Exothermic Reactions

What does exothermic mean?

An exothermic change or reaction is one in which heat energy is given out. The energy is transferred from the reacting chemicals to their surroundings.

The surroundings may include the solution containing the reaction, the container and the nearby air. As these surroundings receive energy, their temperature usually rises. This is why a thermometer can provide evidence of an exothermic change.

Connecting energy transfer to temperature

Imagine measuring a reaction mixture at $20\,^{\circ}\mathrm{C}$. If its temperature rises to $28\,^{\circ}\mathrm{C}$ during the reaction, the surroundings have become warmer. This indicates that heat energy has been given out, so the reaction is exothermic.

The temperature rise is an effect of the energy transfer. “Exothermic” does not simply mean “hot”: it describes the direction in which energy is transferred during the change.

Recognising exothermic reactions

Combustion is a familiar example. When a fuel burns, energy is given out to the surroundings, which may be noticed as heating and light. Neutralisation reactions are also usually exothermic.

A reaction can be exothermic whether it happens quickly or slowly. Its rate describes how quickly it occurs; exothermic describes its heat-energy change.

Core recall

• Exothermic = heat energy is given out.
• Energy is transferred from the reacting chemicals to the surroundings.
• The surroundings usually show a temperature increase.

Key distinction

Exothermic describes the direction of energy transfer, not how fast a reaction happens.

Evidence

If the measured temperature of the surroundings rises during a change, this provides evidence that the change is exothermic.