistry (NCEA Level 3 content unspecified by source)

Which factor increases the rate of a chemical reaction between two gases?

  1. Increasing the pressure by compressing the mixture Correct answer.
  2. Lowering the temperature of the mixture
  3. Adding a catalyst that is consumed in the reaction
  4. Reducing the surface area of the reactants

Explanation

Compressing a gas mixture increases the concentration of the particles, so they collide more frequently per unit time, raising the rate. Temperature, catalysts and surface area also affect rate, but only a genuine increase in collision frequency or successful collisions raises it.

Question 6Chemistry (NCEA Level 3 content unspecified by source)

Which statement about a catalyst is correct?

  1. It lowers the activation energy by providing an alternative reaction pathway. Correct answer.
  2. It is used up permanently during the reaction.
  3. It increases the activation energy so fewer collisions succeed.
  4. It changes the enthalpy change of the reaction.

Explanation

A catalyst speeds a reaction by offering a pathway with a lower activation energy, so a greater fraction of collisions are successful at a given temperature. It is regenerated at the end and does not alter the enthalpy change, which depends only on reactants and products.

Question 7Chemistry (NCEA Level 3 content unspecified by source)

Which pair of species are conjugate acid–base pairs?

  1. H3O+ and OH-
  2. NH4+ and NH3 Correct answer.
  3. HCl and NaCl
  4. NH3 and H2O

Explanation

A conjugate acid–base pair differs by exactly one proton (H+). NH4+ is the acid that donates a proton to become its conjugate base NH3, so the two form a conjugate pair. The other pairs differ by more than a single proton or are not related in this way.

Question 8Chemistry (NCEA Level 3 content unspecified by source)

Which observation is expected when a piece of magnesium is added to dilute hydrochloric acid?

  1. A colour change to blue as copper is displaced
  2. Effervescence as hydrogen gas is produced Correct answer.
  3. A white precipitate of magnesium chloride forming immediately
  4. No reaction because magnesium is less reactive than hydrogen

Explanation

Magnesium is above hydrogen in the activity series, so it displaces hydrogen from the acid, producing magnesium chloride in solution and hydrogen gas, seen as bubbles. The reaction is exothermic and the metal dissolves.

Question 9Chemistry (NCEA Level 3 content unspecified by source)

A reaction has an equilibrium constant Kc much greater than 1 at a given temperature. What does this indicate?

  1. The equilibrium position lies well towards the reactants.
  2. The reaction is endothermic.
  3. The equilibrium position lies well towards the products. Correct answer.
  4. The reaction is very fast.

Explanation

A large Kc means the product concentrations raised to their powers greatly exceed the reactant concentrations at equilibrium, so the position lies to the right. Kc is a thermodynamic quantity and gives no information about reaction rate or enthalpy.

Question 10Chemistry (NCEA Level 3 content unspecified by source)

A buffer solution is prepared from ethanoic acid and sodium ethanoate. Which statement best explains its buffering action?

  1. The sodium ions react with both added acid and added base.
  2. The acid neutralises added base while the conjugate base neutralises added acid. Correct answer.
  3. The buffer resists changes in pH by completely preventing any reaction.
  4. The solution has a very high pH so added acid is diluted.

Explanation

A buffer contains a weak acid and its conjugate base in appreciable amounts. Added H+ is removed by the conjugate base (ethanoate), and added OH- is removed by the weak acid, so the pH changes only slightly. Both components must be present for buffering in either direction.

Question 11Chemistry (NCEA Level 3 content unspecified by source)

For the reaction 2SO2(g) + O2(g) ⇌ 2SO3(g), which change would increase the equilibrium yield of SO3?

  1. Raising the temperature given that the forward reaction is exothermic
  2. Increasing the total pressure Correct answer.
  3. Decreasing the concentration of O2
  4. Adding a catalyst

Explanation

There are three moles of gas on the left and two on the right, so increasing pressure shifts the equilibrium towards the side with fewer gas moles, increasing SO3. Removing a reactant or raising temperature for an exothermic reaction shifts the position away from products, and a catalyst changes neither yield nor position.

Question 12Chemistry (NCEA Level 3 content unspecified by source)

Which reagent and condition are suitable for oxidising a primary alcohol to a carboxylic acid?

  1. Acidified potassium dichromate(VI) under reflux Correct answer.
  2. Concentrated sulfuric acid at room temperature
  3. Sodium borohydride in methanol
  4. Bromine water at room temperature

Explanation

Acidified dichromate(VI) is a strong oxidising agent that converts a primary alcohol first to an aldehyde and then, under reflux, to a carboxylic acid. Distillation would allow the aldehyde to be collected instead.

Question 13Chemistry (NCEA Level 3 content unspecified by source)

Which isomerism is shown by butan-1-ol and butan-2-ol?

  1. Positional isomerism Correct answer.
  2. Optical isomerism
  3. Geometric (cis–trans) isomerism
  4. Chain isomerism

Explanation

Both compounds share the molecular formula C4H10O and the same functional group, but the hydroxyl group is attached to a different carbon in the chain. This is positional isomerism. Chain isomers would have different carbon skeletons, and the others require restricted rotation or chirality.

Question 14Chemistry (NCEA Level 3 content unspecified by source)

Which reagent is used to distinguish an aldehyde from a ketone?

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

Explanation

Fehling's solution contains copper(II) ions that are reduced to a brick-red copper(I) oxide precipitate by an aldehyde but not by a ketone. This makes it a test for the aldehyde group specifically.

Question 15Chemistry (NCEA Level 3 content unspecified by source)

A radioactive isotope has a half-life of 8.0 days. What fraction of the original sample remains after 24 days?

  1. 1/6
  2. 1/24
  3. 1/8 Correct answer.
  4. 1/3

Explanation

24 days equals three half-lives (24 ÷ 8.0 = 3). Each half-life halves the amount, so the fraction remaining is (1/2)^3 = 1/8. Half-life is the time for half the nuclei to decay and is independent of the starting amount.

Question 16Chemistry (NCEA Level 3 content unspecified by source)

Which statement about the spontaneity of a reaction is correct at constant temperature and pressure?

  1. A reaction is spontaneous when the entropy change is negative.
  2. A reaction is spontaneous only when the enthalpy change is negative.
  3. A reaction is spontaneous when the Gibbs free energy change is negative. Correct answer.
  4. A reaction is spontaneous when the Gibbs free energy change is positive.

Explanation

The Gibbs free energy change combines enthalpy and entropy: ΔG = ΔH − TΔS. A negative ΔG indicates a spontaneous process at constant temperature and pressure. A negative ΔH favours spontaneity but is not required if the entropy term is large enough.

Question 17Chemistry (NCEA Level 3 content unspecified by source)

Which species acts as a Lewis acid in the reaction BF3 + NH3 → F3B–NH3?

  1. BF3 Correct answer.
  2. F3B–NH3
  3. NH3
  4. H+

Explanation

A Lewis acid is an electron-pair acceptor. BF3 has an incomplete octet and accepts the lone pair from NH3 to form a dative bond, so BF3 is the Lewis acid and NH3 is the Lewis base. The product is the adduct.

Question 18Chemistry (NCEA Level 3 content unspecified by source)

Which change would shift the equilibrium of a reaction with a negative enthalpy change in the forward direction towards the products?

  1. Lowering the temperature Correct answer.
  2. Adding a catalyst
  3. Raising the temperature
  4. Increasing the volume of the container for a reaction with equal moles of gas on both sides

Explanation

For an exothermic forward reaction, lowering the temperature removes heat, and by Le Chatelier's principle the system shifts in the exothermic direction to oppose the change, favouring products. The equilibrium constant increases as temperature falls.

Question 19Chemistry (NCEA Level 3 content unspecified by source)

Which statement about the strength of intermolecular forces is correct?

  1. Intermolecular forces are stronger than covalent bonds.
  2. Dipole–dipole forces require a permanent dipole and are stronger than hydrogen bonds.
  3. London dispersion forces are always stronger than hydrogen bonding.
  4. Hydrogen bonding is stronger than dipole–dipole forces, which are stronger than London dispersion forces of similar molecular size. Correct answer.

Explanation

Hydrogen bonds, involving H attached to N, O or F interacting with a lone pair, are the strongest common intermolecular force. Permanent dipole–dipole attractions come next, and London dispersion forces are weakest for molecules of comparable size and polarisability. All of these are far weaker than covalent bonds.

Question 20Chemistry (NCEA Level 3 content unspecified by source)

Which half-equation is correctly balanced for the reduction of dichromate in acidic solution?

  1. Cr2O7^2- + 14H+ → 2Cr3+ + 7H2O + 6e-
  2. Cr2O7^2- + 14H+ + 6e- → 2Cr3+ + 7H2O Correct answer.
  3. Cr2O7^2- + 7H2O + 6e- → 2Cr3+ + 14OH-
  4. Cr2O7^2- + 6e- → 2Cr3+ + 7O2

Explanation

In acidic solution, balance oxygen with water and hydrogen with H+, then balance charge with electrons. Two dichromate ions require 14 H+ and yield 2 Cr3+ plus 7 H2O, with 6 electrons consumed because each Cr goes from +6 to +3.

Question 21Chemistry (NCEA Level 3 content unspecified by source)

The pH of a 0.10 mol L^-1 solution of a weak acid with Ka = 1.0 × 10^-5 is closest to which value?

  1. 1.0
  2. 3.0 Correct answer.
  3. 7.0
  4. 5.0

Explanation

For a weak acid, [H+] ≈ √(Ka × c) when the acid is only slightly ionised. Here Ka × c = 1.0 × 10^-5 × 0.10 = 1.0 × 10^-6, so [H+] ≈ 1.0 × 10^-3 mol L^-1 and pH ≈ 3.0. The approximation is valid because ionisation is small.

Question 22Chemistry (NCEA Level 3 content unspecified by source)

A galvanic cell is built from Zn2+/Zn (E° = −0.76 V) and Cu2+/Cu (E° = +0.34 V). Which statement about the cell is correct?

  1. Copper is oxidised at the anode and the standard cell potential is +1.10 V.
  2. Zinc is reduced at the cathode and the standard cell potential is −1.10 V.
  3. The cell potential is +0.42 V because the electrode potentials are added directly.
  4. Zinc is oxidised at the anode and the standard cell potential is +1.10 V. Correct answer.

Explanation

The more negative electrode potential belongs to zinc, which is therefore oxidised at the anode. The standard cell potential is E°(cathode) − E°(anode) = 0.34 − (−0.76) = +1.10 V, and a positive value confirms the reaction is spontaneous.

Question 23Chemistry (NCEA Level 3 content unspecified by source)

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

  1. Magnesium has a lower first ionisation energy, so its lattice is more stable.
  2. Magnesium has more delocalised electrons per atom and a higher charge, strengthening metallic bonding. Correct answer.
  3. Magnesium forms covalent bonds between atoms, whereas sodium is metallic.
  4. Sodium has a larger atomic radius, which increases the strength of its metallic bonds.

Explanation

Both are metals, but Mg contributes two valence electrons per atom to the delocalised sea and has a 2+ cation, giving stronger electrostatic attraction than Na's single electron and 1+ cation. Stronger metallic bonding means more energy is needed to melt the solid.

Question 24Chemistry (NCEA Level 3 content unspecified by source)

An organic reaction converts propan-2-ol into propanone. Which description of the change is correct?

  1. It is a substitution in which the hydroxyl group is replaced by a chlorine atom.
  2. It is an elimination in which an alkene is formed.
  3. It is a reduction in which a ketone becomes a secondary alcohol.
  4. It is an oxidation in which a secondary alcohol becomes a ketone. Correct answer.

Explanation

Propan-2-ol is a secondary alcohol, and oxidation removes two hydrogen atoms from the carbon bearing the OH and its neighbour to give the ketone propanone. Acidified dichromate(VI) is a suitable oxidant. The reverse process would be reduction.

Question 25Chemistry (NCEA Level 3 content unspecified by source)

Which equation correctly represents the ionisation of a weak monoprotic acid, HA, in water?

  1. HA(aq) + OH−(aq) ⇌ H2O(l) + A−(aq)
  2. HA(aq) ⇌ H+(aq) + A−(aq) with no water shown as a reactant
  3. HA(aq) + H2O(l) ⇌ H3O+(aq) + A−(aq) Correct answer.
  4. HA(aq) + H2O(l) → H3O+(aq) + A−(aq)

Explanation

A weak acid ionises only partially, so an equilibrium arrow is required, and in aqueous solution the proton is transferred to a water molecule to give the hydronium ion H3O+. Writing the equilibrium with H3O+ as the product correctly shows both the partial ionisation and the species actually present in water.

Question 26Chemistry (NCEA Level 3 content unspecified by source)

In the reaction N2(g) + 3H2(g) ⇌ 2NH3(g), the equilibrium constant expression Kc is written as which ratio?

  1. [NH3]^2 / ([N2][H2]^3) Correct answer.
  2. [NH3] / ([N2][H2])
  3. [NH3]^2 / ([N2]^3[H2])
  4. [N2][H2]^3 / [NH3]^2

Explanation

For an equilibrium, Kc equals the product of product concentrations each raised to its stoichiometric coefficient, divided by the same product for the reactants. Here NH3 has coefficient 2 in the numerator, while N2 and H2 have coefficients 1 and 3 respectively in the denominator.

Question 27Chemistry (NCEA Level 3 content unspecified by source)

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

  1. The potential measured at any temperature and concentration of the ion
  2. The potential measured when the half-cell is connected to a hydrogen electrode under standard conditions Correct answer.
  3. The voltage generated when the half-cell is short-circuited with itself
  4. The potential measured when the half-cell is connected to a silver electrode under any conditions

Explanation

Standard electrode potentials are defined relative to the standard hydrogen electrode, whose potential is assigned zero, with all solutions at 1 mol L−1, gases at standard pressure and a temperature of 298 K. Comparing a half-cell to this reference under standard conditions gives its E° value.

Question 28Chemistry (NCEA Level 3 content unspecified by source)

Which functional group is present in the compound CH3CH2COOCH3?

  1. Carboxylic acid
  2. Ketone
  3. Aldehyde
  4. Ester Correct answer.

Explanation

The structure contains a carbonyl carbon bonded to an oxygen atom that also links to a methyl group, giving the –COO– linkage characteristic of an ester. The alkyl group on each side of the –COO– group confirms it is an ester rather than a carboxylic acid.

Question 29Chemistry (NCEA Level 3 content unspecified by source)

Which particle is emitted when carbon-14 decays to nitrogen-14?

  1. A gamma photon only
  2. An alpha particle
  3. A beta particle Correct answer.
  4. A neutron

Explanation

Carbon-14 has 6 protons and 8 neutrons, while nitrogen-14 has 7 protons and 7 neutrons, so the mass number stays the same while a neutron is converted into a proton. This conversion releases a beta particle (an electron) and an antineutrino, so the atomic number increases by one.

Question 30Chemistry (NCEA Level 3 content unspecified by source)

Which type of bonding is present throughout a crystal of sodium chloride?

  1. Covalent bonding between all ions
  2. Hydrogen bonding between chloride ions
  3. Ionic bonding between Na+ and Cl− ions Correct answer.
  4. Metallic bonding between sodium atoms

Explanation

Sodium chloride is a giant ionic lattice in which each sodium ion is attracted electrostatically to surrounding chloride ions and vice versa. The strong electrostatic attractions in all directions give the high melting point and brittleness typical of ionic solids.

Question 31Chemistry (NCEA Level 3 content unspecified by source)

Which statement about the entropy change, ΔS, for the sublimation of solid iodine, I2(s) → I2(g), is correct?

  1. ΔS is zero because the same substance is present on both sides
  2. ΔS is positive because the reaction is exothermic
  3. ΔS is positive because the gaseous state is more disordered than the solid state Correct answer.
  4. ΔS is negative because energy is absorbed during the change

Explanation

Entropy is a measure of dispersal of energy and matter, and a gas has far more accessible microstates than a solid. Sublimation therefore increases the disorder of the system, giving a positive ΔS.

Question 32Chemistry (NCEA Level 3 content unspecified by source)

Which pair of reagents would convert propene into propan-1-ol?

  1. Bromine water followed by sodium hydroxide
  2. Concentrated sulfuric acid followed by water Correct answer.
  3. Acidified potassium permanganate
  4. Hydrogen with a nickel catalyst

Explanation

Reacting propene with concentrated sulfuric acid gives an alkyl hydrogen sulfate, which on hydrolysis with water yields an alcohol. Markovnikov addition places the –OH on the more substituted carbon, but the direct hydration route is the standard method described here.

Question 33Chemistry (NCEA Level 3 content unspecified by source)

A solution has a hydrogen ion concentration of 1.0 × 10^−3 mol L−1. What is its pH?

  1. −3.0
  2. 11.0
  3. 3.0 Correct answer.
  4. 1.0 × 10^−3

Explanation

pH is defined as −log10[H3O+], so taking the negative logarithm of 1.0 × 10^−3 gives exactly 3.0. The pH scale is logarithmic, so a tenfold change in hydrogen ion concentration changes the pH by one unit.

Question 34Chemistry (NCEA Level 3 content unspecified by source)

In the reaction 2Na(s) + Cl2(g) → 2NaCl(s), which species is oxidised?

  1. Chlorine, because its oxidation number decreases from 0 to −1
  2. Sodium, because it gains electrons
  3. Sodium, because its oxidation number increases from 0 to +1 Correct answer.
  4. Chlorine, because it gains electrons

Explanation

Oxidation is an increase in oxidation number, and sodium goes from 0 in the element to +1 in sodium chloride, so it is oxidised and loses electrons. Chlorine is reduced because its oxidation number falls from 0 to −1.

Question 35Chemistry (NCEA Level 3 content unspecified by source)

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

  1. The conjugate base in the buffer reacts with the added H3O+ ions, converting them into the weak acid Correct answer.
  2. The buffer dilutes the added acid so its concentration becomes negligible
  3. The buffer neutralises the acid by raising the temperature of the solution
  4. The buffer precipitates the added acid as an insoluble salt

Explanation

A buffer contains a weak acid and its conjugate base in comparable amounts, and the conjugate base reacts with added H3O+ to form more weak acid. Because the added acid is consumed by this reaction rather than remaining as free H3O+, the pH shifts only slightly.

Question 36Chemistry (NCEA Level 3 content unspecified by source)

Which product forms when propan-2-ol is heated under reflux with acidified potassium dichromate(VI)?

  1. Propanal
  2. Propanoic acid
  3. Propene
  4. Propanone Correct answer.

Explanation

Propan-2-ol is a secondary alcohol, and oxidation of a secondary alcohol gives a ketone, so propanone is produced. Further oxidation to a carboxylic acid would require breaking a carbon–carbon bond, which does not occur under these conditions.

Question 37Chemistry (NCEA Level 3 content unspecified by source)

Which statement about the reaction quotient Q compared with the equilibrium constant Kc is correct?

  1. If Q < Kc, the reaction proceeds in the forward direction until Q equals Kc Correct answer.
  2. If Q = Kc, the reaction has stopped completely
  3. If Q < Kc, the reaction proceeds in the reverse direction until Q equals Kc
  4. If Q > Kc, the reaction is at equilibrium

Explanation

The reaction quotient has the same form as Kc but uses current concentrations. When Q is less than Kc, the ratio of products to reactants is too small, so the forward reaction is favoured until Q rises to equal Kc.

Question 38Chemistry (NCEA Level 3 content unspecified by source)

Which change would increase the rate of the reaction between marble chips (calcium carbonate) and dilute hydrochloric acid?

  1. Cooling the reaction mixture in an ice bath
  2. Diluting the hydrochloric acid with water
  3. Using larger marble chips of the same total mass
  4. Using powdered calcium carbonate of the same total mass Correct answer.

Explanation

Reaction rate depends on the frequency of effective collisions, which increases with surface area. Powdered calcium carbonate exposes far more surface area than large chips of the same mass, so collisions with acid particles occur more frequently and the rate increases.

Question 39Chemistry (NCEA Level 3 content unspecified by source)

Which structural feature makes a molecule chiral?

  1. Two identical groups attached to the same carbon atom
  2. A carbon atom bonded to four different groups Correct answer.
  3. A plane of symmetry passing through the molecule
  4. A carbon–carbon double bond that can show cis–trans isomerism

Explanation

A chiral centre is a carbon atom bonded to four different groups, which makes the molecule non-superimposable on its mirror image. This lack of symmetry is what produces optical isomerism.

Question 40Chemistry (NCEA Level 3 content unspecified by source)

Which observation would be expected when aqueous silver nitrate is added to a solution containing chloride ions?

  1. A white precipitate that dissolves in dilute ammonia solution Correct answer.
  2. A cream precipitate that dissolves only in concentrated ammonia
  3. No precipitate forms
  4. A yellow precipitate that is insoluble in ammonia

Explanation

Chloride ions react with silver ions to form silver chloride, a white precipitate. Silver chloride dissolves readily in dilute ammonia solution because ammonia removes silver ions by forming a soluble complex, distinguishing it from bromide and iodide.

Question 41Chemistry (NCEA Level 3 content unspecified by source)

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

  1. It decreases the enthalpy change of the reaction
  2. It increases the rate of the forward reaction only, shifting equilibrium to the right
  3. It lowers the activation energy for both directions equally, so equilibrium position is unchanged Correct answer.
  4. It increases the value of the equilibrium constant at constant temperature

Explanation

A catalyst provides an alternative pathway with lower activation energy for both the forward and reverse reactions by the same amount. Both rates increase equally, so equilibrium is reached faster but the position and the value of Kc remain unchanged at a given temperature.

Question 42Chemistry (NCEA Level 3 content unspecified by source)

Which statement explains why the melting point of ice is lower than that of liquid water's boiling point in terms of intermolecular forces?

  1. Boiling involves breaking covalent bonds within water molecules
  2. Melting requires only partial disruption of hydrogen bonds, while boiling requires complete disruption of them Correct answer.
  3. Melting and boiling involve identical changes in intermolecular forces
  4. Melting requires breaking covalent O–H bonds, which needs more energy than boiling

Explanation

Both processes involve overcoming hydrogen bonds between water molecules, but melting only allows molecules to move past one another while boiling requires molecules to escape completely from the liquid. More energy is therefore needed to boil water than to melt ice.

Question 43Chemistry (NCEA Level 3 content unspecified by source)

A galvanic cell has E°cell = +1.10 V. Which statement about the reaction in this cell is correct?

  1. The reaction is at equilibrium and ΔG° is zero
  2. The reaction is non-spontaneous and ΔG° is positive
  3. The reaction is spontaneous under standard conditions and ΔG° is negative Correct answer.
  4. The reaction is spontaneous but ΔG° is positive

Explanation

The relationship ΔG° = −nFE°cell shows that a positive E°cell gives a negative ΔG°, which indicates a spontaneous reaction under standard conditions. The larger the positive cell potential, the more negative the free energy change.

Question 44Chemistry (NCEA Level 3 content unspecified by source)

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

  1. It generally decreases because atomic radius increases
  2. It generally increases because nuclear charge increases while shielding stays roughly constant Correct answer.
  3. It stays constant because the number of electron shells is the same
  4. It increases only because atomic radius increases

Explanation

Across a period, protons are added to the nucleus while electrons enter the same shell, so the effective nuclear charge increases and the outer electrons are held more tightly. This generally raises the first ionisation energy, with small dips at aluminium and sulfur due to subshell and electron-pairing effects.

Question 45Chemistry (NCEA Level 3 content unspecified by source)

Which equation represents the reduction of MnO4− in acidic solution?

  1. MnO4− + 8H+ + 5e− → Mn2+ + 4H2O Correct answer.
  2. MnO4− + 4H+ + 3e− → MnO2 + 2H2O
  3. MnO4− → Mn2+ + 4H2O + 5e−
  4. MnO4− + 8H+ + 3e− → Mn2+ + 4H2O

Explanation

In acidic solution permanganate is reduced to Mn2+, and the half-equation must balance atoms and charge. Adding 8H+ and 4H2O balances the oxygen and hydrogen, and 5 electrons are needed so that the total charge is +2 on both sides.

Question 46Chemistry (NCEA Level 3 content unspecified by source)

A 0.10 mol L−1 solution of a weak acid has Ka = 1.0 × 10^−5. Which value is closest to the pH of this solution?

  1. 1.0
  2. 3.0 Correct answer.
  3. 7.0
  4. 5.0

Explanation

For a weak acid, [H3O+] is approximately the square root of Ka multiplied by the acid concentration, giving the square root of 1.0 × 10^−6, which is 1.0 × 10^−3 mol L−1. The pH is therefore about 3.0, showing only partial ionisation compared with a strong acid of the same concentration.

Question 47Chemistry (NCEA Level 3 content unspecified by source)

Which statement correctly describes the role of entropy and enthalpy in determining whether a reaction is spontaneous at constant temperature and pressure?

  1. Spontaneity requires ΔG to be negative, where ΔG = ΔH − TΔS Correct answer.
  2. Spontaneity requires both ΔH and ΔS to be negative
  3. Spontaneity requires ΔS to be negative regardless of ΔH
  4. Spontaneity requires ΔH to be negative regardless of ΔS

Explanation

The Gibbs free energy change combines enthalpy and entropy through ΔG = ΔH − TΔS, and a process is spontaneous when ΔG is negative. This means an endothermic reaction can still be spontaneous if the entropy increase is large enough at the given temperature.

Question 48Chemistry (NCEA Level 3 content unspecified by source)

The half-life of a radioisotope is 12 hours. What fraction of an original sample remains after 48 hours?

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

Explanation

The number of half-lives is 48 divided by 12, which equals 4. Each half-life halves the remaining amount, so the fraction remaining is (1/2)^4, which equals 1/16.

Question 49Chemistry (NCEA Level 3 content unspecified by source)

In the reaction between ethanal and hydrogen cyanide, followed by hydrolysis, which type of reaction is the first step?

  1. Electrophilic addition to the carbonyl oxygen
  2. Elimination of water to form an alkene
  3. Nucleophilic addition to the carbonyl carbon Correct answer.
  4. Free radical substitution at the methyl group

Explanation

The carbonyl carbon is electron-poor and is attacked by the cyanide ion, which acts as a nucleophile, forming a cyanohydrin. This nucleophilic addition is characteristic of aldehydes and ketones and is followed by hydrolysis of the nitrile to a carboxylic acid.

Question 50Chemistry (NCEA Level 3 content unspecified by source)

Which Lewis structure feature best accounts for the trigonal planar geometry of the BF3 molecule?

  1. Four bonding pairs arranged tetrahedrally on boron
  2. Three bonding pairs and no lone pairs on boron Correct answer.
  3. Two bonding pairs and two lone pairs on boron
  4. Three bonding pairs and one lone pair on boron

Explanation

VSEPR theory predicts that electron regions around a central atom repel each other and spread as far apart as possible. Boron in BF3 has three bonding regions and no lone pairs, giving a trigonal planar arrangement with bond angles of about 120° rather than a bent or tetrahedral shape.