Definitions Glossary — Equilibrium Key Terms — Notes

Term	Definition	Context
Dynamic Equilibrium	State where forward and backward reaction rates are equal; concentrations constant but not zero	Foundation of all equilibrium calculations
Equilibrium Constant (Kc)	Ratio of product concentrations to reactant concentrations at equilibrium, each raised to stoichiometric coefficient	Pure solids and liquids excluded
Kp	Equilibrium constant in terms of partial pressures of gaseous species	Related to Kc by Kp = Kc(RT)^ $\Delta n$
$\Delta n$	Difference: (moles of gaseous products) − (moles of gaseous reactants)	Used in Kp = Kc(RT)^ $\Delta n$
Reaction Quotient (Q)	Same mathematical form as K but using non-equilibrium concentrations	Q vs K comparison predicts direction of shift
Le Chatelier's Principle	If a stress is applied to a system at equilibrium, the system shifts to partially relieve that stress	Basis of industrial process optimisation
Ka (Acid dissociation constant)	Equilibrium constant for weak acid ionisation: HA ⇌ $H^{+}$ + $A^{-}$ ; Ka = [ $H^{+}$ ][ $A^{-}$ ]/[HA]	Higher Ka → stronger weak acid
Kb (Base dissociation constant)	Equilibrium constant for weak base ionisation: B + $H_{2}O$ ⇌ B $H^{+}$ + $OH^{-}$	Ka × Kb = Kw for conjugate pairs
pH	−log[ $H^{+}$ ]; measure of acidity	pH 0–6 acidic; 7 neutral (25°C); 8–14 basic
pKa	−log Ka; measure of acid strength	Lower pKa → stronger acid
Henderson-Hasselbalch equation	pH = pKa + log([ $A^{-}$ ]/[HA]); gives pH of buffer	Valid when [salt]/[acid] ratio between 0.1 and 10
Buffer solution	Solution that resists pH change upon addition of small amounts of acid or base	Contains weak acid + conjugate base (or weak base + conjugate acid)
Common ion effect	Suppression of ionisation of a weak electrolyte by adding an ion common to the equilibrium	Reduces solubility of sparingly soluble salts
Ksp (Solubility product)	Equilibrium constant for dissolution of sparingly soluble salt	Ksp = product of ion concentrations at saturation
Ionic product (Qsp)	Product of ion concentrations at any given moment	Qsp > Ksp → precipitation; Qsp < Ksp → unsaturated
Salt hydrolysis	Reaction of a salt's ions with water to produce acidic or basic solution	Type depends on parent acid/base strengths
Conjugate acid-base pair	Pair differing by one proton ( $H^{+}$ ): HA and $A^{-}$	Ka(HA) × Kb( $A^{-}$ ) = Kw
Autoionisation of water	$H_{2}O$ ⇌ $H^{+}$ + $OH^{-}$ ; Kw = [ $H^{+}$ ][ $OH^{-}$ ] = 10^{-14} at 25°C	Basis of pH scale and neutrality definition