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Standard Enthalphy of a reaction

Posted: Fri Jan 08, 2016 10:49 pm
by Miriam Velez 1I
In the course reader, there is an example about standard enthalphy of combustion of methane. The problem says to "make sure everything is in standard state..." but I am a little bit confused by this statement. What if the compounds weren't in standard state? How do we convert the enthalphy we are given to standard state?

Re: Standard Enthalphy of a reaction

Posted: Fri Jan 08, 2016 11:13 pm
by Brandon Truong 2G
In lecture, he said that if the compounds were not in standard state then we were suppose to treat it as if the compounds were in standard state and then adjust for the phase changes. I'm not sure how to do this but I think it involves taking difference between the amount of energy in the compounds not in standard state and the amount of energy in the compounds in standard state. Maybe he'll talk about it more in lecture on Monday.

Re: Standard Enthalphy of a reaction

Posted: Fri Jan 08, 2016 11:36 pm
by VivianYang2A
I think they would give us information to replace what is given with "standard state".

Re: Standard Enthalphy of a reaction

Posted: Fri Jan 08, 2016 11:41 pm
by Chem_Mod
Using the method of "products minus reactants" looking up values in the Appendix will give the standard enthalpy of reaction at 1 atm and 298 K. If some component of the reaction is not in standard state, you can always adjust for it by adding some other reactions via Hess's Law. You will soon encounter several of these types of homework problems (for example, find the enthalpy of combustion of methane at 400 K which is not standard temperature!) and it will also be explained further in lecture.