WJEC GCSE Chemistry
WJEC GCSE Chemistry 2.4 Chemical Reactions and Energy Exam Questions
Free WJEC GCSE Chemistry revision for 2.4 Chemical Reactions and Energy: what you need to know, common mistakes and exam-style questions with answers. Covers exothermic and endothermic reactions, temperature changes, energy profile diagrams, activation energy and bond energy calculations.
Practise 2.4 Chemical Reactions and Energy with past papers
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About WJEC GCSE Chemistry 2.4 Chemical Reactions and Energy
This short Unit 2 topic is about the energy changes that accompany reactions. You need to classify reactions as exothermic or endothermic from temperature data, knowing that combustion and neutralisation are exothermic. You must sketch and label energy profile diagrams showing reactants, products, activation energy and the overall energy change, and define activation energy. The main calculation is using bond energies to find the overall energy change, which higher tier candidates may also be asked to reverse to find an unknown bond energy. It links to rates in 1.5 and fuels in 2.5. Bond energy calculations are a regular feature of Unit 2 papers.
What you need to know
- Exothermic reactions Transfer energy to the surroundings, so the temperature rises. Combustion and neutralisation are exothermic.
- Endothermic reactions Take in energy from the surroundings, so the temperature falls. Classify reactions from experimental temperature readings.
- Energy profiles Draw reactants and products as horizontal lines with a curve between them. For exothermic reactions products are lower than reactants; for endothermic reactions products are higher. Label the activation energy and the overall energy change.
- Activation energy The minimum energy needed for a reaction to start. Catalysts lower it.
- Bond energies Breaking bonds takes in energy and making bonds releases energy. Overall energy change = energy to break bonds - energy released making bonds. A negative value means exothermic.
- Working backwards On the higher tier you may be given the overall energy change and asked to find one unknown bond energy by rearranging the same calculation.
Common mistakes in 2.4 Chemical Reactions and Energy
- Saying bond breaking releases energy. Breaking bonds always needs energy; forming bonds releases it.
- Drawing the activation energy arrow from the products. It goes from the reactants up to the top of the curve.
- Forgetting to multiply bond energies by the number of bonds, e.g. using one H-Cl bond when the equation makes two.
- Getting the sign wrong. If more energy is released making bonds than is needed to break them, the overall change is negative and the reaction is exothermic.
Try three 2.4 Chemical Reactions and Energy exam-style questions
1. Use the bond energies H-H 436, Cl-Cl 242 and H-Cl 431 kJ/mol to calculate the energy change for H2 + Cl2 → 2HCl. (3 marks)
Show the mark scheme answer
Bonds broken = 436 + 242 = 678 kJ; bonds made = 2 x 431 = 862 kJ; 678 - 862 = -184 kJ/mol (exothermic).
2. Sketch an energy profile diagram for an exothermic reaction, labelling the activation energy and energy change. (3 marks)
Show the mark scheme answer
Reactants line higher than products line; curve rising to a peak between them with activation energy shown from reactants to peak; overall energy change shown as difference between reactants and products (downwards).
3. When ammonium nitrate dissolves in water, the temperature falls from 21 °C to 15 °C. State and explain whether this is exothermic or endothermic. (2 marks)
Show the mark scheme answer
Endothermic; energy is taken in from the surroundings so temperature decreases.
2.4 Chemical Reactions and Energy FAQs
What does WJEC GCSE Chemistry 2.4 Chemical Reactions and Energy cover?
It covers exothermic and endothermic reactions, interpreting temperature changes, energy profile diagrams, activation energy and calculations of energy changes using bond energies.
Which unit is Chemical Reactions and Energy examined in?
It is examined in Unit 2: Chemical Bonding, Application of Chemical Reactions and Organic Chemistry, a 1 hour 45 minute written paper worth 45% of the GCSE, sat at foundation or higher tier. The Unit 3 practical assessment draws on the same practical skills.
How do I set out a bond energy calculation?
Draw or list every bond in the reactants and products, total the energy for bonds broken and for bonds made separately, then subtract made from broken. Check the sign matches whether the reaction is exothermic.
