[MODI]

7.   The protolysis (or neutralization) of hydronium ion by hydroxide ion yields water, as shown below, and has an enthalpy of -55.7 kJ/mol.

H3O+ + OH- → 2H2O

Given that the neutralization of HCN by NaOH may be viewed as the sum of the protolysis of HCN and neutralization of H3O+ with OH-, calculate the enthalpy for deprotonation of HCN in water under standard state conditions.  The enthalpy for the overall reaction is -12.1 kJ/mol.

HCN + H₂O → H₃O⁺ + CN^-   deprotonation (or protolysis) of HCN

Is the protolysis of HCN endothermic or exothermic?

[nj_bartel]

What's your enthalpy?  Are negative enthalpies endothermic or exothermic?

[Yggdrasil]

First write out the formula for the neutralization of HCN by NaOH.  Then add/subtract the equation for the neutralization of hydronium ion and the protolysis of HCN to get the equation for the deprotonation of HCN.  This procedure should tell you how to add/subtract the entalpies of the two reactions to figure out the enthalpy for the deprotonation of HCN.