Question

Why do we normally not quote \(K_{\mathrm{a}}\) values for strong acids such as \(\mathrm{HCl}\) and \(\mathrm{HNO}_{3}\) ? Why is it necessary to specify temperature when giving \(K_{\mathrm{a}}\) values?

Short answer

Strong acids like HCl and HNO3 fully ionise in solution giving a very large Ka value which is often not quoted due to its impracticality for comparison or calculations. Ka values depend on temperature because the acid dissociation is a chemical reaction and its equilibrium can be influenced by temperature changes. Therefore it's necessary to specify the temperature at which Ka is measured.

Step-by-step solution

Understanding Ka values for Strong Acids

Strong acids such as Hydrochloric acid (HCl) and Nitric acid (HNO3) are fully ionised in solution. This complete ionisation indicates that there is a complete transfer of protons from the acid to the water molecules. As a result, the ratio of the concentration of the resultant ions to the very minimal concentration of the undissociated acid molecules results in a very large Ka value. Since this Ka value is impractically large for calculations or comparisons, it is often not quoted.

Temperature Dependency of Ka

The Ka value is not a constant, but is rather temperature-dependent. This is because the dissociation of an acid is a chemical reaction, and the equilibrium of such reactions is known to be affected by temperature changes according to Le Châtelier's principle. Thus, when the Ka value of an acid is given, the temperature at which this value was measured must also be specified, as the Ka value can change with temperature.