Chemical Equilibrium

Reversible Reactions and Dynamic Equilibrium

In many chemical reactions, products form continuously, and the reaction reaches a point where it seems to stop progressing. However, this is not always because all reactants are used up. Instead, the system may have reached a dynamic equilibrium.

Reversible Reactions

These are reactions where the products can reform the reactants. They are represented by a double arrow ($⇌$) to show that both forward and reverse reactions are happening.

Dynamic Equilibrium

This occurs when:

• The rate of the forward reaction (reactants $→$ products) equals the rate of the reverse reaction (products $→$ reactants).
• The concentrations of reactants and products remain constant, but not necessarily equal.
• The system is dynamic, meaning both reactions continue to happen simultaneously, but their rates balance each other out.

Equilibrium Law and the Equilibrium Constant ($K_c$)

The equilibrium constant ($K_c$) provides a quantitative measure of the position of equilibrium in a reaction. It expresses the ratio of the concentrations of the products to the concentrations of the reactants at equilibrium.

For a general reaction:

$$aA + bB ⇌ cC + dD$$

The equilibrium constant, $K_c$, is given by:

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

Where:

• [A], [B], [C], [D] represent the concentrations of the substances in moles per litre (mol/L) at equilibrium.
• The powers a, b, c, and d correspond to the coefficients of the balanced chemical equation.

Key Things to Remember about $K_c$

• A large $K_c$ value (>>1) means the reaction favours products at equilibrium (more products than reactants).
• A small $K_c$ value (<<1) means the reaction favours reactants at equilibrium (more reactants than products).
• $K_c$ is temperature-dependent; temperature changes will shift the position of equilibrium and alter the value of $K_c$.
• Catalysts do not affect the $K_c$ value—they only speed up the rate at which equilibrium is reached.

Calculations Involving the Equilibrium Constant ($K_c$)

To perform calculations with $K_c$, follow these steps:

1. Write the balanced equation for the reaction and the expression for $K_c$.
2. Set up an ICE table (Initial, Change, Equilibrium) to determine the concentrations of reactants and products at equilibrium.

• Initial: The starting concentrations of reactants and products.
• Change: The change in concentrations as the system moves towards equilibrium.
• Equilibrium: The final concentrations when the system has reached equilibrium.

3. Substitute the equilibrium concentrations into the $K_c$ expression and solve for the unknown.

Important Factors Affecting Equilibrium

Temperature

Changing the temperature alters the equilibrium position and the value of $K_c$. An increase in temperature for an exothermic reaction shifts the equilibrium to the left (more reactants), while for an endothermic reaction, it shifts to the right (more products).

Concentration

Altering the concentration of a reactant or product shifts the position of equilibrium (Le Chatelier's principle), but $K_c$ remains constant unless the temperature changes.

Catalysts

They do not affect the equilibrium position or the value of $K_c$. They only help the system reach equilibrium faster.