Why do some reactions release energy and others take it in?
Exothermic and endothermic reactions and their everyday uses, reaction profiles and activation energy, and the energy changes involved in breaking and making bonds.
A focused answer to the AQA GCSE Combined Science: Trilogy Energy changes topic, covering exothermic and endothermic reactions and their uses, reaction profiles and activation energy, and the energy changes when bonds are broken and made.
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What this topic is asking
AQA wants you to distinguish exothermic and endothermic reactions and give everyday uses, draw and interpret reaction profiles including activation energy, and calculate the overall energy change using bond energies.
Exothermic and endothermic reactions
The energy is conserved overall: in an exothermic reaction the energy stored in the chemical (potential) store falls and is transferred to the surroundings, raising their temperature, while in an endothermic reaction energy is taken in from the surroundings and stored, lowering their temperature. Everyday uses to know:
- Exothermic: self-heating cans, hand warmers, combustion of fuels, neutralisation reactions, and the oxidation reactions in respiration.
- Endothermic: sports-injury cold packs (which take in heat to cool the injury), and thermal decomposition reactions such as breaking down a metal carbonate.
A simple way to measure an energy change in the lab is to carry out the reaction in an insulated container (such as a polystyrene cup) and record the temperature change of the solution.
Reaction profiles and activation energy
The overall energy change is the difference in height between the reactants and the products on the profile. A catalyst provides an alternative reaction pathway with a lower activation energy, so a higher proportion of collisions are successful and the rate increases, but it does not change the reactant or product energy levels and so does not change the overall energy change.
Bond energies
If the overall value is negative, more energy was released making bonds than was absorbed breaking them, so the reaction is exothermic. If it is positive, more energy was needed to break bonds than was released, so the reaction is endothermic. Bond energies are given in the question (in kJ per mole), and you multiply each by the number of that bond in the molecules.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20204 marksA reaction breaks bonds that require a total of 2740 kJ and forms bonds that release a total of 3100 kJ. Calculate the overall energy change for the reaction, and state with a reason whether the reaction is exothermic or endothermic.Show worked answer →
A Chemistry Paper 1 bond-energy calculation. Method: overall energy change energy to break bonds energy released making bonds kJ. Because the value is negative, more energy is released forming bonds than is taken in breaking them, so the reaction is exothermic. Markers credit the correct subtraction in the right order, the negative answer with its unit, and the correct conclusion linked to the sign. The common error is to subtract the wrong way round and get plus 360, then call it endothermic; stress that bonds broken come first.
AQA 20224 marksExplain, in terms of activation energy, how a catalyst increases the rate of a reaction, and describe how a reaction profile for the catalysed reaction differs from the uncatalysed one.Show worked answer →
A Chemistry Paper 1 explanation linking energy changes to rate. Reward: a catalyst provides an alternative reaction pathway that has a lower activation energy, so a greater proportion of colliding particles have at least the activation energy and more collisions are successful, increasing the rate. On the reaction profile, the reactant and product energy levels are unchanged (the overall energy change is the same), but the activation energy hump is lower for the catalysed pathway. Markers credit the lower activation energy, the higher proportion of successful collisions, and the correct description of the profile (lower hump, same start and end levels).
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