Question

Which of the following statement is not true about a catalyst?

• A. It catalyses the spontaneous reactions
• B. A small amount of the catalyst can catalyse large amount of reactants
• C. It does not alter the Gibbs energy of a reaction
• D. It catalyses the non-spontaneous reactions
• E. It does not change the equilibrium constant of a reaction

Hint:

Catalyst lowers activation energy, not Gibbs free energy.

Answer 1

The Correct Option is D

Step 1: Understanding the Role of a Catalyst:
A catalyst is a substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change. It works by providing an alternative reaction pathway with a lower activation energy (\(E_a\)).
Step 2: Analyzing the Statements:
(A) It catalyses the spontaneous reactions: A spontaneous reaction is one that is thermodynamically feasible (\(\Delta G<0\)). A catalyst can only speed up a reaction that is already possible; it cannot make an impossible reaction happen. So, catalysts work on spontaneous reactions. This statement is true.
(B) A small amount of the catalyst can catalyse the large amount of reactants: Catalysts are regenerated at the end of the reaction. Because they are not consumed, a small amount can facilitate the conversion of a large amount of reactants over time. This statement is true.
(C) It does not alter the Gibbs energy of a reaction: The Gibbs free energy change (\(\Delta G\)) depends only on the initial (reactants) and final (products) states, not on the path taken. A catalyst only changes the path (lowers \(E_a\)), so it does not affect the overall \(\Delta G\) of the reaction. This statement is true.
(D) It catalyses the non-spontaneous reactions: A non-spontaneous reaction has \(\Delta G>0\). It is thermodynamically unfavorable and will not proceed on its own. A catalyst cannot change the thermodynamics (\(\Delta G\)) of a reaction, so it cannot make a non-spontaneous reaction occur. This statement is not true.
(E) It does not change the equilibrium constant of a reaction: A catalyst speeds up both the forward and reverse reactions by the same factor. As a result, the position of equilibrium is reached faster, but the equilibrium constant (\(K_{eq}\)), which is the ratio of the rate constants (\(k_f/k_r\)), remains unchanged. This statement is true.
Step 3: Final Answer:
The statement that is not true about a catalyst is that it catalyses non-spontaneous reactions.