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Chapter 14: Problem 130

Using your results from Exercise 129, 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

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

Step by step solution

01

a. Ordering OH-, SH-, SeH- in increasing base strength

The order of increasing base strength for \(\mathrm{OH}^{-}, \mathrm{SH}^{-}, \mathrm{SeH}^{-}\) can be determined by comparing the stability of their conjugate acids. The conjugate acids are \(\mathrm{H}_{2}\mathrm{O}, \mathrm{H}_{2}\mathrm{S}, \mathrm{H}_{2}\mathrm{Se}\). The stronger the acid, the weaker its conjugate base. Comparing the bond strengths of these acids, we find that the O-H bond is stronger than the S-H bond, which is stronger than the Se-H bond. So, \(\mathrm{H}_{2}\mathrm{O}\) is the strongest acid, followed by \(\mathrm{H}_{2}\mathrm{S}\), and then \(\mathrm{H}_{2}\mathrm{Se}\). Therefore, the base strength order is: \(\mathrm{SeH}^{-} < \mathrm{SH}^{-} < \mathrm{OH}^{-}\).
02

b. Ordering NH3, PH3 in increasing base strength

For \(\mathrm{NH}_{3}, \mathrm{PH}_{3}\), we can compare the stability of their conjugate bases. The conjugate bases are \(\mathrm{NH}_2^-, \mathrm{PH}_2^-\). Nitrogen is more electronegative than phosphorus, which means that nitrogen is better able to stabilize a negative charge. Thus, the conjugate base of \(\mathrm{NH}_{3}\) is more stable than that of \(\mathrm{PH}_{3}\), making \(\mathrm{NH}_{3}\) a weaker base compared to \(\mathrm{PH}_{3}\). Therefore, the base strength order is: \(\mathrm{NH}_{3} < \mathrm{PH}_{3}\).
03

c. Ordering NH3, HONH2 in increasing base strength

For \(\mathrm{NH}_{3}, \mathrm{HONH}_{2}\), we can compare their ability to accept a proton. When \(\mathrm{HONH}_{2}\) gains a proton, it forms \(\mathrm{H}_{2}\mathrm{ONH}_{3}^{+}\) in which the positive charge is shared between oxygen and nitrogen, due to resonance. So, the positive charge is distributed over a larger volume in \(\mathrm{H}_{2}\mathrm{ONH}_{3}^{+}\) than in \(\mathrm{NH}_{4}^{+}\). This resonance stabilization makes \(\mathrm{H_{2}ONH_3^{+}}\) more stable than \(\mathrm{NH_4^{+}}\). Therefore, the base strength order is: \(\mathrm{NH}_{3} < \mathrm{HONH}_{2}\).

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Most popular questions from this chapter

Calculate the \(\mathrm{pH}\) of an aqueous solution containing \(1.0 \times 10^{-2} M\) \(\mathrm{HCl}, 1.0 \times 10^{-2} \mathrm{M} \mathrm{H}_{2} \mathrm{SO}_{4}\), and \(1.0 \times 10^{-2} \mathrm{M} \mathrm{HCN}\).

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