Equilibrium (NEET Chemistry)
Why This Chapter Matters
Equilibrium has 6-10 marks in NEET Chemistry. Kc, Kp, Le Chatelier's principle, pH calculations, buffer solutions, and Ksp are all tested extensively.
Core Concepts
1. Chemical Equilibrium
For: aA + bB ⇌ cC + dD
Kc = [C]^c[D]^d / [A]^a[B]^b (equilibrium constant in terms of concentration)
Kp = Kc(RT)^Δn (Δn = moles gas product - moles gas reactant)
Kp = Kc when Δn = 0
2. Le Chatelier's Principle
System opposes any change:
Add reactant → forward reaction | Add product → backward reaction
Increase pressure → side with fewer gas moles
Increase temperature → endothermic direction
Catalyst → no shift (both directions equally)
3. Ionic Equilibrium — pH
Kw = [H⁺][OH⁻] = 10⁻¹⁴ at 25°C
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14
Strong acid HCl (0.1M): [H⁺] = 0.1M → pH = 1
Weak acid HA: Ka = [H⁺][A⁻]/[HA] | [H⁺] = √(Ka × C) | pH = ½(pKa - log C)
Buffer: pH = pKa + log([A⁻]/[HA]) (Henderson-Hasselbalch)
4. Solubility Product (Ksp)
For AgCl ⇌ Ag⁺ + Cl⁻:
Ksp = [Ag⁺][Cl⁻] = s² where s = solubility
Common ion effect: adding Ag⁺ or Cl⁻ reduces solubility of AgCl
For Ag₂CrO₄ ⇌ 2Ag⁺ + CrO₄²⁻:
Ksp = [Ag⁺]²[CrO₄²⁻] = (2s)²(s) = 4s³
PYQs (NEET)
NEET 2023: pH of 0.001M HCl solution?
HCl fully ionises → [H⁺] = 0.001 = 10⁻³ M → pH = 3
NEET 2022: For the reaction N₂ + 3H₂ ⇌ 2NH₃, Kp < Kc because:
Δn = 2-4 = -2 (negative) → Kp = Kc(RT)^(-2) → Kp < Kc
NEET 2021: A buffer solution contains weak acid and its salt. This resists pH change because:
When H⁺ added → reacts with A⁻ (conjugate base). When OH⁻ added → reacts with HA (weak acid). Both neutralise without significant pH change.