Predict which is most likely to be the stronger acid: \(\mathrm{H}_{2} \mathrm{O}\) or \(\mathrm{H}_{2} \mathrm{~S} .\) Explain your reasoning.

One might think that oxygen would make a stronger acid because it is more electronegative than sulfur and can therefore more effectively stabilize the extra negative charge on the OH- ion. So, based purely on electronegativity, one might predict that water is a stronger acid than hydrogen sulfide.

However, there are other factors to consider such as the size of the atom and the distribution of the charge. Sulfur is in the same group as oxygen but in the third period of the periodic table, which means that it is larger than oxygen. Because sulfur's valence electrons are further from the nucleus, they are held less tightly and it is easier for sulfur to accommodate the extra negative charge on the HS- ion. This means that sulfur can stabilize the extra negative charge just as effectively, if not more so, than oxygen. In addition, water molecule, being smaller has less space to distribute the charge resulting in more dense regions of negative charge causing instablity. Thus, based on atom size and charge distribution, one might predict that hydrogen sulfide is a stronger acid than water.

Neither water nor hydrogen sulfide have any resonance structures that would spread the negative charge over more than one atom. Therefore, resonance structures do not affect the acidity in this case.

We concluded that, based purely on electronegativity, water would be a stronger acid but when taking into consideration the larger radius and distribution of the charge, hydrogen sulfide would be the stronger acid. Since the effect of atomic radius on stability of the ion is greater than the effect of electronegativity, the final prediction is that hydrogen sulfide is the stronger acid.