Question

\(\mathrm{H}_{2} \mathrm{SO}_{4}\) is a strong acid, but \(\mathrm{HSO}_{4}^{-}\) is a weak acid. Account for the difference in strength of these two related species.

Short answer

H2SO4 is a strong acid as it completely dissociates in solution, releasing two protons. However, HSO4-, formed after the first dissociation of sulfuric acid, is a weak acid because further dissociation of HSO4- is less favorable due to the resulting ion (SO4^2-) having a higher negative charge, which is less stable.

Step-by-step solution

Understand Acid and Conjugate Base

Acids are substances that donate protons, while their conjugate bases are the substances that form after an acid has donated a proton. In this case, H2SO4 is a strong acid, which means it completely ionizes/dissociates in water, donating two protons (H+) and forming a sulfate ion (SO4^2-). The first dissociation step of H2SO4 is H2SO4 => H+ + HSO4-, resulting in HSO4-.

Understand Weak Acids

A weak acid does not completely ionize or dissociate in a solution, which means it still holds onto some of its protons. In this case, HSO4- is a weak acid. It can donate a proton (H+), but doing so is harder because the species is already negatively charged. When it does, it forms the sulfate ion: HSO4- => H+ + SO4^2-.

Compare Acid Strength

The difference in acid strength can be accounted for by comparing the stability of these acids and their conjugate bases. Sulfuric acid is a strong acid because it's stable; once it has lost a proton, the resulting HSO4- ion is also stable. On the other hand, HSO4- is a weak acid because removing another proton to create sulfate (SO4^2-) creates a species with a high negative charge, which destabilizes the ion.