Acid Dissociation Constant Ka Definition

The acid dissociation constant is the equilibrium constant of the dissociation reaction of an acid and is denoted by Ka. This equilibrium constant is a quantitative measure of the strength of an acid in a solution. Ka is commonly expressed in units of mol/L. There are tables of acid dissociation constants, for easy reference. For an aqueous solution, the general form of the equilibrium reaction is: HA H2O ⇆ A- H3O where HA is an acid which dissociates in the conjugate base of the acid A- and a hydrogen ion that combines with water to form the hydronium ion H3O. When the concentrations of HA, A-, and H3O no longer change over time, the reaction is at equilibrium and the dissociation constant may be calculated: Ka [A-][H3O] / [HA][H2O] where the square brackets indicate concentration. Unless an acid is extremely concentrated, the equation is simplified by holding the concentration of water as a constant: HA ⇆ A- HKa [A-][H]/[HA] The acid dissociation constant is also known as the acidity constant or acid-ionization constant. Relating Ka and pKa A related value is pKa, which is the logarithmic acid dissociation constant: pKa -log10Ka Using Ka and pKa To Predict Equilibrium and Strength of Acids Ka may be used to measure the position of equilibrium: If Ka is large, the formation of the products of the dissociation is favored.If Ka is small, the undissolved acid is favored. Ka may be used to predict the strength of an acid: If Ka is large (pKa is small) this means the acid is mostly dissociated, so the acid is strong. Acids with a pKa less than around -2 are strong acids.If Ka is small (pKa is large), little dissociation has occurred, so the acid is weak. Acids with a pKa in the range of -2 to 12 in water are weak acids. Ka is a better measure of the strength of an acid than pH because adding water to an acid solution doesnt change its acid equilibrium constant, but does alter the H ion concentration and pH. Ka  Example The acid dissociation constant, Ka  of the  acid  HB is: HB(aq) ↔ H(aq) B-(aq)Ka   [H][B-] / [HB] For the dissociation of ethanoic acid: CH3COOH(aq)   H2O(l)   CH3COO-(aq)   H3O(aq)Ka   [CH3COO-(aq)][H3O(aq)] / [CH3COOH(aq)] Acid Dissociation Constant From pH The acid dissociation constant may be found it the pH is known. For example: Calculate the acid dissociation constant Ka for a 0.2 M aqueous solution of propionic acid (CH3CH2CO2H) that is found to have a pH value of 4.88. To solve the problem, first, write the chemical equation for the reaction. You should be able to recognize propionic acid is a weak acid (because its not one of the strong acids and it contains hydrogen). Its dissociation in water is: CH3CH2CO2H H2 ⇆ H3O CH3CH2CO2- Set up a table to keep track of the initial conditions, change in conditions, and equilibrium concentration of the species. This is sometimes called an ICE table: CH3CH2CO2H H3O+ CH3CH2CO2- Initial Concentration 0.2 M 0 M 0 M Change in Concentration -x M +x M +x M Equilibrium Concentration (0.2 - x) M x M x M x [H3O Now use the pH formula: pH -log[H3O]-pH log[H3O] 4.88[H3O 10-4.88 1.32 x 10-5 Plug in this value for x to solve for Ka: Ka [H3O][CH3CH2CO2-] / [CH3CH2CO2H]Ka x2 / (0.2 - x)Ka (1.32 x 10-5)2 / (0.2 - 1.32 x 10-5)Ka 8.69 x 10-10