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Chemistry (university) · Part 07

Chemical equilibrium and solubility

Equilibrium constants, reaction quotients, Le Chatelier's principle and the solubility product.

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  1. Dynamic equilibriumA reversible reaction reaches equilibrium when the forward and reverse rates are equal. Concentrations then stay constant, but the reactions have not stopped. Both directions keep going at the same speed.
  2. Equilibrium constantFor aA + bB ⇌ cC + dD, the equilibrium constant is K = [C]ᶜ[D]ᵈ / ([A]ᵃ[B]ᵇ). Pure solids and liquids are left out. K is large if products are favoured and small if reactants are favoured. It changes only with temperature.
  3. Reaction quotientThe reaction quotient Q has the same form as K but uses the concentrations right now. Compare Q to K. If Q < K, the reaction goes forward. If Q > K, it goes in reverse. If Q = K, it is at equilibrium.
  4. Kp and KcFor gas reactions, K can be written with partial pressures (Kp) or concentrations (Kc). They are related by Kp = Kc(RT)^Δn, where Δn is moles of gas on the product side minus the reactant side.
  5. ICE tableAn ICE table tracks Initial, Change and Equilibrium amounts to find an unknown equilibrium concentration. Let x be the change, use the coefficients for each species, then plug the equilibrium row into the K expression and solve for x.
  6. Le Chatelier's principleIf a system at equilibrium is disturbed, it shifts to partly cancel the change. Adding reactant shifts it toward products. Raising pressure shifts it toward fewer gas moles. Heating shifts it toward the endothermic direction.
  7. Free energy and the equilibrium constantThe standard free energy change and K are linked: ΔG° = −RT ln K. A negative ΔG° means K > 1 and products are favoured. A positive ΔG° means K < 1. At equilibrium, ΔG = 0.
  8. Solubility productFor a slightly soluble salt, Ksp is the equilibrium constant for its dissolving. For AgCl(s) ⇌ Ag⁺ + Cl⁻, Ksp = [Ag⁺][Cl⁻]. A smaller Ksp means a less soluble salt (compare only salts with the same ion ratio).
  9. Common ion effectAdding an ion that is already part of a dissolving equilibrium pushes the equilibrium back toward the solid. The salt becomes less soluble. This is Le Chatelier's principle applied to Ksp.