As we have explained from the collision theory, the kinetic energy of a gas is directly proportional to temperature. As temperature increases, molecules gain energy and move faster and faster resulting in higher probability that molecules will be moving with the necessary activation energy for a reaction to occur upon collision.
Activation energy is the minimum energy with which reactants must collide in order for a reaction to occur. The source of the activation energy needed to push reactions forward is typically heat energy from the surroundings.
The Heat of Reaction is the change in the enthalpy of a chemical reaction. In endothermic reactions, (ΔH>0), heat is absorbed with the reactants. For example:
This means more energy is needed to overcome the forces of attraction between molecules and to separate them from one another (the activation energy) than the energy gained when new bonds are formed.
In exothermic reactions, (ΔH<0), heat is released with the products. When separated molecules join together, enough energy is released to overcompensate for the energy required to break reactant bonds. In this oxidation reaction:
The forward reaction is exothermic because energy is released when CaO(s) and H2O(l) combine to form Ca(OH)2(s) . The energy to break the bonds of each reactant is lower than the amount of energy released from forming the product, and the net difference is observed as heat on the right side of the equation.
In summary, a temperature increase favors an endothermic reaction, and a temperature decrease favors an exothermic reaction.