Whats the enthalpy of formation of C2H2?

Answer 1

Enthalpy change of formation of 1 mole of C2H2 is the energy change when 1 mole of it is formed from its elements (2C(s) and H2(g)) in their standard states.

2C(s) + H2(g) ---> C2H2(g)

enthalpy of formation of C2H2=
energy absorbed to break H-H bond
+
energy released to form 1 C=C bond and 4 C-H bonds
= 610 + 4(-410)
= -1030 KJ/mol
(If you are taking A level Chemistry, refer to your data booklet for these values under the section of bond energies)

Hope this helps.