A-level Chemistry extended-response trainer

AQA 7405

ORIGINAL

Original AQA A-level Chemistry extended-response practice across physical, inorganic and organic chemistry — Maxwell-Boltzmann and catalysts, Kp and Le Chatelier, electrode potentials, buffers, entropy and feasibility, Period 3 oxides, transition-metal colour and catalysis, aqueous ion reactions, halogenoalkane substitution versus elimination, alkene addition and carbocation stability, aromatic substitution, polymer disposal, spectroscopic structure determination and multi-step synthesis. Plan the answer, self-mark against the level descriptors, then read a model response.

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How to use it: write your full answer in the box, then reveal the mark scheme and tick the points you made. AQA marks these by level — a detailed, linked answer — not by counting points, so the tick count is only a rough guide. Read the model answer to see what a Level 3 looks like.

Physical — kinetics and the Maxwell-Boltzmann distribution

Explain

Year 1

A gas-phase reaction has a high activation energy. Explain, using the Maxwell-Boltzmann distribution of molecular energies, why a small rise in temperature produces a large increase in the rate of this reaction, and why adding a catalyst also increases the rate.

[6 marks]

Total for this question: 6

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Physical — equilibria, Kp and industrial conditions

Explain

Years 1 and 2

Ammonia is manufactured by the reversible reaction N₂(g) + 3H₂(g) <=> 2NH₃(g), for which the enthalpy change of the forward reaction is -92 kJ mol⁻¹. A typical plant operates at about 450 °C and 200 atm with an iron catalyst. Explain how temperature, total pressure and the catalyst each affect the equilibrium yield of ammonia and the value of Kp, and justify the conditions chosen.

[6 marks]

Total for this question: 6

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Physical — electrode potentials and electrochemical cells

Describe and explain

Year 2

A student builds a cell from a Zn²⁺/Zn half-cell and a Cu²⁺/Cu half-cell under standard conditions. The standard electrode potentials are E(Zn²⁺/Zn) = -0.76 V and E(Cu²⁺/Cu) = +0.34 V. Describe how the cell is assembled and measured, explain which electrode is negative and calculate the emf, and evaluate one advantage and one drawback of a hydrogen-oxygen fuel cell compared with a petrol engine.

[6 marks]

Total for this question: 6

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Physical — acids, bases and buffer action

Explain

Year 2

A buffer solution is prepared by dissolving sodium ethanoate in aqueous ethanoic acid. Explain how this solution resists a change in pH when a small amount of a strong acid is added and when a small amount of a strong alkali is added, and explain how its pH can be calculated from Ka.

[6 marks]

Total for this question: 6

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Physical — entropy and Gibbs free-energy change

Explain

Year 2

Solid ammonium chloride decomposes on heating: NH₄Cl(s) → NH₃(g) + HCl(g). For this reaction the enthalpy change is +176 kJ mol⁻¹ and the entropy change is +285 J K⁻¹ mol⁻¹. Explain what these two values tell you about the reaction, and explain, with a calculation, why the reaction becomes feasible only above a particular temperature.

[6 marks]

Total for this question: 6

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Inorganic — Period 3 elements and their oxides

Describe and explain

Years 1 and 2

Separate samples of sodium oxide, magnesium oxide, silicon dioxide and sulfur dioxide are each added to water. Describe what happens in each case and the approximate pH of the mixture produced, and explain the differences in terms of the bonding and structure of the oxides.

[6 marks]

Total for this question: 6

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Inorganic — transition metals: colour and catalysis

Explain

Year 2

Explain why aqueous solutions of most transition metal ions are coloured while solutions containing Sc³⁺ or Zn²⁺ are colourless, and explain, with an example of each, how transition metals are able to act as heterogeneous and as homogeneous catalysts.

[6 marks]

Total for this question: 6

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Inorganic — reactions of ions in aqueous solution

Describe and explain

Year 2

Separate aqueous solutions containing [Fe(H₂O)₆]²⁺, [Fe(H₂O)₆]³⁺ and [Al(H₂O)₆]³⁺ are each treated with aqueous sodium hydroxide, added until in excess, and then with aqueous sodium carbonate. Describe what would be observed in each case and explain why the 3+ ions are more acidic than the 2+ ion.

[6 marks]

Total for this question: 6

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Inorganic — ligand substitution and the shapes of complex ions

Describe and explain

Year 2

Aqueous copper(II) sulfate contains the [Cu(H₂O)₆]²⁺ ion. Describe and explain what is observed when an excess of concentrated aqueous ammonia is added to it, and separately when an excess of concentrated hydrochloric acid is added, referring to the colour, coordination number and shape of each complex formed.

[6 marks]

Total for this question: 6

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Q10

Organic — halogenoalkanes: substitution and elimination

Compare

Year 1

2-Bromopropane, CH₃CHBrCH₃, reacts with potassium hydroxide. Under one set of conditions the main organic product is propan-2-ol; under another it is propene. Compare the two reactions. In your answer, give the conditions that favour each product, name the type of reaction in each case and outline its mechanism, and explain the different roles of the hydroxide ion.

[6 marks]

Total for this question: 6

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Q11

Organic — alkenes: electrophilic addition and carbocation stability

Explain

Year 1

Propene reacts with hydrogen bromide at room temperature to give a mixture of two structural isomers with the formula C₃H₇Br. Explain why propene reacts with hydrogen bromide, outline the mechanism of the reaction, and explain why 2-bromopropane is the major product and 1-bromopropane is the minor product.

[6 marks]

Total for this question: 6

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Q12

Organic — aromatic chemistry and electrophilic substitution

Explain

Year 2

Explain, in terms of its bonding, why benzene reacts by electrophilic substitution rather than by electrophilic addition, and describe how the electrophile is generated and used in the nitration of benzene.

[6 marks]

Total for this question: 6

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Q13

Organic — addition and condensation polymers and their disposal

Compare and explain

Years 1 and 2

Poly(propene) is made from propene, and a polyester is made from benzene-1,4-dicarboxylic acid and ethane-1,2-diol. Compare how the two polymers are formed and explain why the polyester can be broken down by hydrolysis and is more readily biodegradable, while poly(propene) is not.

[6 marks]

Total for this question: 6

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Q14

Organic — structure determination by spectroscopy

Deduce and explain

Years 1 and 2

A colourless liquid X has the molecular formula C₃H₆O₂. Its mass spectrum shows a molecular ion peak at m/z = 74. Its infrared spectrum has a broad absorption between 2500 and 3000 cm⁻¹ and a strong absorption at 1710 cm⁻¹. Its proton NMR spectrum shows three peaks with an integration ratio of 3 : 2 : 1, one of them at about 11.5 ppm. X effervesces when aqueous sodium carbonate is added to it. Deduce the structure of X and explain how each piece of evidence supports your deduction.

[6 marks]

Total for this question: 6

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Q15

Organic — multi-step synthesis and reaction pathways

Devise and explain

Years 1 and 2

Devise a synthesis of propan-1-amine, CH₃CH₂CH₂NH₂, starting from ethene. Give the reagents and conditions for each step and identify the type of reaction in each, and explain why the conditions chosen for one of the steps must be specified particularly carefully.

[6 marks]

Total for this question: 6

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