4.5.1.2 - Reaction profiles and activation energy

Reactant and product energy levels

A reaction profile compares the energy of the reactants with the energy of the products as a reaction proceeds.

The vertical axis is energy. The horizontal axis is progress of reaction, representing the change from reactants to products. It is not time, distance travelled by a particle, or temperature. The graph is a model of the energy changes along a reaction pathway, rather than a temperature trace measured by a thermometer.

Reactants appear at the left and products at the right. An exothermic reaction has products at a lower energy level than reactants. The difference is transferred to the surroundings, consistent with the surroundings warming. An endothermic reaction has products at a higher level; energy has been taken in from the surroundings, consistent with cooling.

The overall energy change is the vertical difference between product and reactant levels. Moving down from reactants to products represents an energy decrease for the reaction system. Moving up represents an increase. The height of a curve's highest point is a different feature: it describes the barrier crossed on the way, not the final energy difference.

Use the same vertical energy scale to compare levels. The length of the horizontal line does not measure the amount of energy, and starting at a higher position on the page does not mean there is “more reaction”. Reactant and product labels establish which direction is being considered.

The activation-energy barrier

Reacting particles must collide with sufficient energy for a chemical reaction to occur.

Particles may collide without reacting. A collision needs enough energy to start the changes that rearrange the reactants into products. The minimum energy that reacting particles must have is the activation energy. On a simple reaction profile, the curved line rises from the reactant level to a peak before falling to the product level. The peak represents the highest-energy stage along the pathway.

Draw an activation-energy arrow vertically from the reactant level to the peak. It does not start automatically at the horizontal axis, because the reactants may already be above that axis. It is not the difference between reactants and products. The minimum collision energy is a barrier, whereas the overall energy change is the balance between the start and finish.

An exothermic reaction can therefore require a spark or initial heating. Its reactants are above the products, but there is still a barrier above the reactants. The initial spark helps some collisions cross that barrier. Once combustion begins, its energy transfer can keep further fuel reacting while fuel and oxygen remain available.

Consider energy levels: reactants 40 units, peak 100 units, products 10 units. The activation-energy rise is from 40 to 100, while the overall change is the fall from 40 to 10. These refer to different vertical separations. Supplying activation energy does not determine whether the reaction's overall energy change is exothermic or endothermic.

Exothermic and endothermic profiles

Both exothermic and endothermic profiles have an activation-energy barrier, but their product levels differ relative to their reactant levels.