Problem 25 - A Complete Resource Book in Chemistry for JEE Main

Question

A chemical reaction is catalysed by a catalyst \(\mathrm{X}\). Hence, \(\mathrm{X}\) (a) increases activation energy of the reaction (b) does not affect equilibrium constant of the reaction (c) decreases rate constant of the reaction (d) reduces enthalpy of the reaction

Step-by-Step Solution

Answer

Option (b) is correct: a catalyst does not affect the equilibrium constant.

Step 1: Understanding Catalysts

Catalysts are substances that increase the rate of a chemical reaction without being consumed in the process. They work by providing an alternative reaction pathway with lower activation energy.

Step 2: Assessing Activation Energy

A catalyst does not increase the activation energy; instead, it lowers the activation energy required for a reaction to proceed. Hence, option (a) is incorrect.

Step 3: Examining Equilibrium Constant

Catalysts affect the rate of both forward and backward reactions equally, so they do not change the position of equilibrium or the equilibrium constant. Therefore, option (b) is correct.

Step 4: Analyzing Rate Constant

Catalysts increase the rate of a reaction by lowering its activation energy, which effectively increases the rate constant, not decreases it. Thus, option (c) is incorrect.

Step 5: Considering Enthalpy

A catalyst does not change the reactants and products involved in a reaction, so it does not alter the enthalpy change of a reaction. Hence, option (d) is incorrect.

Key Concepts

Activation Energy

Every chemical reaction requires a certain amount of energy to get started. This energy is known as the "activation energy".
In simple terms, think of it as the energy barrier that the reactants have to overcome in order to transform into products. When a catalyst is introduced, it lowers this energy barrier, making it easier and quicker for the reaction to occur.
Catalysts provide an alternate pathway for the reaction which has a lower activation energy.
This means that while the activation energy with a catalyst is lower, the overall energy change of the reaction remains the same. Importantly, since the barrier is lowered, the same reaction can happen more swiftly or under milder conditions.

Equilibrium Constant

The equilibrium constant is a measure that reflects the ratio of the concentrations of products to reactions at equilibrium.
It's a reflection of the balance between the forward and backward reactions in a chemical process. A catalyst speeds up both the forward and backward reactions equally, which means that it does not change the equilibrium position or the equilibrium constant itself.
This is because equilibrium depends on the concentrations of products and reactants, which are not altered just by speeding up the reaction. Instead, the presence of a catalyst simply helps reach equilibrium faster, without affecting the actual "balance point" of the reaction.

Rate Constant

The rate constant is a proportionality constant in the rate equation of a chemical reaction. It is an essential part of the equation that defines how fast a reaction takes place under specific conditions.
When a catalyst is introduced, it increases the rate of the reaction by lowering the activation energy.
This means the molecules can convert more quickly from reactants to products. As a result, the rate constant actually increases when a catalyst is added, contrary to some beliefs. In essence, with a catalyst, the reaction "gets a boost," making the transformation process more efficient.

Enthalpy

Enthalpy refers to the total heat content of a system.
During chemical reactions, changes in enthalpy determine if a reaction absorbs or releases heat. When a catalyst is involved, it doesn't change the enthalpy of the reaction.
This is because a catalyst doesn't alter the nature of the reactants or the products but rather offers an alternative path for the reaction to progress. Therefore, the overall heat absorbed or released during the reaction remains unchanged. It only affects how fast the reaction gets to its final state.