Question

Heat of formation of acetylene is xj/mole and that of ethylene is yj/mole what is heat of hydrogenation

Answer 1

The heat of hydrogenation is the enthalpy of hydrogenation of a saturated hydrocarbon.

Here the hydrogenation of ethylene is being carried out i.e.

C2H2 + H2 → C2H4

We have

heat of formation of acetylene

2C(s) + H2 (g) → C2H2         x kJ/mol

heat of formation of ethylene

2C(s) + 2 H2 → C2H4           y kJ/mol

According to Hess law, regardless of single step or multiples step reaction the total change in enthalpy would be the sum of the all change

heat of hydrogenation can be found out by  subtracting y-x.

C2H2 → 2C (s) + H2 (g)

2C(s) + 2 H2 → C2H4

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C2H2 + H2 → C2H4       (y-x) kJ/mol

Thus heat of hydrogenation is (y-x) kJ/mol