Hess Law help

[Preciouzgeek]

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Question:
Find the energy released when a C-H bond forms, given the following info.

CH4 + O2 (g) --> CO2 (g) + 2H2O (g)

H'rxn = -802 kj/mol
Hf' CO2 = -393 kj/mol
Hf' H2O = -242 kj/mol

Answer:
Hess law = Hf' products - Hf' reactants
-802 = -393 + (2)(-242) - x
x = -75

One C-H bond = -75/4 bonds = -19 kj/mol

My understanding:

Based on the reaction, wouldn't the equation be the reverse? Meaning the Hf' products is now C-H and the Hf' reactants is CO2 and H2O, thus changing the Hess law to this:

+802 = x - (393 + 2(242) )

I'm confused because in other problems with Hess Law, I'd use Hrxn = H1 + H2 + H3 etc -- changing the signs of the H-formed and H-broken depending on what was given. Please help !😕

 

[movax]

I'm confused because in other problems with Hess Law, I'd use Hrxn = H1 + H2 + H3 etc -- changing the signs of the H-formed and H-broken depending on what was given. Please help !😕

This is more for the problems where they've broken down the reaction into separate elementary steps. For this one...'C-H' bond formation I think is really a tricky way of asking, "what's the Hf of methane?".

So, given Hf = Hf prods - Hf reacts, it's algebra:

-802 (overall Hf) = [(-393) + (2*-242)] - x
and x = -75

In a rush, I'd totally miss that you have to divide by 4 to find each bond's energy, heh.

 

[Preciouzgeek]

Agreed! Thanks 🙂