Question

Use Hess’s law to calculate ∆H° for the reaction:

C(s) + 2H₂(g) + ½O₂(g) → CH₃OH(l)   ∆H°∘= ?

using only the following data:

H₂(g) + ½O₂(g) → H₂O(l)                   ∆H°= -285.8 kJ

C(s) + O₂(g) → CO₂(g)                    ∆H°= -393.5 kJ

2CH₃OH(g) + 3O₂(g) → 2CO₂(g) + 4H₂O(l)                ∆H°= -1452.8 kJ  

Answer 1

The given reactions are as follows :

(1) H2(g) + ½O2(g) → H2O(l) ∆H° = -285.8 kJ

(2) C(s) + O2(g) → CO2(g)        ∆H° = -393.5 kJ

(3) 2CH3OH(g) + 3O2(g) → 2CO2(g) + 4H2O(l) ∆H° = -1452.8 kJ

For required reaction: (2) + 2(1) - 0.5(3)

Then required delta H is as follows :

Delta H = -393.5 + 2(-285.8) - 0.5(-1452.8)

Delta H = 238.7 KJ

Therefore, answer is 238.7 KJ.

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