Energy Changes is a core part of GCSE Chemistry. Revise the key concepts and common mistakes below, then lock them in with the free games.
Key concepts
ExothermicReaction that releases energy to surroundings (T rises)
EndothermicReaction that takes in energy (T falls)
Activation energyMinimum energy needed for a reaction to start
Bond breakingEndothermic step — energy must be put
Bond makingExothermic step — energy is given out
Reaction profileGraph of energy vs progress of reaction
Hand warmerUses an exothermic reaction to release heat
Cold packUses an endothermic reaction to absorb heat
CalorimetryMeasuring heat change by recording temperature rise
Energy per gramHeat released ÷ mass of fuel burned (J/g or kJ/g)
Hess's lawTotal ΔH is independent of the path taken
Reusable warmerUses a reversible exothermic crystallisation
Bond energyEnergy to break 1 mole of a specific bond (kJ/mol)
EnergyTotal energy to break all reactant bonds
Common mistakes to avoid
Questions where students often pick the tempting wrong answer — make sure you know the right one:
In an exothermic reaction, what happens energetically with the bonds?✗ Bond breaking releases energy and bond making absorbs energy. ✓ Energy released forming new bonds exceeds energy absorbed breaking existing bonds.
How does a catalyst speed up a reaction?✗ A catalyst lowers the activation energy of the original reaction pathway. ✓ It provides an alternative reaction pathway with a lower activation energy.
Which statement about bonds is correct?✗ Breaking bonds releases energy ✓ Breaking bonds takes energy in; making bonds releases energy
A sports cold pack contains ammonium nitrate and water. When squeezed, it gets cold. The reaction is.✗ Exothermic — releases heat ✓ Endothermic — takes in heat
Why is the experimental value for a fuel's energy per gram usually lower than the true value?✗ The thermometer reads too high ✓ Heat is lost to the surroundings
Practise Energy Changes — free games
Test yourself with these quick revision games for this topic:
See all 17 games in the Subjects Arcade →
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