Activation Energy

Often a reaction rate may be increased or decreased by affecting the activation energy, that is, the energy necessary to cause a reaction to occur. This is shown graphically in Figure 9.1 for the forward reaction.

Reaction graph showing potential energy on the vertical axis and reaction coordinate on the horizontal axis. Reactants A + B make a curve of activation energy which then decline in potential energy to from products AB.
Figure 9.1Potential Energy Diagram For An Exothermic Reaction

Tip

Activation energy is the energy necessary to cause a reaction to occur.

A catalyst, as explained in the preceding section, is a substance that is introduced into a reaction to speed up the reaction by changing the amount of activation energy needed. The effect of a catalyst used to speed up a reaction can be shown as follows in Figure 9.2:

Reaction graph showing potential energy on the vertical axis and reaction coordinate on the horizontal axis. Reactants A + B make a curve of activation energy. Curve is shorter with the addition of a catalyst. The differences between the two curves is the difference in activation energy. Curve then declines in potential energy to from products AB.
Figure 9.2Potential Energy Diagram For Uncatalyzed/Catalyzed Reaction

Tip

A catalyst speeds up the reaction but is not consumed itself.