Reaction Profiles

The reactants begin at one energy level and the products finish at another. Between them, the curve rises to a peak and then falls. The peak represents the energy barrier that must be overcome before the reaction can proceed.

Identifying activation energy

Even a reaction that eventually gives out energy usually needs an initial energy input. For example, a fuel may release energy as it burns but must first be ignited.

The activation energy, often labelled $E_a$, is shown by a vertical arrow from the energy level of the reactants to the top of the curve. The lower tip must end exactly on the reactants' level. Activation energy is never measured from the bottom of the graph, and it is not simply the height of the whole curve above the horizontal axis.

Exothermic reaction profiles

An exothermic reaction transfers heat energy to the surroundings. The products therefore have less energy than the reactants.

To draw its profile:

1. label the vertical axis energy and the horizontal axis progress of reaction;
2. draw and label a reactants energy level;
3. make the curve rise to a peak and then fall;
4. finish at a lower level labelled products;
5. draw the activation-energy arrow from the reactants' level to the peak.

The difference between the reactant and product levels represents the overall energy released. The temperature of the surroundings may rise, but the products' energy level is lower. Temperature and the energy level on a reaction profile are not the same thing.

Endothermic reaction profiles

An endothermic reaction takes in heat energy from the surroundings. The products therefore have more energy than the reactants.

The curve still rises to a peak because activation energy is still required. However, it finishes at a product level above the reactant level. The difference between the two levels represents the overall energy taken in, and the surroundings may consequently cool.

Reading or constructing any profile

First compare the two horizontal energy levels:

• products below reactants means exothermic;
• products above reactants means endothermic.

Then identify the activation energy by starting at the reactants' level and moving vertically to the peak. The overall heat energy change is represented by the vertical difference between the reactant and product levels, whereas activation energy is represented by the distance from the reactant level to the peak. These are different quantities.