[kevinzak]

We have been working on this problem for two days straight, and cannot figure out our error. Here is the problem we are given:

Calculate the standard enthalpy of formation for the reaction HCL (g) + NH3 (g) ---> NH4CL (s), given the following thermochemical equations:

H2 (g) + Cl2 (g) ---> 2HCL (g), standard enthalpy of formation: -184kJ (-92kJ/mol)
N2 (g) + 3H2 (g) ---> 2NH3 (g), standard enthalpy of formation: -92kJ (-46kJ/mol)
N2 (g) + 4H2 (g) + Cl2 (g) ---> 2NH4CL (s), standard enthalpy of formation: -628kJ (-314kJ/mol)

It wants us to combine the equations using Hess's Law to arrive at the desired equation. We did so, and we arrive at the desired equation given above ( HCL (g) + NH3 (g) ---> NH4CL (s) ). However, the number we have arrived at time and time again (-176kj/mol) is not the accepted standard enthalpy of formation for NH4CL, -314.43kJ/mol.

Someone please show us our error before we bash our head against the wall any further! Thanks!

Edit: Note, the book that gives the problem lists the answer we get (-176kJ/mol) as correct. We are looking for why this is not the accepted value.

[kevinzak]

It occurred to me that we are using a college text, perhaps I should have posted this there? Although reading past threads, I see a wealth of information, so I imagine I am in good hands regardless.

[Borek]

Not sure what the problem is.

Are you asking why enthalpy of the NH₄Cl(s) formation in reaction

HCL (g) + NH3 (g) ---> NH4CL (s)

is not -314 kJ/mol?

[kevinzak]

Yes. It was brought to my attention by someone that I may be confusing the enthalpy of reaction with the enthalpy of formation, but I copied the question verbatim from my text and it definitely says find the enthalpy of formation. Was it simply erroneously worded?

[Borek]

I think wording is a little bit ambiguous. This is reaction of formation of ammonium chloride, not doubt about it, but not from standard substances.

[kevinzak]

I think I was so focused on finding an error in the numbers given, I did not even consider verifying that the concepts would work. This makes sense, now. -176kJ/mol may be the enthalpy of that reaction, but because that reaction does not consist of ammonium chloride's constituent elements in their standard form, it will not coincide with the standard enthalpy of formation for NH4Cl, correct?

[Borek]

Exactly.