delta S of reaction

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Maayan Epstein 14B
Posts: 39
Joined: Fri Sep 28, 2018 12:20 am

delta S of reaction

Postby Maayan Epstein 14B » Sun Feb 17, 2019 8:42 pm

Why is delta S of reaction different than delta S molar/delta S of formation of a substance?
For example, in problem 9.55 (6th edition), we use an appendix to look up the delta S of formation (listed as delta S molar) of a substance, i.e. NH3, and subtract from it the delta S molar of its reactants and their coefficients, to determine the delta S of reaction. I don't understand why you need to do this, and why these values are different.
Can someone explain this conceptually?

Thank you so much!

Te Jung Yang 4K
Posts: 63
Joined: Fri Sep 28, 2018 12:18 am

Re: delta S of reaction

Postby Te Jung Yang 4K » Sun Feb 17, 2019 9:59 pm

Delta S of formation of a substance is the entropy change when one MOLE (hence the two different names) of the substance in its standard state is made from its constituent elements in their standard states. By this definition, you can see that not every reaction will be a substance being formed from its constituent elements, hence the difference in delta S of the reaction and delta S of formations.

The equation to find the entropy change of the reaction from entropy change of formations is the following:

Delta_S_Reaction = Sum(Delta_S_Formation_Products) - Sum(Delta_S_Formation_Reactants)

You can do this because entropy is a state function.

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