Unit 8: Chemical Equilibrium

Unit 8: Chemical Equilibrium (3 Teaching Hours)

1. Physical and Chemical Equilibrium

   • Physical Equilibrium: Occurs in physical processes like evaporation, melting, and dissolving, where the rate of the forward process equals the rate of the reverse process.
     • Example: Water evaporating and condensing at the same rate in a closed system.
   • Chemical Equilibrium: In a reversible chemical reaction, when the rate of the forward reaction equals the rate of the reverse reaction, and the concentrations of reactants and products remain constant.
     • Example: $N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$ (Haber's process).

2. Dynamic Nature of Chemical Equilibrium

   • Even though the concentrations of reactants and products do not change at equilibrium, both the forward and reverse reactions continue to occur at equal rates. This means that chemical equilibrium is dynamic, not static.

3. Law of Mass Action

   • States that the rate of a chemical reaction is proportional to the product of the concentrations of the reactants, each raised to the power of their respective stoichiometric coefficients in the balanced equation.
   • For a reaction: $aA + bB \rightleftharpoons cC + dD$, $$\text{Rate forward} \propto [A]^a [B]^b \quad \text{and} \quad \text{Rate reverse} \propto [C]^c [D]^d$$

4. Expression for Equilibrium Constant and Its Importance

   • The equilibrium constant ($K$) is a ratio of the concentrations (or partial pressures for gases) of products to reactants, each raised to their stoichiometric coefficients.
   • For a reaction: $aA + bB \rightleftharpoons cC + dD$, $$K_c = \frac{[C]^c [D]^d}{[A]^a [B]^b}$$
   • Importance:
     • $K_c$ indicates the extent of a reaction at equilibrium.
     • If $K_c > 1$, products are favored at equilibrium.
     • If $K_c < 1$, reactants are favored at equilibrium.

5. Relationship Between $K_p$ and $K_c$

   • For reactions involving gases, the equilibrium constant can also be expressed in terms of partial pressures ($K_p$).
   • The relationship between $K_p$ and $K_c$ is given by: $$K_p = K_c (RT)^{\Delta n}$$ where:
     • $R$ is the gas constant,
     • $T$ is the temperature in Kelvin,
     • $\Delta n$ is the change in the number of moles of gas (moles of products minus moles of reactants).

6. Le Chatelier’s Principle

   • This principle states that if a system at equilibrium is disturbed by a change in concentration, pressure, or temperature, the system will adjust to counteract the disturbance and restore a new equilibrium.
     • Concentration: Increasing the concentration of reactants will shift the equilibrium toward the products, and vice versa.
     • Pressure: For reactions involving gases, increasing pressure shifts equilibrium toward the side with fewer gas molecules.
     • Temperature: For exothermic reactions, increasing temperature shifts equilibrium toward the reactants, while for endothermic reactions, it shifts toward the products.