hi!  this is my first time to be here.  

would anyone be kind enough to explain to me why a weak acid is usually less negative than for strong acids and bases?  My answer is: because weak acids are not completely ionized in the solution, energy has to be used to break down the bond between the H+ ions and A- ions.  Hence, the amount of energy released in the reaction, i.e. the heats of neutralization, becomes smaller.  Am I getting into the wrong way?  

Besides, I don't understand why experimental results for hydrochloric acid are usually a little less negative than -57.6 kJmol-1, while the others are not.  (That's what my teacher told me, without explanation.)   Should they be the same?

Please *delete me*  Thank you!

the magnitude for enthalpy of neutralisation for a weak acid seem to be less because less H+ are available per mole of acid.

the enthalpy change for H+ + OH- -> H2O is always the same value at the same reaction conditions, eg. 1atm, 30C. However, you have to use more moles of weak acid to achieve the same extent of neutralisation compared to using a strong acid, assuming both acids are of the the same basicity. Hence, for a weak acid, the enthalpy change per mole of weak acid is less because you used more weak acid than strong acid to achieve the same extent of neutralisation.

not exactly.

1mol of H+ is available for 1mol of HCl solution.

however, 1mol of H+ is not necessary available from 1mole of acetic acid solution. This is because of partial ionisation. Unless the neutralisation reaction proceed to completion (which normally don't happen), only a fraction of the 1mole of acetic acid ionises to produce H+ for neutralisation.