5 This question is about sulfur and compounds of sulfur. (a) Sulfur is a non-metal. Describe three physical properties which are typical of non-metals. 1 … 2 … 3 … [3] (b) Name one source of sulfur. … [1] (c) When carbon is heated with sulfur, carbon disulfide, CS2, is produced. C + 2S → CS2 (i) Complete the energy level diagram for the production of carbon disulfide by writing these formulae on the diagram: ● C + 2S ● CS2. energy progress of reaction [1] (ii) Explain, using information on the energy level diagram, how you know that this reaction is endothermic. … [1] (d) Carbon disulfide is a liquid at room temperature. Describe the separation and motion of the particles in carbon disulfide liquid. separation … motion … [2] (e) Hydrogen sulfide is a gas which turns filter paper soaked in aqueous lead(II) ethanoate from white to black. Hydrogen sulfide is slightly soluble in water. A long glass tube is set up as shown. long glass tube cotton wool soaked in aqueous hydrogen sulfide filter paper soaked in aqueous lead(II) ethanoate At first, the filter paper soaked in aqueous lead(II) ethanoate does not turn black. After a short time, the filter paper soaked in aqueous lead(II) ethanoate turns black. Explain these observations using the kinetic particle model. … … … … … [3] (f) Sulfur dioxide is a pollutant in the air. (i) Give one adverse effect of sulfur dioxide on buildings. … [1] (ii) Sulfur dioxide is used to bleach paper. Give one other use of sulfur dioxide. … [1] [Total: 13]

5(a) 1 mark each for any 3 of: • does not conduct electricity / poor conductor of electricity / insulator • does not conduct heat / poor conductor of heat • brittle / not malleable / not ductile • not sonorous / dull sound when hit • not lustrous / not shiny • has (relatively) low melting point / low boiling point 3 5(b) in underground deposits / sulfide ores / volcanoes 1 5(c)(i) C + (2)S on lower line to left AND CS2 on upper line to right (1) 1 5(c)(ii) the product energy level is above the reactant energy level 1 5(d) separation: particles close together / small spaces between the particles (1) motion: limited (movement) / restricted (movement) / sliding over each other / random (motion) (1) 2 5(e) 1 mark each for any 3 of: • evaporation of hydrogen sulfide from the cotton wool / hydrogen sulfide molecules escape from solution • diffusion • molecules in (constant) movement / molecules collide / molecules travel • (movement of) molecules is random / in every direction • molecules spread out / molecules mix • molecules hit filter paper • (molecules spread) from higher concentration to lower concentration 3 5(f)(i) (chemical) weathering / ironwork corrodes 1 5(f)(ii) food preservation / food preservative 1 To earn marks, you must list three distinct physical properties typical of non-metals, name a correct source of sulfur, correctly label the reactants and products on the energy level diagram, explain that the reaction is endothermic because the product energy is higher than the reactant energy, describe the separation and motion of particles in a liquid, explain the process of diffusion using at least three key ideas, and state one use for sulfur dioxide.

4 Dinitrogen tetroxide, N2O4, decomposes into nitrogen dioxide, NO2. The reaction is reversible. N2O4(g) 2NO2(g) colourless gas brown gas A gas syringe containing a mixture of dinitrogen tetroxide and nitrogen dioxide gases was sealed and heated. After reaching equilibrium the mixture was a pale brown colour. end sealed plunger mixture of dinitrogen tetroxide and nitrogen dioxide (a) State what is meant by the term equilibrium. … … … [2] (b) The plunger of the gas syringe is pushed in. The temperature does not change. The mixture initially turns darker brown. After a few seconds the mixture turns lighter brown because the equilibrium shifts to the left. N2O4(g) 2NO2(g) colourless gas brown gas (i) Explain why the mixture initially turns darker brown. … [1] (ii) Explain why the position of equilibrium shifts to the left. … [1] (c) The forward reaction is endothermic. (i) State what happens to the position of equilibrium when the temperature of the mixture is increased. … [1] (ii) State what happens to the rate of the forward reaction and the rate of the backward reaction when the temperature of the mixture is increased. rate of the forward reaction … rate of the backward reaction … [2] [Total: 7]

4(a) the rate of forward reaction equals the rate of the reverse reaction (1) concentrations of reactants and products are constant (1) 2 4(b)(i) (increased pressure) nitrogen dioxide particles or molecules (forced) closer together OR same number of nitrogen dioxide particles or molecules in a smaller volume 1 4(b)(ii) fewer number of gas moles or molecules on left hand side or reactant side (of the equation) ORA 1 4(c)(i) shifts to the right 1 4(c)(ii) increase / faster (1) increase / faster (1) 2 (a) To earn these two marks, you must state that the rate of the forward reaction equals the rate of the reverse reaction, and that the concentrations of reactants and products are constant. (b)(i) To earn this mark, you must explain that the nitrogen dioxide particles are forced closer together, or that the same number of particles are in a smaller volume. (b)(ii) To earn this mark, you must explain that there are fewer gas moles or molecules on the left-hand side (reactant side) of the equation. (c)(i) To earn this mark, you must state that the equilibrium shifts to the right. (c)(ii) To earn these two marks, you must state that both the rate of the forward reaction and the rate of the reverse reaction increase or become faster.

4 Hydrogen iodide, HI, decomposes into iodine and hydrogen. The reaction is reversible. 2HI(g) I2(g) + H2(g) colourless gas purple gas colourless gas A gas syringe containing a mixture of hydrogen iodide, iodine and hydrogen gases was sealed. After reaching equilibrium the mixture was a pale purple colour. end sealed plunger equilibrium mixture of hydrogen iodide, iodine and hydrogen (a) State what is meant by the term equilibrium. … … … [2] (b) The plunger of the gas syringe is pushed in. The position of equilibrium does not change. The colour of the gaseous mixture turns darker purple. The temperature remains constant. 2HI(g) I2(g) + H2(g) colourless gas purple gas colourless gas (i) Explain why the position of equilibrium does not change. … [1] (ii) Suggest why the colour of the gaseous mixture turns darker purple even though the position of equilibrium does not change. … [1] (c) The forward reaction is endothermic. (i) State what happens to the position of equilibrium when the temperature is decreased. … … [1] (ii) State what happens to the rate of the forward reaction and the rate of the backward reaction when the temperature of the mixture is decreased. rate of the forward reaction … rate of the backward reaction … [2] [Total: 7]

4(a) the rate of forward reaction equals (the rate of the) reverse reaction (1) concentrations of reactants and products are constant (1) 2 4(b)(i) same number of gas moles on both sides of the equilibrium / same number of gas molecules on both sides of the equilibrium 1 4(b)(ii) iodine particles or molecules (forced) closer together / same number of iodine particles or molecules in a smaller volume 1 4(c)(i) shifted to the left 1 4(c)(ii) decrease / slower (1) decrease / slower (1) 2 For part (a), you must state that the rates of the forward and reverse reactions are equal AND that the concentrations of reactants and products are constant. For (b)(i), you need to explain that the number of gas moles/molecules is the same on both sides of the equation. For (b)(ii), you must suggest that the iodine particles are closer together or in a smaller volume. For (c)(i), state that the equilibrium shifts to the left. For (c)(ii), state that both the forward and reverse reaction rates decrease/slow down.

4 Chlorine reacts with carbon monoxide to produce phosgene gas, COCl 2(g). A catalyst is used. Cl 2(g) + CO(g) COCl 2(g) The reaction is exothermic. (a) Explain why the reaction is exothermic in terms of the energy changes of bond breaking and bond making. … … … [3] (b) (i) Complete the energy level diagram for this reaction. On your diagram show: ● the product of the reaction ● an arrow representing the energy change, labelled ∆H ● an arrow representing the activation energy, labelled A. energy progress of reaction Cl 2(g) + CO(g) [3] (ii) State why a catalyst is used. … [1] (c) Describe and explain the effect, if any, on the position of equilibrium when: (i) the pressure is increased … … [2] (ii) the temperature is increased. … … [2] (d) The reaction between chlorine and carbon monoxide can be represented as shown. Cl Cl Cl + CO C O Cl When one mole of chlorine reacts with one mole of carbon monoxide, 230 kJ of energy is released. Some bond energies are shown in the table. bond bond energy in kJ / mol Cl –Cl 240 C=O 745 C–Cl 400 Use the information to calculate the energy of the bond between the C and the O in carbon monoxide, CO. bond energy in carbon monoxide, CO = … kJ / mol [3] (e) Complete the dot-and-cross diagram to show the electron arrangement in a molecule of COCl 2. Cl Cl C O Show outer electrons only. C Cl Cl O [3] [Total: 17]

4(a) M1 E of making bonds > breaking bonds M2 bond making releases energy M3 bond breaking requires energy 3 4(b)(i) M1 exothermic mark horizontal line below energy level to R.H.S. of reactants line and labelled COCl 2(g) (1) M2 activation E mark activation energy ‘hump’ with upward arrow labelled A / activation energy (1) M3 energy change mark one downward arrow labelled ΔH and energy change starting from E level of reactants and finishing at E level of products (1) 3 4(b)(ii) increases rate of reaction 1 4(c)(i) equilibrium shifts to right hand side (1) fewer moles (of gas) on right hand side (1) 2 4(c)(ii) equilibrium shifts to left hand side (1) (forward) reaction is exothermic (1) 2 4(d) M1 bond energy in making bonds = [(2 × 400) + 745] = 1545 (kJ mol–1) M2 use of total E change –230 = [240 + E(C≡O)] – 1545 OR [240 + E(C≡O)] = –230 + 1545 = (+1315) M3 E(C≡O) = [–230 + 1545] – 240 = 1075 (kJ mol–1) 3 4(e) M1 all single bonding dot and cross pairs correct M2 double C=O bond dot and cross pairs are correct M3 complete diagram is correct 3 (a) You earn one mark for stating that the energy released from making bonds is greater than the energy required for breaking bonds. You earn a second mark for explaining that bond making releases energy, and a third mark for explaining that bond breaking requires energy. (b)(i) You earn one mark for drawing a horizontal line for the product below the reactants' energy level and labeling it COCl2(g). You earn a second mark for drawing an activation energy 'hump' with an upward arrow labeled 'A'. You earn a third mark for drawing a downward arrow labeled ΔH, starting from the reactants' energy level and ending at the products' energy level. (b)(ii) You earn this mark by stating that a catalyst increases the rate of reaction. (c)(i) You earn one mark for stating that the equilibrium shifts to the right-hand side, and a second mark for explaining that there are fewer moles of gas on the right-hand side. (c)(ii) You earn one mark for stating that the equilibrium shifts to the left-hand side, and a second mark for explaining that the forward reaction is exothermic. (d) You earn one mark for correctly calculating the total energy of making bonds. You earn a second mark for correctly setting up the equation using the total energy change. You earn a third mark for correctly calculating the bond energy of C≡O. (e) You earn one mark for all single bonding dot and cross pairs being correct, a second mark for the double C=O bond dot and cross pairs being correct, and a third mark for the complete diagram being correct.

11 The temperature decreases when aqueous ethanoic acid reacts with solid sodium carbonate to form a salt. Which type of reaction and energy change occur? type of reaction energy change A neutralisation endothermic B neutralisation exothermic C redox endothermic D redox exothermic

Answer: A To earn this mark, a student must correctly identify that the reaction between an acid and a carbonate is neutralisation and that a decrease in temperature indicates an endothermic energy change.

Cambridge IGCSE Chemistry · 0620

IGCSE Chemistry (0620) Chemical energetics past paper questions

Energetics questions test exothermic and endothermic changes and bond-energy calculations.

266 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 IGCSE Chemistry 0620 exams, 2020–2025

Across 2020–2025, Chemical energetics accounted for 256 questions worth 1,409 marks in Cambridge IGCSE Chemistry (0620) papers.

By year
Year	Questions	Marks
2020	54	297
2021	37	195
2022	47	233
2023	34	191
2024	38	206
2025	46	287

Most-tested subtopics

5.1 Exothermic and endothermic reactions1116 marks
5.2 Energy level diagrams292 marks
5.3 Bond energies199 marks

Appears in

Paper 1 · 68 qs
Paper 2 · 96 qs
Paper 3 · 27 qs
Paper 4 · 29 qs
Paper 5 · 18 qs
Paper 6 · 18 qs

All most-tested Chemistry topics

2021 Feb/Mar · Paper 3 · Q5 · 13 marks
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2021 May/Jun · Paper 4 · Q4 · 7 marks
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2021 May/Jun · Paper 4 · Q4 · 7 marks
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2021 Oct/Nov · Paper 4 · Q4 · 17 marks
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2021 Oct/Nov · Paper 1 · Q11 · 1 mark
Marking scheme
Mark scheme — 2021 Paper 1 Q11
Answer: A
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