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Expertise: I can answer questions about biochemistry, molecular biology, physics, chemistry, biology, genetics, applying for graduate school, assays, protocols, grants, experiments. I do answer questions at night since I am in the lab working during the day. Also, to be fair to every one, I do one follow ups and if you are interested in more, please contact me under: http://www.liveperson.com/tutor09. It will allow me to be in a live chat with you so we can communicate more effeciently, and I can understand your issues and help you better.

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Biochemistry and Molecular Biology, but I do my best to help you with any general science questions at any level.

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please visit me at http://www.liveperson.com/tutor09. It will allow me to be in a live chat with you so we can communicate more effeciently, and I can understand your issues and help you better. Credentials
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You are here: Experts > Science > Science/Nature for Kids > Science for Kids > enthalpy

Science for Kids - enthalpy

Expert: ScienceAdviser - 3/17/2009

Question
OK - here is the set - up. Question is way at the bottom but you need this set-up to understand my question.
ΔHf methane is represented by the equation:

C(s) + H2(g) CH4 (g)

This equation can be constructed using the equations for combustion of the reactants (carbon and hydrogen) and the product (methane)
C(s) + O2(g) CO2(g)    -394kj    enthalpy 1

H2(g) + 1/2O2(g) H2O(l)    -242kj    enthalpy 2

   multiply this equation by 2 to get



2H2(g) + O2(g) 2H2O(l)    -484kj    enthalpy 3

   Add the first and third equation together to get:


C(s) + 2O2(g) + 2H2(g) CO2(g) + 2H2O(l)    -878kj    enthalpy (1 + 3) = 4

   Now take away the equation for the combustion of methane


CH4 (g) + 2O2(g) CO2(g) + 2H2O(l)    -891kj    enthalpy 5


   And after rearrangement ( take the CH4 to the right hand side) the result is the equation for the formation of methane


C(s) + H2(g) CH4 (g)    +13kj    enthalpy ( 4 - 5)

I cannot figure out why I have to multiply H2(g) + 1/2O2(g) H2O(l) by two? And why do I start with 1/2 O2 rather than just using O2. THANKS!

Answer
Hi there, you usually multiply because you need the equiblrium between the number of atoms that you have in each site. If you use the 1/2 of oxyen you are not able to get one mole of CH4. If you go back and use only 1/2 of oxygen, you will notice that it won't add up to methan that you are looking for.

Please get back to me if my answer is not very clear and you would like me to go back and explain it step by step. I will do that for you.

Best,
Gloria

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