Problem 28 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following favours the backward reaction in a chemical equilibrium? (a) decreasing the concentration of one of the reactants (b) increasing the concentration of one of the reactants (c) increasing the concentration of one or more of the products (d) removal of at least one of the products at regular intervals

Step-by-Step Solution

Answer

Options (a) and (c) favour the backward reaction.

Step 1: Understand the concept of chemical equilibrium

Chemical equilibrium occurs in a reversible reaction when the rate of the forward reaction equals the rate of the backward reaction. At this point, the concentrations of reactants and products remain constant over time.

Step 2: Apply Le Chatelier's Principle

Le Chatelier's Principle states that if a dynamic equilibrium is disturbed by a change in conditions, the position of equilibrium shifts to counteract the change and re-establish equilibrium. Possible changes include concentration, temperature, and pressure.

Step 3: Analyze the effect of changing reactant concentration

(a) Decreasing the concentration of one of the reactants will shift the equilibrium to the left (backward reaction) to increase the reactant that was decreased. (b) Increasing the concentration of one of the reactants will shift the equilibrium to the right (forward reaction) to consume the added reactant.

Step 4: Analyze the effect of changing product concentration

(c) Increasing the concentration of one or more products will shift the equilibrium to the left to decrease the added product(s), thus favoring the backward reaction. (d) Removal of a product will shift the equilibrium to the right (forward reaction) to produce more of the removed product.

Step 5: Identify the conditions that favor the backward reaction

Based on the analysis: (a) Decreasing the concentration of reactants and (c) Increasing the concentration of products both favor the backward reaction as they both cause the equilibrium to shift to the left.

Key Concepts

Le Chatelier's Principle

Le Chatelier's Principle is a fundamental rule in chemistry that tells us how a system at equilibrium reacts to changes. Imagine you have a balance and someone adds or removes a weight. The balance would tip, and you'd have to adjust it back to equilibrium. This is similar to what happens in chemical reactions that are at equilibrium.

When a change in conditions such as concentration, temperature, or pressure is applied to a system in equilibrium, the system adjusts to counteract that change and restore equilibrium. For example, adding more reactant or product will cause the equilibrium to shift.

The equilibrium will shift in the direction that opposes the change:

• If you add more reactants, the equilibrium shifts to use up added reactants—often moving forward.
• Increasing products pushes the reaction backward to consume the added product.
• The system aims to restore equilibrium by reversing the imposed change.

Understanding this principle helps predict the outcome of changing conditions on reversible reactions.

Reversible Reactions

Reversible reactions are those that can proceed in both directions: from reactants to products and from products back to reactants. Unlike irreversible reactions that run to completion, these reactions reach a point where the rate of making products equals the rate of forming reactants. At this point, the concentrations of products and reactants remain constant over time—a condition we refer to as chemical equilibrium.

In a chemical equation, reversible reactions are usually represented with a double arrow, like this: \[ A + B \rightleftharpoons C + D \].

What's fascinating is that at equilibrium, the reaction continues to occur, but because the rates are equal, there's no net change in the concentration of reactants and products:

• The forward reaction converts reactants to products.
• The backward reaction converts products back to reactants.
• Both reactions happen at the same speed at equilibrium.

Reversible reactions are central to understanding dynamic processes in chemistry, where stability is maintained despite ongoing changes.

Effect of Concentration Changes on Equilibrium

The concentration of substances in a reaction can dictate the direction in which equilibrium shifts. By applying Le Chatelier's Principle, we can predict how equilibrium will adjust when concentrations change. This is instrumental in chemical engineering and industry where reaction conditions are optimized for maximum yield.

When you decrease the concentration of a reactant, the system compensates by shifting the equilibrium to the left. This encourages the backward reaction, effectively increasing the concentration of the reactant you reduced. Similarly, increasing the concentration of a reactant shifts the equilibrium to the right, boosting the forward reaction.

Let's break down these effects:

• Decreasing reactants' concentration favors the backward reaction, shifting equilibrium left.
• Increasing reactants' concentration promotes the forward reaction, moving equilibrium right.
• Adding more products pushes equilibrium left, enhancing the backward reaction.
• Removing products drives equilibrium right, favoring the forward reaction.

By controlling the concentration of reactants or products, one can influence the direction and extent of a chemical reaction, a key strategy in synthetic chemistry and industrial applications.