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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: jennielynn_1980 on August 13, 2006, 09:25:57 PM

Title: calculating heat of combustion
Post by: jennielynn_1980 on August 13, 2006, 09:25:57 PM
Hi all.

I have been given the following question and I am trying to figure out if I am missing something or there is a typo in the question because if there is not a typo, I have no idea how to solve this.  So here it is:

calculate the heat of combustion for C2H6 from the following information:

C2H4 + 3O2 --> 2CO2 + 2H2O  heat of formation = -1409.5 kJ
C2H4 + H2 --> C2H2  heat of formation = -136.7 kJ
H2 + 1/2O2 --> H2O  heat of formation = -285.5 kJ

Thanks :)
Title: Re: calculating heat of combustion
Post by: Will on August 13, 2006, 09:43:45 PM
Quote from: jennielynn_1980 on August 13, 2006, 09:25:57 PM
calculate the heat of combustion for C2H6 from the following information:

C2H4 + 3O2 --> 2CO2 + 2H2O  heat of formation = -1409.5 kJ
C2H4 + H2 --> C2H2  heat of formation = -136.7 kJ
H2 + 1/2O2 --> H2O  heat of formation = -285.5 kJ

AFAIK the second equation is complete rubbish and most definately a typo/mistake. And I don't get what compound the heat of formation is referring to. I think the question means just the enthalpy change (with the last being heat of formation of H2O).
If you made the assumption that the second equation is supposed to be C2H2 + H2 --> C2H4 then you can estimate the answer.
Title: Re: calculating heat of combustion
Post by: jennielynn_1980 on August 13, 2006, 09:57:50 PM
Okay thanks.  I am glad there is a type because I was getting extremely confused trying to wrap my head around this question  :P
Title: Re: calculating heat of combustion
Post by: jennielynn_1980 on August 23, 2006, 10:20:18 AM
I am still stuck on this question.  If someone could give me a hint that would be great.  Here is what I have so far:

The combustion equation is:
2C2H6 + CO2 --> 4CO2 + 4H2O


equation #1 C2H4 + 3O2 --> 2CO2 + 2H2O  heat of formation = -1409.5 kJ
equation #2 C2H4 + H2 --> C2H6  heat of formation = -136.7 kJ
equation #3 H2 + 1/2O2 --> H2O  heat of formation = -285.5 kJ


So from these equations we can tell that the heat of formation for:
C2H6 is = 136.7 kJ (got this by reversing equation #2)
H2O is = -285.5 kJ (from equation#3)
O2 I don't need to know the heat of formation because it is assumed to be 0

So if I multiply #1 by 2 then:
2C2H4 + 6O2 --> 4CO2 + 4H2O
Then mulitply #2 by -2 then:
2C2H6 --> 2C2H4 + 2H2
Then multiply #3 by 2:
2H2 + O2 --> 2H2O

Then add all three together to get:

2C2H4 + 6O2 + 2C2H6 + 2H2 + O2 --> 4CO2 + 4H2O + 2C2H4 + 2H2 + 2H2O

Now that I have this, I am unsure of the next step.

Sorry for how long this is.

Title: Re: calculating heat of combustion
Post by: Borek on August 23, 2006, 11:29:58 AM
Quote from: jennielynn_1980 on August 23, 2006, 10:20:18 AM
2C2H4 + 6O2 + 2C2H6 + 2H2 + O2 --> 4CO2 + 4H2O + 2C2H4 + 2H2 + 2H2O

No idea if you did everything correctly, but obvious step now is to sort and cancel out.
Title: Re: calculating heat of combustion
Post by: jennielynn_1980 on August 23, 2006, 11:54:45 AM
So I can cancel out to simplify the equation?  I wasn't sure I could do that.  I think I have the answer then.

If I mulitply the heat of formations for each equation by 2 and simplify the long equation like you said, then I should have my final equation and heat of combustion.