AQA GCSE Chemistry (8462) · Paper 1

🔥 Energy Changes

Revision notes written to the specification, with examiner tips and the required practicals. Every point here has flashcards in the Stickwise app.

Exothermic & endothermic

ExothermicAn exothermic reaction transfers energy out to the surroundings, so the temperature rises. Combustion, neutralisation and most oxidation reactions are exothermic, and this is used in hand warmers and self-heating cans.

EndothermicAn endothermic reaction takes in energy from the surroundings, so the temperature falls. Thermal decomposition and the reaction between citric acid and sodium hydrogencarbonate are endothermic, and this is used in instant sports injury packs.

Energy is never created or destroyed in reactions, only transferred between the chemicals and the surroundings.

🧪 Required practical: In this required practical, an acid is reacted with an alkali, a metal or a carbonate in an insulated polystyrene cup, and the temperature change is measured with a thermometer. Only one variable should be changed at a time, such as the concentration or the metal used, while the volumes and starting temperature are kept the same. The biggest temperature change, ΔT, shows the biggest transfer of energy.

Profiles & bond energies

A reaction profile plots how energy changes over the course of a reaction. In an exothermic reaction, the products finish below the reactants, and the gap between them is the energy released. In an endothermic reaction, the products finish above the reactants. The hump in both profiles is the activation energy, which is the minimum energy that colliding particles need before they can react.

Higher tier only

Bond energies (Higher Tier)

Higher tier only

Breaking bonds requires energy, so it is endothermic, while making bonds releases energy, so it is exothermic. The overall energy change for a reaction is calculated as follows:

Higher tier only

energy change = energy to break bonds − energy released making bonds

Higher tier only

A negative answer means more energy was released than was put in, so the reaction is exothermic. Work through the calculation bond by bond: list every bond broken in the reactants and every bond formed in the products, multiply each by the energy value given, and then subtract the totals.

Cells & fuel cells (Triple) Separate science only

A simple cell consists of two different metals in an electrolyte. The bigger the reactivity difference between the two metals, the bigger the voltage produced. A battery is simply a number of cells joined together in series.

Non-rechargeable cells, such as alkaline cells, stop working once a reactant is used up, because the chemical reaction only goes in one direction. Rechargeable cells can run the reaction backwards when an electric current is fed into them, which restores the original chemicals.

A hydrogen fuel cell never goes flat as long as fuel keeps flowing into it: hydrogen is oxidised by oxygen, releasing electrical energy, and the only product is water. Overall: 2H₂ + O₂ → 2H₂O.

Fuel cells never need recharging, produce only water, and are lighter than large battery packs.
However, hydrogen is a flammable gas that is hard to store safely, and most hydrogen is currently produced using fossil fuels.
Batteries are convenient and improving quickly, but they are heavy, slow to charge, and have a limited lifetime.

🎯 Examiner tip: Evaluation questions want both sides of the argument, plus a final judgement. For example, a full-mark answer might say that fuel cells only emit water, but that the hydrogen still has to be made and stored somehow, before reaching a conclusion.

Turn these notes into memory. 14 flashcards cover this topic. Short daily sessions, spaced repetition up to your exam date, quizzes with friends. Free during the beta.Practise Energy Changes in the app ↗

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