Question

A catalyst is a substance which:
A.Increases the average kinetic energy of reacting molecules
B.Alters the reaction mechanism
C.Increases the frequency of collisions of reaction species
D.None of these

Answer 1

To answer this question you must recall the purpose of a catalyst. We know that a catalyst is used to alter the rate of a reaction. It increases the rate of reaction by decreasing the activation energy required for the reaction.

Complete answer:
A catalyst is a substance that when added to a reaction promotes it thereby increasing the rate of the reaction. A catalyst is not used up in the reaction. However it does take a part in reaction providing a suitable surface or situation or forming an essential complex or intermediate to help the reactants to react effectively. Later, on completion of the reaction, the catalyst is obtained back along with the product. Catalysts are generally of two types, namely positive catalyst and negative catalysts. A catalyst that increases the rate of the reaction is positive catalyst while a catalyst that retards the reaction is known as the negative catalyst
Addition of a catalyst provides an additional and more feasible method to the reaction. Thus a catalyst accelerates a reaction. The alternate mechanism involving the catalyst has a lower activation energy and thus the reaction proceeds at a faster rate. The activation energy is the minimum amount of energy required by the reactants to react and form the product.
So we can say that a catalyst alters the reaction mechanism.

Thus, the correct answer is B.
Note:
While studying a chemical reaction, it is important that we consider other factors affecting the reaction other than just the chemical properties of the reactants, for instance, the reaction conditions, the mechanism of the reaction, the equilibrium toward it is approaching, and the rate at which it is occurring.