Chemical Equilibrium deals with reversible reactions, equilibrium constants, Le Chatelier’s principle and factors affecting equilibrium (JEE Main focus).
1) Reversible & Irreversible Reactions
- Irreversible reaction: Proceeds only in one direction until completion.
- Reversible reaction: Proceeds in both forward and backward directions.
Example:
H2 + I2 ⇌ 2HI
2) State of Chemical Equilibrium
- Rate of forward reaction = Rate of backward reaction
- Equilibrium is dynamic in nature
- Macroscopic properties (pressure, concentration, colour) remain constant
- Equilibrium can be attained from reactants or products
- Catalyst does not change equilibrium position
3) Homogeneous & Heterogeneous Equilibrium
- Homogeneous equilibrium: All species in same phase
- Heterogeneous equilibrium: More than one phase present
CaCO3(s) ⇌ CaO(s) + CO2(g)
4) Law of Chemical Equilibrium
For the reaction:
A + B ⇌ C + D
At equilibrium:
K = [C][D] / [A][B]
5) Properties of Equilibrium Constant (K)
- Independent of initial concentration
- Depends only on temperature
- K > 1 → Products favoured
- K < 1 → Reactants favoured
- Unaffected by catalyst
Reversal of reaction:
Kreverse = 1 / K
6) Relationship between Kp and Kc
Kp = Kc(RT)Δn
where:
- Δn = moles of gaseous products − moles of gaseous reactants
- Δn = 0 → Kp = Kc
- Δn > 0 → Kp > Kc
- Δn < 0 → Kp < Kc
7) Le Chatelier’s Principle
If a system at equilibrium is subjected to a change, the equilibrium shifts in a direction that opposes the applied change.
- Concentration: Increase shifts equilibrium to consume added species
- Pressure: Increase favours side with fewer gas moles
- Temperature:
- Endothermic → Increase T favours forward reaction
- Exothermic → Increase T favours backward reaction
8) Units of Equilibrium Constant
- Kc unit = (mol/L)Δn
- Kp unit = (atm or bar)Δn
- In modern chemistry, equilibrium constants are treated as dimensionless
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Last modified: December 14, 2025
