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Hess's Law

The solid compound, urea, NH2CONH2, is manufactured in the following reaction:

2NH3(g) + CO2(g) = NH2CONH2(s) +H2O(l)

Calculate the standard enthalpy change for the above reaction using following data:

Enthalpy of combustion (kjmol-1)
NH3(g) = -46
CO2(g) = -394
NH2CONH2(s) = -333
H2O(l) = -286

Thanks a lot! I'm getting confused as by using Enthalpy of combustion, i can only find enthalpy of formation, right?
(edited 8 years ago)
Original post by 123chem
The solid compound, urea, NH2CONH2, is manufactured in the following reaction:

2NH3(g) + CO2(g) = NH2CONH2(s) +H2O(l)

Calculate the standard enthalpy change for the above reaction using following data:

Enthalpy of combustion (kjmol-1)
NH3(g) = -46
CO2(g) = -394
NH2CONH2(s) = -333
H2O(l) = -286

Thanks a lot! I'm getting confused as by using Enthalpy of combustion, i can only find enthalpy of formation, right?


I think the figures you have given are enthalpy of formations. This makes the problem solvable.

Answer you should get is -133
(edited 8 years ago)
Reply 2
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123chem
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In the worksheet, it said the symbol for enthalpy of combustion (triangle H with subscript c). Also, a quick google search showed that those were combustion enthalpies
(edited 8 years ago)
Original post by 123chem
In the worksheet, it said the symbol for enthalpy of combustion (triangle H with subscript c). Also, a quick google search showed that those were combustion enthalpies



Formation!


https://en.m.wikipedia.org/wiki/Standard_enthalpy_of_formation
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123chem
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Original post by TeachChemistry


image.jpg Also, searching up combustion values gives 285.5~ too.
(edited 8 years ago)
Original post by 123chem
image.jpg Also, searching up combustion values gives 285.5~ too.


Your worksheet is incorrect. These are formation values not combustion.
Think about this. How can CO2 have a combustion value? It's already burned and can't burn any further!

What you might have found on the web is the combustion enthalpy for carbon which is EXACTLY the same as the formation value for CO2.

C + O2 --> CO2

This represents ΔHf (CO2) OR ΔHc (C)
(edited 8 years ago)
Reply 6
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123chem
OP
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Original post by TeachChemistry
Your worksheet is incorrect. These are formation values not combustion.
Think about this. How can CO2 have a combustion value - it's already burned and can't burn any further?

What you might have found on the web is the combustion enthalpy for carbon which is EXACTLY the same as the formation value for CO2.

C + O2 --> CO2

This represents dHf (CO2) OR dHc (C)


Ok, cheers.

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