This mini-lesson walks you through the whole of AQA Topic 4.5 — Energy changes: exothermic and endothermic reactions, their everyday uses, the required practical, reaction profiles, working out energy changes from bond energies, and chemical cells, batteries and fuel cells.
Make a guess: why does a match need a strike to start burning?
Mission 1Why does a burning match need a push?
Mission 2How do hydrogen and oxygen make electricity?
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you're ready.
Exothermic & endothermic
Energy in, or energy out?
Energy is conserved: the amount of energy in the universe is the same before and after a reaction. What changes is whether energy is transferred to or from the surroundings.
An exothermic reaction transfers energy to the surroundings, so the temperature of the surroundings increases.
An endothermic reaction takes in energy from the surroundings, so the temperature of the surroundings decreases.
Hold the test tube: exothermic feels warm, endothermic feels cold.
Watch out: "exothermic" means energy leaves the reaction and goes to the surroundings — so the surroundings warm up. Don't muddle the direction.
Examples to learn
Which reactions are which?
AQA expects you to recognise specific examples of each type:
Everyday uses — Exothermic: self-heating cans and hand warmers. Endothermic: some sports injury (cold) packs.
Quick check
Read the temperature
?A student adds citric acid to sodium hydrogencarbonate solution. The thermometer reading falls from 21 °C to 14 °C. What does this tell you about the reaction?
Everyday applications
Putting energy changes to work
You should be able to evaluate uses of exothermic and endothermic reactions when given information:
Self-heating cans — an exothermic reaction warms a drink without a power source.
Hand warmers — an exothermic reaction (e.g. oxidation of iron) releases energy to keep hands warm.
Sports injury packs — an endothermic reaction takes in energy, so the pack feels cold and is held against a sprain or bruise.
Required practical 4: investigate the variables that affect temperature changes in reacting solutions (e.g. acid + metal, acid + carbonate, neutralisation, displacement). Mix the reactants in an insulated cup, record the starting temperature and the highest (or lowest) temperature reached, and find the change. Only the temperature change is measured — ΔH calculations are not needed here.
Sort it
Exothermic or endothermic?
Tap whether each process gives energy out (exothermic) or takes energy in (endothermic).
Reaction profiles
Reading a reaction profile
Reactions only happen when particles collide with enough energy. The minimum energy needed to react is the activation energy. A reaction profile shows the relative energies of reactants and products, the activation energy, and the overall energy change.
Exothermic: products sit lower than reactants — energy is released. The rise from the reactants to the top of the hump is the activation energy.Endothermic: products sit higher than reactants — energy is taken in.
Watch out: the activation energy is the height of the hump measured from the reactants up to the peak — not the overall energy change between reactants and products.
Quick check
Which profile?
?On a reaction profile, the products are drawn at a higher energy than the reactants. What does this tell you?
Mission 1 check
Why does a burning match need a push?
?A match head burns in an exothermic reaction. Yet a match left in its box never lights by itself. Why must it be struck first?
Higher tier only
Energy change from bond energies
During a reaction, bonds in the reactants break and new bonds form in the products:
Breaking bonds is endothermic — energy must be supplied.
Making bonds is exothermic — energy is released.
ΔH = (bonds broken) − (bonds made)overall energy change = sum of energies to break reactant bonds − sum of energies released making product bonds
A negative ΔH means energy is given out: more energy is released making bonds than is used breaking them.
Remember the signs: in an exothermic reaction the energy released forming bonds is greater than the energy needed to break bonds (so ΔH is negative); in an endothermic reaction it's the other way round (ΔH positive).
Higher tier · Calculate
Your turn — bond energies
1For H₂ + Br₂ → 2HBr, use these bond energies (kJ/mol): H–H = 436, Br–Br = 193, H–Br = 366. Calculate the overall energy change ΔH in kJ/mol. (Give the sign.)
Make a guess: how can hydrogen make electricity without burning?
Chemistry only
Chemical cells & batteries
Cells contain chemicals that react to produce electricity. A simple cell is made by connecting two different metals in contact with an electrolyte.
The voltage (potential difference) a cell produces depends on factors including the type of electrode (the metals used) and the electrolyte.
The bigger the difference in reactivity between the two metals, the larger the voltage.
A battery is two or more cells connected in series to give a greater voltage.
Non-rechargeable cells/batteries (e.g. alkaline) stop when a reactant is used up.
Rechargeable cells/batteries can be recharged because the reactions are reversed by an external current.
Two different metals in an electrolyte set up a potential difference, driving a current.
You only need to interpret data on relative reactivity and evaluate the use of cells — not memorise specific cell chemistry beyond this.
Chemistry only
Fuel cells
A fuel cell is supplied with an external fuel (e.g. hydrogen) and oxygen (or air). The fuel is oxidised electrochemically to produce a potential difference — it doesn't run down like a battery, it keeps going as long as fuel is supplied.
2H₂ + O₂ → 2H₂Ooverall reaction in a hydrogen fuel cell: hydrogen is oxidised to produce water
Hydrogen and oxygen go in; the only product is water; a potential difference drives the circuit.
Evaluate vs rechargeable batteries: fuel cells make only water (no CO₂ at point of use), don't lose charge or need recharging while fuel lasts, and are lighter for the energy stored — but hydrogen is hard to store and transport safely, and is often made using fossil fuels. (HT only: half equations — negative electrode: 2H₂ → 4H⁺ + 4e⁻; positive electrode: O₂ + 4H⁺ + 4e⁻ → 2H₂O.)
Quick check
The hydrogen fuel cell
?What is the only product of the overall reaction in a hydrogen fuel cell?
Mission 2 check
How do hydrogen and oxygen make electricity?
?A hydrogen fuel cell lights a lamp. No flame is seen, and only water leaves the cell. Where does the electricity come from?
Recap
The key facts to know
Exothermic: energy out → surroundings warm (combustion, oxidation, neutralisation).
Endothermic: energy in → surroundings cool (thermal decomposition, citric acid + sodium hydrogencarbonate).
Reaction profile: activation energy = rise from reactants to the peak; exo products lower, endo products higher.
Bond energies (HT): ΔH = bonds broken − bonds made. Breaking is endo, making is exo.
Cells & fuel cells (Chemistry only): two metals + electrolyte make a p.d.; hydrogen fuel cell: 2H₂ + O₂ → 2H₂O.
You've covered the key ideas in AQA 4.5 — exothermic & endothermic reactions, reaction profiles, bond-energy calculations (HT), and chemical cells & fuel cells (Chemistry only). Press Finish to see your score.
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Mini-lesson complete!
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You've worked through Energy changes for AQA GCSE Chemistry. 🎉