Total marks: 70 · Duration: 75 minutes · Tier: Higher
Instructions to candidates
- Use black ink or black ball-point pen. Draw diagrams in pencil.
- Answer all questions in the spaces provided.
- In all calculations, show clearly how you work out your answer.
- You may use a calculator.
- Where a question describes apparatus or a graph in words, you may find it helpful to sketch it.
Information
- The maximum mark for this paper is 70.
- The marks for questions are shown in brackets.
- You are expected to use good English and to organise information clearly, especially in questions marked with an asterisk (*).
- This paper covers AQA GCSE Combined Science: Trilogy chemistry topics 5.6 to 5.10: the rate and extent of chemical change; organic chemistry; chemical analysis; chemistry of the atmosphere; and using resources.
- Relative atomic masses (Ar): C = 12, O = 16.
Paper
Question 1 — Rate of reaction (12 marks)
A student investigates the reaction between marble chips (calcium carbonate) and dilute hydrochloric acid:
CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)
The student puts excess marble chips and 50 cm³ of acid into a conical flask on a balance. A loose plug of cotton wool is placed in the neck of the flask. The student records the total loss in mass every 30 seconds.
| Time (s) | 0 | 30 | 60 | 90 | 120 | 180 | 240 |
|---|---|---|---|---|---|---|---|
| Loss in mass (g) | 0.00 | 0.62 | 1.04 | 1.30 | 1.44 | 1.54 | 1.54 |
(a) Explain why the mass of the flask and its contents decreases. [1 mark]
(b) Suggest the purpose of the cotton wool plug. [1 mark]
(c) Calculate the mean rate of reaction over the first 60 seconds, in g/s. Give your answer to 2 significant figures. [2 marks]
(d) The student plots a graph of loss in mass against time and draws a tangent to the curve at 90 s. The tangent passes through the points (30 s, 0.94 g) and (150 s, 1.66 g).
Calculate the rate of reaction at 90 s. [2 marks]
(e) The student repeats the experiment using the same mass of marble powder instead of chips. All other conditions are kept the same.
Explain, in terms of particles, why the reaction is faster. [3 marks]
(f) Explain why the final loss in mass is the same when marble powder is used. [1 mark]
(g) Calculate the number of moles of carbon dioxide given off in the original experiment. [2 marks]
Question 2 — Reversible reactions and equilibrium (9 marks)
Brown nitrogen dioxide gas, NO₂, forms colourless dinitrogen tetroxide gas, N₂O₄, in a reversible reaction:
2NO₂(g) ⇌ N₂O₄(g) brown colourless
(a) Equilibrium is only reached if the reaction happens in a closed system. What is meant by a closed system? [1 mark]
(b) A sealed syringe contains the gas mixture at equilibrium. The plunger is pushed in, which increases the pressure. When the mixture reaches equilibrium again, its colour is paler.
Explain why. [2 marks]
(c) The syringe is placed in iced water. The colour of the mixture becomes paler.
Is the forward reaction exothermic or endothermic? Explain your answer. [2 marks]
(d) Hydrogen reacts with iodine vapour in a reversible reaction:
H₂(g) + I₂(g) ⇌ 2HI(g)
Predict the effect of increasing the pressure on the amount of hydrogen iodide at equilibrium. Explain your answer. [2 marks]
(e) A catalyst is added to a reaction at equilibrium. Explain the effect, if any, on the amount of product and on the time taken to reach equilibrium. [2 marks]
Question 3 — Crude oil and hydrocarbons (10 marks)
(a) Give the molecular formula of the alkane with 20 carbon atoms. [1 mark]
(b) Describe how crude oil is separated into fractions by fractional distillation. [4 marks]
(c) Butane, C₄H₁₀, is used as a camping fuel. Write a balanced symbol equation for the complete combustion of butane. [2 marks]
(d) Dodecane, C₁₂H₂₆, is cracked to form hexane, C₆H₁₄, and propene, C₃H₆, only.
(i) Write a balanced symbol equation for this reaction. [1 mark]
(ii) Describe a test to show that propene is an alkene. Give the result. [1 mark]
(e) Give one reason why oil companies crack long-chain hydrocarbons. [1 mark]
Question 4 — Chemical analysis (10 marks)
(a) A student measures the melting points of two samples of aspirin. Sample A melts at 136 °C. Sample B melts gradually between 128 °C and 133 °C.
Which sample is pure? Explain your answer. [2 marks]
(b) A cough medicine is a formulation. Explain what is meant by a formulation. [2 marks]
(c) A student uses paper chromatography to analyse an ink. The solvent front moves 6.4 cm from the start line.
(i) One spot moves 4.8 cm from the start line. Calculate its Rf value. [2 marks]
(ii) The table shows the Rf values of three dyes in the same solvent. Identify the dye in the spot from part (i). [1 mark]
| Dye | A | B | C |
|---|---|---|---|
| Rf value | 0.30 | 0.75 | 0.52 |
(iii) One spot of the ink stays on the start line. Suggest why. [1 mark]
(d) Describe the test for each gas and give the result.
(i) oxygen [1 mark]
(ii) carbon dioxide [1 mark]
Question 5 — The atmosphere (12 marks)
(a) Give the approximate percentages of nitrogen and oxygen in the Earth's atmosphere today. [1 mark]
(b) Explain how the amount of oxygen in the atmosphere increased. [2 marks]
(c) Give one way in which the amount of carbon dioxide in the early atmosphere decreased. [1 mark]
(d)* Burning fossil fuels in vehicles and power stations releases carbon monoxide, sulfur dioxide, oxides of nitrogen and particulates into the atmosphere.
Describe how each of these pollutants is produced and explain the problems it causes. [6 marks]
(e) Give two possible effects of global climate change. [2 marks]
Question 6 — Using resources (10 marks)
(a) Water from rivers and reservoirs is treated to make it potable. Name two stages in this treatment and give the purpose of each. [2 marks]
(b) Name a method of desalinating sea water, other than distillation. [1 mark]
(c) Copper can be obtained from low-grade ores by bioleaching. Describe how bioleaching works, and how copper metal is then obtained. [3 marks]
(d) A dairy is choosing between glass bottles and plastic (PET) bottles for its milk. The table shows some life cycle assessment data.
| Glass bottle | PET bottle | |
|---|---|---|
| Raw materials | sand, limestone, sodium carbonate (quarried) | crude oil |
| Energy to make one bottle (MJ) | 7.5 | 2.4 |
| Mass of one bottle (g) | 350 | 40 |
| Number of times used before disposal | 20 (returned and washed) | 1 |
| End of life | widely recycled | can be recycled, often landfilled |
Evaluate the two types of bottle. Use the data and your own knowledge. [4 marks]
Question 7 — Analysing a water sample (7 marks)
A student is given a sample of water from a stream and is asked to find the mass of solid dissolved in it.
(a) Describe how the student could find the mass of dissolved solid in 50.0 cm³ of the water. [3 marks]
(b) The student's results are shown.
| Mass of empty evaporating basin | 32.41 g |
|---|---|
| Mass of basin and solid after evaporation | 32.47 g |
Calculate the mass of dissolved solid in 1 dm³ of the water. [2 marks]
(c) The student also measures the pH of the water. Suggest one advantage of using a pH meter instead of universal indicator. [1 mark]
(d) The student distils some of the water. How could the student show that the distilled water is pure? [1 mark]