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Equilibrium - Le Chatelier's Principle

Grade 11CBSEChemistry

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Definition: Le Chatelier's Principle states that if a system at equilibrium is subjected to a change in concentration, pressure, or temperature, the equilibrium is shifted in a direction that tends to undo the effect of the change.

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Effect of Concentration: Increasing the concentration of reactants (or decreasing products) shifts the equilibrium in the forward direction since Qc<KcQ_c < K_c. Conversely, adding products shifts it in the reverse direction.

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Effect of Pressure: An increase in pressure (by decreasing volume) shifts the equilibrium toward the side with the fewer number of moles of gas. If Δng=0\Delta n_g = 0, pressure has no effect.

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Effect of Temperature: For exothermic reactions (ΔH<0\Delta H < 0), increasing temperature shifts the equilibrium to the left (reactants). For endothermic reactions (ΔH>0\Delta H > 0), increasing temperature shifts the equilibrium to the right (products).

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Effect of Inert Gas: Adding an inert gas at constant volume has no effect on the equilibrium. Adding it at constant pressure shifts the equilibrium toward the side with more moles of gas.

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Effect of Catalyst: A catalyst increases the rate of both forward and backward reactions equally. It helps attain equilibrium faster but does not change the position of the equilibrium or the value of KcK_c.

📐Formulae

Qc=[C]c[D]d[A]a[B]bQ_c = \frac{[C]^c [D]^d}{[A]^a [B]^b}

Kp=Kc(RT)ΔngK_p = K_c (RT)^{\Delta n_g}

ΔG=ΔG∘+RTln⁡Q\Delta G = \Delta G^\circ + RT \ln Q

log⁡K2K1=ΔH∘2.303R(T2−T1T1T2)\log \frac{K_2}{K_1} = \frac{\Delta H^\circ}{2.303 R} \left( \frac{T_2 - T_1}{T_1 T_2} \right)

💡Examples

Problem 1:

For the synthesis of ammonia: N2(g)+3H2(g)⇌2NH3(g)N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g); ΔH=−92.4 kJ mol−1\Delta H = -92.4 \text{ kJ mol}^{-1}. Predict the effect of increasing pressure and increasing temperature on the yield of NH3NH_3.

Solution:

  1. Increasing pressure favors the forward reaction because the number of gaseous moles decreases from 44 (reactants) to 22 (products). 2. Increasing temperature favors the reverse reaction because the process is exothermic (ΔH<0\Delta H < 0).

Explanation:

According to Le Chatelier's Principle, the system tries to counteract high pressure by moving to the side with fewer moles (NH3NH_3). Since the reaction releases heat, adding more heat (increasing TT) forces the system to consume it by moving in the endothermic (reverse) direction.

Problem 2:

Predict the effect of adding Cl2(g)Cl_2(g) to the following equilibrium: PCl5(g)⇌PCl3(g)+Cl2(g)PCl_5(g) \rightleftharpoons PCl_3(g) + Cl_2(g).

Solution:

The equilibrium will shift to the left (toward the formation of PCl5PCl_5).

Explanation:

Adding Cl2Cl_2 increases the concentration of the products. To restore equilibrium, the system must consume the excess Cl2Cl_2 by reacting it with PCl3PCl_3 to form more PCl5PCl_5 (reverse reaction).