Heat Energy Changes in Chemical ReactionsEdexcel GCSE Chemistry: Revision notes
Section 1
Which reactions involve heat energy changes?
Changes in heat energy accompany many chemical and physical changes, including:
- Salts dissolving in water
- Neutralisation reactions
- Displacement reactions
- Precipitation reactions
When these happen in solution, the temperature change can be measured (e.g. with a thermometer in an insulated cup) to reflect the heat energy change taking place.
Section 2
What are exothermic and endothermic reactions?
- An exothermic reaction or change is one in which heat energy is given out to the surroundings — the temperature of the surroundings increases
- An endothermic reaction or change is one in which heat energy is taken in from the surroundings — the temperature of the surroundings decreases
Combustion, neutralisation and most oxidation reactions are exothermic; dissolving ammonium nitrate in water is a well-known endothermic example.
Section 3
How do bond breaking and bond making explain energy changes?
During a chemical reaction:
- Breaking bonds in the reactants requires energy — this is endothermic
- Making bonds in the products releases energy — this is exothermic
Whether the overall reaction is exothermic or endothermic depends on the balance between these two processes:
- If more energy is released forming bonds in the products than is required to break bonds in the reactants → overall exothermic
- If less energy is released forming bonds in the products than is required to break bonds in the reactants → overall endothermic
Higher tier — calculating energy change:
energy change = (energy to break bonds in reactants) − (energy released making bonds in products)
Using bond energies given in kJ mol⁻¹, add up the energy needed to break all bonds in the reactants, then subtract the total energy released making all the bonds in the products. A positive result means the reaction is endothermic overall; a negative result means it is exothermic overall.
When asked to explain why a reaction is exothermic in terms of bonds, always compare the two energy quantities explicitly — 'more energy released making bonds than needed breaking bonds' — not just 'bonds are made'.
Section 4
What is activation energy, and how are reaction profiles drawn?
Activation energy is the minimum amount of energy that colliding particles need for a reaction to occur.
A reaction profile is a graph of energy (y-axis) against progress of reaction (x-axis):
- Exothermic reaction profile: products are drawn at a lower energy level than reactants (energy is released overall); there is a 'hump' above the reactants showing the activation energy needed to start the reaction
- Endothermic reaction profile: products are drawn at a higher energy level than reactants (energy is absorbed overall); the hump still represents the activation energy
In both profiles, the activation energy is labelled as the height of the hump above the reactants' energy level (not the full height of the hump from the x-axis).
A common error is labelling activation energy as the whole height of the hump from the bottom of the graph — it must be measured from the reactants' energy level to the peak of the hump.
Must Know
- Exothermic: gives out heat energy, surroundings warm up. Endothermic: takes in heat energy, surroundings cool down
- Dissolving, neutralisation, displacement and precipitation reactions can all involve a measurable temperature change
- Breaking bonds is endothermic; making bonds is exothermic
- Overall reaction is exothermic if more energy is released making bonds than is needed breaking them, and endothermic if less energy is released
- Activation energy = minimum energy needed for particles to react on collision
- On a reaction profile, exothermic reactions end with products at lower energy than reactants; endothermic reactions end with products at higher energy; activation energy is measured from reactants up to the peak
That's the notes covered.
Carry on to the next subtopic.