5 Magnesium undergoes oxidation when it reacts with aqueous silver nitrate. Mg + 2Ag+ Mg2+ + 2Ag (a) Explain how this equation shows that magnesium is oxidised. … … [1] (b) The reaction of magnesium with silver nitrate is exothermic. Complete the energy profile diagram for this reaction. Label reactants, products and ∆H. progress of reaction energy [2] (c) Acidified aqueous silver nitrate is added to a solution containing halide ions. A yellow precipitate is observed. State the name of the halide ion present in the solution. … [1] (d) Silver is a metal. Use your knowledge of the structure of metals to explain why silver is malleable. … … … [2] (e) The table gives information about the thermal decomposition of some metal carbonates. metal carbonate ease of thermal decomposition calcium carbonate decomposes at 900 °C magnesium carbonate decomposes at 540 °C sodium carbonate does not decompose below 1000 °C Describe how the ease of thermal decomposition depends on the position of these metals in the reactivity series. … … [1] [Total: 7]

5(a) (magnesium) loses electrons (1) 1 5(b) reactants on the left and products on the right and reactant line above product line (1) arrow downwards between reactants and products with ∆H label (1) 2 5(c) iodide 1 5(d) layers made of ions (1) layers slide (1) 2 5(e) more reactive the metal (in the carbonate) the harder it is to break down (the carbonate) 1 (a) To earn the mark, you must explain that magnesium loses electrons. (b) To earn the first mark, you must draw reactants on the left and products on the right, with the reactant line above the product line. To earn the second mark, you must draw a downward arrow between reactants and products and label it ∆H. (c) To earn the mark, you must state 'iodide' as the halide ion. (d) To earn the first mark, you must state that metals have layers of ions. To earn the second mark, you must explain that these layers can slide. (e) To earn the mark, you must state that the more reactive the metal in the carbonate, the harder it is to break down the carbonate.

6 Chlorine reacts with phosphorus to form phosphorus(III) chloride. 3Cl2 + 2P 2PCl3 The reaction is exothermic. (a) Explain in terms of bond breaking and bond forming why the reaction is exothermic. … … … [2] (b) (i) When chlorine reacts with aqueous potassium iodide a brown solution is formed. Name the products of this reaction. … and … [1] (ii) When aqueous bromine is added to potassium chloride there is no reaction. Explain why there is no reaction. … [1] (c) Phosphorus(III) chloride reacts with water to form a mixture of phosphorous acid, H3PO3, and hydrochloric acid. Construct the equation for this reaction. … [1] [Total: 5] Section B Answer three questions from this section in the spaces provided. The total mark for this section is 30.

6(a) bond breaking endothermic AND bond making exothermic / energy absorbed to break bonds AND energy released on making bonds (1) more energy released than absorbed (1) 2 6(b)(i) iodine AND potassium chloride 1 6(b)(ii) chlorine is more reactive than bromine / bromine is less reactive than chlorine 1 6(c) PCl3 + 3H2O → H3PO3 + 3HCl 1 (a) To earn the first mark, you must state that bond breaking is endothermic and bond making is exothermic (or energy is absorbed to break bonds and energy is released on making bonds). To earn the second mark, you must explain that more energy is released than absorbed. (b)(i) To earn the mark, you must name iodine and potassium chloride as the products. (b)(ii) To earn the mark, you must explain that chlorine is more reactive than bromine, or bromine is less reactive than chlorine. (c) To earn the mark, you must construct the balanced equation: PCl3 + 3H2O → H3PO3 + 3HCl.

6 Phosphorus reacts with hydrogen to produce phosphine, PH3. 2P + 3H2 2PH3 The reaction is endothermic. (a) Complete the energy profile diagram for this reaction. Label reactants, products and ∆H. progress of reaction energy [2] (b) Explain, in terms of bond breaking and bond forming, why the reaction is endothermic. … … … [2] (c) Phosphine reacts with oxygen to form phosphorus(V) oxide, P4O10, and water. Complete the equation for this reaction. … PH3 + … O2 P4O10 + … H2O [1] [Total: 5] Section B Answer three questions from this section in the spaces provided. The total mark for this section is 30.

6(a) reactants on the left and products on the right and reactant line below product line (1) arrow upwards between reactants and products with ΔH label (1) 2 6(b) bond breaking endothermic and bond making exothermic / (thermal) energy absorbed to break bonds and (thermal) energy released on making bonds (1) more energy absorbed than released (1) 2 6(c) 4PH3 + 8O2 → P4O10 + 6H2O 1 (a) To earn the first mark, you must draw the reactants on the left and products on the right, with the reactant line below the product line. To earn the second mark, you must draw an upward arrow between reactants and products and label it ΔH. (b) To earn the first mark, you must state that bond breaking is endothermic and bond making is exothermic (or energy is absorbed to break bonds and released to make bonds). To earn the second mark, you must explain that more energy is absorbed than released. (c) To earn this mark, you must correctly balance the equation as 4PH3 + 8O2 → P4O10 + 6H2O.

2 A student investigates the effect of concentration on the energy change in a neutralisation reaction. The student: • measures 25.0 cm3 of aqueous sodium hydroxide, NaOH(aq), into a beaker • measures the temperature of this solution • adds 25.0 cm3 of 0.10 mol / dm3 hydrochloric acid, HCl (aq), to the beaker • stirs the mixture • measures and records the highest temperature of the solution in the beaker • repeats the experiment with different concentrations of hydrochloric acid. (a) The diagram shows the highest temperatures reached for 0.10 mol / dm3 and 0.50 mol / dm3 HCl (aq). 19 20 21 22 23 24 25 19 20 21 22 23 24 25 0.10 mol / dm3 0.50 mol / dm3 The initial temperature of NaOH(aq) and the HCl (aq) for all experiments is 20.0 °C. Record the highest temperatures and the temperature changes in the table. concentration of HCl (aq) in mol / dm3 highest temperature in °C temperature change in °C 0.10 0.50 [2] Another student repeats the experiment for eight different concentrations of HCl (aq) and records the temperature changes. The results are shown in the table. concentration of HCl (aq) in mol / dm3 temperature change in °C 0.10 0.6 0.30 2.0 0.50 3.4 0.70 2.6 0.90 6.2 1.30 6.9 1.50 6.9 1.60 6.9 (b) The temperature change for 0.70 mol / dm3 HCl (aq) is anomalous. Suggest what the student should do to check that this temperature change is anomalous. … [1] (c) Plot the points of temperature change (y-axis) against concentration of HCl (aq) (x-axis) on the grid. 0 0 1 2 3 4 5 6 7 8 0.4 0.2 0.6 0.8 1.2 1.4 1.6 1.0 temperature change in °C concentration of HCl (aq) in mol / dm3 [2] (d) Draw one straight line of best fit for concentrations up to 0.90 mol / dm3 and a second straight line through the other three points. Extend both straight lines until they intersect. [2] (e) Use the graph to determine the concentration of HCl (aq) which gives a temperature change of 5.4 °C. concentration … mol / dm3 [1] (f) Use the graph to determine the temperature change for 1.40 mol / dm3 HCl (aq). temperature change … °C [1] (g) Use the graph to determine the minimum concentration of HCl (aq) that neutralises all the NaOH(aq). concentration … mol / dm3 [1] (h) A major source of error in this experiment is heat loss. Suggest how the student can reduce the heat loss. … … [1] [Total: 11]

2(a) M1 20.8 inserted in table for 0.100 mol / dm3 AND 23.6 inserted in table for 0.500 mol / dm3 (1) M2 0.80 inserted for maximum temperature change for 0.100 mol / dm3 AND 3.60 inserted for maximum temperature change for 0.100 mol / dm3 (1) 2 2(b) Repeat the experiment 1 2(c) 7 or 8 points plotted correctly ± half a square (2 marks) 6 points plotted correctly ± half a square (1 mark) 2 2(d) M1 Single line of best fit within one square of all the non-anomalous points up to 0.9 mol / dm3 (1) M2 Single line within one square of last 3 points and both lines extended to intersect. (1) 2 2(e) Correct value from student’s line of best fit ± half a square 1 2(f) Correct value from student’s line of best fit ± half a square 1 2(g) Correct value from intersection of student’s lines of best fit ± half a square 1 2(h) Any one from: insulation / lagging (the beaker) use a polystyrene cup / use insulating material for the beaker use a lid / bung / cover over the beaker 1 (a) To earn the first mark, you must correctly record the highest temperatures for both 0.10 mol/dm³ and 0.50 mol/dm³ HCl(aq) in the table. To earn the second mark, you must correctly calculate and record the temperature changes for both concentrations in the table. (b) To earn this mark, you must state that the student should repeat the experiment.

16 Which process is endothermic? A atoms bonding to form molecules B the chemical reaction occurring in a fuel cell C the reaction of carbon dioxide and water to produce glucose and oxygen D the reaction of methane with oxygen to produce water and carbon dioxide

Answer: C To earn the mark, a student must identify the process that absorbs energy from its surroundings, which is photosynthesis (the reaction of carbon dioxide and water to produce glucose and oxygen).

Cambridge O Level Chemistry · 5070

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

Revise O-Level Chemistry (5070) — Chemical energetics 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.

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

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

Across 2020–2025, Chemical energetics accounted for 81 questions worth 560 marks in Cambridge O Level Chemistry (5070) papers.

By year
Year	Questions	Marks
2020	9	75
2021	9	43
2022	17	98
2023	14	98
2024	15	123
2025	17	123

Most-tested subtopics

5.1 Exothermic and endothermic reactions528 marks

Appears in

Paper 1 · 32 qs
Paper 2 · 26 qs
Paper 3 · 7 qs
Paper 4 · 16 qs

All most-tested Chemistry topics

2021 Oct/Nov · Paper 2 · Q5 · 7 marks
Marking scheme
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2021 Oct/Nov · Paper 2 · Q6 · 5 marks
Marking scheme
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2021 Oct/Nov · Paper 2 · Q6 · 5 marks
Marking scheme
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2021 May/Jun · Paper 4 · Q2 · 11 marks
Marking scheme
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2021 Oct/Nov · Paper 1 · Q16 · 1 mark
Marking scheme
Mark scheme — 2021 Paper 1 Q16
Answer: C
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