Whats the literature value for enthalpy of neutralization weak acid and weak base?

i know its less that -57kJ... but what is it?|||It depends on the weak acid and weak base. Usually, the stronger each is, the more negative the enthalpy of neutralization will be (and the closer to the strong acid-base value of around -58 kJ/mol, or -57 as you stated).





The enthalpy of neutralization value is the enthalpy change for:


H+(aq) + OH-(aq) -----%26gt; H2O(l)


For a weak acid/base HA and B, the enthalpy of neutralization may be written as the sum of three reaction steps:


1. HA(aq) -----%26gt; H+(aq) + A-(aq)


2. B(aq) + H2O(l) -----%26gt; BH+(aq) + OH-(aq)


3. H+(aq) + OH-(aq) -----%26gt; H2O(l)


gives


HA(aq) + B(aq) -----%26gt; A-(aq) + HB+(aq)


Reactions 1 and 2 usually have an unfavorable (positive) enthalpy change. * By Hess's Law, the enthalpy change for the sum of the three reactions is the sum of the enthalpy changes of the three individual reactions, so the result is going to be greater than the -58 kJ/mol value for reaction 3 - that is, not as exothermic. To find the actual value, you will have to obtain delta-H values for reactions 1 and 2, which vary depending on the identities of HA and B.





* Reactions 1 and 2 will have unfavorable free energy changes (delta-G). In acid-base reactions this usually means that the delta-H value is unfavorable, but not always: HF, for example, has a favorable enthalpy change but an unfavorable free energy change, since the entropy change is highly negative.