Equilibrium

1.0What is an Equilibrium?

Equilibrium means a state of balance. In chemistry, equilibrium is when the observable properties (like colour, temperature, pressure, or concentration) do not change over time.

• It occurs when the rate of the forward reaction is equal to the rate of the backward reaction.
• It can be physical or chemical.

2.0Physical Equilibrium

Physical equilibrium exists when a substance exists in more than one phase, and there is no chemical change. Examples include:

(i) Solid–Liquid Equilibrium

H₂O(solid) ⇌ H₂O(liquid)
Rate of melting = Rate of freezing

(ii) Liquid–Vapour Equilibrium

H₂O(liquid) ⇌ H₂O(vapour)

Rate of vaporization = Rate of condensation

NOTE: Triple point of water is the temperature and pressure where solid, liquid, and gas states of water coexist: 273.15 K

(iii) Solid–Vapour Equilibrium

NH₄Cl(solid) ⇌ NH₄Cl(vapour)
This process is sublimation, and reverse is called deposition or desublimation.

(iv) Solid–Solution Equilibrium

Sugar(solid) ⇌ Sugar(solution)
In a saturated solution, the rate of dissolution = rate of precipitation

(v) Gas–Liquid Equilibrium

Solubility of a gas in a liquid depends on:

• Nature of gas and solvent
• Temperature of the liquid
• Pressure of the gas above the liquid

3.0Henry's Law

At constant temperature, the mass of a gas dissolved in a liquid is directly proportional to the pressure of the gas above the liquid. Gas solubility decreases with increasing temperature. Occurs during physical changes.

Examples:

4.0Chemical Equilibrium

Chemical equilibrium occurs in reversible chemical reactions.

Conditions for Chemical Equilibrium

• Reaction must be reversible
• System must be closed
• No reactants or products should be lost
• Temperature and pressure must be constant
• Rate of forward reaction = Rate of backward reaction
• Concentration of reactants and products remains constant

Examples:

• 2HI ⇌ H₂ + I₂
• H₂ + I₂ ⇌ 2HI

Characteristics of Equilibrium

Dynamic in nature – both forward and backward reactions continue at equal rates

1. Closed system: It only happens in a closed container.
2. Constant properties: Pressure, colour, concentration stay unchanged.
3. Same state: Whether you start from reactants or products, final state is the same.
4. Catalysts: Help in reaching equilibrium faster but do not affect the final state.
5. Dynamic nature: Reactions continue in both directions, but no net change is observed.
6. ΔG = 0 at equilibrium

5.0Ionic Equilibrium

• Happens in ionic compounds like acids, bases, and salts dissolved in water.
• Balance exists between unionised molecules and ions.

Electrolytes

Examples of weak electrolyte equilibrium:

• CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺
• NH₄OH + H₂O ⇌ NH₄⁺ + OH⁻

6.0Types of Equilibrium (Beyond Chemical)

7.0Equilibrium Constant (Kₑq)

For a general reaction: aA + bB ⇌ cC + dD

The equilibrium constant is: Kₑq = [C]^c [D]^d / [A]^a [B]^b

• Square brackets [ ] mean concentration (mol/L).
• Exponents come from the coefficients in the balanced equation.

Interpreting Kₑq Values:

Note: Kₑq is constant at a given temperature, but changes if the temperature changes.