10 free IGCSE Chemistry Chemical Energetics questions written in the style of Cambridge Chemistry papers: 4 multiple-choice and 6 structured, each with its answer and mark scheme. 6 of them are A* challenge questions, built to catch the mistakes strong students still make.
This topic covers Exothermic & endothermic reactions. Work through the questions below, then generate a full Chemical Energetics paper with a fresh set every time.
Generate a full Chemical Energetics paper →A catalyst is added to a reaction.
What is the effect on the activation energy and on the enthalpy change, ΔH, of the reaction?
| activation energy | ΔH | |
|---|---|---|
| A | decreases | decreases |
| B | increases | no change |
| C | no change | decreases |
| D | decreases | no change |
Answer: D
The energy level diagram is for a reaction. Which statement is correct?
Answer: D
H2 + Cl2 → 2HCl
Bond energies in kJ/mol: H–H 436; Cl–Cl 243; H–Cl 432.
What is the energy change for the reaction, in kJ/mol?
Answer: B
Which processes are endothermic?
1 thermal decomposition of calcium carbonate
2 respiration
3 neutralisation of hydrochloric acid by sodium hydroxide
Answer: C
Hydrogen burns in chlorine: H–H + Cl–Cl → 2 H–Cl. Bond energies (kJ/mol): H–H 436, Cl–Cl 242, H–Cl 431.
Calculate the energy needed to break the bonds.
Calculate the energy released when the bonds form.
Calculate the enthalpy change of the reaction and state whether it is exothermic or endothermic.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | 678 kJ 436 + 242 = 678 kJ B1 | 1 |
| (b) | 862 kJ 2 × 431 = 862 kJ B1 | 1 |
| (c) | −184 kJ/mol, exothermic 678 − 862 = −184 kJ/mol C1 exothermic A1 | 2 |
A student adds a metal to 150 cm³ of an acid. The temperature rises by 8 °C. (specific heat capacity of solution = 4.2 J/g °C)
State whether the reaction is exothermic or endothermic.
Calculate the energy released.
0.05 mol of metal reacted. Calculate the enthalpy change, ΔH, in kJ/mol.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | exothermic exothermic B1 | 1 |
| (b) | 5040 J Q = mcΔT = 150 × 4.2 × 8 C1 5040 J A1 | 2 |
| (c) | −101 kJ/mol 5.04 ÷ 0.05 C1 −101 kJ/mol (negative sign) A1 | 2 |
The equation for hydrogen burning is
2H2 + O2 → 2H2O
| bond | H–H | O=O | O–H |
|---|---|---|---|
| bond energy in kJ/mol | 436 | 498 | 464 |
Calculate the energy needed to break all the bonds in the reactants.
Calculate the energy released when the bonds in the products are formed.
Calculate the energy change, ΔH, for the reaction and state whether it is exothermic or endothermic.
Draw a labelled reaction pathway diagram for this reaction. Show the activation energy and ΔH.
Explain, in terms of bond breaking and bond making, why this reaction is exothermic.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | 1370 kJ 2 × 436 + 1 × 498 M1 1370 A1 | 2 |
| (b) | 1856 kJ 4 × 464 M1 1856 A1 | 2 |
| (c) | −486 kJ/mol, exothermic −486 (sign required) B1 exothermic B1 | 2 |
| (d) | See diagram products below reactants, axis labelled energy B1 activation energy shown as arrow from reactants to peak B1 ΔH shown as arrow between reactant and product levels B1 | 3 |
| (e) | More energy is released making bonds in the products than is taken in breaking bonds in the reactants bond breaking is endothermic and bond making is exothermic B1 energy released (making) > energy absorbed (breaking) B1 | 2 |
This question is about methane as a fuel.
Write the equation for the complete combustion of methane.
Bond energies, in kJ/mol, are given in the table.
| bond | C–H | O=O | C=O | O–H |
|---|---|---|---|---|
| bond energy / kJ/mol | 412 | 498 | 805 | 464 |
Calculate the energy change for the complete combustion of one mole of methane.
Use your answer to (b)(i) to calculate the energy released when 8 g of methane is burned. [Mᵣ: CH₄, 16]
Write an equation for the incomplete combustion of methane to form carbon monoxide.
Explain why a gas boiler should be serviced regularly, with reference to carbon monoxide.
Methane is a greenhouse gas. Give one source of methane in the atmosphere other than natural gas leaks.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | CH₄ + 2O₂ → CO₂ + 2H₂O correct formulae B1 balanced B1 | 2 |
| (b)(i) | −822 kJ/mol bonds broken = 4(412) + 2(498) = 2644 M1 bonds formed = 2(805) + 4(464) = 3466 M1 −822 kJ/mol A1 | 3 |
| (ii) | 411 kJ 8/16 = 0.5 mol M1 411 kJ A1 | 2 |
| (c) | 2CH₄ + 3O₂ → 2CO + 4H₂O CO and H₂O as products B1 balanced B1 | 2 |
| (d) | blocked air supply causes incomplete combustion; CO toxic, colourless, odourless a poor air supply causes incomplete combustion B1 carbon monoxide is toxic and colourless / odourless so it cannot be detected B1 | 2 |
| (e) | cattle / decomposition / rice paddies / landfill digestion in animals (cattle) / decomposition of vegetation / landfill B1 | 1 |
Reactions can be exothermic or endothermic.
Define an endothermic reaction.
Explain, in terms of bond breaking and bond making, why a reaction is exothermic.
Describe how the reaction pathway diagram for an exothermic reaction shows that it is exothermic.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | takes in thermal energy a reaction that takes in thermal energy from the surroundings B1 | 1 |
| (b) | more energy released making bonds than used breaking bond breaking is endothermic and bond making is exothermic B1 more energy is released making new bonds than is used breaking bonds B1 | 2 |
| (c) | products lower than reactants the products are at a lower energy level than the reactants B1 | 1 |
A student dissolves 8.0 g of ammonium nitrate, NH₄NO₃, in 100 cm³ of water. The temperature falls by 6.1 °C.
State whether dissolving ammonium nitrate is exothermic or endothermic. Explain your answer.
Calculate the energy change in J. Assume 4.2 J raise the temperature of 1 cm³ of water by 1 °C.
Use your answer to (b) to calculate the energy change in kJ per mole of ammonium nitrate. [Mᵣ: NH₄NO₃, 80]
Draw a labelled reaction pathway diagram for an endothermic reaction. Label the activation energy and the enthalpy change.
Add to your diagram the pathway with a catalyst, and explain its effect on the rate.
Suggest a use for this reaction and explain why polystyrene cups are used in the experiment.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | endothermic; temperature falls, energy taken in from surroundings endothermic B1 the temperature decreases because thermal energy is taken in from the surroundings B1 | 2 |
| (b) | 2560 J 100 × 4.2 × 6.1 M1 2560 J A1 | 2 |
| (c) | +25.6 kJ/mol moles = 8.0/80 = 0.100 M1 2.56 kJ ÷ 0.100 M1 +25.6 kJ/mol (positive sign) A1 | 3 |
| (d) | products above reactants; Ea from reactants to peak; ΔH up arrow products higher than reactants B1 activation energy shown from reactants to the top of the curve B1 ΔH shown as an upward arrow from reactants to products B1 | 3 |
| (e) | lower peak; lower activation energy so more successful collisions a lower peak drawn B1 lower activation energy so a greater proportion of collisions are successful B1 | 2 |
| (f) | cold packs; polystyrene is an insulator, reduces energy transfer with surroundings instant cold packs (sports injuries) B1 polystyrene is a poor thermal conductor so less energy is transferred from the surroundings B1 | 2 |
In the 0620 syllabus, Chemical Energetics covers Exothermic & endothermic reactions.
This page has 10 Chemical Energetics questions with answers. For more, the free generator on mentorriyaz.in builds a new Chemical Energetics paper with a full mark scheme each time you click.
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Meet Riyaz →Original Cambridge-style practice questions written for mentorriyaz.in. These are not Cambridge past paper questions, and this site is not affiliated with or endorsed by Cambridge University Press & Assessment. Updated 2026-11-04. Paper generator · mentorriyaz.in