Question

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

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

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: \(\mathrm{SeH}^{-} < \mathrm{SH}^{-} < \mathrm{OH}^{-}\).

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: \(\mathrm{NH}_{3} < \mathrm{PH}_{3}\).

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: \(\mathrm{NH}_{3} < \mathrm{HONH}_{2}\).

Key concepts

Conjugate Acids

A conjugate acid is what remains after a base gains a proton. They are integral to understanding base strength. This concept helps us predict which bases are stronger or weaker.
Conjugate acids relate to their respective bases. A strong acid forms a weak conjugate base, while a weak acid forms a strong conjugate base.

• For example, the conjugate acid of \(\mathrm{OH}^{-}\) is water \(\mathrm{H}_2\mathrm{O}\), which is a weak acid, thereby making \(\mathrm{OH}^{-}\) a strong base.
• On the other hand, the conjugate acid of \(\mathrm{SeH}^{-}\) is \(\mathrm{H}_{2}\mathrm{Se}\), a stronger acid compared to water, hence \(\mathrm{SeH}^{-}\) is a weaker base.

Understanding the relationship between conjugate acids and bases allows chemists to organize species by their base strength by predicting how freely an acid gives off its proton.

Bond Strengths

In chemistry, bond strengths indicate how strongly atoms in a molecule hold onto each other. This directly impacts the acid strength and, by extension, the base strength.
Stronger bonds are harder to break, making their corresponding acids weaker. This is because stronger bonds don't easily give up a proton.

• The O-H bond in water is stronger than the S-H bond in \(\mathrm{H}_2\mathrm{S}\), meaning \(\mathrm{H}_2\mathrm{O}\) is a weaker acid than \(\mathrm{H}_2\mathrm{S}\).
• Thus, \(\mathrm{OH}^{-}\) is stronger as a base than \(\mathrm{SH}^{-}\).

Examining bond strengths is a way of sorting how effective a substance is as a base or an acid. It tells us about how easily these substances can donate or accept protons.

Electronegativity

Electronegativity is the tendency of an atom to attract electrons towards itself. This property affects the stability of molecules and ions, thereby influencing their basicity.
An atom with a higher electronegativity can stabilize a negative charge more effectively than one with lower electronegativity.

• Nitrogen, being more electronegative than phosphorus, can stabilize a negative charge better in \(\mathrm{NH}_2^{-}\) than phosphorus can in \(\mathrm{PH}_2^{-}\).
• Therefore, \(\mathrm{NH}_3\) is a weaker base compared to \(\mathrm{PH}_3\).

Electronegativity helps determine the stability of conjugate bases, making it a crucial factor when assessing base strength.

Resonance Stabilization

Resonance stabilization occurs when a positive or negative charge can be delocalized over multiple atoms, which increases the stability of molecules or ions.
This effect is significant when comparing the basicity of different molecules.

• In \(\mathrm{HONH}_2\), when it accepts a proton, the resulting positive charge can be spread between oxygen and nitrogen, creating resonance stabilization.
• This delocalization in \(\mathrm{H}_2\mathrm{ONH}_3^{+}\) makes it more stable than \(\mathrm{NH}_4^{+}\), in which the charge is localized on the nitrogen.

Resonance stabilization helps to decrease the energy of a compound, making the compound or ion featuring this phenomenon less reactive, hence affecting its ability to act as a base. By reviewing resonance, one can predict the strength of basicity for different molecules.