Chemical Equilibrium — 60 Terms Defined

Activity Chemical Equilibrium Dimensionless The effective concentration of a species, corrected for non-ideal interactions with its surroundings. a = γ c / c°

Activity Coefficient Chemical Equilibrium Dimensionless The factor by which a species' concentration must be multiplied to give its thermodynamic activity. γ = a / c

Approach to Equilibrium Chemical Equilibrium s The time-dependent process by which a reacting system moves towards its equilibrium composition.

Association Constant Chemical Equilibrium L/mol The equilibrium constant for the combination of two species into a single associated entity, the reciprocal of the dissociation constant. Ka = 1 / Kd

Chemical Equilibrium Chemical Equilibrium Dimensionless The state of a reversible reaction in which the forward and reverse reactions proceed at equal rates, so no net change in composition occurs. aA + bB ⇌ cC + dD

Combining Equilibrium Constants Chemical Equilibrium Various The rule that when two reactions are added, their equilibrium constants multiply. K_overall = K₁ × K₂

Common Ion Effect Chemical Equilibrium mol/L The suppression of the ionisation or dissolution of a substance by adding a second substance that supplies one of the same ions.

Complex Ion Equilibrium Chemical Equilibrium mol/L The equilibrium between a metal ion, its ligands and the resulting complex ion in solution. M + nL ⇌ MLₙ

Concentration Equilibrium Constant Chemical Equilibrium Various The equilibrium constant expressed in terms of the molar concentrations of the species involved. Kc = [C]^c[D]^d / [A]^a[B]^b

Coupled Equilibria Chemical Equilibrium Dimensionless Two or more equilibria that share a common species, so that shifting one necessarily shifts the others.

Debye–Hückel Limiting Law Chemical Equilibrium Dimensionless The theoretical relation predicting activity coefficients of ions in very dilute solution from their charge and the ionic strength. log γ = −A z² √I

Degree of Dissociation Chemical Equilibrium Dimensionless The fraction of the original substance that has broken apart at equilibrium. α = amount dissociated / initial amount

Dissociation Constant Chemical Equilibrium mol/L The equilibrium constant for the breaking apart of a compound or complex into smaller species. Kd = [A][B] / [AB]

Distribution Law Chemical Equilibrium Dimensionless The principle that a solute distributes itself between two immiscible solvents in a constant ratio at fixed temperature. K_D = C_organic / C_aqueous

Dynamic Equilibrium Chemical Equilibrium Dimensionless An equilibrium in which opposing processes continue at the molecular level but at equal and opposite rates. rate_forward = rate_reverse

Effect of a Catalyst on Equilibrium Chemical Equilibrium Dimensionless A catalyst changes how quickly equilibrium is reached but never where that equilibrium lies. K unchanged

Effect of an Inert Gas Chemical Equilibrium Pa The influence of adding a non-reacting gas on the position of a gaseous equilibrium.

Effect of Concentration Change Chemical Equilibrium mol/L The shift in equilibrium position caused by adding or removing a reactant or product. Q ≠ K → shift until Q = K

Effect of Pressure Change Chemical Equilibrium Pa The shift in a gaseous equilibrium caused by changing the total pressure through a volume change. shift towards fewer gas moles when p rises

Effect of Temperature Change Chemical Equilibrium K The shift in equilibrium caused by heating or cooling, which uniquely also changes the equilibrium constant. exothermic: K falls as T rises

Equilibrium Constant Chemical Equilibrium Various The fixed ratio of product to reactant terms, each raised to its stoichiometric power, that characterises a reaction at a given temperature. K = Π[products]^ν / Π[reactants]^ν

Equilibrium Constant of a Reversed Reaction Chemical Equilibrium Various The constant obtained when a reaction is written in the opposite direction, equal to the reciprocal of the original. K_reverse = 1 / K_forward

Equilibrium Constant of a Scaled Reaction Chemical Equilibrium Various The constant obtained when every coefficient in an equation is multiplied by the same factor. K_new = K_old^n

Equilibrium in Closed Systems Chemical Equilibrium Dimensionless The requirement that no matter enters or leaves for a true chemical equilibrium to be established.