simple enthalpy of formation question

[wall1two]

what is the heat of formation for the formation of NO2

1/2 N2 + 1/2 O2 -> NO +21.6 kj

NO_2_ -> NO + 1/2 O2 +13.5kj


isnt the answer (-13.5) + (-21.6) giving -34.1?

the answer this test gives is 8.1. what did i do wrong

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[wizi]

what is the heat of formation for the formation of NO2

1/2 N2 + 1/2 O2 -> NO +21.6 kj (1)

NO_2_ -> NO + 1/2 O2 +13.5kj (2)


isnt the answer (-13.5) + (-21.6) giving -34.1?

the answer this test gives is 8.1. what did i do wrong

Step 1: 1/2N2 + O2 --> NO2

Step 2: (1) is going to be the same but we have to switch (2) to (-2) which is NO + 1/2 O2 -> NO2 (-13.5)

Step 3: (1) + (-2): 1/2 N2 + 1/2 O2 + NO + 1/2 O2 -> NO + NO2

1/2N2 + O2 -> NO2 which is 21.6 - 13.5 = 8.1 kj.

 

[wall1two]

Step 1: 1/2N2 + O2 --> NO2

Step 2: (1) is going to be the same but we have to switch (2) to (-2) which is NO + 1/2 O2 -> NO2 (-13.5)

Step 3: (1) + (-2): 1/2 N2 + 1/2 O2 + NO + 1/2 O2 -> NO + NO2

1/2N2 + O2 -> NO2 which is 21.6 - 13.5 = 8.1 kj.

how come we don't reverse the sign for number 1?, we are decomposing NO not forming it?

 

[wizi]

how come we don't reverse the sign for number 1?, we are decomposing NO not forming it?

If you look at step 1 and how we form NO2. We need 1/2N2 + O2 on the left side right?

If you reverse the sign for (1): 1/2 N2 + 1/2 O2 would end up on the right side, NO would be on the left but we don't need NO (its not in the original equation), we have to find a way to cancel NO.

We want NO on 2 sides of the net equation so we can cancel NO out.

 

[invictusx]

how come we don't reverse the sign for number 1?, we are decomposing NO not forming it?

Because you're FORMING NO2, so the first equation stays the same as they are the reactants that later form NO2. The second equation needs to be flipped, putting NO2 on the product side since this IS your product, otherwise... what would you make? So, reverse the second equation, which will eventually reverse the sign of the second equation and you get your answer 21.6 - 13.5 = 8.1

I think you may have to familiarize yourself with these kind of questions because they show up on the DAT and a couple of times in DAT Destroyer--it can be helpful but my DAT didn't require us to know it but hey, it's worth it!

 

[wall1two]

If you look at step 1 and how we form NO2. We need 1/2N2 + O2 on the left side right?

If you reverse the sign for (1): 1/2 N2 + 1/2 O2 would end up on the right side, NO would be on the left but we don't need NO (its not in the original equation), we have to find a way to cancel NO.

We want NO on 2 sides of the net equation so we can cancel NO out.

AH sorry! thanks for the help i was doing hess' law calculations though it clearly asks for heat of formation (1 mole @ stp) so yeah

prod-reactants = 8.1 i need to be more careful and thanks a lot!