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 diagram 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 5.1 to 5.5: atomic structure and the periodic table; bonding, structure and the properties of matter; quantitative chemistry; chemical changes; and energy changes.
- Relative atomic masses (Ar): H = 1, C = 12, O = 16, Na = 23, Al = 27, Cl = 35.5, Fe = 56.
Paper
Question 1 — Atomic structure (9 marks)
(a) Niels Bohr adapted the nuclear model of the atom. Describe the change Bohr made to the model, and the evidence that supported it. [2 marks]
(b) Give the relative charge and the relative mass of a proton, a neutron and an electron. [3 marks]
(c) The radius of an atom is about 0.1 nm. The radius of its nucleus is about 1/10 000 of the radius of the atom.
Calculate the radius of the nucleus in metres. Give your answer in standard form. (1 nm = 1 × 10⁻⁹ m) [2 marks]
(d) Copper has two isotopes. 69% of copper atoms are copper-63 and 31% are copper-65.
Calculate the relative atomic mass of copper. Give your answer to 1 decimal place. [2 marks]
Question 2 — The periodic table (8 marks)
(a) When Mendeleev arranged the elements, he left gaps in his table. Explain why he left gaps, and why later discoveries showed that his table was correct. [2 marks]
(b) Explain why the Group 0 elements are unreactive. [1 mark]
(c) Astatine is below iodine in Group 7.
Predict whether astatine would displace iodine from potassium iodide solution. Explain your answer. [2 marks]
(d) Explain why all the elements in Group 7 have similar chemical reactions. [1 mark]
(e) Sodium burns in chlorine to form sodium chloride. Write a balanced symbol equation for this reaction. [2 marks]
Question 3 — Bonding, structure and properties (11 marks)
(a) Calcium fluoride is an ionic compound with the formula CaF₂. Calcium is in Group 2 and fluorine is in Group 7.
(i) Give the formula of each ion in calcium fluoride. [1 mark]
(ii) Explain why the formula of calcium fluoride is CaF₂. [1 mark]
(iii) Explain why solid calcium fluoride does not conduct electricity but molten calcium fluoride does. [2 marks]
(b) How many pairs of electrons are shared in one molecule of ammonia, NH₃? [1 mark]
(c) Silicon dioxide melts at about 1700 °C. Explain why silicon dioxide has a very high melting point. [2 marks]
(d) Explain why copper is a good conductor of electricity. [2 marks]
(e) Brass is an alloy of copper and zinc. Explain why brass is harder than pure copper. [2 marks]
Question 4 — Quantitative chemistry (12 marks)
(a) A student puts marble chips (calcium carbonate) and dilute hydrochloric acid in an open conical flask on a balance. The reading on the balance goes down during the reaction.
Explain why the mass goes down, even though mass is conserved in chemical reactions. [2 marks]
(b) Iron reacts with chlorine to form iron(III) chloride:
2Fe + 3Cl₂ → 2FeCl₃
A student reacts 11.2 g of iron with 28.4 g of chlorine.
(i) Show that iron is the limiting reactant. [3 marks]
(ii) Calculate the maximum mass of iron(III) chloride that can be made. [2 marks]
(c) 1.08 g of aluminium reacts with 0.96 g of oxygen, O₂, to form 2.04 g of aluminium oxide, Al₂O₃.
Use these masses to balance the equation:
___ Al + ___ O₂ → ___ Al₂O₃
You must show your working. [3 marks]
(d) A student dissolves 5.0 g of sodium hydroxide in water and makes the solution up to 250 cm³.
Calculate the concentration of the solution in g/dm³. [2 marks]
Question 5 — Chemical changes (11 marks)
(a) Iron is extracted from its ore by heating it with carbon. Explain why aluminium cannot be extracted in this way. [1 mark]
(b) Zinc reacts with copper sulfate solution. The ionic equation is:
Zn + Cu²⁺ → Zn²⁺ + Cu
(i) Identify the species that is oxidised. Explain your answer in terms of electrons. [2 marks]
(ii) Write a half equation for the change that happens to zinc. [1 mark]
(c) Write the ionic equation for the reaction that happens in every neutralisation of an acid by an alkali. [1 mark]
(d) Hydrochloric acid and ethanoic acid solutions have the same concentration. The hydrochloric acid has a lower pH.
Explain why. [2 marks]
(e) A solution has a pH of 2. A second solution has a hydrogen ion concentration 1000 times lower than the first. What is the pH of the second solution? [1 mark]
(f) A student makes a soluble salt by adding zinc oxide to dilute hydrochloric acid until no more dissolves.
(i) Name the salt formed. [1 mark]
(ii) Explain why the student adds zinc oxide until some is left undissolved. [1 mark]
(iii) Name the method used to remove the excess zinc oxide. [1 mark]
Question 6 — Electrolysis (10 marks)
(a)* A student is given sodium chloride solution, two inert carbon electrodes, a low-voltage d.c. power supply and the usual laboratory apparatus.
Describe how the student could electrolyse the solution, collect the products and identify the product formed at each electrode. In your answer, explain why sodium is not formed at the negative electrode. [6 marks]
(b) Molten lead bromide, PbBr₂, is electrolysed using inert electrodes.
(i) Explain why the lead bromide must be molten. [1 mark]
(ii) Write a half equation for the reaction at the negative electrode. [1 mark]
(iii) Bromine is formed at the positive electrode. Explain why this is an oxidation reaction. [2 marks]
Question 7 — Energy changes (9 marks)
(a) A student adds 1.0 g of a metal powder to 25 cm³ of copper sulfate solution in a polystyrene cup and records the temperature rise. The results are shown.
| Metal | Temperature rise (°C) |
|---|---|
| Magnesium | 18.5 |
| Zinc | 9.0 |
| Iron | 6.5 |
| Silver | 0.0 |
(i) Explain why there was no temperature rise with silver. [1 mark]
(ii) Suggest why the student puts a lid on the polystyrene cup. [1 mark]
(iii) Is the reaction between magnesium and copper sulfate exothermic or endothermic? Give a reason. [1 mark]
(b) Describe what the reaction profile of an exothermic reaction shows about the energy of the reactants and the energy of the products. [1 mark]
(c) Methane burns in oxygen:
CH₄ + 2O₂ → CO₂ + 2H₂O
The table shows some bond energies.
| Bond | Bond energy (kJ/mol) |
|---|---|
| C–H | 412 |
| O=O | 498 |
| C=O | 805 |
| O–H | 464 |
(i) Calculate the overall energy change for the reaction. [3 marks]
(ii) Explain, in terms of bond breaking and bond making, why this reaction is exothermic. [2 marks]