Question 16.20: Arrange the compounds in each series in order of increasing ...
Arrange the compounds in each series in order of increasing acid strength: (a) AsH_{3} , HBr , KH , H_{2}Se ; (b) H_{2}SO_{4} ,H_{2}SeO_{3} , H_{2}SeO_{4}.
Analyze We are asked to arrange two sets of compounds in order from weakest acid to strongest acid. In (a), the substances are binary compounds containing H, and in (b) the substances are oxyacids.
Plan For the binary compounds, we will consider the electronegativities of As, Br, K, and Se relative to the electronegativity of H. The higher the electronegativity of these atoms, the higher the partial positive charge on H and so the more acidic the compound. For the oxyacids, we will consider both the electronegativities of the central atom and the number of oxygen atoms bonded to the central atom.
Solve
(a) Because K is on the left side of the periodic table, it has a very low electronegativity (0.8, from Figure 8.8, p. 383). As a result, the hydrogen in KH carries a negative charge. Thus, KH should be the least acidic (most basic) compound in the series. Arsenic and hydrogen have similar electronegativities, 2.0 and 2.1, respectively. This means that the As—H bond is nonpolar, and so AsH_{3} has little tendency to donate a proton in aqueous solution. The electronegativity of Se is 2.4, and that of Br is 2.8. Consequently, the H—Br bond is more polar than the H—Se bond, giving HBr the greater tendency to donate a proton. (This expectation is confirmed by Figure 16.19, where we see that H_{2}Se is a weak acid and HBr a strong acid.) Thus, the order of increasing acidity is KH < AsH_{3} < H_{2}Se < HBr.
(b) The acids H_{2}SO_{4} and H_{2}SeO_{4} have the same number of O atoms and the same number of OH groups. In such cases, the acid strength increases with increasing electronegativity of the central atom. Because S is slightly more electronegative than Se (2.5 vs 2.4), we predict that H_{2}SO_{4} is more acidic than H_{2}SeO_{4}. For acids with the same central atom, the acidity increases as the number of oxygen atoms bonded to the central atom increases. Thus, H_{2}SeO_{4} should be a stronger acid than H_{2}SeO_{3} We predict the order of increasing acidity to be H_{2}SeO_{3} < H_{2}SeO_{4} < H_{2}SO_{4}.
