# Salters AS question

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#1
Could anyone please help me with this please, I have never got this and my exam is tomorrow Hf values

CH3NHNH2 +54.0
N2O4 -20.0
CO2 -393
H2O -286

i) Write the equation, that corresponds to the standard enthalpy change of formation of N2O4

ii) Theres an enthaply cycle and the enthalpty change of reaction needs to be found

4CH3NHNH2 + 5N2O4 ----H---> 4CO2 +12H2O +9N2
^ ^ (1st is H1, 2nd is H2)
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4C + 9N2 + 12H2 +10O2

Well thats sort of the cycle lol

iii) I need to use this and the table to calculate values for H1 and H2
iv) Sub values into the equation to find a value for h

I have no clue what to do for this

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#2
Anyone?

Sorry I'm slightly despirate to know this 0
11 years ago
#3
OKay...

i) N2 + 2O2 ---> N2O4 (all gases)

iii) H1 = (4 x 54) + (5 x -20)
H2 = (4 x -393) + (12 x -286) + (9 x 0)

iv) H = H2 - H1

make sense?
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#4
(Original post by EierVonSatan)
OKay...

i) N2 + 2O2 ---> N2O4 (all gases)

iii) H1 = (4 x 54) + (5 x -20)
H2 = (4 x -393) + (12 x -286) + (9 x 0)

iv) H = H2 - H1

make sense?
Thank you,

I get i) and iii) but not ii) 0
11 years ago
#5
(Original post by Loz17)
Thank you,

I get i) and iii) but not ii) what's the question though? You've just drawn a diagram 0
#6
The actual question is:

Use the data in the table and the diagram to calculate values for H1 and H2 following the steps given
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11 years ago
#7
(Original post by Loz17)
The actual question is:

Use the data in the table and the diagram to calculate values for H1 and H2 following the steps given
Ahh okay, well the top equation is the main equation you want to find the enthalpy change for

The bottom part of the diagrams are the elements in their standard states, the enthalpy of formation of a compound (e.g. H2)/CO2 etc) is defined by the energy required to form it from these elements in their standard states.

The diagram is showing the hess cycle, with H1 and H2 being the enthalpy of formations for the reagents and products of the main equation.
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#8
(Original post by EierVonSatan)
Ahh okay, well the top equation is the main equation you want to find the enthalpy change for

The bottom part of the diagrams are the elements in their standard states, the enthalpy of formation of a compound (e.g. H2)/CO2 etc) is defined by the energy required to form it from these elements in their standard states.

The diagram is showing the hess cycle, with H1 and H2 being the enthalpy of formations for the reagents and products of the main equation.
Ahhh I get it So to calculate I basically multiply the amount of substance in moles by the standard enthalpy change to form the products and the same for the reactants?

Then for change in H, would it always be change in hf in products - change in H in reactants? Or does it change?
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11 years ago
#9
(Original post by Loz17)
Ahhh I get it So to calculate I basically multiply the amount of substance in moles by the standard enthalpy change to form the products and the same for the reactants?

Then for change in H, would it always be change in hf in products - change in H in reactants? Or does it change?
For formation, always products - reactants 0
#10
(Original post by EierVonSatan)
For formation, always products - reactants Cool, thank you some much,

Just one more quick question...

Is this the same for combustion as well? and reaction ? (not sure if thats the other one but I know it begins with an r)
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11 years ago
#11
(Original post by Loz17)
Cool, thank you some much,

Just one more quick question...

Is this the same for combustion as well? and reaction ? (not sure if thats the other one but I know it begins with an r)
I think it's better if you do it using the cycle, than learning the rules. It's very difficult for me to explain over the internet how to do that, but I'll try You want to go from the top left to the top right by going down through the bottom part. The H1 arrow is pointing in the wrong direction to do this, so you need to reverse the direction. this has the effect of changing the sigh i.e. H1 to - H1. Now add H2 since it is pointing in the right direction.

Overall H = - H1 + H2 = H2 - H1
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#12
(Original post by EierVonSatan)
I think it's better if you do it using the cycle, than learning the rules. It's very difficult for me to explain over the internet how to do that, but I'll try You want to go from the top left to the top right by going down through the bottom part. The H1 arrow is pointing in the wrong direction to do this, so you need to reverse the direction. this has the effect of changing the sigh i.e. H1 to - H1. Now add H2 since it is pointing in the right direction.

Overall H = - H1 + H2 = H2 - H1
No I get that Thank you so much

I'll rep you when my list goes down lol Thanks again 0
6 years ago
#13
why did you times 9 by 0??
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