Le Chatelier's Principle - Pressure Effect
For the reaction CO(g) + 3H_2(g) \rightleftharpoons CH_4(g) + H_2O(g), what will happen to the equilibrium position when the total pressure is increased at constant temperature by reducing the volume?
Select the correct option:
Solution
Equilibrium shifts to the right because moles decrease in forward direction
Le Chatelier's principle for pressure changes states that when pressure is increased by decreasing volume, the equilibrium shifts toward the side with fewer moles of gas. For CO(g) + 3H_2(g) \rightleftharpoons CH_4(g) + H_2O(g): moles of gaseous reactants = 1 + 3 = 4; moles of gaseous products = 1 + 1 = 2. Since the product side has fewer moles (2 vs 4), increasing pressure shifts equilibrium to the right, favouring CH_4 and H_2O formation. Option B is wrong because the reverse direction would have 4 moles of gas, which would make the pressure problem worse, not better; Le Chatelier's principle opposes the disturbance, not amplifies it. Option C is partially true — K_c is indeed independent of pressure — but the position of equilibrium (not K_c) does shift with pressure when \Delta n_g \neq 0. Option D is incorrect; temperature and pressure effects are independent, and pressure alone can shift equilibrium position when \Delta n_g \neq 0. This concept is fundamental in NCERT and tested in JEE Main for industrial processes like methanation. Plausibility check: \Delta n_g = 2 - 4 = -2 < 0, confirming that increased pressure favours the forward (lower mole) direction.
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About This Question
- Subject
- chemistry
- Chapter
- equilibrium
- Topic
- le chatelier's principle - pressure effect
- Difficulty
- Medium
- Year
- 2025
Solution
Correct Answer:
Equilibrium shifts to the right because moles decrease in forward direction
Le Chatelier's principle for pressure changes states that when pressure is increased by decreasing volume, the equilibrium shifts toward the side with fewer moles of gas. For CO(g) + 3H_2(g) \rightleftharpoons CH_4(g) + H_2O(g): moles of gaseous reactants = 1 + 3 = 4; moles of gaseous products = 1 + 1 = 2. Since the product side has fewer moles (2 vs 4), increasing pressure shifts equilibrium to the right, favouring CH_4 and H_2O formation. Option B is wrong because the reverse direction would have 4 moles of gas, which would make the pressure problem worse, not better; Le Chatelier's principle opposes the disturbance, not amplifies it. Option C is partially true — K_c is indeed independent of pressure — but the position of equilibrium (not K_c) does shift with pressure when \Delta n_g \neq 0. Option D is incorrect; temperature and pressure effects are independent, and pressure alone can shift equilibrium position when \Delta n_g \neq 0. This concept is fundamental in NCERT and tested in JEE Main for industrial processes like methanation. Plausibility check: \Delta n_g = 2 - 4 = -2 < 0, confirming that increased pressure favours the forward (lower mole) direction.
This medium difficulty chemistry question is from the chapter equilibrium, covering the topic of le chatelier's principle - pressure effect. It appeared in the 2025 exam.
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