Problem 169 - A Complete Resource Book in Chemistry for JEE Main

Question

Match the following Column-I (a) \(\mathrm{H}_{2}(\mathrm{~g})+\mathrm{I}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{HI}(\mathrm{g})\) (b) \(\mathrm{CaCO}_{3}(\mathrm{~s}) \rightleftharpoons \mathrm{CaO}(\mathrm{s})+\mathrm{CO}_{2}(\mathrm{~g})\) (c) \(\mathrm{N}_{2}(\mathrm{~g})+3 \mathrm{H}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{NH}_{3}(\mathrm{~g})\) (d) \(\mathrm{PCl}_{5}(\mathrm{~g}) \rightleftharpoons \mathrm{PCl}_{3}(\mathrm{~g})+\mathrm{Cl}_{2}(\mathrm{~g})\) Column-II (p) Unaffected by inert gas addition at constant volume (q) Forward shift by rise in pressure (r) Unaffected by increase in pressure (s) Backward shift by rise in pressure (t) reaction has \(\Delta n_{g}>0\)

Step-by-Step Solution

Answer

(a) - (p), (b) - (r), (c) - (q), (d) - (s)

Step 1: Analyze the Reaction in Column-I

Look at each chemical reaction and determine the change in the number of moles of gas (9_g) for each reaction. (a) 9_g = 2 - (1 + 1) = 0 (b) 9_g = 1 - 0 = 1 (c) 9_g = 2 - (1 + 3) = -2 (d) 9_g = 2 - 1 = 1 These values will help us match the reactions with the descriptions in Column-II.

Step 2: Match Reaction (a)

Reaction (a) is 9_g = 0. This indicates that the reaction is unaffected by the addition of an inert gas at constant volume because the pressure change won't favor either direction. Match it with (p). Selection: (a) - (p)

Step 3: Match Reaction (b)

Reaction (b) has 9_g = 1. This suggests that it is unaffected by an increase in pressure due to the presence of a solid reactant and product. Therefore, it matches the situation where gas-centric reactions are examined. Match it with (r). Selection: (b) - (r)

Step 4: Match Reaction (c)

For reaction (c), 9_g = -2, which indicates a decrease in gas moles. Increasing pressure will shift the equilibrium to the forward direction, favoring the production of fewer gas moles. Match with (q). Selection: (c) - (q)

Step 5: Match Reaction (d)

In reaction (d), 9_g = 1, indicating an increase in moles of gas. Increasing pressure would typically shift the equilibrium backward to reduce gas moles. Additionally, 9_g>0 also holds true, matching it with (s) and (t). However, the backward shift by pressure change is more immediate to select. Match it with (s). Selection: (d) - (s)

Key Concepts

Le Chatelier's Principle

Le Chatelier's Principle is a fundamental concept in chemical equilibrium that helps predict how a system at equilibrium will respond to various changes. It essentially states that if a dynamic equilibrium is disturbed by changing the conditions, such as concentration, temperature, or pressure, the system will adjust itself to counteract the effect of the disturbance and restore a new equilibrium state.

For instance, if the concentration of a reactant in an equilibrium mixture is increased, the system will shift to consume the added reactant and produce more products. Conversely, if the temperature is increased in an exothermic reaction, the equilibrium position will shift towards the reactants, as the system attempts to absorb the excess heat.

Understanding Le Chatelier's Principle is crucial for predicting how changes in a reaction environment will influence the concentrations of reactants and products at equilibrium. By applying this principle, students can determine which way a reaction will shift under different circumstances.

Equilibrium Shift

An equilibrium shift occurs when an external change impacts a chemical reaction at equilibrium, prompting a re-adjustment to reach a new equilibrium position. According to Le Chatelier's Principle, the direction of this shift depends on the nature of the disturbance.

When discussing equilibrium shifts, it's important to consider the type of reaction and the changes being imposed, such as:

• Concentration: Adding or removing a reactant or product will cause the equilibrium to shift towards the side that will neutralize the change.
• Temperature: For endothermic reactions, an increase in temperature results in a shift towards the products, while for exothermic reactions, it shifts towards the reactants.
• Pressure: An increase in pressure will favor the direction that produces fewer moles of gas.

Recognizing these shifts is fundamental for understanding how equilibrium systems react to changes, whether it's in an industrial process or a lab setting.

Reaction Moles Change

In chemical equilibrium, the change in moles of gas (9_g) is a crucial factor that determines how equilibrium will react to changes in pressure. It refers to the difference in the number of moles of gaseous products and reactants, and it helps predict the shift in chemical equilibrium.

Calculating 9_g involves subtracting the total moles of reactants from the total moles of products. For example, in the equation for reaction (a), 9_g is calculated as:\[\Delta n_g = (2 \text{ moles of } HI) - (1 + 1 \text{ moles of } H_2 + I_2) = 0\]This zero change implies no shift in equilibrium with pressure changes when an inert gas is added at constant volume, as no side is favored.

Understanding 9_g is imperative because it directly influences how a reaction responds to changes in pressure:

• If 9_g = 0, reactions are typically unaffected by pressure changes.
• When 9_g > 0, increasing pressure often shifts the equilibrium towards the side with fewer moles of gas.
• If 9_g < 0, the equilibrium may shift towards the side that reduces the overall pressure, thus yielding fewer gas molecules.

Therefore, the number of gaseous moles and their change plays a pivotal role in predicting and understanding equilibrium shifts due to pressure changes.