Unit 7 · Equilibrium

Reversible process ● (core concept) — A process that can proceed in both the forward and reverse directions.

Dynamic equilibrium ● (core concept) — The state in which the forward and reverse rates of a process are equal, so there is no net change and concentrations remain constant.

Net direction of a reversible reaction ● (core concept) — At any moment, the faster of the forward and reverse reactions determines the net direction of conversion; when the rates are equal, the system is at equilibrium.

Reaction quotient (Q) ● (core concept) — A value with the same form as the equilibrium constant expression but calculated with current (not necessarily equilibrium) concentrations or partial pressures: Q_c uses concentrations, Q_p uses partial pressures.

Equilibrium constant (K) ● (core concept) — The value of the reaction quotient when the system is at equilibrium; K_c uses equilibrium concentrations and K_p uses equilibrium partial pressures. Pure solids and pure liquids are omitted from the expression.

Law of mass action ● (core concept) — For aA + bB ⇌ cC + dD, K = [C]^c[D]^d / ([A]^a[B]^b).

Determining K from measurements ● (core concept) — The equilibrium constant can be calculated from measured equilibrium concentrations or partial pressures, which is how K is determined experimentally.

Magnitude of K ● (core concept) — A large K (K >> 1) means the reaction proceeds essentially to completion, favoring products; a small K (K << 1) means it barely proceeds, favoring reactants.

Manipulating equilibrium constants ● (core concept) — Reversing a reaction gives 1/K; multiplying the coefficients by a number c gives K^c; adding reactions multiplies their K values.

Manipulating the reaction quotient (Q) ● (core concept) — Since K and Q have identical mathematical forms, the same algebraic rules apply to Q: reversing a reaction inverts Q, scaling coefficients by c raises Q to the power c, and adding reactions multiplies their Q values.

Predicting direction with Q vs. K ● (core concept) — If Q < K, the reaction shifts toward products; if Q > K, it shifts toward reactants; if Q = K, the system is at equilibrium.

Particulate representations of equilibrium ● (core concept) — Particle-level diagrams of an equilibrium mixture show constant macroscopic concentrations arising from continuous forward and reverse reactions at the molecular level.

Le Châtelier's principle ● (core concept) — When a system at equilibrium is disturbed (by changing concentration, temperature, or pressure/volume), it shifts to counteract the disturbance. Le Châtelier's principle also predicts how stresses affect pH, temperature, and color of the system.

Q vs. K after a disturbance ● (core concept) — A concentration disturbance changes Q (not K); a temperature change changes K itself. The system then shifts until Q = K again.

Solubility-product constant (K_sp) ● (core concept) — The equilibrium constant for a slightly soluble salt dissolving: for M_aX_b(s) ⇌ aM⁺ + bX⁻, K_sp = [M⁺]^a[X⁻]^b.

Molar solubility ● (core concept) — The moles of salt that dissolve per liter of saturated solution; it can be calculated from K_sp and used to calculate K_sp.

K_sp and solubility ● (core concept) — The solubility rules can be quantitatively related to K_sp: salts with K_sp values greater than 1 are considered soluble.

Common-ion effect ● (core concept) — The solubility of a salt decreases when the solution already contains one of its ions; the added ion shifts the dissolution equilibrium toward the solid (Le Châtelier's principle).