# DAT question:

This forum made possible through the generous support of SDN members, donors, and sponsors. Thank you.

#### toyotasupra

##### Full Member
10+ Year Member

I ran into a problem on Topscore.

Using Hess's law what is the H rxn for 2C + O2 -> 2CO

C+O2 -> CO2 H = -393.7 kj/mol
CO + 1/2O2 -> CO2 H= -283.3 kj/mol

Answer: 2C + O2 -> 2CO; 2(-393.7kj) - (-(2(283.3kj))) = -220.8kj

I think Topscore has this WRONG. Correct me if I am wrong, but isn't Hess's law Hrxn = H sum(products) - H sum(reactants)?

2CO is the product and 2C + O2 is the reactants. So it should be:

2(283.3 kj) - 2(-393.7 kj) = 1354 kj

I flipped the negative on the CO because the reaction above show's it in the forward, however I am using it in reverse so the sign should be flipped.

Anyway, let me know if I am wrong. Just getting frustrated.

#### katwoman1688

##### Full Member
I ran into a problem on Topscore.

Using Hess's law what is the H rxn for 2C + O2 -> 2CO

C+O2 -> CO2 H = -393.7 kj/mol
CO + 1/2O2 -> CO2 H= -283.3 kj/mol

Answer: 2C + O2 -> 2CO; 2(-393.7kj) - (-(2(283.3kj))) = -220.8kj

I think Topscore has this WRONG. Correct me if I am wrong, but isn't Hess's law Hrxn = H sum(products) - H sum(reactants)?

2CO is the product and 2C + O2 is the reactants. So it should be:

2(283.3 kj) - 2(-393.7 kj) = 1354 kj

I flipped the negative on the CO because the reaction above show's it in the forward, however I am using it in reverse so the sign should be flipped.

Anyway, let me know if I am wrong. Just getting frustrated.

So there are usually two kinds of ways of answering all these questions. In this question, you must use Hess's Law that states that reactions and enthalpies can be additive. So I use the formula your using when it ask me for a specific enthalpy of a reactant, product, or overall enthalpy of the reaction. However in this case you are finding the enthalpy of an entirely new reaction. What they did is you must add the two reactions together to get the "given".

R1 - C+O2 -> CO2 H = -393.7 kj/mol
R2- CO + 1/2O2 -> CO2 H= -283.3 kj/mol

Sorry - super wordy. So to get this 2C + O2 -> 2CO ...I take reaction 1 and multiply its enthalpy by 2. Than I know that R2 must be flipped so instead of -283.3 I take + 283.3 and add that to the R1x2.

To simplify: its all additive when your making a new reaction (ignore Hess Law). Use hess law for finding enthalpy of specifically a product reactant or a reaction already given not one that you must make.

#### toyotasupra

##### Full Member
10+ Year Member
R1 - C+O2 -> CO2 H = -393.7 kj/mol
R2- CO + 1/2O2 -> CO2 H= -283.3 kj/mol

Sorry - super wordy. So to get this 2C + O2 -> 2CO ...I take reaction 1 and multiply its enthalpy by 2. Than I know that R2 must be flipped so instead of -283.3 I take + 283.3 and add that to the R1x2.

I get -504.1 kj using your method.... Topscore is saying it's -220.8 kj. #### Thn0017

##### New Member
5+ Year Member
Top score is correct on this one.

For this problem... all you need to do is be able to adjust and add the rxns together for it to to equal the main rxn that they are looking for (2C + O2 --> 2CO).

They give you two equations and their H values:
1) C+O2 -> CO2 H = -393.7 kj/mol
2) CO + 1/2O2 -> CO2 H= -283.3 kj/mol

Looking at rxn #2 you see that CO is on the reactants side, you wanna flip it so that it is on the products side like the 2C + O2 --> 2CO. Making it a +283.3 and multiply by 2 to get rid of the 1/2 O2.

this would lead to: 2(CO + 1/2O2 -> CO2) = 2CO + O2 -> 2CO2
H= 2(+283.3 kj/mol)=566.6kj/mol

For the rxn #1 you wanna get rid of CO2 bc it's not in the actual eqn so you multiply it by 2 as well.
2(C+O2 -> CO2) = 2C +2O2 -->2CO2
H = 2(-393.7 kj/mol) = -747.4 kj/mol

Now you just add them together (-747.4)+(566.6)= -220.8 kj.

You are right that the hess's law is products - reactants but this problem does not need to do that. Unless they give you the main problem and give you the H's of CO and O2 and C separately... that's why you plug in and solve by Products - reactants.

Hope that helps... sorry if confusing. #### katwoman1688

##### Full Member
I get -504.1 kj using your method.... Topscore is saying it's -220.8 kj. ASorry! I forgot to say you have to multiply both equations by 2. REason being you need R1 and R2 to equal the final reaction once you add them together, cancel out spectators

so topscore is right - sorry my explanation isn't better

#### katwoman1688

##### Full Member
I get -504.1 kj using your method.... Topscore is saying it's -220.8 kj. Oh there you go haha thats a way better explanation sorry I didn't just do that for u

#### toyotasupra

##### Full Member
10+ Year Member
So if I'm only given the overall reaction heat, then I just add them up?

But if I'm given specific heats of the individual molecules/compounds then I use the Hess's products - reactants rule?

Thanks.