Paper 4 Energetics Structured Practice
5 structured exam questions on chemical energetics for IGCSE Chemistry Paper 4. Covers exothermic and endothermic reactions, energy level diagrams, bond energy calculations, and activation energy. Total: 25 marks.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
Five structured questions in the style of IGCSE Chemistry Paper 4, totalling 25 marks. Write your answers in full before checking the mark scheme.
Question 1 (5 marks)
The combustion of methane is an exothermic reaction:
CH₄ + 2O₂ → CO₂ + 2H₂O
(a) State what is meant by the term exothermic. [1]
(b) Draw a labelled energy level diagram for this reaction. Your diagram should show: the energy of the reactants, the energy of the products, the activation energy, and the overall energy change. [3]
(c) State one everyday use of an exothermic reaction. [1]
Mark scheme
(a) A reaction that releases / gives out energy (heat) to the surroundings [1].
(b) Diagram should show:
- Reactants (CH₄ + 2O₂) labelled at a higher energy level [1]
- Products (CO₂ + 2H₂O) labelled at a lower energy level [1]
- Activation energy shown as an arrow from reactants to the peak of the curve, and overall energy change (delta H) shown as a downward arrow from reactants to products, labelled as negative / exothermic [1]
(c) Any suitable example: hand warmers / self-heating cans / combustion of fuels for heating or cooking [1].
Total: 5 marks
Question 2 (6 marks)
Use the bond energies below to calculate the energy change for the reaction:
H₂ + Cl₂ → 2HCl
| Bond | Bond energy (kJ/mol) |
|---|---|
| H-H | 436 |
| Cl-Cl | 242 |
| H-Cl | 431 |
(a) Calculate the total energy required to break all the bonds in the reactants. [1]
(b) Calculate the total energy released when all the bonds in the products are formed. [1]
(c) Calculate the overall energy change for the reaction. [1]
(d) State whether this reaction is exothermic or endothermic. Explain your answer using your calculated value. [1]
(e) Explain, in terms of bond breaking and bond making, why some reactions are exothermic. [2]
Mark scheme
(a) Bonds broken: 1 x H-H + 1 x Cl-Cl = 436 + 242 = 678 kJ/mol [1]
(b) Bonds formed: 2 x H-Cl = 2 x 431 = 862 kJ/mol [1]
(c) Energy change = bonds broken - bonds formed = 678 - 862 = -184 kJ/mol [1]
(d) Exothermic [1]. The value is negative, meaning more energy is released making bonds than is absorbed breaking bonds.
(e) In an exothermic reaction, the total energy released when new bonds are formed in the products is greater than the total energy required to break the bonds in the reactants [1]. The excess energy is released to the surroundings as heat [1].
Total: 6 marks
Question 3 (5 marks)
Thermal decomposition of calcium carbonate is an endothermic reaction:
CaCO₃ → CaO + CO₂
(a) State what is meant by the term endothermic. [1]
(b) State what you would observe about the temperature of the surroundings during this reaction. [1]
(c) On an energy level diagram for this reaction, would the products be drawn at a higher or lower energy level than the reactants? Explain your answer. [2]
(d) State one other example of an endothermic process. [1]
Mark scheme
(a) A reaction that absorbs / takes in energy (heat) from the surroundings [1].
(b) The temperature of the surroundings decreases / the surroundings cool down [1].
(c) Higher [1]. The products have more chemical energy than the reactants because energy has been absorbed from the surroundings and stored as chemical energy in the products [1].
(d) Any suitable example: photosynthesis / dissolving ammonium nitrate in water / melting ice / electrolysis [1].
Total: 5 marks
Question 4 (4 marks)
A catalyst is added to a reaction.
(a) Define the term catalyst. [2]
(b) Describe the effect of a catalyst on an energy level diagram. [1]
(c) State the effect of a catalyst on the overall energy change of the reaction. [1]
Mark scheme
(a) A substance that increases the rate of a chemical reaction [1] without itself being chemically changed at the end of the reaction / without being used up [1].
(b) The activation energy peak is lower / the catalyst provides an alternative pathway with a lower activation energy [1].
(c) No effect --- the overall energy change (enthalpy change) remains the same [1]. The catalyst only affects the activation energy, not the energy levels of reactants or products.
Total: 4 marks
Question 5 (5 marks)
The reaction between nitrogen and hydrogen to form ammonia is reversible:
N₂ + 3H₂ ⇌ 2NH₃ (forward reaction is exothermic)
(a) State what the symbol ⇌ means. [1]
(b) Using bond energies: N≡N = 944 kJ/mol, H-H = 436 kJ/mol, N-H = 388 kJ/mol. Calculate the energy change for the forward reaction. [3]
(c) State whether the reverse reaction (decomposition of ammonia) is exothermic or endothermic. [1]
Mark scheme
(a) The reaction is reversible / can proceed in both directions (forward and backward) [1].
(b) Bonds broken: 1 x N≡N + 3 x H-H = 944 + (3 x 436) = 944 + 1308 = 2252 kJ/mol [1] Bonds formed: 2 x 3 x N-H = 6 x 388 = 2328 kJ/mol [1] Energy change = 2252 - 2328 = -76 kJ/mol [1]
(c) Endothermic [1]. If the forward reaction is exothermic, the reverse reaction must be endothermic by the same amount of energy.
Total: 5 marks
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