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Chapter 17: Problem 22

The deciding factor on why HF is a weak acid and not a strong acid like the other hydrogen halides is entropy. What occurs when HF dissociates in water as compared to the other hydrogen halides?

Short Answer

Expert verified
In summary, HF is a weak acid because its strong H-F bond and high electronegativity of fluorine make dissociation into ions difficult, limiting the increase in entropy that would favor dissociation. In contrast, other hydrogen halides have weaker H-X bonds and larger halide ions, making their dissociation more favorable in terms of increasing entropy, leading to higher acid strengths.

Step by step solution

01

Understand the terms weak and strong acids

First, let's clarify the meaning of weak and strong acids. A strong acid is one that completely ionizes in a solution, donating all of its protons (H+) to the solvent. In contrast, a weak acid only partially ionizes, donating only a small fraction of its protons. This means that strong acids have a higher concentration of H+ ions in the solution, leading to a lower pH.
02

Write the dissociation reactions for HF and other hydrogen halides

The general dissociation reaction for a hydrogen halide (HX) can be written as: \[ \mathrm{HX}\, (aq) \rightleftharpoons \mathrm{H^+} \, (aq) + \mathrm{X^-} \, (aq) \] Now, let's write the dissociation reactions for HF and another hydrogen halide (HCl, for example) in water: \[ \mathrm{HF}\, (aq) \rightleftharpoons \mathrm{H^+} \, (aq) + \mathrm{F^-} \, (aq) \] \[ \mathrm{HCl}\, (aq) \rightleftharpoons \mathrm{H^+} \, (aq) + \mathrm{Cl^-} \, (aq) \]
03

Analyze the bonds formed in the dissociation

When HF dissociates in water, it forms a hydrogen ion (H+) and a fluoride ion (F-). However, the H-F bond is highly polar, as fluorine is highly electronegative. The H-F bond is also relatively strong due to the small size of F-, compared to Cl- and other halide ions. Because of this strong bond, when the HF molecule splits into ions, the fluoride ion strongly attracts the proton, so less dissociation occurs in HF than in the other hydrogen halides. It makes HF a weak acid, despite its high polarity and ability to donate protons.
04

Understand the role of entropy

Entropy is a measure of the degree of disorder or randomness in a system. The dissociation of an acid into its ions increases the entropy of the system. However, the strong H-F bond makes it more difficult for the system to reach the higher entropy state associated with the ionized form. Comparing HF to other hydrogen halides, they have weaker H-X bonds and larger halide ions, making their dissociation more favorable in terms of increasing the entropy of the system.
05

Conclusion

In conclusion, HF is a weak acid compared to other hydrogen halides due to its strong polar H-F bond and high electronegativity of fluorine. These factors make it difficult for the HF molecule to fully dissociate into ions, limiting the increase in entropy that would be more favorable for dissociation. This is in contrast with other hydrogen halides, whose H-X bonds are weaker and more easily dissociated, leading to higher acid strengths.

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