Cambridge IGCSE Chemistry
Cambridge IGCSE Chemistry 5 Chemical Energetics Exam Questions
Free Cambridge IGCSE Chemistry revision for 5 Chemical Energetics: what you need to know, common mistakes and exam-style questions with answers. Covers exothermic and endothermic reactions, temperature changes, reaction pathway diagrams, enthalpy change, activation energy and bond energy calculations.
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About Cambridge IGCSE Chemistry 5 Chemical Energetics
Chemical energetics in Cambridge IGCSE Chemistry 0620 is about the transfer of thermal energy during reactions. You need to identify exothermic and endothermic reactions from temperature changes, and draw and interpret reaction pathway diagrams. Extended candidates also define enthalpy change and activation energy, explain energy changes in terms of bond breaking and bond making, and calculate enthalpy changes from bond energies. The topic links to fuels and combustion in organic chemistry, to rates of reaction through activation energy, and to fuel cells. Examiners regularly ask you to label pathway diagrams correctly, give the sign of the enthalpy change and carry out bond energy calculations.
What you need to know
- Exothermic reactions An exothermic reaction transfers thermal energy to the surroundings, so the temperature of the surroundings increases. Examples include combustion, neutralisation and many displacement reactions.
- Endothermic reactions An endothermic reaction takes in thermal energy from the surroundings, so the temperature of the surroundings decreases. Examples include thermal decomposition and dissolving some salts, such as ammonium nitrate.
- Reaction pathway diagrams Show reactants, products and the overall energy change. For an exothermic reaction the products are lower in energy than the reactants; for an endothermic reaction they are higher.
- Enthalpy change (Supplement) The enthalpy change, ΔH, is the transfer of thermal energy during a reaction. It is negative for exothermic reactions and positive for endothermic reactions.
- Activation energy (Supplement) Activation energy, Ea, is the minimum energy that colliding particles must have to react. On a pathway diagram it is drawn from the reactants up to the top of the energy hump.
- Bond energies (Supplement) Bond breaking is endothermic and bond making is exothermic. ΔH = energy needed to break bonds in reactants minus energy released making bonds in products. If more energy is released than taken in, the reaction is exothermic.
Common mistakes in 5 Chemical Energetics
- Saying bond breaking releases energy. Breaking bonds always takes in energy; making bonds releases it.
- Subtracting the wrong way round in bond energy calculations. Use bonds broken minus bonds made, so an exothermic reaction gives a negative answer.
- Forgetting to count every bond. Multiply each bond energy by the number of those bonds in the balanced equation, for example two H-Cl bonds in 2HCl.
- Drawing activation energy from the products or to the wrong level. It goes from the reactants to the peak of the curve.
- Saying an endothermic reaction feels hot. It takes in energy, so the surroundings, including the reaction container, get colder.
Try three 5 Chemical Energetics exam-style questions
1. When ammonium nitrate dissolves in water the temperature falls from 21 °C to 14 °C. State whether the process is exothermic or endothermic and explain your answer. (2 marks)
Show the mark scheme answer
Endothermic; thermal energy is taken in from the surroundings so the temperature decreases.
2. H2 + Cl2 → 2HCl. Bond energies in kJ/mol: H-H 436, Cl-Cl 242, H-Cl 431. Calculate the enthalpy change for the reaction. (3 marks)
Show the mark scheme answer
Bonds broken = 436 + 242 = 678 kJ; bonds made = 2 x 431 = 862 kJ; ΔH = 678 - 862 = -184 kJ/mol.
3. Sketch and label a reaction pathway diagram for an exothermic reaction. (3 marks)
Show the mark scheme answer
Products drawn at a lower energy level than reactants; enthalpy change shown as a downward arrow from reactants to products labelled ΔH; activation energy shown from reactants to the top of the curve labelled Ea.
5 Chemical Energetics FAQs
What does Cambridge IGCSE Chemistry 5 Chemical Energetics cover?
It covers exothermic and endothermic reactions, temperature changes, reaction pathway diagrams and, for Extended, enthalpy change, activation energy and bond energy calculations.
Which papers is 5 Chemical Energetics examined in?
Every topic in the 0620 syllabus can be tested in Paper 1 and Paper 3 (Core) or Paper 2 and Paper 4 (Extended), with Supplement content only in the Extended papers. Practical skills linked to the topic can also appear in Paper 5 (Practical Test) or Paper 6 (Alternative to Practical).
How do I set out a bond energy calculation?
List the bonds broken in the reactants and the bonds made in the products, including how many of each. Add each set, then calculate bonds broken minus bonds made and give the sign and units, kJ/mol.
