AQA GCSE Chemistry
AQA GCSE Chemistry 4.5.1 Exothermic and Endothermic Reactions Exam Questions
Free AQA GCSE Chemistry exam questions for 4.5.1 Exothermic and Endothermic Reactions, with a mark scheme for every pack. Covers energy transfer in reactions, exothermic and endothermic examples, the temperature change required practical, reaction profiles and bond energy calculations.
4.5.1 Exothermic and Endothermic Reactions exam questions
4 question packs of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About AQA GCSE Chemistry 4.5.1 Exothermic and Endothermic Reactions
AQA GCSE Chemistry 4.5.1 explains why some reactions heat their surroundings while others cool them. You learn the difference between exothermic and endothermic reactions, draw reaction profiles showing activation energy, and carry out the required practical measuring temperature changes. At Higher tier you calculate the overall energy change using bond energies. The topic sits in section 4.5 Energy changes on Paper 1 and links to rates (activation energy and catalysts) in 4.6 and fuels in 4.7. Examiners often ask you to interpret a reaction profile, plan the polystyrene cup practical, and complete bond energy calculations with the correct sign.
What you need to know
- Conservation of energy Energy is conserved in chemical reactions. The amount of energy in the universe at the end of a reaction is the same as before; energy is transferred between the reacting chemicals and the surroundings.
- Exothermic reactions Transfer energy to the surroundings, so the temperature of the surroundings increases. Examples: combustion, many oxidation reactions and neutralisation. Uses include self-heating cans and hand warmers.
- Endothermic reactions Take in energy from the surroundings, so the temperature of the surroundings decreases. Examples: thermal decomposition and the reaction of citric acid with sodium hydrogencarbonate. Uses include some sports injury cold packs.
- Required practical Investigate the variables that affect temperature changes in reacting solutions, for example acid plus metal or acid plus alkali, using a polystyrene cup with a lid and a thermometer, recording the maximum temperature change.
- Reaction profiles Show the relative energies of reactants and products, the activation energy (the minimum energy particles need to react) and the overall energy change. In an exothermic profile the products are lower than the reactants.
- Bond energies (Higher tier only) Breaking bonds is endothermic; making bonds is exothermic. Overall energy change = energy needed to break bonds in reactants minus energy released forming bonds in products. A negative value means the reaction is exothermic.
Common mistakes in 4.5.1 Exothermic and Endothermic Reactions
- Saying an endothermic reaction 'gives out cold'. It takes in energy from the surroundings, so the surroundings cool down.
- Thinking bond making needs energy. Breaking bonds requires energy; forming bonds releases energy.
- Subtracting the wrong way round in bond energy calculations. Use bonds broken minus bonds made.
- Forgetting to count all bonds, for example the 4 C-H bonds in CH4 or the 2 O=O molecules in the equation.
- Drawing the activation energy arrow from the reactants to the products. It goes from the reactants to the peak of the curve.
Try three 4.5.1 Exothermic and Endothermic Reactions exam-style questions
1. Hydrogen reacts with chlorine: H2 + Cl2 → 2HCl. Bond energies in kJ/mol: H-H = 436, Cl-Cl = 243, H-Cl = 432. Calculate the overall energy change. (3 marks) Higher tier only
Show the mark scheme answer
Bonds broken = 436 + 243 = 679 kJ/mol; bonds made = 2 x 432 = 864 kJ/mol; energy change = 679 - 864 = -185 kJ/mol.
2. Explain how a reaction profile shows that a reaction is exothermic. (2 marks)
Show the mark scheme answer
The products are at a lower energy than the reactants; so energy has been released to the surroundings.
3. Describe how to investigate the temperature change when different masses of magnesium react with hydrochloric acid. (4 marks)
Show the mark scheme answer
Measure a fixed volume of acid into a polystyrene cup (in a beaker for stability); record the starting temperature; add a known mass of magnesium, stir and put on a lid; record the highest temperature reached and calculate the change; repeat with different masses, keeping acid volume and concentration the same.
4.5.1 Exothermic and Endothermic Reactions FAQs
What does AQA GCSE Chemistry 4.5.1 cover?
It covers energy transfer, exothermic and endothermic reactions and their uses, the temperature change required practical, reaction profiles and activation energy, and bond energy calculations (Higher tier only).
Which paper is 4.5.1 on?
Section 4.5 Energy changes is the last section assessed on AQA GCSE Chemistry Paper 1.
How do I set out a bond energy calculation?
Draw or list every bond in the reactants and products, add up bonds broken and bonds made separately, then calculate bonds broken minus bonds made. A negative answer is exothermic and a positive answer is endothermic.
