Reaction profiles

A reaction profile, or energy level diagram, shows how energy changes as a reaction proceeds. It compares the energy levels of the reactants and products and shows a pathway between them.

Energy is shown on the vertical axis, and reaction progress is shown on the horizontal axis. The reactants are placed on the left and the products on the right. A curve rises from the reactants to a peak, then falls to the products.

Reading energy changes from a reaction profile

The peak represents the top of the activation energy barrier. The activation energy is the energy needed to reach that peak from the reactants. On a diagram, it is shown by a vertical arrow from the reactant energy level to the peak.

The overall energy change is the difference between the energy levels of the reactants and products. It is shown by a vertical arrow between those two levels. It is not the difference between the reactants and the peak.

Exothermic and endothermic profiles

In an exothermic reaction, the products are at a lower energy level than the reactants. Energy is released overall.

Energy
  ↑           /\
  |          /  \
  | Reactants    \
  |               \_____ Products
  +----------------------------→ Reaction progress

In an endothermic reaction, the products are at a higher energy level than the reactants. Energy is taken in overall.

Energy
  ↑           /\
  |          /  \_____ Products
  | Reactants
  |
  +----------------------------→ Reaction progress

To draw either profile, label the axes and place the reactants and products at the appropriate relative energy levels. Connect them with a curve that rises to a peak before falling to the products. To identify the reaction type from a profile, compare the product level with the reactant level: lower means exothermic; higher means endothermic.

Activation energy and particle collisions

For a chemical reaction to occur, reacting particles must collide. A collision provides an opportunity for the atoms in the particles to rearrange, but not every collision produces a reaction.

A collision must have at least the activation energy: the minimum energy needed for the reaction to occur. If the particles collide with less than this energy, they do not react. A collision with sufficient energy can allow bonds to break and atoms to rearrange into products.

The activation energy is the energy barrier to starting a reaction. It is not the same as the overall energy change between the reactants and products: a reaction needs enough energy to get started even when the products have less energy than the reactants.