Equilibrium

Dynamic equilibrium: forward and reverse reactions occur at equal rates. Concentrations of reactants and products remain CONSTANT (not equal). Equilibrium is dynamic — both reactions still happening. Reached in closed system. Symbol ⇌ shows reversible reaction. Example: N₂ + 3H₂ ⇌ 2NH₃.

For aA + bB ⇌ cC + dD: Kc = [C]^c[D]^d / [A]^a[B]^b. Square brackets = molar concentration at equilibrium. Kp uses partial pressures: Kp = Kc(RT)^Δn where Δn = moles of products - moles of reactants. Large K: equilibrium favours products. Small K: favours reactants.

If a system at equilibrium is disturbed, it shifts in the direction that reduces the disturbance. Increase concentration of reactant → shifts forward. Increase concentration of product → shifts backward. Increase pressure → shifts toward fewer moles of gas. Increase temperature → shifts in endothermic direction. Catalyst: speeds up reaching equilibrium, does NOT change K or shift position.

Weak acids partially dissociate: HA ⇌ H⁺ + A⁻. Ka = [H⁺][A⁻]/[HA]. pH = -log[H⁺]. pOH = -log[OH⁻]. pH + pOH = 14 at 25°C. Strong acids: HCl, H₂SO₄, HNO₃ (fully dissociate). Weak acids: CH₃COOH, H₂CO₃ (partially dissociate). Buffer: resists pH change (weak acid + its salt).