Exothermic Reaction

What is Exothermic Reaction?

A reaction that transfers thermal energy from the reacting chemicals to their surroundings, so the temperature of the surroundings increases.

This definition is part of the Chemical Energetics chapter in Cambridge IGCSE Chemistry.

Concepts related to Exothermic Reaction

• surroundings
• thermal transfer
• system

Common mistakes with Exothermic Reaction

• C4. “This profile shows an exothermic reaction, and its products are above the reactants.” Repair Exothermic means energy left the system, so what is left behind — the products — must sit lower. Products above reactants is the endothermic picture. Establish the level order before you name the reaction type, not after.
• S4. “This reaction has a big activation energy, so \(\Delta H\) must be large and positive.” Repair The two are independent. The barrier height says how much energy colliding particles need; \(\Delta H\) says where the products ended up relative to the reactants. A strongly exothermic reaction can have a very large barrier, and a barely endothermic one can have a small barrier. You cannot infer either from the other.
• S9. “\(\Delta H = \) bonds made \(-\) bonds broken.” Repair Reversed. It is \(\Delta H = \sum E(\text{broken}) - \sum E(\text{made})\). Energy in comes first because it happens first and because it is the positive contribution. Reverse it and every exothermic reaction in your answer becomes endothermic.