Enthalpy Question?

2016-07-03 12:40 pm

Hey, I need some help with a hand-in. I have been able to do most of the questions but this one about enthalpy is tricky I think:

Calculate the enthalpy change, ΔH4, for the reaction

C + 2H2 + 0.5O2 -> CH3OH

using Hess' Law and the following information:
CH3OH + 1.5O2 -> CO2 + 2H2O (ΔH1 = -676 kJ/mol)
C + O2 -> CO2 (ΔH2 = -394 kJ/mol)
H2 + 0.5O2 -> H2O (ΔH3 = -242 kJ/mol)

更新1:

And how do I know if the reactants or the products are more stable?

回答 (1)

micatkie

2016-07-03 12:52 pm

✔ 最佳答案

Rewrite the three given thermochemical equations as :
CO₂ + 2H₂O → CH₃OH + 1.5O₂ ...... ΔH = -ΔH₁ = -(-6.76) kJ = +676 kJ
C + O₂ → CO₂ ...... ΔH = ΔH₂ = -394 kJ
2H₂ + O₂ → 2H₂O ...... ΔH = 2ΔH₂ = 2(-242 kJ) = -484 kJ

Add the above three thermochemical equations, and cancel CO₂, O₂ and 2H₂O on the both sides.
C + 2H₂ + 0.5O₂ → CH₃OH ...... ΔH₄ = 676 - 394 - 484 kJ = -202 kJ

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