Problem 16 - Chemistry

Question

Is the conjugate base of a weak acid a strong base? Explain. Explain why \(\mathrm{Cl}^{-}\) does not affect the \(\mathrm{pH}\) of an aqueous solution.

Step-by-Step Solution

Answer

The conjugate base of a weak acid is not a strong base because the strength of an acid and its conjugate base are inversely related. The chloride ion (\(\mathrm{Cl}^{-}\)) does not affect the pH of an aqueous solution since it is the conjugate base of a strong acid (\(\mathrm{HCl}\)) and a very weak base that doesn't cause significant pH changes, essentially acting as a spectator ion.

Step 1: Conjugate Acid-Base Pairs

In an acid-base reaction, an acid donates a proton (H+) to a base, which accepts the proton. The species that remains after the acid donates a proton is called the conjugate base, while the species that remains after the base accepts a proton is called the conjugate acid. The conjugate base of a weak acid is the ion formed after the acid donates a proton; conversely, the conjugate acid of a weak base is the ion formed after the base accepts a proton.

Step 2: Properties of Conjugate Bases of Weak Acids

A weak acid is an acid that does not fully ionize in solution, so it has a relatively low concentration of H+ ions. As a result, it has a higher pH compared to a strong acid. The conjugate base of a weak acid is typically a weak base, which means that it does not readily accept protons in an aqueous solution. This is because the strength of an acid and its conjugate base are inversely related – the weaker the acid, the weaker its conjugate base. In summary, the conjugate base of a weak acid is not considered a strong base.

Step 3: Behavior of \(\mathrm{Cl}^{-}\) in Aqueous Solutions

\(\mathrm{Cl}^{-}\), or the chloride ion, is the conjugate base of a strong acid, hydrochloric acid (\(\mathrm{HCl}\)). Due to the strength of the \(\mathrm{HCl}\) acid, the chloride ion is a very weak base and does not readily accept protons when dissolved in water. Consequently, the presence of \(\mathrm{Cl}^{-}\) ions in an aqueous solution does not cause a significant change to the pH value. Moreover, in aqueous solutions, \(\mathrm{Cl}^{-}\) ions do not react with water molecules to form \(\mathrm{OH}^{-}\) ions (which would make the solution more basic) nor with \(\mathrm{H}^{+}\) ions (which would make the solution more acidic). In other words, \(\mathrm{Cl}^{-}\) is a spectator ion – it does not participate in any chemical reactions that would alter the pH of the solution.

Step 4: Conclusion

The conjugate base of a weak acid is not a strong base as the strength of the weak acid and its conjugate base are inversely related. The chloride ion (\(\mathrm{Cl}^{-}\)) does not affect the pH of an aqueous solution because it is the conjugate base of a strong acid (\(\mathrm{HCl}\)) and thus, a very weak base that does not cause any significant pH changes.