Hess's Law

[Dr.9999]

Here is the problem:

The differing structures of graphite and diamond each have a different ΔH of combustion. The ΔH of combustion for two reactions is given below. Find the ΔH of combustion for the 3rd reaction.

C(graphite)+O2(g)⟶CO2(g)Δ H of combustion -393.5 kJ/mol
C(diamond)+O2(g)⟶CO2(g)Δ H of combustion -395.4 kJ/mol
C(graphite)⟶C(diamond) Δ H of combustion= ????

I thought you take dH product - dH reactant. Since we flipped the second equation -395.4 becomes positive. So 395.4 - (-393.5) = 788.9 kJ/mol

But the answer is 1.9 kJ/mol :/
Isn't a negative - negative = positive?

I don't understand this and its driving me crazy because its soooo simple!!!! (

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[steppingstone]

For these sorts of problems, I find it easiest to just set it up like a series of equations. Once you flip the C(diamond), dH= +395.4. Therefore you take -393.5 + 395.4 = 1.9 kJ/mol.

For Hess's Law, it is best to line the equations up like above and add them together. Hope that helps, let me know if you still don't get it!

 

[Dr.9999]

Okay thanks that might be easier. I was taught to just take the product and subtract it from the reactant but obviously it didn't work in this example.