State where forward and reverse reactions occur at equal rates

• Concentrations of reactants and products remain constant
• Reaction continues at molecular level (not stopped)
• Can be approached from either direction

For reaction: $aA + bB \rightleftharpoons cC + dD$ $K_{eq} = \frac{[C]^c[D]^d}{[A]^a[B]^b}$

Pure solids and pure liquids are NOT included in K expression.

If stress applied to system at equilibrium, system shifts to relieve stress

Stresses and responses:

• Add reactant -> shifts right (toward products)
• Add product -> shifts left (toward reactants)
• Increase pressure -> shifts toward fewer moles of gas
• Increase temperature -> shifts toward endothermic direction

Kc: Based on concentrations Kp: Based on partial pressures

$K_p = K_c(RT)^{\Delta n}$ where $\Delta n = moles\ gas_{products} - moles\ gas_{reactants}$

Method for solving equilibrium problems:

Reaction quotient (Q): Same expression as K, but with current concentrations

For dissolving: $A_xB_y(s) \rightleftharpoons xA^{n+}(aq) + yB^{m-}(aq)$ $K_{sp} = [A^{n+}]^x[B^{m-}]^y$

Solubility (s): Concentration of dissolved compound

• Use Ksp and stoichiometry to find solubility

Common-ion effect: Solubility decreases when common ion already present

• Shifts equilibrium toward solid (less dissolution)
• Can be used for selective precipitation