Thursday, September 27, 2007

Chemistry Question - Bonding

Q) Since HCL has strong covalent bonds, why is it that they dissociate easily into H+ ions and Cl- ions when put in water?


In GCE O Level education, this question is usually not posed. In Junior Collleges, detailed explaination will be given in general chemistry classes.

Suggested Answer:

The hydrogen chloride molecule HCl is a simple diatomic molecule consisting of a hydrogen atom H and a chlorine atom Cl connected with a covalent single bond. Since the chlorine atom is much more electronegative than the hydrogen atom, the covalent bond between the atoms is quite polar. Consequently the molecule has a large dipole moment with a negative partial charge δ- at the chlorine atom and a positive partial charge δ+ at the hydrogen atom. In part due to its high polarity, HCl is very soluble in water (and in other polar solvents).
Upon contact with water, it immediately ionizes, forming hydronium cations H3O+ and chloride anions Cl- through a reversible chemical reaction with the water:
HCl + H2O → H3O+ + Cl−
The resulting solution is called hydrochloric acid and is a strong acid.