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 the reverse is called deposition or desublimation.
(iv) Solid–Solution Equilibrium
Sugar(solid) ⇌ Sugar(solution)
In a saturated solution, the rate of dissolution = the 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
- The 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
- The 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
- Closed system: It only happens in a closed container.
- Constant properties: Pressure, colour, and concentration stay unchanged.
- Same state: Whether you start from reactants or products, the final state is the same.
- Catalysts: Help in reaching equilibrium faster, but do not affect the final state.
- Dynamic nature: Reactions continue in both directions, but no net change is observed.
- 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.
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