How Do I Reconcile The Fact That HF Is A Weak Acid In Aqueous Solution, Yet It Exhibits A Relatively High Acidity In Non-aqueous Solvents Like Acetic Acid Or Dimethyl Sulfoxide, And What Role Does The Solvent's Ability To Stabilize The Conjugate Base Play In This Phenomenon?
The acidity of hydrofluoric acid (HF) varies depending on the solvent due to the solvent's ability to stabilize the conjugate base, fluoride ion (F⁻). Here's a concise explanation:
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In Aqueous Solution: HF is a weak acid because water, a polar solvent with a high dielectric constant, effectively stabilizes F⁻ through solvation. This stabilization reduces the need for HF to dissociate extensively, resulting in weak acidity.
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In Non-Aqueous Solvents (e.g., Acetic Acid, DMSO):
- Acetic Acid: As a solvent with a lower dielectric constant than water, it stabilizes ions less effectively. This reduced stabilization of F⁻ encourages HF to dissociate more, increasing its acidity.
- DMSO: Although DMSO is polar and can stabilize ions, its lower dielectric constant compared to water means F⁻ is less stabilized. Consequently, HF dissociates more, exhibiting stronger acidity than in water.
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Role of Solvent: The solvent's ability to stabilize the conjugate base (F⁻) is crucial. Solvents that stabilize F⁻ less than water force HF to dissociate more, enhancing its acidity. Thus, in solvents where F⁻ is less stabilized, HF acts as a stronger acid.
In summary, HF's acidity increases in solvents that poorly stabilize F⁻, as the acid dissociates more to relieve the instability of the undissociated form.