Given
2C(s)+O_2(g)->2CO(g)color(red)(......[1]), Delta H_1= -221.0 kJ
2C(s)+O_2(g)+4H_2(g)-> 2CH_3OH(g)color(red)(......[2]), Delta H_2= -402.4 kJ
To calculate DeltaH_3 for
CO(g) + 2H_2(g)->CH_3OH(g)color(red)(......[3]),DeltaH_3=?
1/2(Eq[2]-Eq[1]) gives
DeltaH_3=1/2(DeltaH_2-DeltaH_1)=1/2(-402+221)=-90.5kJ