Reaction Coordinate Diagrams

Let's consider a general reaction where a reactant or set of reactants, A, is transformed into a product or set of products, B. The diagram below is called a reaction coordinate diagram. It shows how the energy of the system changes during a chemical reaction.
In this example, B is at a lower total energy than A. This is an exothermic reaction (heat is given off) and should be favorable from an energy standpoint. The energy difference between A and B is E in the diagram.

However some energy is required for A to convert to B. This energy is called the activation energy or Eact. Sometimes this is also called the activation barrier. Perhaps one or more bonds in A must be stretched or even broken. Perhaps the molecule(s) must bend to form a less stable structure before the reaction can take place. Any of these changes requires energy.

The energy of the system at the beginning of the reaction is the same as the energy of the reactant(s), A. Energy must be added to the system as the reaction proceeds to the transition state. The transition state is the point of highest energy between the reactant(s) and product(s). It is not a stable molecule and can't be isolated.

After the transition state, the energy of the system gradually decreases as the high energy structure relaxes and becomes product(s), B. The final energy of the system is the energy of B.

An amount of energy equal to Eact must be added to the system or no reaction will take place. Energy equal to the sum of Eact and E is released from the system as B is produced. The total energy change is equal to E.

The rate of the reaction, that is, how fast the reactants change over into products depends on the height of the activation barrier NOT the energy difference between reactants and products.

Imagine that there are 2 different ways for A to transform into B. Pathway 1 has a high activation barrier (high Eact). It requires a great deal of added energy for the reaction to take place so it proceeds very slowly if it proceeds at all.

This is something like the reaction between H2 and O2. It is favorable according to thermodynamics but the activation barrier is very high.

Pathway 2 has a much lower activation barrier. Only a small amount of energy is required and this amount of energy can easily be added from the surroundings. The rate of the reaction will be very much faster for pathway 2 than for 1.

A catalyst, such as the platinum metal added to the H2/O2 mixture, can cause the reaction to occur through a series of small steps and reduce the activation barrier. A catalyst is any substance that increases the rate of a chemical reaction by changing the reaction pathway without being changed itself at the end of the reaction. A catalyst can't change the energy difference between reactants and products. It can reduce Eact but can't change E.

Reaction Coordinate Diagram of Ozone Photolysis

The reaction coordinate diagram for the ozone photolysis reaction is a little different from those above because this is an endothermic reaction.
Together, the products O2 and atomic O, have a higher energy than the reactant O3 and energy must be added to the system for this reaction. This is primarily due to the very high energy (low stability) of the oxygen atom that is produced. The oxygen atom produced has a higher energy than ground state oxygen.

Energy, either light or heat energy, added to the ozone will cause one O-O bond to break.

When molecular oxygen and oxygen atom are formed some energy is released (Eact - E). Where does it go? The two products have a greater kinetic energy than the ozone they came from, so the overall temperature of the gas increases.

The O2 and O move quickly away from each other due to their higher kinetic energy after the reaction.

Professor Patricia Shapley, University of Illinois, 2012