7.9 - Exothermic & Endothermic Reactions

Exothermic and endothermic reactions

• In an exothermic reaction, energy is transferred to the surroundings, so the temperature of the surroundings rises.
• In an endothermic reaction, energy is taken in from the surroundings, so the temperature of the surroundings falls.

Energy is conserved

• Energy can't be created or destroyed, only transferred.
• In an exothermic reaction, the products hold less energy than the reactants because energy was released to the surroundings.
• In an endothermic reaction, the products hold more energy than the reactants because energy was taken in from the surroundings.

Sign of the energy change

• Negative (−) - Energy given out (exothermic).
• Positive (+) - Energy taken in (endothermic).

Exothermic and endothermic examples

Exothermic reactions

• Combustion - Burning a substance in oxygen gives out a large amount of energy.
• Oxidation - Reactions such as iron rusting in air give out energy.
• Neutralisation - An acid reacting with a base or an alkali gives out energy.

Endothermic reactions

• Thermal decomposition - Breaking down a compound by heating (e.g. calcium carbonate decomposing into calcium oxide and carbon dioxide).
• Citric acid and sodium hydrogencarbonate - Reacting these together causes the temperature of the surroundings to fall.

Everyday uses

• Hand warmers - Use the exothermic oxidation of iron in air to release heat.
• Self-heating cans - Use an exothermic reaction in the base to warm a drink.
• Cold packs - Use an endothermic reaction to cool an injury immediately without needing a freezer.