Unit 7: Equilibrium

Executive Summary

Reversible reactions reach equilibrium when forward and reverse rates match. Concentrations stop changing, but reactions continue in both directions.

Equilibrium constants

For aA + bB ⇌ cC + dD, K = [C]^c[D]^d / [A]^a[B]^b (solutions and gases only — omit solids and pure liquids). K is temperature-dependent only. K >> 1 favors products; K << 1 favors reactants. Kp uses partial pressures.

Reaction quotient and direction

Q has the same form as K but uses current concentrations. Q < K: forward shift (products form). Q > K: reverse shift. Q = K: at equilibrium.

ICE tables

Set up Initial / Change / Equilibrium rows. Change rows follow stoichiometry (x, 2x, …). Substitute into K and solve — use the 5% rule or the quadratic when needed. Small-K problems let you neglect x in the denominator.

Le Chatelier's principle

A system at equilibrium responds to stress by countering it: adding reactant shifts toward products; removing product shifts forward; increasing pressure (by volume decrease) shifts toward fewer gas moles; temperature is special — increasing T favors the endothermic direction. Adding inert gas at constant volume changes nothing (no concentration change). Catalysts change no position, only speed.

Solubility equilibria

Ksp = ion product at saturation, e.g., AgCl(s) ⇌ Ag⁺ + Cl⁻, Ksp = [Ag⁺][Cl⁻]. Molar solubility (s) converts via stoichiometry: for AB salts Ksp = s²; for AB₂ salts Ksp = 4s³. Common-ion effect lowers solubility; pH can raise it for basic or acidic anions.

Quantitative skill-set

K and Kp from equilibrium data; ICE tables; Q vs. K predictions; Ksp ↔ molar solubility conversions; comparing Ksp magnitudes only between same-stoichiometry salts.

Exam traps

Solids and liquids never appear in K. Only temperature changes K. Ksp comparisons across different formulas need the molar-solubility conversion, not raw Ksp values. "Shifts right" means Q > K or Le Chatelier response — always justify with Q where numbers allow.