Law of Chemical Equilibrium | Class 11 Chemistry

Law of Chemical Equilibrium

Mass Action & Equilibrium Constants | Class 11 Chemistry

1. Law of Mass Action

Proposed by Guldberg and Waage (1864). It states that the rate of a chemical reaction is directly proportional to the product of the active masses (molar concentrations) of the reactants, each raised to the power of their stoichiometric coefficient.

                    Chemical Equilibrium: A dynamic state in a reversible reaction where the rate of the forward reaction equals the rate of the backward reaction, and concentrations of reactants and products remain constant.
                

2. The Equilibrium Constant ($K$)

For a general reversible reaction:

$$ aA + bB \rightleftharpoons cC + dD $$

A. In terms of Concentration ($K_c$)

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

Where $[A]$ represents molar concentration (mol/L).

B. In terms of Partial Pressure ($K_p$)

Used for gaseous reactions.

$$ K_p = \frac{(p_C)^c (p_D)^d}{(p_A)^a (p_B)^b} $$

                    Important Rule: The concentration of pure solids and pure liquids is taken as unity (1) and does not appear in the $K$ expression.
                

3. Relation between $K_p$ and $K_c$

Assuming ideal gas behavior ($PV = nRT \Rightarrow P = \frac{n}{V}RT = [C]RT$):

$$ K_p = K_c (RT)^{\Delta n_g} $$

Where:

$R$ = Gas constant ($0.0821 \, L \cdot atm \cdot K^{-1} \cdot mol^{-1}$)
$T$ = Temperature in Kelvin
$\Delta n_g$ = (Sum of gaseous moles of products) - (Sum of gaseous moles of reactants).

Conditions:

If $\Delta n_g = 0$, then $K_p = K_c$.
If $\Delta n_g > 0$, then $K_p > K_c$.
If $\Delta n_g < 0$, then $K_p < K_c$.

4. Characteristics of $K$

Temperature Dependent: $K$ changes *only* with temperature. It does NOT change with concentration, pressure, or catalyst.
Direction (Reaction Quotient $Q$):
- $Q < K$: Reaction moves Forward (Reactants $\to$ Products).
- $Q > K$: Reaction moves Backward.
- $Q = K$: Reaction is at Equilibrium.
Stoichiometry:
- Reverse reaction: $K' = 1/K$.
- Multiply by 2: $K' = K^2$.
- Divide by 2: $K' = \sqrt{K}$.

Practice Quiz

Test your knowledge on Equilibrium Constants.

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