A student investigates two chemical reactions, Reaction X and Reaction Y. In Reaction X, the temperature of the surroundings increases during the reaction. In Reaction Y, the temperature of the surroundings decreases during the reaction. Explain, in terms of bond breaking and bond making, why the overall energy change is different for each reaction, and state which reaction releases more energy when bonds form in the products than is required to break bonds in the reactants.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Reaction X: magnesium burns in oxygen to form magnesium oxide. Reaction Y: citric acid dissolves and reacts in water, causing the solution to cool.
Model answer (5 marks)
Reaction X is exothermic – the temperature of the surroundings rises because energy is released to the surroundings.
Reaction Y is endothermic – the temperature of the surroundings falls because energy is absorbed from the surroundings.
Breaking bonds requires an input of energy (endothermic). Making bonds releases energy (exothermic).
In Reaction X, the energy released when new Mg–O bonds are formed is greater than the energy required to break the Mg–Mg and O=O bonds, so the net ΔE is negative.
In Reaction Y, the energy required to break the bonds in citric acid and water exceeds the energy released by forming new bonds, giving a positive ΔE.
Therefore, Reaction X releases more energy when bonds form in the products than is required to break bonds in the reactants.
Reaction Y is endothermic – the temperature of the surroundings falls because energy is absorbed from the surroundings.
Breaking bonds requires an input of energy (endothermic). Making bonds releases energy (exothermic).
In Reaction X, the energy released when new Mg–O bonds are formed is greater than the energy required to break the Mg–Mg and O=O bonds, so the net ΔE is negative.
In Reaction Y, the energy required to break the bonds in citric acid and water exceeds the energy released by forming new bonds, giving a positive ΔE.
Therefore, Reaction X releases more energy when bonds form in the products than is required to break bonds in the reactants.
Examiner tips
- Use the terms exothermic/endothermic and link to temperature change. Show the energy flow: bond breaking = +ΔE, bond making = –ΔE. State clearly that Reaction X has a larger negative ΔE.
- Use the exact wording from the mark scheme: "energy is transferred to the surroundings" and "energy is taken in from the surroundings".
Common mistakes
- Confusing the direction of heat flow – saying the reaction releases heat in Y. Forgetting to mention that bond breaking is endothermic and bond making is exothermic. Using vague terms like "more energy" without specifying which reaction is exothermic or endothermic.
Mark scheme (5 marks)
- Reaction X is exothermic because energy is transferred to the surroundings / the temperature of the surroundings increases
- Reaction Y is endothermic because energy is taken in from the surroundings / the temperature of the surroundings decreases
- Bond breaking is an endothermic process (requires energy input)
- Bond making is an exothermic process (releases energy)
- In Reaction X (exothermic), more energy is released during bond making in the products than is required to break bonds in the reactants, so Reaction X releases more energy when bonds form
Key terms in this question
Related
- All OCR A-Level Chemistry A (H432) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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