Final answer:
Activation energy is necessary to start an exothermic reaction by overcoming the energy barrier, and once provided, it can lead to a self-sustaining reaction. Catalysts can lower the required activation energy, allowing the reaction to happen more easily without affecting the overall energy change of the reaction.
Step-by-step explanation:
The role of activation energy in an exothermic reaction is critical as it provides the necessary 'push' to start the reaction. Even though an exothermic reaction results in a net release of energy, it still requires this initial input of energy to overcome the energy barrier and proceed with the reaction. Once the activation energy is provided, for instance, by heat from a spark, the reaction can continue to release energy, which may in turn provide the activation energy needed for surrounding reactants to react, creating a self-sustaining process.
For many chemical reactions such as the burning of fuel, a catalyst might be used to lower the activation energy, allowing the reaction to proceed more quickly at a given temperature. This is because a catalyst offers an alternative pathway with a lower energy barrier. However, the catalyst does not affect the overall energy of the reactants or products or the overall energy change of the reaction (ΔH).
Overall, activation energy is a kind of gatekeeper for chemical reactions, determining the rate and the conditions under which a reaction will proceed, especially in vital processes such as those occurring in living cells.