| The large bond enthalpy of the H-F bond is compensated for by the large hydration enthalpy of the fluoride ion. There is a very strong attraction between the very small fluoride ion and the water molecules. This releases a lot of heat (the hydration enthalpy) when the fluoride ion becomes wrapped in water molecules. The fact that there isn't much difference between the overall values means that we still aren't looking in the right place for the explanation of why hydrofluoric acid is weak! Looking at other attractions in the system The mistake we are making so far is in starting from the wrong place! The energy terms we have been looking at start from HX as a gas. In fact, we should be thinking of it starting in solution - but not yet reacted with the water. The proper equation we should be working from is:
That then needs to be incorporated into an improved energy cycle:
Unfortunately, at this point I've been unable to find any figures for that first stage. However, in each case, that initial separation of the HX from water molecules will be endothermic. Energy is needed to break the intermolecular attractions between the HX molecules and water. That energy will be much greater in the case of hydrogen fluoride because it forms hydrogen bonds with the water molecules. The other hydrogen halides only form the weaker van der Waals dispersion forces or dipole-dipole attractions. | 
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y bond energy dec from HCl to HI....?PLZ XPLAIN
