Cambridge O Level Chemistry · Syllabus 5070 · Chemical Energetics
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 and the enthalpy change of the reaction is negative.
This definition is part of the Chemical Energetics chapter in Cambridge O Level Chemistry.
Exothermic Reaction in context
Chemical energetics is the study of the thermal energy transferred when a chemical reaction takes place. An exothermic reaction transfers thermal energy to the surroundings, so the temperature of the surroundings increases; an endothermic reaction takes in thermal energy from the surroundings, so their temperature decreases. The transfer of thermal energy during a reaction is called the enthalpy change, \(\Delta H\): it is negative for an exothermic reaction because energy leaves the system, and positive for an endothermic reaction because energy enters it. The thermometer sits in the surroundings, so whatever it does, the system did the opposite.
Common mistakes with Exothermic Reaction
- 8. “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.
- 13. “\(\Delta H = \) 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.
Questions students ask about Exothermic Reaction
What is the difference between an exothermic and an endothermic reaction?
An exothermic reaction transfers thermal energy to the surroundings, so the temperature of the surroundings increases and \(\Delta H\) is negative. An endothermic reaction takes in thermal energy from the surroundings, so the temperature of the surroundings decreases and \(\Delta H\) is positive. Exothermic does not mean the mixture contains a lot of heat: a chemical does not contain heat. The word describes a transfer, so always say where the energy went, and remember that the thermometer reading is evidence about the surroundings, not the system.
Why is the enthalpy change negative for an exothermic reaction?
Because the sign is a direction label, not a statement that energy is negative. There is no negative energy. The minus sign records that thermal energy left the system and went into the surroundings. If a reaction has \(\Delta H = -184\;\mathrm{kJ\,mol^{-1}}\), then 184 kJ of thermal energy is transferred to the surroundings for every mole of reaction; the sign says which way it went and the number says how much. On a reaction pathway diagram the products sit below the reactants.
Does a large activation energy mean a large enthalpy change?
No. The two are independent. The activation energy says how much energy colliding particles need to cross the barrier; the enthalpy change 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, so two reactions with very different peaks can have exactly the same \(\Delta H\). You cannot infer either quantity from the other, and a question that gives only one does not let you conclude the other.