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Ch.17 - Additional Aspects of Aqueous Equilibria
Chapter 17, Problem 120

In nonaqueous solvents, it is possible to react HF to create H2F+. Which of these statements follows from this observation? (a) HF can act like a strong acid in nonaqueous solvents, (b) HF can act like a base in nonaqueous solvents, (c) HF is thermodynamically unstable, (d) There is an acid in the nonaqueous medium that is a stronger acid than HF.

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1
Identify the reaction: HF is reacting to form H2F+. This suggests that HF is donating a proton (H+).
Recognize that when a substance donates a proton, it is acting as an acid according to the Brønsted-Lowry acid-base theory.
Consider the implications of HF acting as an acid in a nonaqueous solvent. This means there must be a base present to accept the proton.
Evaluate the options: (a) suggests HF acts as a strong acid, which aligns with the observation of proton donation. (b) suggests HF acts as a base, which contradicts the proton donation. (c) suggests thermodynamic instability, which is not directly related to the observed reaction. (d) suggests the presence of a stronger acid, which is necessary for HF to donate a proton.
Conclude that the formation of H2F+ indicates HF is acting as an acid, and for it to donate a proton, there must be a stronger acid present in the nonaqueous medium, supporting option (d).

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Acid-Base Theory

Acid-base theory explains the behavior of substances in terms of proton (H+) transfer. In this context, an acid is a substance that donates protons, while a base accepts them. The ability of HF to form H2F+ in nonaqueous solvents suggests that it can act as a strong acid, donating a proton to another species in the solvent.
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Solvent Effects on Acidity

The nature of the solvent significantly influences the acidity of a compound. In nonaqueous solvents, the solvation of ions differs from that in water, which can stabilize or destabilize certain species. This means that HF, typically a weak acid in water, may exhibit stronger acidic behavior in a nonaqueous environment, allowing it to form H2F+.
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Thermodynamic Stability

Thermodynamic stability refers to the relative energy levels of reactants and products in a chemical reaction. If HF can form H2F+, it implies that the resulting ion is more stable than HF in the nonaqueous medium. This observation can lead to conclusions about the relative strengths of acids in that medium, indicating that there may be stronger acids present that can facilitate this reaction.
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