If you are working in a research laboratory preparing a series of compounds, you might be faced with having to make a choice between acids based on their strengths. Can you decide which to use based only on their formulas? Predict from their molecular structures which acid in each of the following pairs is the stronger:

(a) H₂S and H₂Se;

(b) H₂SO₄ and H₂SO₃;

(c) H₂SO₄ and H₃PO₄.

PLAN Identify the relevant trends in the summary in TABLE 6C.7.

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TABLE 6C.7 Correlations of Molecular Structure and Acid Strength* Acid type Trend binary The more polar the H-A bond, the stronger the acid. oxoacid carboxylic This effect is dominant for acids of the same period. The weaker the H-A bond, the stronger the acid. This effect is dominant for acids of the same group. The greater the number of O atoms attached to the central atom (the greater the oxidation number of the central atom), the stronger the acid. For the same number of O atoms attached to the central atom, then the greater the electronegativity of the central atom, the stronger the acid. The greater the electronegativities of the groups attached to the carboxyl group, the stronger the acid.