Conjugate pairs
In order to function as an acid, a Brønsted Lowry acid requires the presence of a base (and vice versa for a Brønsted Lowry base requiring an acid). In losing a proton, an acid forms a base – its conjugate base. Conversely, in accepting a proton, a base forms an acid – its conjugate acid.
Consider the following examples and questions:
INSERT [PDA_CI_04_ima]
In the above example, what is the Brønsted Lowry acid, Brønsted Lowry base, the conjugate acid and the conjugate base?
INSERT [PDA_CI_05_ima]
Identify the Brønsted Lowry acid, Brønsted Lowry base, the conjugate acid and the conjugate base in HClO4 + H2O ClO4 + H3O+
INSERT [PDA_CI_06_ima]
Consider how might the pH of a salt vary according to its component parts. The part which comes from a strong acid or strong base will remain as ions. The component from the weak acid/base requires some thought.
How can Brønsted Lowry theory explain the pH of a salt of a weak acid and a strong base such as sodium ethanoate, Na+CH3COO- ?
The ethanoate ion is the conjugate base of ethanoic acid, a weak acid: CH3COO-(aq) + H2O CH3COOH(aq) + OH-(aq) The equilibrium can be thought of as a competition between the ethanoate ion and water for the proton. The ethanoate ion 'wins' and accepts a proton from water producing hydroxide ions, hence the pH is above 7.
How can Brønsted Lowry theory explain the pH of a salt of a strong acid and a weak base such as ammonium chloride NH4Cl?
The ammonium ion is the acid and ammonia, the conjugate base: NH4+(aq) + H2O NH3(aq) + H3O+(aq) The ammonium ion donates a proton to water, the base, producing acidic hydronium ions, hence the pH is below 7.
INSERT [PDA_CI_09_ima]