The Equilibrium Constant (Kc) (The Law of Mass Action)

1. What is the Equilibrium Constant $(K_c)$?

• The equilibrium constant $(K_c)$ is a numerical value that represents the ratio of product concentrations to reactant concentrations at equilibrium in a closed system.
• It is based on the Law of Mass Action, which states that the concentrations of reactants and products at equilibrium follow a specific mathematical relationship.

2. Writing the $K_c$ Expression

For a general reaction:

$aA + bB ⇌ cC + dD$

The equilibrium constant (Kc) is expressed as:

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

where:

[ ] = concentration in $mol·dm⁻³$

• $a, b, c, d =$ coefficients from the balanced equation
• Products are in the numerator, reactants in the denominator.
• Concentrations are raised to the power of their coefficients.

3. Important Rules for $K_c$ Calculations

✅ Solids and pure liquids are excludedK_c$$Kc expression. Only gases and aqueous solutions are included.

✅ $K_c$ is constant at a specific temperature—changing temperature affects $K_c.$

✅ A large $K_c$ value $(>1)$ means products are favoured at equilibrium (reaction goes forward).

✅ A small $K_c$ value $(<1)$ means reactants are favoured at equilibrium (reaction goes backward).

✅ $K_c$ does NOT depend on initial concentrations, only on equilibrium concentrations.

5. Summary Table

Factor	Effect on $K_c$
Change in concentration	No effect, but system shifts to restore equilibrium
Change in pressure (gases only)	No effect unless different gas moles on each side
Change in temperature	Affects Kc (depends on exothermic or endothermic reaction)
Addition of a catalyst	No effect on Kc (only speeds up reaction)