How does a covalent molecule like HCl dissociate to form H+ and Cl- ions in Water ?

Question

HCl is polar covalent molecule, but why does it form ions in water? What causes it to happen? Ionic compounds that can dissociate are because of formation of ion-dipole bonds that produces energy greater than that needed to break the ionic lattice. HCl is covalent, but it dissociates because of polar nature of water? If so what is the exact interaction between molecules that casues it to happen?

Answer 1

Polar covalent molecules are the trickiest ones of all. Polar covalent molecules can behave like ionic molecules in a polar solvent. The HCl bond is very polar, which means that there is a large partial positive charge on the H and a large partial negative charge on the Cl. it is "almost" an ionic bond. You put a polar covalent molecule that is almost ionic already in a polar solvent and the water pops off the H from the HCl and you have ions. In a non-polar solvent - such as benzene, the benzene cannot pull the H and Cl away from one another (no affinity for the H or Cl because polar and non-polar species generally don't like one another much) so no ions would form!

Dipole-dipole interactions make this possible

The same things is seen with sulfuric acid. It forms H+ and SO4^-2 ions in solution - so it is also polar covalent but behaves like an ionic species in a polar solvent (like water)

Answer 2

HCl is a polar molecule. This means that the electrons in the adjoining orbital are not being shared equally. In this case the shared electrons spend more time closer to the very electronegative (ability to attract electrons) chlorine atom. This creates a slight negative charge on the chlorine atom and a slight positive charge on the hydrogen. Water has a similar situation where the oxygen is slightly negative and the hydrogen atoms are again slightly positive. However, oxygen is not as electronegative as chlorine so the shared electrons in the water molecule are more evenly shared. When HCl salt is placed in water, the slightly negative oxygen atoms form hydrogen bonds with the positive hydrogen atoms of the HCl, and the slightly positive hydrogen atoms of the water form hydrogen bonds to the partial negative chlorine atoms. More than one of these hydrogen bonds can form on a single molecule and together they will overpower the HCl covalent bond. (If you need an even more detailed explanation with math and numbers let me know, but I will have to go dig up some notes)

Answer 3

The force between two ions is inversely dependent on a parameter called the dielectric constant. In air ,this constant is very near 1. In water it is 80. So , the force between H+ and Cl-is approximately 80 times weaker in water than in air;
For this reason the molecule HCl dissociates