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Equilibrium - Factors Affecting Equilibria

Grade 11CBSEChemistry

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

🔑Concepts

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

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Effect of Concentration: Increasing the concentration of reactants or decreasing the concentration of products shifts the equilibrium in the forward direction. Conversely, increasing product concentration or decreasing reactant concentration shifts it in the backward direction.

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Effect of Pressure: For gaseous reactions, an increase in pressure shifts the equilibrium in the direction of 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: In an exothermic reaction (ΔH<0\Delta H < 0), increasing temperature shifts the equilibrium to the left (reactants). In an endothermic reaction (ΔH>0\Delta H > 0), increasing temperature shifts the equilibrium to the right (products).

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Effect of Inert Gas: At constant volume, adding an inert gas has no effect on equilibrium. At constant pressure, adding an inert gas shifts the equilibrium in the direction where the number of moles of gas increases (Δng>0\Delta n_g > 0).

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Effect of Catalyst: A catalyst increases the rate of both forward and backward reactions equally. It helps in attaining equilibrium faster but does not change the equilibrium composition 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}

Δng=∑nproducts(g)−∑nreactants(g)\Delta n_g = \sum n_{products(g)} - \sum n_{reactants(g)}

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

ΔG∘=−RTln⁡K\Delta G^{\circ} = -RT \ln K

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

💡Examples

Problem 1:

For the reaction: 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}. What is the effect of (i) increasing pressure and (ii) increasing temperature?

Solution:

(i) Increasing pressure shifts equilibrium to the right. (ii) Increasing temperature shifts equilibrium to the left.

Explanation:

In the reaction, Δng=2−(1+3)=−2\Delta n_g = 2 - (1 + 3) = -2. Since the number of moles decreases in the forward direction, high pressure favors the formation of NH3NH_3. Because the reaction is exothermic (ΔH<0\Delta H < 0), increasing temperature provides energy that the system tries to oppose by shifting towards the endothermic (backward) direction.

Problem 2:

Predict the shift in equilibrium for the reaction PCl5(g)⇌PCl3(g)+Cl2(g)PCl_5(g) \rightleftharpoons PCl_3(g) + Cl_2(g) if an inert gas is added at constant pressure.

Solution:

The equilibrium will shift in the forward direction (towards the products).

Explanation:

At constant pressure, addition of an inert gas increases the total volume. To maintain constant pressure, the system shifts to the side with a greater number of moles. Here, Δng=(1+1)−1=+1\Delta n_g = (1 + 1) - 1 = +1. Since the number of moles increases in the forward direction, the equilibrium shifts to the right.