[JW21]

Hello,

This is my first post here. First of all, sorry for my English. Not sure if I'm using terms like combustion enthalpy right. It's ΔHc if that helps.

1. Solved this by myself, so I deleted it!

2. While on the subject, I have another reaction involving enthalpy that I'm also not sure of. I need the reaction enthalpy ΔHr of ureum with water into ammonia and carbondioxide, so that's:

H2N-CO-NH2 + H2O -> 2NH3 + CO2

I'd use formation enthalpy, but the formation enthalpy of ureum isn't given. I only have the ΔHc of ureum. I feel like I can somehow add reactions (again with ΔH1, ΔH2, ΔH3, using the ureum + o2 burning reaction), but I can't seem to find the solution. The burning reaction would be:

H2N-CO-NH2 + 3/2 O2 -> CO2 + 2 H2O + N2 if I'm correct. Could you give me some advice for this question as well?

If someone could help me, it'd be really appreciated! Thanks!

[Burner]

2. While on the subject, I have another reaction involving enthalpy that I'm also not sure of. I need the reaction enthalpy ΔHr of ureum with water into ammonia and carbondioxide, so that's:

H2N-CO-NH2 + H2O -> 2NH3 + CO2

I'd use formation enthalpy, but the formation enthalpy of ureum isn't given. I only have the ΔHc of ureum. I feel like I can somehow add reactions (again with ΔH1, ΔH2, ΔH3, using the ureum + o2 burning reaction), but I can't seem to find the solution. The burning reaction would be:

H2N-CO-NH2 + 3/2 O2 -> CO2 + 2 H2O + N2 if I'm correct. Could you give me some advice for this question as well?

What information are given else? For example, ΔHf of water and carbon dioxide?

[JW21]

I have the formation enthalpy of CO, CO2, H2O (liquid and gas). Formation of N2 is just 0.

[mjc123]

Well, if you have ΔH_c of urea (that's what it's called in English, by the way), and ΔH_f of CO₂ and H₂O, you can work out ΔH_f of urea.

[JW21]

Well, if you have ΔH_c of urea (that's what it's called in English, by the way), and ΔH_f of CO₂ and H₂O, you can work out ΔH_f of urea.

Ah yes, urea! I should know that.

Well, this is the part where I'm stuck. I'm unsure how to calculate the ΔHf of urea from the ΔHc? Can you show me the steps you do? It doesn't even have to involve the numbers, just ΔHc of urea - or + CO2 or xyz etc.

[mjc123]

For any reaction, ΔH = ΣΔH_f(products) - ΣΔH_f(reactants)
Apply that to ΔH_c(urea)

[JW21]

For any reaction, ΔH = ΣΔH_f(products) - ΣΔH_f(reactants)
Apply that to ΔH_c(urea)

But here's where I'm confused. I am only given the ΔHc of urea. I'm not allowed to use that when I'm calculating ΔH through formation right? Sorry if I'm being dumb, but I though those were 2 separate things. How do I get the ΔHf of urea from the ΔHc of urea?

Or do you mean I should say ΔHr of urea = ΔHf of urea products - ΔHf reactants? And not use the ΔHc at all?

[mjc123]

I'm not allowed to use that when I'm calculating ΔH through formation right?

Who says?
What is the reaction for which ΔH = ΔH_c(urea)?
Apply to that reaction ΔH = ΣΔHf(products) - ΣΔHf(reactants)

[JW21]

I'm not allowed to use that when I'm calculating ΔH through formation right?

Who says?
What is the reaction for which ΔH = ΔH_c(urea)?
Apply to that reaction ΔH = ΣΔHf(products) - ΣΔHf(reactants)

Okay I understand less than I did before...

for ΔHc urea = ΔHc reactants - ΔHc products (because it's a combustion reaction it's not products - reactants but reacts - products) right?

But then I'm burning something to make urea... which seems odd?

I'm seriously lost now.

[Corribus]

You've already identified what your unknown is: you need the formation enthalpy of urea to solve the problem. You can determine that by looking at another reaction involving urea for which the formation enthalpies or all other involved species and reaction enthalpy are known.

You are essentially using the combustion reaction to find the heat of formation of urea, and then using the heat of formation of urea to solve the problem. It boils down to two equations and two unknowns similar to:

B - A = C
D - A = E.

Find C if you know B, D and E.