Question

Considcring the influence of a catalyst on a chemical cquilibrium, which onc of the following is wrong? The catalyst (1) increases the speed of both forward and reverse reactions to the same cxtent (2) does not change the relative amount of substanccs present at cquilibrium (3) changes the time required for reacting the cquilibrium (4) changes the equilibrium constant

Short answer

Option (4) is wrong because a catalyst does not change the equilibrium constant.

Step-by-step solution

Understanding the Role of a Catalyst

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

Effect on Forward and Reverse Reactions

A catalyst increases the speed of both the forward and reverse reactions to the same extent. This means it helps the system reach equilibrium faster but does not favor either the forward or reverse reaction. Thus, statement (1) is correct.

Effect on Amount of Substances at Equilibrium

Catalysts do not change the relative amount of substances present at equilibrium. This is because they do not alter the position of the equilibrium; they simply help it to be reached more quickly. Hence, statement (2) is correct.

Effect on Time to Reach Equilibrium

By increasing the speed of the reactions, a catalyst changes the time required to reach equilibrium, generally making it faster. Therefore, statement (3) is correct.

Effect on Equilibrium Constant

A catalyst does not change the equilibrium constant because it does not affect the relative energies of the reactants and products. The equilibrium constant is dependent only on the temperature and inherent properties of the substances involved, not on the presence of a catalyst. Thus, statement (4) is incorrect.

Key concepts

Effect of Catalysts

Catalysts play a crucial role in speeding up chemical reactions. They achieve this by offering an alternative pathway that has a lower activation energy.
It's important to note that catalysts do not get consumed in the reaction. This means that they remain unchanged and can be used repeatedly.
The crucial point is that catalysts speed up both the forward and reverse reactions equally.
Consequently, while they help the system reach equilibrium faster, they do not affect the equilibrium position. The relative concentrations of reactants and products at equilibrium remain unchanged.
By lowering activation energy, catalysts make it easier for reactants to convert to products and vice versa. This is why both reactions are sped up.
In conclusion, the correct statements about catalysts from the exercise are:

• (1) A catalyst increases the speed of both forward and reverse reactions to the same extent.
• (3) A catalyst changes the time required to reach equilibrium.

Chemical Equilibrium

Chemical equilibrium occurs in reversible reactions, where the forward and reverse reactions happen at the same rate. At equilibrium, the concentrations of reactants and products remain constant over time.
This dynamic state is described as 'dynamic' because the reactions are still occurring, but there are no net changes in concentrations.
When a system is at equilibrium, changes such as temperature, concentration, or pressure can influence the equilibrium position, shifting it either towards products or reactants according to Le Chatelier's Principle.
It is essential to understand that a catalyst does not shift the equilibrium position. It merely helps to achieve equilibrium faster by speeding up both reactions equally.
The exercise states correctly that:

• (2) A catalyst does not change the relative amount of substances present at equilibrium.

Equilibrium Constant

The equilibrium constant (\[\begin{equation} K_{eq} \text{ or } K_c \text{ for concentrations} \text{ or } K_p \text{ for partial pressures} \end{equation}\]) is a measure of the relative concentrations of reactants and products at equilibrium.
This constant is derived from the balanced chemical equation and depends only on temperature. It does not depend on the presence of a catalyst.
For a generic reaction \[\begin{equation} aA + bB \rightleftarrows cC + dD, \end{equation}\] the equilibrium constant \[\begin{equation} K_c \end{equation}\] is expressed as:
\[\begin{equation} K_c = \frac{{[C]^c[D]^d}}{{[A]^a[B]^b}} \end{equation}\]
It is crucial to realize that adding a catalyst will not alter these concentrations at equilibrium; hence, it does not change the value of \[\begin{equation} K_c \end{equation}\].
The equilibrium constant remains solely a function of the temperatures and intrinsic properties of the reactants and products. Statement (4) from the exercise is incorrect as a result.
In conclusion, remember:

• A catalyst does not change the equilibrium constant.