Problem 30 - A Complete Resource Book in Chemistry for JEE Main

Question

In the reaction \(\mathrm{H}_{2}+\mathrm{I}_{2}=2 \mathrm{HI}\) at equilibrium, some \(\mathrm{I}_{2}\) is added. What happens to the equilibrium? (a) it gets shifted to the right (b) it remains unchanged (c) it gets shifted to the left (d) first (b) then (c)

Step-by-Step Solution

Answer

(a) it gets shifted to the right.

Step 1: Understanding the Reaction

We are given the chemical equilibrium reaction \(\mathrm{H}_{2} + \mathrm{I}_{2} = 2 \mathrm{HI} \). This indicates a reversible reaction that can shift in either direction.

Step 2: Le Chatelier's Principle

Le Chatelier's Principle states that if a system at equilibrium is subjected to a change, the system will adjust in such a way as to counteract that change and restore a new equilibrium.

Step 3: Analyzing the Addition of \( \mathrm{I}_{2} \)

Adding more \( \mathrm{I}_{2} \) increases the concentration of iodine gas in the system. According to Le Chatelier's Principle, the reaction will shift to reduce the increase in \( \mathrm{I}_{2} \), thus favoring the forward reaction to form more \( \mathrm{HI} \). This means the equilibrium shifts to the right.

Key Concepts

Le Chatelier's Principle

Le Chatelier's Principle is central to understanding how chemical equilibria respond to changes in their environment. Named after the French chemist Henri Louis Le Chatelier, this principle suggests that systems at equilibrium will adjust to counteract any imposed change. This predictive tool helps chemists anticipate the direction of an equilibrium shift when alterations happen.

Let's break it down further with some key points:

• If the concentration of a reactant or product is changed, the equilibrium will shift to minimize this change.
• Temperature changes will shift the equilibrium depending on whether the reaction is exothermic or endothermic.
• Pressure changes, applicable for gaseous reactions, will shift the equilibrium towards the side with fewer moles of gas when pressure is increased.

By applying Le Chatelier's Principle, one can predict that adding \(\mathrm{I}_2\) to the equilibrium mixture of hydrogen, iodine, and hydrogen iodide will cause the system to shift the equilibrium to the right. This is due to the system's need to consume the added \(\mathrm{I}_2\). As a result, more \(\mathrm{HI}\) will be formed until a new equilibrium state is reached.

Reversible Reactions

Reversible reactions can proceed in both the forward and backward directions. This is symbolized in chemical equations with a double-headed arrow, such as in the reaction \(\mathrm{H}_{2} + \mathrm{I}_{2} \rightleftharpoons 2 \mathrm{HI} \). Reversible reactions are fundamental in the study of chemical dynamics and equilibrium.

Key features of reversible reactions include:

• They never go to completion in a closed system; instead, they arrive at a state of dynamic equilibrium.
• The rates of the forward and reverse reactions are equal at equilibrium, leading to constant concentrations of reactants and products.
• Bang for buck, these types of reactions are incredibly responsive to changes in concentration, temperature, or pressure, as they can shift either way to accommodate these changes according to Le Chatelier's Principle.

In our examination of the \(\mathrm{H}_{2} + \mathrm{I}_{2} = 2 \mathrm{HI}\) reaction, the reversible nature means that alterations like adding \(\mathrm{I}_{2}\) can prompt a shift to the right or left, maintaining a delicate balance that defines equilibrium.

Equilibrium Shift

An equilibrium shift occurs when the position of equilibrium changes due to an external influence on the system. This shift can result from changes in concentration, temperature, or pressure. The equilibrium in a chemical system is not static but dynamic, meaning substances continue to react, albeit at equal rates for the forward and reverse directions at equilibrium.

Factors influencing equilibrium shifts include:

• Concentration: Increasing a reactant shifts the equilibrium towards the products, while increasing a product shifts it towards the reactants.
• Temperature: Increasing the temperature of an endothermic reaction shifts the equilibrium towards the products. For exothermic reactions, increased temperature shifts it towards the reactants.
• Pressure: Applicable for gaseous systems, an increase in pressure favors the direction that produces fewer moles of gas.

The statement, "the equilibrium shifts to the right," specifically means that the forward reaction rate increases, producing more products. Thus, when \(\mathrm{I}_{2}\) is added, the equilibrium of \(\mathrm{H}_{2} + \mathrm{I}_{2} = 2 \mathrm{HI}\) shifts right to counteract the change, resulting in an increased concentration of \(\mathrm{HI}\) until equilibrium is reestablished.