Question

(a) What is the common-ion effect? (b) Give an example of a salt that can decrease the ionization of \(\mathrm{HNO}_{2}\) in solution.

Short answer

(a) The common-ion effect is the suppression of the ionization of a weak electrolyte by the presence of a common ion from a strong electrolyte in the solution. This effect occurs because adding a common ion shifts the equilibrium of the weak electrolyte's ionization reaction backward, decreasing its ionization. (b) An example of a salt that can decrease the ionization of \(\mathrm{HNO}_{2}\) is \(\mathrm{KNO}_{2}\) (potassium nitrite). This salt introduces the common ion \(\mathrm{NO}_{2}^{-}\) into the solution, suppressing the ionization of \(\mathrm{HNO}_{2}\).

Step-by-step solution

Define the common-ion effect

The common-ion effect is the suppression of the ionization of a weak electrolyte, such as a weak acid or a weak base, by the presence of a common ion from a strong electrolyte in the solution. This effect occurs because adding a common ion shifts the equilibrium of the weak electrolyte's ionization reaction backward, decreasing the ionization of the weak electrolyte.

Consider the ionization of \(\mathrm{HNO}_{2}\)

The ionization reaction for \(\mathrm{HNO}_{2}\) in water is given by: \[\mathrm{HNO}_{2} \rightleftharpoons \mathrm{H}^{+} + \mathrm{NO}_{2}^{-}\] Here, \(\mathrm{HNO}_{2}\) is a weak acid and does not ionize completely, meaning that the equilibrium lies to the left. We're looking for a salt that can decrease the ionization of \(\mathrm{HNO}_{2}\) by introducing a common ion into the solution.

Choose a salt that introduces the common ion

In order to suppress the ionization of \(\mathrm{HNO}_{2}\), we need to choose a salt that has a common ion with the ionization reaction of \(\mathrm{HNO}_{2}\). The ions present in the reaction are \(\mathrm{H}^{+}\) and \(\mathrm{NO}_{2}^{-}\). A salt that contains either of these ions will decrease the ionization of \(\mathrm{HNO}_{2}\). A good option for this purpose is \(\mathrm{KNO}_{2}\) (potassium nitrite), which dissociates in water as \(\mathrm{KNO}_{2} \rightarrow \mathrm{K}^{+} + \mathrm{NO}_{2}^{-}\). The common ion with \(\mathrm{HNO}_{2}\) ionization is \(\mathrm{NO}_{2}^{-}\). So, the example of a salt that can decrease the ionization of \(\mathrm{HNO}_{2}\) is \(\mathrm{KNO}_{2}\).