Anhydrous hydrogen fluoride, HF, has a liquid range from 190 to 292.5 K and, sim
ID: 940035 • Letter: A
Question
Anhydrous hydrogen fluoride, HF, has a liquid range from 190 to 292.5 K and, similar to water, it autoionizes with an autoionization constant of 2 ×10–12 .
(a) Write the balanced chemical equilibrium that describes the autoionization of HF(l) using the Brønsted-Lowry perspective. On each side of the chemical equilibrium, identify the (conjugate) acid and the (conjugate) bases.
(b) Write the mass-action expression for the autoionization of HF(l).
(c) Given the autoionization constant, identify the concentration conditions that would make acidic, neutral, and basic solutions of HF(l).
(d) Few protic (Brønsted) acids are able to exhibit acidic behavior in HF(l). Perchloric acid, which behaves as a very strong acid in aqueous solution, exhibits only amphoteric character in HF(l). Define the term amphoteric. Write the two chemical equilibria that demonstrates the amphoteric character of perchloric acid in HF(l).
(e) HF differs from the other acidic halides, HCl, HBr, and HI in two ways: (i) by engaging in hydrogen bonding, so that it has properties related to those of water; and (ii) by forming the weak acid, hydrofluoric acid, HF(aq), in water. By considering bond enthalpies (see Table 8.4), halide sizes, and electronegativities, provide a brief (2-3 sentence) explanation for the fact that HF is a weak acid in water, but that HCl, HBr, and HI are strong acids in water.
Explanation / Answer
(a): 2HF(liq) <-----> H2F+(solv) + F-(aq)
Here H2F acts both as both acid and base.
H2F+ is a conjugate acid to base HF and HF2- is a conjuate base to acid HF.
(b): The mass action expression is
K = [H2F+]*[HF2-] / [HF]2