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Help. Having a problem with my assignment.

The reaction given is as below.

n-heptane -> toluene + hydrogen

Now given standard heats of formation at 25 degree celcius is.

n-heptane = -187.8
toluene = 50.0

However the reaction takes place at 500 degree celcius.

The main question, do I have to use hess law to determine the entalphy of reaction at that specific temperature (500 celcius) or I can just use the calculated heats of formation which is 25 celcius for my reaction that takes place at 500 celcius.

[djack77494]

CB,
You must adjust for the temperature not being 25C. Begin with the heat of reaction at 25C = difference in the heats of formation of the products - reactants (all at 25C). Then adjust for the sensible heat effects in heating the products and reactants from 25C to 500C. That will give you the heat of reaction at 500C.