Question

Consider this combination reaction:

2Mg(s)+O2(g)→2MgO(s) ΔH=−1204 kJ
What is the enthalpy for the decomposition of 1 mole of MgO(s) into Mg(s) and O2(g)?

Answer 1

Since the decomposition of MgO is the reverse of the reaction you have given us. The sign will be reversed:

2MgO -> 2Mg + O2 = 1204 kJ

Divide by two:

MgO -> Mg + 1/2 O2 =

602 kJ