Question

Using your results from Exercise \(133,\) place the species in each of the following groups in order of increasing base strength. a. \(\mathrm{OH}^{-}, \mathrm{SH}^{-}, \mathrm{SeH}^{-}\) b. \(\mathrm{NH}_{3}, \mathrm{PH}_{3}\) c. \(\mathrm{NH}_{3}, \mathrm{HONH}_{2}\)

Short answer

The order of increasing base strength for the given species is as follows: a. \(\mathrm{OH}^{-} < \mathrm{SH}^{-} < \mathrm{SeH}^{-}\) b. \(\mathrm{NH}_{3} < \mathrm{PH}_{3}\) c. \(\mathrm{HONH}_{2} < \mathrm{NH}_{3}\)

Step-by-step solution

Analyze the conjugate acids

Analyze the conjugate acids of \(\mathrm{OH}^{-}, \mathrm{SH}^{-}, \mathrm{SeH}^{-}\), which are \(\mathrm{H}_{2}\mathrm{O}\), \(\mathrm{H}_{2}\mathrm{S}\), and \(\mathrm{H}_{2}\mathrm{Se}\), respectively. The stability of these acids can be examined by looking at the bond strength and electronegativity of the atoms involved.

Compare bond strength

Compare the bond strength between hydrogen and each of the heavier elements, oxygen, sulfur, and selenium. The bond strength decreases down the periodic table, so the O-H bond is stronger than the S-H bond, which in turn is stronger than the Se-H bond.

Compare electronegativity

Compare the electronegativity of the heavier elements, oxygen, sulfur, and selenium. Electronegativity also decreases down the periodic table, so oxygen is more electronegative than sulfur, which in turn is more electronegative than selenium.

Combine the observations

Combining the observations from Steps 2 and 3, we can infer that the conjugate acid of \(\mathrm{OH}^{-}\) is the most stable, followed by the conjugate acid of \(\mathrm{SH}^{-}\), and then the conjugate acid of \(\mathrm{SeH}^{-}\). Since the stability of the conjugate acid is inversely correlated to the base strength, the order of increasing base strength should be: \(\mathrm{OH}^{-} < \mathrm{SH}^{-} < \mathrm{SeH}^{-}\) b. Arranging \(\mathrm{NH}_{3}, \mathrm{PH}_{3}\) in order of increasing base strength

Analyze the conjugate acids

Analyze the conjugate acids of \(\mathrm{NH}_{3}\) and \(\mathrm{PH}_{3}\), which are \(\mathrm{NH}_{4}^{+}\) and \(\mathrm{PH}_{4}^{+}\), respectively. As in the previous case, we can compare the stability of these acids by looking at the bond strength and electronegativity of the elements involved.

Compare bond strength and electronegativity

Comparing the bond strength and electronegativity between nitrogen and phosphorus, we observe that the N-H bond is stronger and nitrogen is more electronegative than phosphorus.

Combine the observations

Since the bond strength of N-H is stronger and nitrogen is more electronegative, we can infer that the conjugate acid of \(\mathrm{NH}_{3}\) is more stable than the conjugate acid of \(\mathrm{PH}_{3}\). Therefore, the order of increasing base strength should be \(\mathrm{NH}_{3} < \mathrm{PH}_{3}\). c. Arranging \(\mathrm{NH}_{3}, \mathrm{HONH}_{2}\) in order of increasing base strength

Analyze the conjugate acids

Analyze the conjugate acids of \(\mathrm{NH}_{3}\) and \(\mathrm{HONH}_{2}\), which are \(\mathrm{NH}_{4}^{+}\) and \(\mathrm{H}_{2}\mathrm{ONH}_{3}^{+}\), respectively. Comparing the stability of these acids can be done by examining the electron withdrawing properties.

Compare electron withdrawing properties

The oxygen atom in \(\mathrm{H}_{2}\mathrm{ONH}_{3}^{+}\) can stabilize the positive charge on the nitrogen atom through resonance. This electron withdrawing effect makes the conjugate acid of \(\mathrm{HONH}_{2}\) more stable than the conjugate acid of \(\mathrm{NH}_{3}\).

Combine the observations

Since the conjugate acid of \(\mathrm{HONH}_{2}\) is more stable, the order of increasing base strength should be \(\mathrm{HONH}_{2} < \mathrm{NH}_{3}\).