The energy level of the bonds in the products is lower than the energy level of the bonds in the reactants. Energy has to be supplied to start the reactions as the bonds in the reactants have to be broken before the reaction can start. An example of this would be striking a match, the heat from the friction is need to start the reaction, once it begins the energy released by the reaction can break the remaining bonds and so the reaction continues with no further input from the surroundings.

The diagram show difference in the energy levels of the reactants compared to the products. The difference is the amount of energy released to the environment.

Exothermic reactions results in an increase in the temperature of the surrounds and/or the emission of light or sound.

Examples:

1. Combustion: releases thermal, light and sometime sound energy too (as in an explosion).
2. Thermite reaction
3. Rusting of iron to form iron oxide, (slow process)
4. Neutralization reactions (acid + base → salt + water and heat energy)
5. Respiration, (energy released from sugars in the form of high energy bonds of ATP)
6. Dissolving of copper sulfate into water
7. Nuclear fission, (see physics section of this course)

Activation Energy

All reactions whether they are endothermic or exothermic have to take in some energy from the surroundings to break the bonds of the reactants for the reaction to start. This initial energy is known as the activation energy. If the energy supplied is less than this activation energy the reaction will not start.

In an exothermic reaction this activation energy only has to supplied from the surroundings at the being of the reaction. After that the energy released by the reaction is more than required to supplied the activation energy for the rest of the reactants. The reaction is self-sustaining, for example a candle needs to be lite from a match to provide the activation energy for the wax to burn. Once burning the heat from the flame is more than enough to sustain the burning of the rest of the wax.

In an endothermic reaction, no surplus energy is released from the reaction, therefore, for the reaction to continue energy must be continually supplied from the surroundings. If this is not supplied the reaction will stop. Photosynthesis, for example, will only occur as long as there is a light source of a high enough intensity to drive the reaction.