[CocoaBear]

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.

[eugenedakin]

Hi CocoaBear,

This is a great question. 

Although Hess's law (http://en.wikipedia.org/wiki/Hess's_law) is only concerned with the beginning and end step, there does need to be enough energy (possibly in the form of heat) to ensure that the 'middle steps' of the reactions take place. This is my guess as to the reasoning behind the elevated temperature.

I hope this helps,

Eugene

[Yggdrasil]

Do you know the specific heats of the reactants and products?  If so, you can take the heat of cooling the reactants from 500 to 25 deg C, add the heat of reaction at 25 deg C, then add the heat of heating the reactants from 25 to 500 deg C.

[Donaldson Tan]

n-heptance, 500C -> toulene + hydrogen, 500C

Using Hess' Law, the above reaction can be resolved into 3 steps:
1. n-heptance, 500C -> n-heptane, 25C
2. n-heptane, 25C -> toulene + hydrogen, 25C
3. toulene + hydrogen, 25C -> toulene + hydrogen, 500C

the dH of step 1 can be found by using the heat capacity of n-heptance
the dH of step 2 can be found by directly using the standard heat of formation
the dH of step 3 can be found by using the heat capacities of toulene and hydrogen

the overall dH of the reaction at 500C is the sum of dH for steps 1,2 and 3 .