which of the following does NOT exist as an equilibrium? HF + water or HCl + water?

Question

I don't see how either of them differ being that hydrofluoric acid and hydrochloric acid both contain an H+ and an F-, Cl- respectively. However, the answer that I'm reading is that HCl does not exist as an equilibrium. Little help

Answer 1

The reason HCl does not exist as an equilibrium is that it is one of eight strong acids. This means that most of the HCl molecules break apart, or dissociate, in water. HF on the other hand is not a strong acid. This means that in water, only a small portion of the HF molecules break apart in the ions, H+ and F-. This happens because the HF bond is stronger than the HCl bond (F has a smaller atomic radius than Cl). Because the bond in HF is strong, as soon as some molecules break apart into H+ and F-, others reform into the HF molecule. Because HF exists as both molecules and ions in a solution at the same time, it is said to be in equilibrium, contrary to the HCl which exists almost exclusively as ions in solution.

For future reference, the eight "common" strong acids are:

HCl - hydrochloric acid
HBr - hydrobromic acid
HI - hydroiodic acid
HClO4 - perchloric acid
HClO3 - chloric acid
HNO3 - nitric acid
HIO4 - periodic acid
H2SO4 - sulfuric acid

These all almost completely ionize in water. All other acids can be considered weak acids which will set up equilibria in solution.

Answer 2

Not all ions are created equal, especially those in the first full octect of elements. Although HF is an ACTIVE chemical, as it is used to etch glass, that property is due to its power to oxidize things, which is NOT the major attribute of an acid in water.

Both acids do have equilibrium Ka, but the one for HCl is so high that for all intent purposes, it exists as ions in water.

Answer 3

This is because HCl is a strong acid and dissociates completely in water and therefore has no equilibrium state.

HCl + H20 ---> H+ and OH- and Cl-

HF is considered a weak acid when dilute and does not dissociate completely.

HF + H2O <--->H3O+ and F-