Problem 12 - A Complete Resource Book in Chemistry for JEE Main

Question

A reversible reaction is said to have attained equilibrium, when (a) backward reaction stops (b) both backward and forward reactions take place at equal speed (c) both backward and forward reactions stop (d) concentration of each of the reactants and products becomes equal

Step-by-Step Solution

Answer

(b) Both backward and forward reactions take place at equal speed.

Step 1: Understanding Equilibrium

In a chemical reaction, equilibrium is a state where the concentrations of reactants and products remain constant over time. This occurs because the rate of the forward reaction equals the rate of the backward reaction, not because the reactions stop.

Step 2: Evaluating Option (a)

Option (a) suggests that equilibrium is reached when the backward reaction stops. This is incorrect because both reactions continue to occur at equilibrium, just at equal rates.

Step 3: Evaluating Option (b)

Option (b) suggests equilibrium occurs when both backward and forward reactions take place at equal speed. This is correct, as the rate of the forward reaction equals the backward reaction's rate, maintaining the concentrations of reactants and products.

Step 4: Evaluating Option (c)

Option (c) suggests that equilibrium occurs when both backward and forward reactions stop. This is incorrect because reactions don't stop at equilibrium; they continue at equal pace.

Step 5: Evaluating Option (d)

Option (d) suggests that equilibrium is achieved when the concentration of reactants and products becomes equal. This is incorrect, as equilibrium does not require equal concentrations, only constant concentrations over time due to equal reaction rates.

Key Concepts

Reversible Reactions

A reversible reaction is a chemical reaction where the products of the reaction can reform into the original reactants. This means the reaction can proceed in both forward and backward directions. In chemical notation, this is usually represented by a double-headed arrow like this: \( A + B \rightleftharpoons C + D \). Reversible reactions are a key factor in reaching equilibrium. When both the forward and backward reactions are occurring, but at equal rates, the system is said to be in chemical equilibrium. At this point, the concentrations of the reactants and products remain constant over time.

Understanding reversible reactions is crucial because it affects how we view not just single reactions but chemical processes in general. Reversible reactions emphasize the dynamic nature of chemical systems, highlighting that even when macroscopic changes are not observable, molecular-level changes are ongoing.

Reaction Rates

Reaction rates refer to how quickly or slowly reactants are converted into products in a chemical reaction. It's the speed at which a reaction happens. In the context of equilibrium, the rates of the forward and backward reactions become equal.

Several factors influence the rate of a reaction:

• Concentration: Higher concentrations of reactants usually increase the rate of reaction.
• Temperature: Increasing temperature typically increases reaction rates due to higher energy molecules.
• Catalysts: These substances increase reaction rates by lowering the activation energy required for the reaction to proceed.
• Surface Area: More surface area allows for more collisions between particles, speeding up reactions.

Understanding reaction rates is essential for predicting how a reaction will proceed over time and effectively leveraging temperature, catalysts, or other conditions to achieve desired outcomes.

Equilibrium Constant

The equilibrium constant, represented as \( K_{eq} \), is a value that indicates the ratio of product concentrations to reactant concentrations at equilibrium for a given reaction. It is defined specifically for a reversible reaction at a particular temperature and can be expressed using the equation:\[K_{eq} = \frac{[C]^c [D]^d}{[A]^a [B]^b}\]Here, \([A], [B], [C], \) and \([D]\) represent the concentrations of the reactants and products, while \(a, b, c,\) and \(d\) are their respective coefficients in the balanced chemical equation.

A large \( K_{eq} \) indicates a reaction with a high concentration of products relative to reactants, suggesting the equilibrium lies to the right. Conversely, a small \( K_{eq} \) suggests that reactants are more prevalent than products, and the equilibrium lies to the left.

The equilibrium constant is pivotal in chemistry as it provides insight into the composition of a reaction mixture at equilibrium and helps in predicting the direction and extent of chemical reactions.