Given the following standard heats of reactions:
(a) heat of formation of water `= -68.3 kcal`, (b) heat of combustion of `C_(2)H_(2) =- 310.6 kcal`, (c ) heat of combustion of ethylene `=- 337.2 kcal`. Calculate the heat of reaction for the hydrogenation of acetylene at constant volume and at `25^(@)C`.

Explanation

`C_(2)H_(2) +H_(2) to C_(2)H_(4)`
`Delta H^(@) = sum Delta H_("comb")^(@) ("reactants") - sum Delta H_("comb")^(0) ("p r o ducts") = -310.6 - 68.3 - (-337.2) = -41.7 k cal`.