2 Dry air contains nitrogen, oxygen, noble gases and carbon dioxide. (a) State the percentage of oxygen present in dry air. … [1] (b) Carbon dioxide is removed from a sample of air by passing the air through aqueous sodium hydroxide. Explain why aqueous sodium hydroxide removes carbon dioxide from air. … … … [2] (c) Describe how oxygen, nitrogen and the noble gases are separated from each other after carbon dioxide has been removed. … … [2] (d) Describe the test for oxygen. test … observation … [2] (e) Ozone, O3, is formed in the atmosphere by the reaction of nitrogen dioxide with oxygen in the presence of ultraviolet light. (i) State the type of chemical reaction that takes place when ozone is formed in this way. … [1] (ii) Nitrogen dioxide is formed in internal combustion engines. State one other source of nitrogen dioxide in the atmosphere. … [1] (f) A layer of ozone is present high in the atmosphere. State one problem for humans that can arise if the ozone layer is depleted by CFCs. … [1] [Total: 10]

2(a) 21(%) 1 2(b) carbon dioxide is an acidic oxide (1) reacts with base / reacts with alkali / neutralised by sodium hydroxide (1) 2 2(c) air liquefied (1) fractional distillation (1) 2 2(d) glowing splint (1) relights (1) 2 2(e)(i) photochemical / redox 1 2(e)(ii) lightning 1 2(f) (more) skin cancer / (more) sunburn / (more) harm to eyes 1 (a) To earn the mark, you must state the percentage of oxygen in dry air as 21%. (b) To earn the first mark, you must state that carbon dioxide is an acidic oxide. To earn the second mark, you must explain that it reacts with a base/alkali or is neutralised by sodium hydroxide. (c) To earn the first mark, you must state that air is liquefied. To earn the second mark, you must state that fractional distillation is used. (d) To earn the first mark, you must describe the test as using a glowing splint. To earn the second mark, you must state the observation that the splint relights. (e)(i) To earn the mark, you must state 'photochemical' or 'redox'. (e)(ii) To earn the mark, you must state 'lightning'. (f) To earn the mark, you must state a harmful effect like 'more skin cancer', 'more sunburn', or 'more harm to eyes'.

3 There is concern about the disposal of plastics made from non-biodegradable polymers. (a) The partial structure of a non-biodegradable polymer is shown. CH3 C C C H CH3 H Cl H H Cl C (i) Name the type of polymer shown. … [1] (ii) Draw the structure of the monomer used to make this polymer. [1] (iii) This polymer is often disposed of by combustion. Suggest one problem associated with this method of disposal. … … [1] (b) Lactic acid is used to make poly(lactic acid), a biodegradable polymer. The structure of lactic acid is shown. CH3 O C C H H O H O (i) Suggest what is meant by the term biodegradable. … … [1] (ii) Draw the partial structure of poly(lactic acid). Show at least two repeat units. [2] (iii) A factory uses 500 tonnes of lactic acid to make poly(lactic acid). The percentage yield is 100% but the mass of poly(lactic acid) made is less than 500 tonnes. Explain why the mass of poly(lactic acid) made is less than 500 tonnes. … … [1] (iv) Aqueous lactic acid reacts with acidified potassium manganate(VII). There is a colour change from purple to colourless. Suggest what happens to the lactic acid in this reaction. … [1] (v) Aqueous lactic acid is neutralised by aqueous sodium hydroxide. Write the ionic equation for this neutralisation. … [1] (vi) Aqueous lactic acid reacts with magnesium. Name the gas made in this reaction. … [1] [Total: 10]

3(a)(i) addition 1 3(a)(ii) C C H Cl H CH3 1 3(a)(iii) makes poisonous gases / makes poisonous fumes 1 3(b)(i) can be decomposed naturally / can be broken down by bacterial action 1 3(b)(ii) O C H CH3 H C O O C CH3 H C O O H one ester linkage drawn correctly (1) structure with at least two repeat units and extension bonds at both ends (1) 2 3(b)(iii) condensation polymerisation / a small molecule is also made 1 3(b)(iv) oxidised 1 3(b)(v) H+ + OH− → H2O 1 3(b)(vi) hydrogen 1 To earn marks, you must correctly name the polymer type, draw the correct structure of the monomer, and suggest a valid problem with combustion disposal. For part (b), you need to define biodegradable, draw a partial structure of poly(lactic acid) showing at least two repeat units and correct ester linkages with extension bonds, and state that condensation polymerisation occurs or that a small molecule is also made. For part (b)(iv), you must state 'oxidised', for (b)(v) provide the correct ionic equation for neutralization, and for (b)(vi) identify hydrogen.

3 Iron powder reacts with dilute sulfuric acid. Fe(s) + H2SO4(aq) FeSO4(aq) + H2(g) (a) The graph shows the volume of hydrogen gas produced at 20 °C as the reaction proceeds. volume of hydrogen gas / cm3 40 30 20 10 0 4 8 12 16 20 24 28 time / min (i) State how long it takes from the start of the experiment to collect 18 cm3 of hydrogen gas. … [1] (ii) The experiment is repeated at 30 °C. All other conditions are the same. Draw a line on the grid to show how the volume of hydrogen gas produced changes with time when the reaction is done at 30 °C. [2] (b) (i) Describe and explain, using ideas about collisions between particles, how the rate of the reaction changes when the temperature of the reaction mixture is increased. All other conditions are the same. … … … … [2] (ii) Describe and explain, using ideas about collisions between particles, how the rate of the reaction changes when larger pieces of iron are used. All other conditions are the same. … … … … [2] (c) Calculate the maximum volume, in dm3, of hydrogen formed when 3.36 g of iron react with excess dilute sulfuric acid at room temperature and pressure. Give your answer to three significant figures. maximum volume = … dm3 [2] (d) Concentrated sulfuric acid is an oxidising agent. Describe a test for oxidising agents. test … observations … … [2] [Total: 11]

3(a)(i) 6 minutes 1 3(a)(ii) initial gradient steeper and starting from origin (1) line levels off at 34 cm3 gas (1) 2 3(b)(i) rate of reaction increases (no mark) and particles move faster / particles have more kinetic energy (1) more particles have activation energy (or above) / more successful collisions / more energetic collisions (1) 2 3(b)(ii) rate of reaction decreases (no mark) and fewer particles exposed on surface / fewer particles (of iron) per unit area / fewer particles per cm2 (1) collision frequency decreases / fewer particles collide per second / collision rate decreases / collisions less often (1) 2 3(c) mol iron = 3.36 56 OR 0.060 mol (1) volume of hydrogen = 1.44 (dm3) (1) 2 3(d) add (aqueous) potassium iodide (1) colourless to brown solution (1) 2 (a)(i) To earn the mark, you must correctly read the graph and state the time as 6 minutes. (a)(ii) To earn the first mark, you must draw a line starting from the origin with a steeper initial gradient than the original line. To earn the second mark, your drawn line must level off at 34 cm3 of gas. (b)(i) To earn the first mark, you must state that particles move faster or have more kinetic energy. To earn the second mark, you must explain that more particles have activation energy (or above), leading to more successful collisions or more energetic collisions. (b)(ii) To earn the first mark, you must state that fewer particles are exposed on the surface, or fewer particles of iron are present per unit area/cm². To earn the second mark, you must explain that collision frequency decreases, or fewer particles collide per second, or the collision rate decreases. (c) To earn the first mark, you must correctly calculate the moles of iron (3.36/56 or 0.060 mol). To earn the second mark, you must correctly calculate the volume of hydrogen as 1.44 dm3. (d) To earn the first mark, you must state adding aqueous potassium iodide. To earn the second mark, you must state the observation of a colourless to brown solution.

3 Magnesium ribbon reacts with dilute hydrochloric acid. Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g) (a) The graph shows the volume of hydrogen gas produced at 20 °C as the reaction proceeds. The magnesium is in excess. volume of hydrogen gas / cm3 60 50 40 30 20 10 0 1 2 3 4 5 6 7 time / min The experiment is repeated using a lower concentration of hydrochloric acid. The volume of acid used and all other conditions are the same. Draw a line on the grid to show how the volume of hydrogen gas produced changes with time when the reaction is done with a lower concentration of hydrochloric acid. [2] (b) (i) Describe and explain, using ideas about collisions between particles, how the rate of the reaction changes when magnesium powder is used instead of magnesium ribbon. All other conditions are the same. … … … … [2] (ii) Describe and explain, using ideas about collisions between particles, how the rate of the reaction changes when the temperature of the reaction mixture is decreased. All other conditions are the same. … … … … [2] (c) Calculate the maximum volume, in dm3, of hydrogen formed when 1.68 g of magnesium react with excess dilute hydrochloric acid at room temperature and pressure. Give your answer to three significant figures. volume = … dm3 [2] (d) Magnesium is a good reducing agent. Describe a test for reducing agents. test … observations … … [2] [Total: 10]

3(a) initial gradient less steep AND starting from origin (1) line levels off at lower than 45 cm3 gas (1) 2 3(b)(i) rate of reaction increases AND one or both of: more particles exposed on surface / more (magnesium) particles per unit area / more particles per cm2 (1) collision frequency increases / collision rate increases / more collisions per second (1) 2 3(b)(ii) rate of reaction decreases AND one or both of: particles move more slowly / particles have less kinetic energy (1) fewer particles have activation energy (or above) / collisions are less successful (1) 2 3(c) mol magnesium = 1.68 24 OR 0.070 mol (1) volume of oxygen = 1.68 (dm3) (1) 2 3(d) add (aqueous acidified) potassium manganate(VII) (1) turns from purple to colourless (1) 2 (a) To earn the first mark, your drawn line must start at the origin and have a less steep initial gradient than the original line. To earn the second mark, your line must level off at a volume of hydrogen gas lower than 45 cm³. (b)(i) To earn the first mark, you must state that the rate of reaction increases. To earn the second mark, you must explain this by mentioning more particles exposed on the surface or more particles per unit area, leading to an increased collision frequency. (b)(ii) To earn the first mark, you must state that the rate of reaction decreases. To earn the second mark, you must explain this by mentioning that particles move more slowly or have less kinetic energy, resulting in fewer particles having the activation energy or collisions being less successful. (c) To earn the first mark, you must correctly calculate the moles of magnesium as 0.070 mol (or 1.68/24). To earn the second mark, you must correctly state the volume of oxygen as 1.68 dm³. (d) To earn the first mark, you must state that aqueous acidified potassium manganate(VII) is added. To earn the second mark, you must state that the solution turns from purple to colourless.

2 A student follows the rate of the reaction between marble chips, CaCO3, and dilute hydrochloric acid. CaCO3 + 2HCl  CaCl 2 + CO2 + H2O Which diagrams show apparatus that, with a stopwatch, is suitable for this experiment? measuring cylinder hydrochloric acid marble chips hydrochloric acid marble chips 1 2 balance water 3 marble chips hydrochloric acid burette loose plug A 1 only B 1 and 2 only C 2 and 3 only D 1, 2 and 3

Answer: B To earn the mark, you must identify that apparatus 1 (measuring gas volume) and 2 (measuring mass change) are suitable for monitoring the reaction rate.

Cambridge O Level Chemistry · 5070

O-Level Chemistry (5070) Chemical reactions past paper questions

Revise O-Level Chemistry (5070) — Chemical reactions with real Cambridge past-paper questions grouped by topic, each shown with its marking scheme. Working one topic at a time targets exactly the skill each question tests.

226 free questions from 2016–2021, each with its marking scheme. Open the full topical bank to filter by subtopic, paper, and year.

Chemical reactions in O-Level Chemistry 5070 exams, 2020–2025

Across 2020–2025, Chemical reactions accounted for 228 questions worth 1,210 marks in Cambridge O Level Chemistry (5070) papers.

By year
Year	Questions	Marks
2020	40	280
2021	45	229
2022	48	216
2023	40	180
2024	31	182
2025	24	123

Most-tested subtopics

6.2 Rate of reaction526 marks
6.4 Redox500 marks
6.3 Reversible reactions and equilibrium250 marks

Appears in

Paper 1 · 121 qs
Paper 2 · 85 qs
Paper 3 · 7 qs
Paper 4 · 15 qs

All most-tested Chemistry topics

2021 Oct/Nov · Paper 2 · Q2 · 10 marks
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2021 May/Jun · Paper 2 · Q3 · 10 marks
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2021 Oct/Nov · Paper 2 · Q3 · 11 marks
Marking scheme
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2021 Oct/Nov · Paper 2 · Q3 · 10 marks
Marking scheme
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2021 Oct/Nov · Paper 1 · Q2 · 1 mark
Marking scheme
Mark scheme — 2021 Paper 1 Q2
Answer: B
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