Question

A catalyst is a substance which
a) Increases the equilibrium constant of the reaction
b) Increases the equilibrium concentration of the product
c) Does not alter the reaction mechanism
d) Change the activation energy of the reaction

Answer 1

Hint: We need to recall the prime purpose of a catalyst. We essentially know for sure that catalyst has to do something with the reaction rate of the reaction that is happening. The students must try to extend on this concept.

Complete Step by Step Solution:
Let us first understand the phenomenon caused by adding a catalyst to any reaction:
A catalyst is a substance which could be particularly added in a chemical reaction in order to increase the reaction rate. It does not get consumed in the reaction, that is it does actually get reacted itself in the reaction, rather it provides a favourable surface/situation for the other reactants to react effectively.
It typically accelerates the reaction, by essentially reducing the activation energy or changing the reaction mechanism. Now, one needs to know something about the activation energy. It is the minimum amount of extra energy that is required for the reactant to get converted into a product. Hence, Option C could not be correct.
So, considering Option A, there wouldn’t be any change in the equilibrium constant of the reaction as long as there is no change in the temperature of the reaction.
Also, one should know that there won’t be any change in the equilibrium concentration as the mass won’t change without adding any other content in the overall reaction. Hence, it could not be option B.

The answer to this question is Option D. In fact the addition of catalyst would lead to decrease in the overall activation energy of the reaction.

Note: The students should know that there are two types of catalysts: Positive and Negative catalyst. A catalyst which actually increases the rate of reaction is essentially a positive catalyst. But on the contrary, a negative catalyst is a catalyst which decreases or retards the rate of reaction. Negative catalyst actually increases the activation energy. Example of Positive catalyst is $Mn{{O}_{2}}$ and an example of a negative catalyst is Phosphoric acid.