In dilute aqueous solution HF acts as a weak acid. However, pure liquid HF (boiling point =19.5^

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In dilute aqueous solution HF acts as a weak acid. However,...

In dilute aqueous solution $\mathrm{HF}$ acts as a weak acid. However, pure liquid HF (boiling point $=19.5^{\circ} \mathrm{C} )$ is a strong acid. In liquid HF, HNO_ acts like and accepts protons. The acidity of liquid HF can be increased by adding one of several inorganic fluorides that are Lewis acids and accept $\mathrm{F}^{-}$ ion (for example, $\mathrm{BF}_{3}$ or $\mathrm{SbF}_{5} ) .$ Write balanced chemical equations for the reaction of pure $\mathrm{HNO}_{3}$ with pure $\mathrm{HF}$ and of pure $\mathrm{HF}$ with $\mathrm{BF}_{3}$

Key Concepts

Solvent Effects on Acid Strength

Acid strength is not an inherent property of a molecule alone but can vary dramatically with the medium. For instance, a substance like HF behaves as a weak acid in water due to strong solvation of ions, yet in its pure liquid form its self-ionization leads to a very high concentration of reactive proton species, making it act as a strong acid. This concept emphasizes how the surrounding environment can alter acid–base behavior.

Proton Transfer Reactions in Superacid Systems

Superacid systems involve proton transfer reactions that go beyond standard Brønsted acid–base reactions. In such systems, even molecules normally considered strong acids in aqueous solution can reverse roles, with one species donating a proton to another that would typically be acidic in water. This leads to the formation of unique cationic species (such as H2X+, where X is the original acid) and accompanying anions, an equilibrium that is central to understanding the acid chemistry in media like pure HF.

Lewis Acid Behavior and Complex Formation

Lewis acids are species that accept electron pairs; in the context of superacid chemistry, certain inorganic fluorides (e.g., BF3) act as Lewis acids by accepting a fluoride ion. This fluoride ion acceptance shifts equilibria by removing F– from the system, thereby effectively increasing the acidity. The formation of complex ions such as BF4– through the reaction between BF3 and fluoride exemplifies how Lewis acid–base interactions can be used to manipulate acid strength in nonaqueous solvents.

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