Acidity and Basicity constant Chapter 12 – Qualitative Chemical Equilibrium

Acid Dissociation Acids undergo electrolytic dissociation in solution to different degrees.   A strong acid is an acid that dissociates completely into ions in water. Examples of strong acids includes: HCl, HBr, HI, HNO3, H2SO4, and HClO4. Equilibrium is not attained during the dissociation of a strong acid because the reaction is complete.

Acid Dissociation Let’s take HCl as an example. There will be no more HCl molecules remaining at the end of the reaction; the ionization is 100%. The final concentration of the H+ and Cl- ions is equal to the concentration of the hydrochloric acid (1:1 molar ratio).

Weak Acids A weak acid is an acid that does not dissociate completely into ions in water. In fact, they dissociate very little. The percentage of acid that has dissociate can be calculated using the following equation: The state of equilibrium will be attained when a weak acid dissociates in water; not all of the acid will dissociate and the final concentration of each of the ions produced will be equal. The rate of the dissociation of the acid is equal to the rate of the recombination of the ions.

HA(aq) + H2O(l)  H3O+(aq) + A-(aq) Acidity Constant Take a look at the generic acid dissociation reaction below: HA(aq) + H2O(l)  H3O+(aq) + A-(aq) The equilibrium constant for this reaction can be expressed as:

Acidity Constant The acidity constant does not depend on the concentration of the liquid water, because as in all equilibrium calculations we only consider the concentration of gases and aqueous solutions. The acidity constant of a strong acid cannot be described using this expression because their dissociation is complete and there is no equilibrium established between the acid molecules and their ions. The acidity constant is used to classify acids according to their strength; the lower the constant, the weaker the acid. A value close to 1010 is considered a strong acid.

Acidity Constant You can obtain the value of the equilibrium constant of a weak acid if you know all the equilibrium concentrations. If the equilibrium concentrations (H3O+ in particular) are known, you can also calculate the pH of the solution. It helps to organize the information in an ICE table (see class work or textbook for examples)

B(aq) + H2O(l)  HB+(aq) + OH-(aq) Bases Bases can also be classified as weak or strong based on their degree of dissociation. A state of equilibrium will only be obtained for a weak base according to the following expression: B(aq) + H2O(l)  HB+(aq) + OH-(aq) The basicity constant can be written as follows:

Bases The basicity constant is used to rank bases according to their strength; the lower the basicity constant, the weaker the base. Values for the concentrations at equilibrium can be obtained using ICE tables.