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MATSEC AM Chemistry study materials
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MATSEC AM Chemistry practice questions

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Question 1Chemistry (AM syllabus content)

In the reaction Cr2O7^2- + 14 H+ + 6 e- → 2 Cr^3+ + 7 H2O, how does the oxidation state of each chromium atom change?

  1. From +6 to +3 Correct answer.
  2. From +7 to +3
  3. From +3 to +6
  4. From +6 to zero

Explanation

Seven oxygens contribute minus fourteen and the ion totals minus two, so each chromium starts at plus six. The product chromium ion is plus three, a three-unit reduction per atom.

Question 2Chemistry (AM syllabus content)

Which statement correctly describes the bonding in a molecule of ethyne (C2H2)?

  1. Each carbon is sp hybridised and the two carbons share one sigma and two pi bonds. Correct answer.
  2. Each carbon is sp hybridised and the carbons are joined by two sigma and one pi bond.
  3. Each carbon is sp2 hybridised and the carbons are joined by a single sigma bond.
  4. Each carbon is sp3 hybridised and the carbons are joined by one sigma bond.

Explanation

Each carbon in ethyne is bonded to one hydrogen and one carbon, so two electron domains give sp hybridisation and linear geometry. The triple bond consists of one sigma bond formed by head-on overlap of sp orbitals and two pi bonds formed by sideways overlap of the remaining p orbitals.

Question 3Chemistry (AM syllabus content)

A reaction has an activation energy of 50 kJ mol-1 without a catalyst and 30 kJ mol-1 with a catalyst at the same temperature. By what factor does the rate constant increase, given that e^8 ≈ 2980?

  1. About 1.7 times larger
  2. About 8 times larger
  3. About 2980 times larger Correct answer.
  4. About 20 times larger

Explanation

The Arrhenius equation gives k proportional to e^(-Ea/RT), so the ratio of rate constants is e^(ΔEa/RT) where ΔEa = 20 kJ mol-1 = 20000 J mol-1. At 298 K, RT ≈ 2478 J mol-1, so ΔEa/RT ≈ 8.07 and the ratio is approximately e^8 ≈ 2980.

Question 4Chemistry (AM syllabus content)

Which reagent and conditions are most suitable for converting propan-2-ol into propanone?

  1. Acidified potassium dichromate(VI) heated under reflux Correct answer.
  2. Concentrated sulfuric acid heated to 170 °C
  3. Sodium hydroxide in aqueous solution at room temperature
  4. Acidified potassium dichromate(VI) with immediate distillation of the product

Explanation

Propan-2-ol is a secondary alcohol, so oxidation with acidified dichromate(VI) gives the ketone propanone, and reflux ensures complete oxidation. Unlike a primary alcohol, the secondary alcohol cannot be oxidised further to a carboxylic acid, so distillation is unnecessary.

Question 5Chemistry (AM syllabus content)

Which species is the strongest Brønsted-Lowry base in aqueous solution?

  1. Cl-
  2. NH3
  3. NH2- (amide ion) Correct answer.
  4. H2O

Explanation

The amide ion is the conjugate base of ammonia, a very weak acid, so its conjugate base is extremely strong and reacts essentially completely with water. Chloride and water are conjugate bases of strong acids (HCl and H3O+), making them negligible bases in aqueous solution.

Question 6Chemistry (AM syllabus content)

What is the standard enthalpy of formation of a compound?

  1. The enthalpy change when one mole of the compound is formed from its gaseous ions
  2. The enthalpy change when one mole of the compound is completely burned in oxygen
  3. The enthalpy change when one mole of the compound dissolves in excess water
  4. The enthalpy change when one mole of the compound forms from its elements in their standard states Correct answer.

Explanation

By definition, the standard enthalpy of formation refers to one mole of product formed from its constituent elements, each in its standard state at 100 kPa and a stated temperature. Because elements in their standard states are the reference zero, the value is meaningful for comparing compounds.

Question 7Chemistry (AM syllabus content)

Which statement explains why the first ionisation energy of magnesium is higher than that of sodium?

  1. The outer electron of magnesium experiences a greater effective nuclear charge and is in the same shell. Correct answer.
  2. Magnesium has a half-filled 3s subshell that confers extra stability on the atom.
  3. Magnesium has a larger atomic radius, so its outer electron is held less tightly.
  4. Magnesium has a lower nuclear charge than sodium, so its outer electron is removed more easily.

Explanation

Across Period 3 the nuclear charge increases while the outer electrons remain in the same shell, so shielding changes little and the effective nuclear charge rises. The 3s electron of magnesium is therefore held more strongly than that of sodium, giving a higher first ionisation energy.

Question 8Chemistry (AM syllabus content)

In a mass spectrometer, what is the purpose of the ionisation chamber?

  1. To accelerate ions so that all have the same kinetic energy
  2. To detect the abundance of each ion reaching the collector
  3. To convert vaporised sample atoms or molecules into positive ions by electron bombardment Correct answer.
  4. To separate ions according to their mass-to-charge ratio

Explanation

The sample must be ionised before it can be deflected by electric and magnetic fields, and electron impact is the usual method in a simple instrument. The resulting positive ions are then accelerated, deflected and detected in later stages.

Question 9Chemistry (AM syllabus content)

Which pair of half-equations correctly describes the electrode reactions when molten lead(II) bromide is electrolysed using inert electrodes?

  1. Cathode: 2H2O + 2e- → H2 + 2OH-; anode: 2Br- → Br2 + 2e-
  2. Cathode: 2Br- → Br2 + 2e-; anode: Pb2+ + 2e- → Pb
  3. Cathode: Pb2+ + 2e- → Pb; anode: 2Br- → Br2 + 2e- Correct answer.
  4. Cathode: Pb2+ + 2e- → Pb; anode: 2H2O → O2 + 4H+ + 4e-

Explanation

In the molten salt only Pb2+ and Br- ions are present, so reduction of Pb2+ occurs at the cathode and oxidation of Br- occurs at the anode. The absence of water means the water-based half-equations cannot apply.

Question 10Chemistry (AM syllabus content)

Which compound shows cis-trans (E/Z) isomerism?

  1. 2-methylpropene
  2. Propene
  3. But-1-ene
  4. But-2-ene Correct answer.

Explanation

Cis-trans isomerism requires a C=C bond with two different groups attached to each carbon. In but-2-ene each doubly bonded carbon carries a hydrogen and a methyl group, so two distinct arrangements exist; the other alkenes have two identical hydrogens on at least one carbon.

Question 11Chemistry (AM syllabus content)

A buffer is prepared from ethanoic acid and sodium ethanoate. Which change best explains its resistance to a small addition of hydrochloric acid?

  1. Ethanoic acid molecules donate protons to the added chloride ions.
  2. Ethanoate ions remove the added protons, forming more ethanoic acid. Correct answer.
  3. The added acid is diluted by the large volume of water in the buffer.
  4. The added acid is neutralised by sodium ions present in solution.

Explanation

A buffer contains a reservoir of the conjugate base, which reacts with added strong acid: CH3COO- + H+ → CH3COOH. This removes most of the added protons and the pH changes only slightly, because the ratio of acid to conjugate base alters little.

Question 12Chemistry (AM syllabus content)

Which quantum number determines the shape of an atomic orbital?

  1. The magnetic quantum number, m
  2. The azimuthal (subsidiary) quantum number, l Correct answer.
  3. The spin quantum number, s
  4. The principal quantum number, n

Explanation

The value of l fixes the type of subshell (s, p, d or f) and therefore the shape of the orbital. The principal quantum number sets the shell and size, the magnetic quantum number sets orientation, and the spin quantum number describes the electron's spin.

Question 13Chemistry (AM syllabus content)

Which statement about the entropy change is correct for the reaction N2(g) + 3H2(g) → 2NH3(g)?

  1. ΔS is positive because ammonia has a larger molar mass than hydrogen.
  2. ΔS is positive because the reaction is exothermic.
  3. ΔS is negative because four moles of gas form two moles of gas. Correct answer.
  4. ΔS is zero because the number of atoms is conserved.

Explanation

Gases have far higher entropy than liquids or solids, so a decrease in the number of moles of gas greatly reduces the number of accessible microstates. Here the mole count falls from four to two, so the entropy of the system decreases and ΔS is negative.

Question 14Chemistry (AM syllabus content)

Which observation would be expected when chlorine gas is bubbled through aqueous potassium bromide?

  1. The solution turns orange-brown as bromide ions are oxidised to bromine. Correct answer.
  2. A violet vapour is produced as iodide ions are oxidised.
  3. A colourless gas is evolved and the solution remains unchanged.
  4. A white precipitate of potassium chloride forms immediately.

Explanation

Chlorine is a stronger oxidising agent than bromine, so it displaces bromine from bromide ions: Cl2 + 2Br- → 2Cl- + Br2. The liberated bromine colours the solution orange-brown, illustrating the trend in oxidising power down Group 17.

Question 15Chemistry (AM syllabus content)

Which factor increases the lattice enthalpy (makes it more exothermic) of an ionic compound?

  1. Decreasing the ionic radii while keeping the charges the same Correct answer.
  2. Increasing the temperature of the solid without changing its structure
  3. Increasing the ionic radii while keeping the charges the same
  4. Replacing a doubly charged ion with a singly charged ion of similar size

Explanation

Lattice enthalpy depends on the electrostatic attraction between ions, which increases as the charges rise and as the distance between ion centres falls. Smaller ions of the same charge are closer together, so the lattice enthalpy becomes more exothermic.

Question 16Chemistry (AM syllabus content)

Which reagent set will distinguish ethanal from propanone?

  1. Bromine water, which is decolourised by propanone only
  2. Litmus paper, which turns red with ethanal only
  3. Sodium carbonate solution, which gives carbon dioxide with propanone only
  4. Fehling's solution, which gives a red precipitate with ethanal only Correct answer.

Explanation

Fehling's solution contains copper(II) ions that are reduced to copper(I) oxide by an aldehyde but not by a ketone. Ethanal therefore gives a red precipitate on warming, while propanone gives no such change.

Question 17Chemistry (AM syllabus content)

Which expression gives the pH of a 0.010 mol dm-3 solution of a weak acid with Ka = 1.0 x 10^-5 mol dm-3?

  1. pH = -log(√(Ka × c)) = 3.5 Correct answer.
  2. pH = 14 + log(c) = 16.0
  3. pH = -log(Ka) = 5.0
  4. pH = -log(c) = 2.0

Explanation

For a weak acid, [H+] ≈ √(Ka × c) when dissociation is small compared with the initial concentration. Substituting gives √(1.0 x 10^-5 × 0.010) = √(1.0 x 10^-7) ≈ 3.2 x 10^-4 mol dm-3, so pH ≈ 3.5.

Question 18Chemistry (AM syllabus content)

Which mechanism operates when 2-bromopropane reacts with aqueous sodium hydroxide?

  1. Electrophilic addition across the carbon-bromine bond
  2. Nucleophilic addition of hydroxide to the C-Br bond
  3. Nucleophilic substitution by an SN1 pathway via a planar carbocation Correct answer.
  4. Free-radical substitution at the tertiary carbon

Explanation

2-Bromopropane is a secondary, sterically hindered halide and the nucleophile is a neutral solvent, so ionisation to a carbocation is favoured and substitution proceeds mainly by SN1. The planar carbocation intermediate can be attacked from either face, giving some racemisation when the carbon is chiral.

Question 19Chemistry (AM syllabus content)

Which statement about the transition elements is correct?

  1. They act as oxidising agents only and never as catalysts.
  2. They show only one stable oxidation state in all their compounds.
  3. Their ions have completely filled d subshells in every oxidation state.
  4. They form coloured compounds because of d-d electronic transitions. Correct answer.

Explanation

In transition metal complexes, ligands split the d orbitals into sets of different energy, and absorption of visible light promotes an electron between them. This d-d transition gives the characteristic colours, and the energy gap depends on the ligand and oxidation state.

Question 20Chemistry (AM syllabus content)

Which type of isomerism is shown by the pair [Co(NH3)6]Cl3 and [CoCl(NH3)5]Cl2·NH3?

  1. Ionisation isomerism Correct answer.
  2. Optical isomerism
  3. Chain isomerism
  4. Geometric (cis-trans) isomerism

Explanation

Ionisation isomers give different ions in solution because the counter-ion and a ligand exchange places, so they produce different precipitates with test reagents. Optical and geometric isomerism require specific spatial arrangements within the same complex ion, which is not the difference here.

Question 21Chemistry (AM syllabus content)

Which reagent is used in the test for a carbonyl group that gives a silver mirror with an aldehyde?

  1. Fehling's solution
  2. Bromine water
  3. Acidified potassium manganate(VII)
  4. Tollens' reagent (ammoniacal silver nitrate) Correct answer.

Explanation

Tollens' reagent contains the diamminesilver(I) ion, which is reduced to metallic silver by an aldehyde, depositing a mirror on the glass. Ketones do not reduce it under the mild conditions used, so the test distinguishes aldehydes from ketones.

Question 22Chemistry (AM syllabus content)

A first-order reaction has a half-life of 20 minutes. What fraction of the reactant remains after 60 minutes?

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

Explanation

For a first-order reaction the half-life is constant, so 60 minutes corresponds to three half-lives. Each half-life halves the remaining amount, giving (1/2)^3 = 1/8 of the original reactant.

Question 23Chemistry (AM syllabus content)

Which statement correctly compares the standard electrode potentials of the halogens?

  1. E°(F2/F-) is the least positive, so fluoride is easily oxidised.
  2. The electrode potentials of the halogens are all zero by definition.
  3. E°(Cl2/Cl-) is more positive than E°(Br2/Br-), so chlorine is the stronger oxidising agent. Correct answer.
  4. E°(I2/I-) is the most positive, so iodine is the strongest oxidising agent of the halogens.

Explanation

Standard electrode potentials become less positive down Group 17, reflecting the decreasing tendency to gain electrons. A more positive E° means a greater tendency to be reduced, so chlorine oxidises bromide while fluorine is the strongest oxidising halogen of all.

Question 24Chemistry (AM syllabus content)

How many structural isomers (including stereoisomers) have the molecular formula C4H8 and contain a carbon-carbon double bond?

  1. Five
  2. Two
  3. Three
  4. Four Correct answer.

Explanation

The alkenes with formula C4H8 are but-1-ene, 2-methylpropene and the two cis-trans forms of but-2-ene. Counting the stereoisomers of but-2-ene separately gives four structures in total.

Question 25Chemistry (AM syllabus content)

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 the Markovnikov product.
  3. The reaction proceeds by a free-radical mechanism and gives the anti-Markovnikov product. Correct answer.
  4. The reaction proceeds by elimination and gives a shorter alkane.

Explanation

Peroxides generate radicals that add the bromine atom to the less substituted carbon first, placing the radical on the more substituted carbon. Hydrogen abstraction then gives the anti-Markovnikov product, in contrast with the ionic Markovnikov addition observed without peroxides.

Question 26Chemistry (AM syllabus content)

Which property of an element is defined as the energy required to remove one electron from each ion in one mole of gaseous 2+ ions?

  1. Electron affinity
  2. Lattice enthalpy
  3. Second ionisation energy Correct answer.
  4. Third ionisation energy

Explanation

Ionisation energy is always defined per mole of gaseous species, and the ordinal counts the electron being removed. Removing an electron from a 2+ ion gives a 3+ ion, so this is the third ionisation energy, not the second. The second ionisation energy would start from 1+ ions.

Question 27Chemistry (AM syllabus content)

Which species has the electron configuration 1s2 2s2 2p6 3s2 3p6 3d5?

  1. Cr3+
  2. Mn2+ Correct answer.
  3. Fe3+
  4. Fe2+

Explanation

Manganese (Z = 25) loses the two 4s electrons first when forming Mn2+, leaving the 3d5 configuration with 23 electrons. Fe3+ and Cr3+ would each give 3d3 after removing 4s electrons, and Fe2+ would give 3d6. Counting total electrons is the reliable check.

Question 28Chemistry (AM syllabus content)

Which statement about the shape of the PF5 molecule is correct?

  1. It is tetrahedral with bond angles of 109.5°.
  2. It is square pyramidal with bond angles of about 90°.
  3. It is trigonal bipyramidal with bond angles of 90° and 120°. Correct answer.
  4. It is octahedral with bond angles of 90°.

Explanation

Phosphorus in PF5 has five bonding pairs and no lone pairs, so VSEPR predicts a trigonal bipyramid. That geometry contains two axial bonds at 90° to the equatorial plane and three equatorial bonds at 120° to each other. Square pyramidal and octahedral shapes require six electron domains.

Question 29Chemistry (AM syllabus content)

Which statement best explains why the boiling point of hydrogen fluoride is higher than that of hydrogen chloride?

  1. Hydrogen fluoride is ionic whereas hydrogen chloride is covalent.
  2. Hydrogen fluoride molecules form hydrogen bonds with each other. Correct answer.
  3. Hydrogen fluoride has a larger electron cloud and stronger London dispersion forces.
  4. The H–F bond is longer than the H–Cl bond, so more energy is released on boiling.

Explanation

Boiling points of simple molecular hydrides depend on intermolecular forces, not on bond strength. Fluorine is small and highly electronegative, so HF molecules form strong hydrogen bonds, whereas HCl has only dipole–dipole and dispersion forces. Hydrogen bonding raises the boiling point markedly.

Question 30Chemistry (AM syllabus content)

Which statement about the equilibrium constant Kc is correct for an exothermic reaction?

  1. Kc is unaffected by temperature changes.
  2. Kc changes only when a catalyst is added.
  3. Kc increases as temperature increases.
  4. Kc decreases as temperature increases. Correct answer.

Explanation

For an exothermic forward reaction, raising the temperature shifts the position of equilibrium towards the reactants. That reduces the ratio of products to reactants at equilibrium, so Kc falls. Kc depends only on temperature, not on pressure, concentration or catalysts.

Question 31Chemistry (AM syllabus content)

In a titration, 25.0 cm3 of 0.100 mol dm-3 ethanoic acid is neutralised by 0.100 mol dm-3 sodium hydroxide. Which volume of sodium hydroxide is required?

  1. 2.50 cm3
  2. 12.5 cm3
  3. 50.0 cm3
  4. 25.0 cm3 Correct answer.

Explanation

Ethanoic acid reacts with sodium hydroxide in a 1:1 mole ratio. Equal concentrations therefore require equal volumes, so 25.0 cm3 of NaOH neutralises 25.0 cm3 of the acid. Calculating moles (0.00250 mol) and dividing by the alkali concentration confirms the volume.

Question 32Chemistry (AM syllabus content)

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

  1. R has units of Pa m3 K-1 and volume must always be in cm3.
  2. R has units of J K-1 mol-1 and p must be in pascals when V is in m3. Correct answer.
  3. R has units of J mol-1 and temperature must be in degrees Celsius.
  4. R has units of J K-1 mol-1 and p must be in kPa when V is in dm3.

Explanation

The gas constant R is 8.31 J K-1 mol-1, and the equation is consistent only when pressure is in pascals and volume in cubic metres, since 1 Pa x 1 m3 equals 1 J. Temperature must always be in kelvin. Using kPa with dm3 also happens to give joules, but the strict SI statement is the pascals/metres combination.

Question 33Chemistry (AM syllabus content)

Which statement correctly describes the bonding in a molecule of ethyne, C2H2?

  1. Each carbon is sp3 hybridised and the two carbons share a single sigma bond.
  2. Each carbon is sp hybridised and the two carbons share one sigma and two pi bonds. Correct answer.
  3. Each carbon is sp2 hybridised and the two carbons share one sigma and one pi bond.
  4. Each carbon is sp hybridised and the two carbons share two sigma and one pi bond.

Explanation

Each carbon in ethyne forms two sigma bonds (one to hydrogen, one to the other carbon) and has two unhybridised p orbitals. Mixing one s and one p orbital gives sp hybridisation with linear geometry. The triple bond consists of one sigma and two pi bonds.

Question 34Chemistry (AM syllabus content)

Which pair of reagents would convert propan-2-ol into propanone?

  1. Acidified potassium dichromate(VI) heated under reflux Correct answer.
  2. Concentrated sulfuric acid heated to 170 °C
  3. Sodium hydroxide solution warmed
  4. Bromine water 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 turns green as chromium is reduced. Dehydration or substitution conditions would not give a carbonyl compound.

Question 35Chemistry (AM syllabus content)

Which statement about the reaction of propanoyl chloride with water is correct?

  1. It is unreactive because acid chlorides are stable to moisture.
  2. It undergoes hydrolysis to give propanoic acid and hydrogen chloride. Correct answer.
  3. It forms an ester directly with water.
  4. It is reduced to propanal by water.

Explanation

Acid chlorides are highly reactive towards water, which acts as a nucleophile to displace chloride. The products are the corresponding carboxylic acid and hydrogen chloride, often seen as fumes. This vigorous hydrolysis is why acid chlorides must be kept dry.

Question 36Chemistry (AM syllabus content)

Which statement explains why benzene undergoes electrophilic substitution rather than addition?

  1. Benzene has a permanent dipole that repels electrophiles.
  2. Benzene contains alternating single and double bonds that are fixed in place.
  3. The delocalised pi system gives extra stability, which would be lost on addition. Correct answer.
  4. The carbon atoms in benzene are sp3 hybridised, preventing addition.

Explanation

The six p electrons in benzene are delocalised over the ring, giving resonance stabilisation of about 150 kJ mol-1. Addition would break up this delocalisation and cost energy, so substitution, which restores the aromatic ring, is favoured. Benzene does not have localised alternating double bonds.

Question 37Chemistry (AM syllabus content)

Which statement about the relative rates of hydrolysis of the halogenoalkanes C–Cl, C–Br and C–I is correct?

  1. The bromo compound hydrolyses fastest because bromine is intermediate in size.
  2. All three hydrolyse at the same rate because the mechanism is identical.
  3. The chloro compound hydrolyses fastest because chlorine is most electronegative.
  4. The iodo compound hydrolyses fastest because the C–I bond is weakest. Correct answer.

Explanation

The rate of hydrolysis depends on the strength of the carbon–halogen bond. Bond strength falls down the group as the halogen orbital overlaps less effectively with carbon, so C–I is weakest and breaks fastest. Greater polarity of the C–Cl bond is not enough to offset this.

Question 38Chemistry (AM syllabus content)

Which statement about the acidity of phenol compared with ethanol is correct?

  1. Ethanol is more acidic because the alkyl group is electron-withdrawing.
  2. Phenol is more acidic because the phenoxide ion is stabilised by delocalisation into the ring. Correct answer.
  3. Phenol is more acidic because it contains a C=O group.
  4. Ethanol is more acidic because it is more soluble in water.

Explanation

In phenoxide, the negative charge on oxygen is delocalised into the aromatic ring, which stabilises the conjugate base. Ethoxide has no such delocalisation, so ethanol is the weaker acid. Solubility does not determine acid strength.

Question 39Chemistry (AM syllabus content)

Which statement about the reaction of an amine with a carboxylic acid to form an amide is correct?

  1. The reaction is a redox process in which the acid is oxidised.
  2. The reaction is a hydrolysis in which water is consumed.
  3. The reaction is a condensation in which water is eliminated. Correct answer.
  4. The reaction is an addition in which hydrogen is added across the C–N bond.

Explanation

Amide formation from an amine and a carboxylic acid releases water, so it is a condensation. In practice the acid is often converted to an acid chloride first to speed the reaction. No change in oxidation state occurs, and water is a product, not a reactant.

Question 40Chemistry (AM syllabus content)

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

  1. It increases the value of the equilibrium constant for the reaction.
  2. It lowers the enthalpy change of the reaction.
  3. It raises the average kinetic energy of the reactant molecules.
  4. It provides an alternative pathway with lower activation energy, increasing the proportion of collisions that are successful. Correct answer.

Explanation

A catalyst offers a different route with a lower activation energy, so a greater fraction of collisions have sufficient energy to react. It does not change the enthalpy change or the equilibrium constant. Temperature, not a catalyst, changes average kinetic energy.

Question 41Chemistry (AM syllabus content)

Which statement about the pH of a solution of a salt of a weak acid and a strong base, such as sodium ethanoate, is correct?

  1. The solution is neutral because the salt is fully dissociated.
  2. The solution is alkaline because the ethanoate ion hydrolyses to produce hydroxide ions. Correct answer.
  3. The solution is acidic because sodium ions hydrolyse to release hydrogen ions.
  4. The solution is alkaline because ethanoate ions accept protons from water, releasing hydrogen ions.

Explanation

The ethanoate ion is the conjugate base of a weak acid, so it reacts with water to produce ethanoic acid and hydroxide ions. This hydrolysis makes the solution alkaline. Sodium ions do not hydrolyse appreciably, and full dissociation alone does not determine pH.

Question 42Chemistry (AM syllabus content)

Which statement about the entropy change for the dissolution of ammonium nitrate in water is correct?

  1. The entropy of the system increases only if the dissolution is exothermic.
  2. The entropy of the system increases because the ordered ionic lattice is broken up into mobile ions. Correct answer.
  3. The entropy of the system decreases because the ions become hydrated and ordered.
  4. The entropy change is zero because the temperature is constant.

Explanation

Breaking an ordered ionic lattice into freely moving hydrated ions greatly increases the number of available microstates, so the entropy of the system rises. Hydration orders some water molecules, but the overall ionic disorder dominates. Entropy change is independent of whether the process is exothermic.

Question 43Chemistry (AM syllabus content)

Which statement about the standard hydrogen electrode is correct?

  1. It consists of platinum in 1 mol dm-3 H+ with hydrogen gas at 100 kPa and is assigned a potential of 0.00 V at 298 K. Correct answer.
  2. It requires hydrogen gas at 1 kPa and a solution of pH 7 to be standard.
  3. It uses a silver wire in hydrochloric acid and is assigned a potential of +0.80 V.
  4. It uses a zinc electrode in 1 mol dm-3 zinc sulfate and is assigned 0.00 V.

Explanation

The standard hydrogen electrode uses platinised platinum, hydrogen gas at 100 kPa and H+ at 1 mol dm-3, all at 298 K. It is the reference against which all other electrode potentials are measured and is defined as exactly 0.00 V. The platinum provides an inert surface for the electrode reaction.

Question 44Chemistry (AM syllabus content)

Which statement about the use of standard electrode potentials to predict whether a reaction is feasible is correct?

  1. A reaction is feasible if the difference between the two half-cell potentials is zero.
  2. A reaction is feasible only if both half-cell potentials are positive.
  3. A reaction is feasible if the cell potential is negative.
  4. A reaction is feasible if the cell potential calculated from the two half-cells is positive. Correct answer.

Explanation

A positive cell potential corresponds to a negative Gibbs energy change under standard conditions, so the reaction is thermodynamically feasible. The cell potential is obtained by subtracting the potential of the oxidation half-cell from that of the reduction half-cell. A negative value indicates the reverse reaction is favoured.

Question 45Chemistry (AM syllabus content)

Which statement about the bonding in the complex ion [Cu(NH3)4(H2O)2]2+ is correct?

  1. Ammonia and water molecules act as ligands by donating lone pairs into vacant orbitals on copper. Correct answer.
  2. Copper forms covalent bonds by sharing single electrons equally with nitrogen and oxygen.
  3. The ligands are held by ionic attraction to the copper ion without any orbital overlap.
  4. Ammonia and water molecules accept electron pairs from copper into their own orbitals.

Explanation

A ligand must have a lone pair that it donates to a vacant orbital on the central metal ion, forming a dative covalent bond. Ammonia and water both possess lone pairs on nitrogen and oxygen respectively. The copper ion acts as a Lewis acid accepting these electron pairs.

Question 46Chemistry (AM syllabus content)

Which statement about the colour of transition metal complexes is correct?

  1. Colour arises from the metal losing electrons to the ligands.
  2. Colour arises when electrons are promoted between split d orbitals, absorbing light of a complementary colour. Correct answer.
  3. Colour arises from nuclear transitions within the metal ion.
  4. Colour arises because all d orbitals have the same energy in a complex.

Explanation

Ligands split the d orbitals into different energy levels, and an electron can be promoted between them by absorbing visible light. The colour observed is the complement of the wavelength absorbed. If the d subshell were degenerate or empty, no such absorption would occur.

Question 47Chemistry (AM syllabus content)

Which statement about the mechanism of nucleophilic addition to a carbonyl compound is correct?

  1. The nucleophile attacks the oxygen atom first and the pi bond breaks to put a positive charge on carbon.
  2. The nucleophile attacks the electrophilic carbon, and the pi bond opens to place a negative charge on oxygen. Correct answer.
  3. The carbonyl oxygen acts as the electrophile while the carbon acts as the nucleophile.
  4. The reaction proceeds by homolytic fission of the C=O bond into radicals.

Explanation

The C=O bond is polarised with a partial positive charge on carbon, making it the electrophilic site. A nucleophile attacks that carbon, and the pi electrons move onto the more electronegative oxygen, giving an alkoxide intermediate. Protonation then completes the addition.

Question 48Chemistry (AM syllabus content)

Which statement about the rate equation of a reaction is correct?

  1. The orders always equal the stoichiometric coefficients in the balanced equation.
  2. The overall order is always equal to the number of reactants in the equation.
  3. The rate constant k has the same units for every reaction.
  4. The orders in the rate equation must be determined experimentally and do not necessarily match the stoichiometric coefficients. Correct answer.

Explanation

The rate equation reflects the mechanism, so orders can be zero, fractional, or unrelated to the balanced equation. Only for a single-step elementary reaction do the orders match the stoichiometry. The units of k depend on the overall order.

Question 49Chemistry (AM syllabus content)

Which statement about the stereochemistry of an SN1 reaction at a chiral carbon is correct?

  1. The product is a racemic mixture because the leaving group departs and returns repeatedly.
  2. The product has no stereochemistry because the carbocation is tetrahedral.
  3. The product is a racemic mixture because the planar carbocation intermediate can be attacked from either face. Correct answer.
  4. The product is a single enantiomer because attack occurs only from the side opposite the leaving group.

Explanation

In an SN1 reaction the leaving group departs first, leaving a planar sp2 carbocation. A nucleophile can then attack either face with roughly equal probability, giving a racemic mixture if the carbon is chiral. Complete inversion is characteristic of SN2, where attack is backside.

Question 50Chemistry (AM syllabus content)

Which statement about the use of high-resolution proton NMR spectroscopy to distinguish isomers is correct?

  1. The number of proton environments and their splitting patterns differ between structural isomers. Correct answer.
  2. The chemical shift depends only on the number of protons in the molecule.
  3. Splitting patterns arise from coupling with carbon-12 nuclei.
  4. All isomers give identical spectra because they have the same molecular formula.

Explanation

Protons in different chemical environments give separate signals, and neighbouring non-equivalent protons split these signals according to the n+1 rule. Structural isomers differ in the arrangement of their protons, so their spectra differ. Integration also reveals the relative numbers of protons in each environment.