A catalyst is a substance that increases the rate of a chemical reaction without being consumed or permanently altered in the process. It achieves this by lowering the activation energy required for the reaction to proceed, which allows more reactant molecules to have sufficient energy to undergo the reaction. This results in an increased rate of reaction.The effect of a catalyst on the reaction mechanism is that it provides an alternative reaction pathway with a lower activation energy. Catalysts achieve this by forming temporary bonds with the reactants, stabilizing the transition state, and facilitating the formation of products. In some cases, the catalyst may also help to break down the reactants into intermediate species, which can then react more readily to form the final products.In summary, a catalyst increases the rate of a chemical reaction by lowering the activation energy and providing an alternative reaction pathway. This is directly related to the reaction mechanism, as the catalyst interacts with the reactants and transition states to facilitate the formation of products.