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Chapter 4: Problem 40

What is the strongest base that can exist in liquid ammonia as a solvent?

Short Answer

Expert verified
Answer: The strongest base that can exist in liquid ammonia as a solvent is the amide ion (NH2-).

Step by step solution

01

Understanding liquid ammonia as a solvent

Liquid ammonia (NH3) is a polar solvent, meaning it has a net dipole moment due to the presence of different electronegativities between nitrogen and hydrogen atoms in the molecule. As a solvent, liquid ammonia is capable of solvating different ions and promoting various chemical reactions.
02

Acidity of ammonium ion

When ammonia is used as a solvent, it can act as a weak acid by donating a proton (H+) to form its conjugate acid, the ammonium ion (NH4+), according to the following equation: NH3 (aq) + H2O (l) ⇌ NH4+ (aq) + OH- (aq) The equilibrium constant for this reaction, Ka, is 5.6x10^{-10} at 25°C. This gives the pKa value of 9.3, which implies that the acidity of ammonium ion in water is weak. However, the acidity of ammonium ion in ammonia as a solvent is even weaker.
03

Evaluation of possible bases in ammonia as a solvent

In liquid ammonia as a solvent, weak bases present in the solution will compete with ammonia to donate protons and form the conjugate base. To find the strongest base that can exist in liquid ammonia, we need to look for a base that is less basic than ammonia itself, as ammonia will not allow a more basic species to exist in its presence. This means we need to find bases with a higher pKa value than ammonia's (lower Ka value).
04

Finding the strongest base in liquid ammonia

Since the basicity of a substance in ammonia depends on its pKa value, we need to find a base with a pKa value slightly higher than ammonia. Normally, we would look for a base with a pKa value in the range of 9-10, but still less basic than ammonia. A potential candidate for the strongest base that can exist in liquid ammonia is the amide ion (NH2-). The amide ion has a pKa value of around 9.8, which makes it less basic than ammonia but still a relatively strong base. Therefore, the strongest base that can exist in liquid ammonia as a solvent is the amide ion (NH2-).

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Most popular questions from this chapter

For each conjugate acid-base pair, identify the first species as an acid or base and the second species as its conjugate acid or conjugate base. In addition, draw Lewis structures for each species, showing all valence electrons and any formal charges. (a) \(\mathrm{H}_{2} \mathrm{SO}_{4}, \mathrm{HSO}_{4}{ }^{-}\) (b) \(\mathrm{NH}_{3}, \mathrm{NH}_{2}{ }^{-}\) (c) \(\mathrm{CH}_{3} \mathrm{OH}, \mathrm{CH}_{3} \mathrm{O}^{-}\)

Each molecule or ion can function as a base. Write a structural formula of the conjugate acid formed by reaction of each with \(\mathrm{HCl}\). (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) (b) \(\mathrm{HCH}\) (c) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH}\) (d) \(\mathrm{HCO}_{3}{ }^{-}\)

An ester is a derivative of a carboxylic acid in which the hydrogen of the carboxyl group is replaced by an alkyl group (Section 1.3E). Draw a structural formula of methyl acetate, which is derived from acetic acid by replacement of the \(\mathrm{H}\) of its \(-\mathrm{OH}\) group by a methyl group. Determine if proton transfer to this compound from \(\mathrm{HCl}\) occurs preferentially on the oxygen of the \(\mathrm{C}=\mathrm{O}\) group or the oxygen of the \(\mathrm{OCH}_{3}\) group.

For each conjugate acid-base pair, identify the first species as an acid or base and the second species as its conjugate acid or base. In addition, draw Lewis structures for each species, showing all valence electrons and any formal charge. (a) \(\mathrm{HCOOH} \mathrm{HCOO}^{-}\) (b) \(\mathrm{NH}_{4}^{+} \mathrm{NH}_{3}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-} \quad \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) (d) \(\mathrm{HCO}_{3}{ }^{-} \quad \mathrm{CO}_{3}^{2-}\) (e) \(\mathrm{H}_{2} \mathrm{PO}_{4}^{-} \mathrm{HPO}_{4}{ }^{2-}\) (f) \(\mathrm{CH}_{3} \mathrm{CH}_{3} \mathrm{CH}_{3} \mathrm{CH}_{2}{ }^{-}\) (g) \(\mathrm{CH}_{3} \mathrm{~S}^{-} \quad \mathrm{CH}_{3} \mathrm{SH}\)

Which has the larger numerical value? (a) The \(\mathrm{p} K_{\mathrm{a}}\) of a strong acid or the \(\mathrm{p} K_{\mathrm{a}}\) of a weak acid (b) The \(K_{\mathrm{a}}\) of a strong acid or the \(K_{\mathrm{a}}\) of a weak acid
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