Problem 13 - Chemistry: The Central Science

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.

Step-by-Step Solution

Answer

(a) The common-ion effect refers to the suppression of the ionization or dissolution of a weak electrolyte, like a weak acid or weak base, in a solution containing a common ion shared with a strong electrolyte. This occurs due to an increase in the concentration of a particular ion, causing a shift in the equilibrium position according to Le Chatelier's principle. (b) An example of a salt that can decrease the ionization of \(\mathrm{HNO}_{2}\) in a solution is sodium nitrite (\(\mathrm{NaNO}_{2}\)). When dissolved in water, it dissociates into sodium ions (Na⁺) and nitrite ions (NO₂⁻), which have a common ion (NO₂⁻) with the ionization reaction of \(\mathrm{HNO}_{2}\). The increased concentration of nitrite ions shifts the equilibrium towards the left, suppressing the ionization of \(\mathrm{HNO}_{2}\), demonstrating the common-ion effect.

Step 1: Define the common-ion effect

The common-ion effect refers to the suppression of the ionization or dissolution of a weak electrolyte, like a weak acid or weak base, in a solution containing a common ion shared with a strong electrolyte. This effect occurs because of the increase in the concentration of a particular ion, causing a shift in the equilibrium position according to Le Chatelier's principle.

Step 2: Identify the ions present in HNO2

\(\mathrm{HNO}_{2}\) is a weak acid, and when it ionizes in a solution, it produces hydrogen ions (H+) and nitrite ions (NO₂⁻): \[ \mathrm{HNO}_{2} \rightleftarrows \mathrm{H}^{+} + \mathrm{NO}_{2}^- \]

Step 3: Provide an example of a salt that decreases the ionization of HNO2

To find a salt that decreases the ionization of \(\mathrm{HNO}_{2}\), we need to identify a salt that has a common ion with the nitrous acid ionization reaction. In this case, the common ion is the nitrite ion, NO₂⁻. An example of such a salt is sodium nitrite (\(\mathrm{NaNO}_{2}\)). When dissolved in water, sodium nitrite dissociates into sodium ions (Na⁺) and nitrite ions (NO₂⁻): \[ \mathrm{NaNO}_{2} \rightarrow \mathrm{Na}^{+} + \mathrm{NO}_{2}^- \] The presence of additional nitrite ions from the sodium nitrite salt will shift the ionization equilibrium of \(\mathrm{HNO}_{2}\) towards the left, suppressing its ionization according to Le Chatelier's principle. The increased concentration of nitrite ions results in the suppression of HNO2 ionization, which is an example of the common-ion effect.