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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) 鈬� NH4+ (aq) + OH- (aq) The equilibrium constant for this reaction, Ka, is 5.6x10^{-10} at 25掳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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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Polar Solvent
Liquid ammonia is a classic example of a polar solvent. Its polar nature arises because the nitrogen and hydrogen atoms in ammonia have different electronegativities. This means that the nitrogen atom, being more electronegative, pulls the electrons closer to itself, creating an unequal distribution of charge. As a result, each ammonia molecule has a dipole moment, making it capable of engaging in various interactions with other ions or molecules.
  • A polar solvent like ammonia is excellent at dissolving ionic compounds.
  • This property allows it to promote chemical reactions by providing a medium in which ions can move and react.
Understanding the polar character of liquid ammonia helps explain why it is used in many chemical reactions and processes.
Ammonium Ion
When discussing ammonia as a solvent, one must consider the ammonium ion. When ammonia accepts a proton, it transforms into the ammonium ion, NH4^+. This is depicted in the reversible reaction:
  • NH3 + H2O 鈬� NH4^+ + OH^鈭�
This reaction shows how ammonia acts as a weak base by accepting a proton. However, this behavior is greatly influenced by the surrounding medium. In liquid ammonia, the ionization behavior slightly differs from that in water due to ammonia's unique properties as a solvent. The ammonium ion's presence in such a system can affect the acidity and basicity dynamics of the solution, which is crucial for determining the type of reactions that can occur.
Basicity in Chemistry
Basicity in chemistry refers to the ability of a substance to accept protons. In the context of chemical solutions, basicity is represented by the pKa value. The pKa value provides a numeric scale that helps determine a substance's strength as a base or acid. Lower pKa values indicate stronger acids, while higher values indicate weaker acids or stronger bases.
  • In liquid ammonia, understanding basicity is key to determining which species can exist stably in the solution.
  • Bases that are weaker than ammonia itself can exist in its presence.
Knowing the basicity is important, particularly in liquid ammonia solutions, because it tells us about the possible chemical equilibrium and reactions, including which bases will dominate the behavior of the solution.
Amide Ion
Among the various basics ions that can form in liquid ammonia, the amide ion (NH2^鈭�) stands out. It acts as the strongest base that can exist in liquid ammonia without being overwhelmed by ammonia itself. This is because the amide ion has a pKa value of approximately 9.8, which positions it between stronger bases and weaker substances that ammonia typically forms.
  • Its slightly higher pKa value than ammonia but still being less basic allows it to exist stably in liquid ammonia.
  • The amide ion's presence in ammonia-based solutions plays a crucial role in particular synthesis and reaction mechanisms.
Comprehending the role of the amide ion in such scenarios is essential for using liquid ammonia as a solvent in both industrial and laboratory chemical processes.

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Most popular questions from this chapter

For each pair of molecules or ions, select the stronger base, and write its Lewis structure. (a) \(\mathrm{CH}_{3} \mathrm{~S}^{-}\)or \(\mathrm{CH}_{3} \mathrm{O}^{-}\) (b) \(\mathrm{CH}_{3} \mathrm{NH}^{-}\)or \(\mathrm{CH}_{3} \mathrm{O}^{-}\) (c) \(\mathrm{CH}_{3} \mathrm{COO}^{-}\)or \(\mathrm{OH}^{-}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}^{-}\)or \(\mathrm{H}^{-}\) (e) \(\mathrm{NH}_{3}\) or \(\mathrm{OH}^{-}\) (f) \(\mathrm{NH}_{3}\) or \(\mathrm{H}_{2} \mathrm{O}\) (g) \(\mathrm{CH}_{3} \mathrm{COO}^{-}\)or \(\mathrm{HCO}_{3}^{-}\) (h) \(\mathrm{HSO}_{4}^{-}\)or \(\mathrm{OH}^{-}\) (i) \(\mathrm{OH}^{-}\)or \(\mathrm{Br}^{-}\)

In acetic acid, \(\mathrm{CH}_{3} \mathrm{COOH}\), the \(\mathrm{OH}\) hydrogen is more acidic than the \(\mathrm{CH}_{3}\) hydrogens. Explain.

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) \(\mathrm{CH}_{3} \mathrm{OH}, \mathrm{CH}_{3} \mathrm{O}^{-}\)

If the \(\Delta G^{\circ}\) for a reaction is \(-4.5 \mathrm{kcal} / \mathrm{mol}\) at \(298 \mathrm{~K}\), what is the \(K_{\text {eq }}\) for this reaction? What is the change in entropy of this reaction if \(\Delta H^{\circ}=-3.2 \mathrm{kcal} / \mathrm{mol}\) ?

Write an equation to show the proton transfer between each alkene or cycloalkene and HCl. Where two carbocations are possible, show each. (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}=\mathrm{CHCH}_{3}\) (b) C1=CCCCC1 2-Pentene Cyclohexene
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