Problem 35 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following reaction will be favoured at low pressure? (a) \(\mathrm{N}_{2}+3 \mathrm{H}_{2} \rightleftharpoons 2 \mathrm{NH}_{3}\) (b) \(\mathrm{H}_{2}+\mathrm{I}_{2}=2 \mathrm{HI}\) (c) \(\mathrm{PCl}_{5} \rightleftharpoons \mathrm{PCl}_{3}+\mathrm{Cl}_{2}\) (d) \(\mathrm{N}_{2}+\mathrm{O}_{2} \rightleftharpoons 2 \mathrm{NO}\)

Step-by-Step Solution

Answer

Reaction (c) \(\mathrm{PCl}_{5} \rightleftharpoons \mathrm{PCl}_{3}+\mathrm{Cl}_{2}\) is favored at low pressure.

Step 1: Understand the principle of Le Chatelier's

Le Chatelier's principle suggests that a system at equilibrium will adjust to counteract changes in conditions such as pressure, temperature, or concentration. A change in pressure will favor the reaction which produces fewer moles of gas if pressure increases, or more moles of gas if pressure decreases.

Step 2: Analyze each reaction for moles of gas

Look at the number of moles of gaseous reactants and products in each reaction: - Reaction (a): 4 moles (1 N2 + 3 H2) on the left and 2 moles (2 NH3) on the right. - Reaction (b): 2 moles (1 H2 + 1 I2) on the left and 2 moles (2 HI) on the right. - Reaction (c): 1 mole (PCl5) on the left and 2 moles (PCl3 + Cl2) on the right. - Reaction (d): 2 moles (1 N2 + 1 O2) on the left and 2 moles (2 NO) on the right.

Step 3: Determine effect of low pressure

At low pressure, the system will favor the direction that produces more moles of gas. Compare the mole differences from each reaction: - Reaction (a) produces fewer moles. - Reaction (b) maintains equal moles. - Reaction (c) produces more moles. - Reaction (d) maintains equal moles.

Step 4: Identify the favored reaction

From the analysis, reaction (c) \(\mathrm{PCl}_{5} \rightleftharpoons \mathrm{PCl}_{3}+\mathrm{Cl}_{2}\) produces more moles of gas (from 1 to 2 moles) and will be favored at low pressure according to Le Chatelier's principle.

Key Concepts

Equilibrium Reactions

Chemical reactions can reach a state where the rate of the forward reaction equals the rate of the reverse reaction. This balance is known as equilibrium. At this point, the concentration of reactants and products remains constant over time, although they are not necessarily equal.
Reactions at equilibrium respond predictably to changes in conditions thanks to Le Chatelier's principle. This principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium shifts to counteract the change.

• This might mean producing more products if reactants increase.
• It could mean shifting to more reactants if products increase.

The equilibrium state does not imply that the reactions have stopped, but rather that they continue to occur at an equal rate in both directions, thus maintaining a stable mixture in a closed system.

Pressure Effects in Chemical Equilibrium

Pressure changes can significantly shift the equilibrium position in reactions involving gases. When the pressure of a system changes, the system adjusts to minimize this change, in alignment with Le Chatelier's principle. This is particularly noticeable in reactions where the total moles of gas differ between reactants and products.
If pressure increases:

• The equilibrium shifts toward the side with fewer moles of gas, reducing the pressure.

If pressure decreases:

• The equilibrium will shift toward the side with more moles of gas to increase pressure.

This concept is vital for industrial applications where controlling the conditions of reactions can optimize production yield. For example, increasing the pressure in the Haber process, 2 + 3 3 ightleftharpoons 2 3, favors the formation of ammonia.

Moles of Gas in Reactions

In equilibrium reactions involving gases, counting the moles on each side of the reaction is crucial. This count helps predict how changes in pressure will affect the equilibrium position.
Take a reaction like (c) \(3 ightleftharpoons 3 + 2\):

• Begin with 1 mole of gas on the reactant side (\( 5\)),
• and end with 2 moles of gas on the product side (\(3 + 2\)).

Under low pressure conditions, this reaction is favored because it shifts towards creating more moles of gas, thus increasing the pressure. Similarly, understanding the moles of gas in reactions allows chemists to predict and manipulate reaction conditions effectively, in compliance with Le Chatelier's principle, maximizing desired products or reducing unwanted ones.