[krackers]

The bonds between H and Cl are strong, almost close to ionic, right? Thus if that is the case why does HCl readily ionize into H+ and Cl- upon contact with water? It doesn't make sense that such a strongly bonded compound should completely disassociate upon contact with water. Further, why does HF, another strongly bonded compound, not do the same thus making it a weak acid?

On a similar note, why does NaCl do the same? Na and Cl are very strongly ionically bonded, requiring lots of energy to break apart the bond. Yet on contact with water, they easily disassociate. Interactions between the Na, Cl, and the H2O dipole don't seem like they have enough energy to break it apart.

[Kate]

The bond between H and Cl is a mix of ionic and covalent, though I think the ionic is the prevalent one here. The covalent bond isn't strong, because there is poor orbital overlap (Cl is a big atom and H is the smallest). Besides Cl can easily accommodate a negative charge and is more electronegative than H. So all this makes the HCl bond rather weak. HF has a stronger bond, even though F is more electronegative than Cl, because there's better orbital overlap between H and F.

The energy that always contributes to the dissolution of any compound is the increase in entropy of solution. The spontaneity of dissolution (whether a compound is soluble or not) comes from the gibbs energy: a balance of enthalpy, entropy and temperature.