Chemical changes · GCSE Chemistry
Energy changes
Exothermic and endothermic reactions, reaction profiles, activation energy, bond energies and simple cells.
Exothermic transfers energy to the surroundings — temperature rises. Endothermic takes energy in — temperature falls. If more energy is released forming bonds than is used breaking them, the reaction is exothermic.
The important bits
What you need to know
- 1
Exothermic reactions transfer energy to the surroundings. The temperature of the surroundings increases. Combustion, respiration and many neutralisations are exothermic.
- 2
Endothermic reactions take in energy from the surroundings. The temperature of the surroundings decreases. Thermal decomposition and some sports-pack reactions are endothermic.
- 3
A reaction profile shows energy of reactants and products. Exothermic: products lower than reactants. Endothermic: products higher. The hump is the activation energy.
- 4
A catalyst lowers the activation-energy hump on the profile but does not change the energy of reactants or products, so ΔH stays the same.
- 5
Bond breaking is endothermic (energy in). Bond making is exothermic (energy out). Overall ΔH = energy to break bonds − energy released when bonds form.
- 6
If the energy released in making bonds is greater than the energy used to break bonds, the reaction is exothermic and ΔH is negative.
- 7
Simple cells and fuel cells (Higher / Triple on some specifications) transfer chemical energy electrically. Hydrogen fuel cells produce water as the only waste from the cell reaction.
Go deeper
Measure the surroundings, not the chemicals’ feelings
Students say a reaction “gives off cold”. Energy is transferred. If the surroundings get hotter, the reaction is exothermic: chemical stores have fallen, and the difference has gone to the thermal store of the surroundings. If the surroundings get colder, it is endothermic. Reaction profiles make this visible. Draw reactants, a hump for activation energy, then products. The vertical drop or rise between reactants and products is ΔH. Catalysts cut the hump, not the ΔH arrow. In practical work, use a polystyrene cup, a lid and a thermometer, and repeat. Heat loss to the air is the usual accuracy limit.
Go deeper
Bond energies need a careful subtraction
Add up every bond broken in the reactants using the displayed formula, not just the ones that “look like they change”. Add up every bond made in the products. ΔH = broken − made. A negative answer is exothermic. Show the working in two totals; do not try to do it in one line in your head. Units are kJ/mol. If your sign is wrong, you have swapped breaking and making. Combustion of methane is a standard example: break C–H and O=O, make C=O and O–H, large negative ΔH. That is why fuels are useful: forming the product bonds releases more energy than was needed to break the reactant bonds.
See the idea in action
A reaction breaks bonds totalling 1582 kJ/mol and makes bonds totalling 1850 kJ/mol. ΔH = 1582 − 1850 = −268 kJ/mol. The negative sign means exothermic: more energy was released when bonds formed than was taken in to break bonds. On a profile, products sit 268 kJ/mol below reactants, with an activation-energy hump above the reactants.
Exam technique
Turn knowledge into marks
State whether the surroundings heat up or cool down, then name the reaction type. On profiles, label activation energy and ΔH with arrows in the correct direction. For bond energies, total broken minus total made, and include the sign.
Common mistakes
Do not give these marks away
- 01
Swapping bond breaking and bond making, or omitting the sign of ΔH.
- 02
Saying a catalyst changes the energy of the products.
- 03
Describing an endothermic reaction as one that “gives out cold”.
In terms of bonds, when is a reaction exothermic?
AWhen more energy is used to break bonds than is released when bonds form
BWhen more energy is released forming bonds than is used breaking bonds
CWhen all bonds are broken
DWhen a catalyst is added
Show the answer
When more energy is released forming bonds than is used breaking bonds. Bond making releases energy. If that amount is larger than the energy needed to break reactant bonds, the surroundings heat up and ΔH is negative.
Quick questions
If this is the bit you searched
What does a negative ΔH mean?
The reaction is exothermic: energy is transferred to the surroundings and the products are lower in energy than the reactants.
How does a catalyst appear on a reaction profile?
It lowers the activation-energy hump. The reactant and product energies, and therefore ΔH, stay the same.