Thermal energy transfer in exothermic and endothermic reactions

A chemical reaction can transfer thermal energy between the reacting substances and their surroundings, such as the solution or container around them. The direction of this transfer determines whether the reaction is exothermic or endothermic.

In an exothermic reaction, thermal energy is transferred from the reaction to the surroundings. The surroundings gain thermal energy, so their temperature increases.

In an endothermic reaction, thermal energy is transferred from the surroundings to the reaction. The surroundings lose thermal energy, so their temperature decreases.

The two types therefore have opposite effects on the surroundings: an exothermic reaction warms them, while an endothermic reaction cools them.

Interpreting exothermic and endothermic reaction pathway diagrams

A reaction pathway diagram shows how energy changes as a reaction progresses. Energy is shown on the vertical axis, and the progress of the reaction is shown on the horizontal axis. The pathway starts at the energy level of the reactants and ends at the energy level of the products.

To identify the reaction type, compare the start and end levels:

• Exothermic: the products are at a lower energy level than the reactants. Energy is transferred to the surroundings.
• Endothermic: the products are at a higher energy level than the reactants. Energy is taken in from the surroundings.

The vertical difference between the reactant and product levels represents the energy change of the reaction. Its size shows how much energy is transferred; whether the product level is lower or higher shows the direction of the transfer. The reaction type depends on these start and end levels, not on the shape of the pathway between them.