The energy change of reactions (HT only) — AQA GCSE Chemistry
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The energy change of reactions (HT only) explained
The overall energy change of a reaction is found by subtracting the energy released when new bonds form from the energy needed to break existing bonds.
Read the full explanation
In symbols, ΔH = energy needed to break bonds − energy released when bonds form. For H₂ + Cl₂ → 2HCl, breaking bonds needs 436 + 243 = 679 kJ/mol and forming bonds releases 2 × 431 = 862 kJ/mol, so ΔH = 679 − 862 = −183 kJ/mol. A negative result means the reaction is exothermic overall; a positive result means it is endothermic overall. The calculation must use the balanced equation so that the number of each bond is correct, and the sign of the answer must be interpreted, not just calculated.
Your focus
- Calculate the overall energy change of a reaction from bond energies using the correct subtraction order.
- Interpret the sign of an overall energy change as showing an exothermic or endothermic reaction.
- Present a bond energy calculation with correct units, sign and clear working.
The energy change of reactions (HT only) exam tips
Marking Points
- Calculate the total energy needed to break bonds in the reactants by adding the relevant bond energies.
- Calculate the total energy released when bonds form in the products by adding the relevant bond energies.
- Subtract the energy released when bonds form from the energy needed to break bonds to obtain the overall energy change.
- Interpret a negative overall energy change as exothermic and a positive overall energy change as endothermic.
- Include the correct sign and units, usually kJ/mol, with the final answer.
- Use the balanced equation to ensure the number of each bond broken and formed is correct.
Examiner Tips
- 💡Set out the calculation in two labelled stages: energy to break bonds, then energy released when bonds form, before subtracting.
- 💡State the final answer with its sign and units, and add a short sentence interpreting the sign as exothermic or endothermic.
- 💡If the question supplies bond energies in a table, copy each value accurately and show which bond it applies to.
Common Mistakes
- Subtracting in the wrong order, for example energy to break bonds minus energy released when bonds form reversed; correct this by always using energy needed to break bonds minus energy released when bonds form.
- Omitting the sign of the final answer; correct this by stating whether the value is positive or negative and linking that sign to endothermic or exothermic behaviour.
- Forgetting to multiply bond energies by coefficients in the balanced equation; correct this by counting bonds from displayed formulae that match the balanced equation.