Question

How do you use Hess's Law to calculate the enthalpy change for the reaction?

`WO_3(s) + 3H_2(g) -> W(s) + 3H_2O(g)`
from the following equations:
`2W(s) + 3O_2(g) -> 2WO_3(s)`
`Delta H = -1685.4 kJ`
`2H_2(g) + O_2(g) -> 2H_2O(g)3`
`Delta H = -477.84 kJ`

Answer 1

`2WO_3(s)->2W(s)+3O_2(g),DeltaH =+1685.4kJ`
Dividing the eq by 2 we have
`WO_3(s)->W(s)+3/2O_2(g),DeltaH =+842.7kJ....(.1)`
Again
`2H_2(g)+O_2(g)->2H_2O(g),DeltaH=-477.8kJ`
Multiplying this eq by `3/2`
`3H_2(g)+3/2O_2(g)->3H_2O(g),DeltaH=-716.7kJ.....(2)`

Adding Eq(1) with eq (2)

`WO_3(s)+3H_2(g)->W(s)+H_2O(g), Delta H =842.7-716.7=126kJ`