Chemical Equilibrium – Complete Formula & Concept Sheet (JEE Main + Advanced)
Chemical equilibrium deals with reversible reactions where forward and backward reaction rates become equal. Equilibrium is dynamic, not static.
1. Law of Mass Action
For a general reaction:
aA + bB ⇌ cC + dD
The equilibrium constant:
Kc = [C]c[D]d / [A]a[B]b
- Only concentration of gases and solutes appear
- Pure solids and liquids are omitted
2. Equilibrium Constant Kc and Kp
For gaseous reactions:
Kp = Kc(RT)Δn
where Δn = (moles of gaseous products − moles of gaseous reactants)
Trap: Δn counts only gases, not solids or liquids.
3. Reaction Quotient (Q)
Same expression as Kc, but using initial concentrations.
- Q < K → Reaction proceeds forward
- Q > K → Reaction proceeds backward
- Q = K → System at equilibrium
4. Characteristics of Chemical Equilibrium
- Equilibrium can be achieved from either side
- Equilibrium constant depends only on temperature
- Catalyst does not change K, only speeds attainment
5. Le Chatelier’s Principle
If a system at equilibrium is disturbed, it shifts to oppose the disturbance.
| Change | Effect |
|---|---|
| Increase concentration | Shifts to consume added species |
| Increase pressure | Shifts to side with fewer gas moles |
| Increase temperature | Favours endothermic direction |
Classic Trap: Pressure has no effect if Δn = 0.
6. Relation Between K and Thermodynamics
ΔG° = −RT ln K
- K > 1 → ΔG° < 0 → Product favoured
- K < 1 → ΔG° > 0 → Reactant favoured
7. Degree of Dissociation (α)
For reactions like:
AB ⇌ A + B
If initial concentration = C:
Kc = Cα² / (1 − α)
Approximation: If α ≪ 1 → (1 − α) ≈ 1
8. Homogeneous vs Heterogeneous Equilibrium
- Homogeneous: All species in same phase
- Heterogeneous: Multiple phases (solids ignored)
9. Special Standard Cases (Must Remember)
- Reversing reaction → K becomes 1/K
- Multiplying equation by n → K becomes Kⁿ
- Adding reactions → Multiply respective K values
10. JEE-Level Hidden Traps
- Equilibrium constant expression written incorrectly with solids
- Δn miscalculated by including liquids
- Temperature change assumed without knowing reaction type
- Confusing ionic equilibrium ideas here
StudyBeacon Tip:
Chemical equilibrium questions reward clarity, not memorisation.
Every error here is conceptual, not mathematical.
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