AP Chemistry Unit 7 Practice

Equilibrium expressions

aA + bB ⇌ cC + dD

Kc = [C]^c[D]^d / ([A]^a[B]^b)

Use products over reactants, raised to their balanced coefficients. Omit pure solids and pure liquids; retain dissolved species and gases. Kc uses equilibrium concentrations; Kp uses equilibrium gas partial pressures.

All K and Q values are dimensionless here. Choose no units for them. Use M for concentrations and molar solubility. Concentrations approximate activities in ideal dilute solutions; gases are ideal.

Q versus K

Q uses the same expression as K with current values, not necessarily equilibrium values.

• Q < K: shift right; net product formation.
• Q > K: shift left; net reactant formation.
• Q = K: dynamic equilibrium; equal forward and reverse rates.

Changing the reaction equation

Reverse: K′ = 1/K. Multiply coefficients by n: K′ = Kⁿ. Add reactions: K = K₁ × K₂.

Gas equilibria

Kp = Kc(RT)^Δn; R = 0.08206 L·atm/(mol·K).

Δn is the sum of gaseous product coefficients minus the sum of gaseous reactant coefficients. Use kelvin. At fixed temperature, concentration, volume, catalyst and inert-gas changes do not change K. Temperature can change K.

A catalyst changes both reaction rates. An inert gas causes no shift at constant temperature and volume; at constant total pressure it can cause expansion and a shift when gaseous coefficients differ. Heating favors the endothermic direction.

ICE tables and approximations

ICE means initial, change, equilibrium. Use coefficient-weighted changes and substitute the equilibrium row into K. Discard roots that give negative concentrations.

For a small-x approximation, check each neglected concentration change divided by its initial concentration, times 100, is under 5%. Otherwise solve the full equation. Keep extra digits and round only the final answer.

Solubility equilibria

M_aX_b(s) ⇌ aM^b+(aq) + bX^a−(aq); Ksp = [M^b+]^a[X^a−]^b.

AgCl(s) ⇌ Ag⁺(aq) + Cl⁻(aq); Ksp = [Ag⁺][Cl⁻] = s².

CaF₂(s) ⇌ Ca²⁺(aq) + 2F⁻(aq); Ksp = [Ca²⁺][F⁻]² = 4s³.

The s relations apply in pure water with no side reactions. For precipitation, use ion concentrations after mixing: Qsp > Ksp forms precipitate; Qsp = Ksp is saturated equilibrium; Qsp < Ksp is unsaturated.

A common ion lowers molar solubility at fixed temperature without changing Ksp. Include the initial common-ion concentration in the ICE table. Ignore acid-base reactions and complex formation only when the prompt specifies no side reactions.