Exploring the Molecular and Ionic Nature of HCl and Other Acids

[barryj]

I have thought the HCl is an ionic compound since it disassociates in water. However I read in my chemistry book that HCl is molecular. So what is it? Are all acids molecular or just the ones like HF, HCl, HBr, etc?

 

[Bystander]

In solution, they're dissociated to whatever degree. As solids, you're looking at a very small cation which can polarize charge on the anions and lead to that "gray" area between ionic and covalent bonds.

 

[Quantum Defect]

I have thought the HCl is an ionic compound since it disassociates in water. However I read in my chemistry book that HCl is molecular. So what is it? Are all acids molecular or just the ones like HF, HCl, HBr, etc?

HCl is a stable molecule that can exist in the gas phase. As an isolated molecule, it has a permanent dipole moment, so the sharing of electrons in this molecule is not equal -- chemists would say that it shows partial ionic character. Interestingly, as you increase the bond distance of the molecule in the gas phase, it will dissociate into neutral hydrogen and neutral chlorine atoms (lowest energy path) -- i.e. the percent ionic character will decrease with increasing bond length. You can see the same behavior in a gas-phase NaCl molecule, as well. The diatomic molecule shows partial ionic character, but will dissociate to neutral atoms. You can read about some interesting chemical physics about the reverse process if you look up "harpoon mechanism".

If you increase the bond distance in aqueous solution, you will get ions (H+ and Cl-). The reason for this difference is that the ionic pathway is lower energy in solution because the enthalpy of solvation of the ions is very exothermic.

Most Bronsted-Lowry acids would behave the same way. I.e. differing gas-phase and solution-phase behavior, due to the large negative enthalpy of solvation of the ions.