# Why does HCl act as a weak base in HF solvent, even though it is a stronger acid compared to HF?

I've read that $$\ce{HCl}$$ acts as a weak base in $$\ce{HF}$$ solvent, which means $$\ce{HCl}$$ is supposed to lose $$\ce{H+}$$ ion to $$\ce{HF}$$. $$\ce{HF}$$ is a weak acid compared to $$\ce{HCl}$$, then shouldn't $$\ce{HF}$$ be accepting the proton and HCl be losing it. Why doesn't this happen?

• Jun 27 at 6:18
• Jun 27 at 19:20

As Poutnik hints in his reference, it is not accurate to call $$\ce{HF}$$ a weak acid in water. What appears in dilute aqueous solution as a weak acid is the hydrogen-bonded ion pair $$\ce{H3O^+F^-}$$ that forms when $$\ce{HF}$$ reacts with water. If this ion pair is placed in concentrated $$\ce{HF}$$, it will act as a strong base, forming $$\ce{H3O^+}$$ and $$\ce{FHF^-}$$ where the latter is the characteristics basic species in $$\ce{HF}$$. Thus $$\ce{HCl}$$ is a stronger acid than the aqueous $$\ce{H3O^+F^-}$$ solute in water, and $$\ce{HCl}$$ is also less basic than $$\ce{H3O^+F^-}$$ in $$\ce{HF}$$.
The Hammett acidity function of neat $$\ce{HF}$$, unsullied by water, is close to that of 100% $$\ce{H2SO4}$$. So, anything with appreciable acidity in $$\ce{HF}$$ would cross over into superacidity. $$\ce{HCl}$$ does not make it.