Why is HS- a stronger base than HSO4-? (in terms of its ability to donate a pair of electrons)
The correct answer and explanation is:
Correct Answer:
HS⁻ is a stronger base than HSO₄⁻ because it has a greater ability to donate a lone pair of electrons.
Explanation:
The basic strength of a species depends on its ability to donate a pair of electrons to accept a proton. This is often influenced by factors such as the charge on the ion, the stability of the conjugate acid, the electronegativity of the atoms involved, and the extent of resonance stabilization.
HS⁻ (hydrosulfide ion) is the conjugate base of H₂S, a weak acid. This means that HS⁻ is relatively less stable and more reactive, which makes it eager to accept a proton by donating its lone pair of electrons. It holds its negative charge mainly on a sulfur atom, which is larger and less electronegative than oxygen, so it does not stabilize the negative charge as well. As a result, HS⁻ is a better electron-pair donor and therefore a stronger base.
On the other hand, HSO₄⁻ (hydrogen sulfate ion) is the conjugate base of a strong acid, H₂SO₄. The sulfate structure allows for extensive resonance delocalization of the negative charge across multiple oxygen atoms. This makes HSO₄⁻ much more stable and less likely to donate a lone pair of electrons to bind with a proton. Since it is already stabilized by resonance and the electronegative oxygen atoms, its basicity is significantly lower.
In simpler terms, HS⁻ is more reactive and less stable, so it seeks out protons more aggressively, which makes it a stronger base. HSO₄⁻, being very stable and already well-balanced, has little tendency to donate electrons, which makes it a weaker base. Therefore, in terms of donating a pair of electrons, HS⁻ is the stronger base.