Sulfur dioxide (SO2) is a weaker acid than hydrogen sulfide (H2S) due to differences in their molecular structures and the stability of their conjugate bases.
In an acidic solution, both sulfur dioxide and hydrogen sulfide can donate a proton (H+) to act as acids. However, the ease with which they donate a proton and the stability of their resulting conjugate bases determine their relative acid strengths.
Sulfur dioxide (SO2) has the molecular formula of one sulfur atom bonded to two oxygen atoms. It can donate a proton to form the bisulfite ion (HSO3-). The reason SO2 is a weaker acid compared to H2S is primarily due to the electronegativity difference between sulfur and oxygen. Oxygen is more electronegative than sulfur, and the oxygen atoms in SO2 withdraw electron density from the sulfur atom. This electron-withdrawing effect makes it more challenging for SO2 to donate a proton, decreasing its acidity.
On the other hand, hydrogen sulfide (H2S) consists of two hydrogen atoms bonded to a sulfur atom. It can donate a proton to form the hydrosulfide ion (HS-). In H2S, the electronegativity difference between sulfur and hydrogen is relatively smaller than that between sulfur and oxygen in SO2. As a result, the electron density is less affected by electronegative forces, and H2S can more easily donate a proton, making it a stronger acid compared to SO2.
Additionally, the stability of the resulting conjugate bases also influences the relative acid strengths. The bisulfite ion (HSO3-) formed from the deprotonation of SO2 is relatively more stable compared to the hydrosulfide ion (HS-) formed from the deprotonation of H2S. This increased stability of the bisulfite ion further contributes to the weaker acidity of SO2 compared to H2S.
In summary, the weaker acidity of sulfur dioxide (SO2) compared to hydrogen sulfide (H2S) can be attributed to the electron-withdrawing effect of the oxygen atoms in SO2, which makes it more difficult to donate a proton. Additionally, the stability of the resulting conjugate bases also influences their acid strengths, with the bisulfite ion (HSO3-) being more stable than the hydrosulfide ion (HS-).