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### Topic: Heats of Combustion and Heats of Formation Questions  (Read 8349 times)

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#### nepenthean

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##### Heats of Combustion and Heats of Formation Questions
« on: August 29, 2008, 03:15:10 AM »
Straightaway, thank you for providing this website in that I can see I will frequent its url ad infinitum owing to my inaugural science regimen!

That said, here we go...

1) I understand the equation to find the enthalpy of a reaction, H of rxn= H of products - H of reactants. Yet the following question does not offer the H of formation for the first reactant. Is this unusual? My chapter's practice questions had all values, so I do not understand.

C2H4 (g)  +  3O2 (g) ---> 2CO2 (g)   +   2H2O (L)

2(-313.5) + 2(-285.  - (0  +  ?)

Am I missing something?

2)   CaCo3 (s)  --> CaO (s)  +  CO2 (g)

Calculate the change in enthalpy if 66.8g of CO2 are formed.

So I am dealing with 1.52 mol of CO2, which I multiplied by -393.5 (heat of formation of CO2) resulting in -598 KJ/mol.

So -635.6 +  -598 = -1234KJ/mol

CaCo3 heats of formation =  -1206.9 KJ/mol

So I found the difference = 27.1KJ/mol; however, the book says 2.70 X 10^2

I do not see my oversight.

Many sincere thanks to ya'll!

As an aside, my experience with chemistry thus far has been frustrating insofar as I thoroughly read the chapter, follow along with the practice questions yet, when I enter the end of chapter problem sets, I get 1/5 correct. I am using Chang's 9th edition. I do not get it.

#### Astrokel

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##### Re: Heats of Combustion and Heats of Formation Questions
« Reply #1 on: August 29, 2008, 07:29:59 AM »
hey nepenthean!

1) not necessary to use formation. You could use either of the following

:delta:Hr(Reaction) =  :delta:Hf(Products) -  :delta:Hf(Reactants)
=  :delta:Hc(reactants) -  :delta:Hc(Products)
= Sum of energies of bonds broken - sum of energies of bonds made
= Hess' law

Which do you think is applicable in your question?

2) Did you multiply the mole to the rest of the values beside formation of CO2?

My only advice is that understand your concepts well before attempting any questions, otherwise no point doing it. Try hard, and i believe you can do it! Good luck!!
No matters what results are waiting for us, it's nothing but the DESTINY!!!!!!!!!!!!

#### Yggdrasil

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##### Re: Heats of Combustion and Heats of Formation Questions
« Reply #2 on: August 29, 2008, 11:02:50 AM »
1)  The problem should provide you with the heat of formation of C2H4 in order to solve the problem.  Perhaps it is in an appendix in the back of the book?

2)  First find the heat of reaction for the given reaction, then multiply this heat of reaction by the number of moles of CO2.  The way you're doing the problem the units don't work.  For example, 1.52 mol x -393.5 kJ/mol = -598 kJ, not -598 kJ/mol.  There is no way for you to add -598 kJ to -635.6 kJ/mol.  If you wanted to take this route, you would have to multiply the heats of formation of CaO and CaCO3 by the number of moles to get all of the heats in kJ, then perform the calculations.

Units are of the utmost importance in solving chemistry problems.  Do not ignore them.

#### nepenthean

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##### Re: Heats of Combustion and Heats of Formation Questions
« Reply #3 on: August 29, 2008, 07:06:56 PM »
Thank you two very much.

I see your explanations.

1)  No heat of formation for C2H4 in the appendix. That is what is confusing me.
Otherwise, I would use the difference of the sums of the products and the reactants. I looked it up online, and the heat of formation of C2H4 is the answer.
?

2) Ok, I see what you are saying. It is algebraically consistent to assert what I do to one term must be done to all the others. Gotcha.

#### Astrokel

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##### Re: Heats of Combustion and Heats of Formation Questions
« Reply #4 on: August 30, 2008, 12:52:43 AM »
hey nepenthean,

1) I would think that you should use bond energies in this case to calculate the enthalpy change of reaction. Bond energies are given in data booklet, or did the question only gives two of the products enthalpy change but it does not give the C2H4?

Quote
the heat of formation of C2H4 is the answer
I've looked up the standard  :delta:Hf of ethene, and it is positive. How can the standard enthalpy change of reaction (combustion) be positive?

Quote
2(-313.5) + 2(-285.  - (0  +  ?)
Check your value for -313.5
No matters what results are waiting for us, it's nothing but the DESTINY!!!!!!!!!!!!

#### nepenthean

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##### Re: Heats of Combustion and Heats of Formation Questions
« Reply #5 on: August 30, 2008, 09:57:24 PM »
Quote
hey nepenthean,

1) I would think that you should use bond energies in this case to calculate the enthalpy change of reaction. Bond energies are given in data booklet, or did the question only gives two of the products enthalpy change but it does not give the C2H4?

THe book I currently use does not even cover bond energies in the chapter offering the question! The question  says to access the appendix, but it does not have the heat of formation for C2H4. That value is pertinent to this question! Its heat of formation is not the answer; I think I misread another post. The answer is -1411KJ/mol. I just do not know how to arrive at that answer without the value supplied. Geez.

Quote
Check your value for -313.5

Gotcha

#### Astrokel

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##### Re: Heats of Combustion and Heats of Formation Questions
« Reply #6 on: August 31, 2008, 03:14:07 AM »
hey nepenthean,

if you go to this http://www.google.com.sg/search?hl=en&q=standard+enthalpy+change+of+formation+ethylene&meta= and on the 4th one on "Chatper 7", download it and question 2 is similar to yours, check it out.
No matters what results are waiting for us, it's nothing but the DESTINY!!!!!!!!!!!!

#### nepenthean

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##### Re: Heats of Combustion and Heats of Formation Questions
« Reply #7 on: August 31, 2008, 05:34:14 PM »
I saw that on yahoo answers. Thank you very much for your contributions.

Regards,
The Nep