Volume: $\Delta S = nR\ln \frac{V_{2}}{V_{1}}$
Temperature: $\Delta S = nC\ln \frac{T_{2}}{T_{1}}$

Dakota_Campbell_1C
Posts: 51
Joined: Fri Sep 28, 2018 12:15 am

Why do we calculate separate reaction entropies and why is that final accurate? For example in question 5 of Lydon's review packet we did 3 separate change in entropy equations then added them to get the final answer. I guess what I'm not understanding is how is the sum of those entropies an accurate representation of the entropy of the reaction as a whole?

Chem_Mod
Posts: 17501
Joined: Thu Aug 04, 2011 1:53 pm
Has upvoted: 393 times

Because entropy is a state function and is path independent

Raj_Bains_2C
Posts: 66
Joined: Fri Sep 28, 2018 12:25 am
Been upvoted: 1 time

This is because entropy is a state function, so the path taken does not matter. Just the final and initial values matter.

Karina Jiayu Xu 4E
Posts: 58
Joined: Fri Sep 28, 2018 12:29 am

It is allowed because entropy is a state function. A lot of what we are working on are state functions (like enthalpy, gibbs free energy), except for work and heat.

chris_tsai_4H
Posts: 52
Joined: Fri Sep 28, 2018 12:26 am

Since entropy is a state function and is dependent on other situations occurring in the reaction, we have to calculate each change in entropy separately and then add all of them together to find total entropy.

Andrew Bennecke
Posts: 62
Joined: Fri Sep 28, 2018 12:15 am

An example of a state function Lavelle gave in class was going up in altitude level. To go from Altitude A to Altitude B, we could just take any combination of pathways to get from A to B. This is the same as calculating entropies, we can use a wide range of methods to obtain the total change of entropy.

Also, all the state functions we know so far fit into this acronym that Lyndon said at his review session:
I'm under PRESSURE and feeling DENSE, I just want to watch TV and have some HUGS.
-Pressure, Density, Temperature, Volume, Enthalpy, Internal Energy, Gibb's Free Energy, Entropy

armintaheri
Posts: 68
Joined: Fri Sep 28, 2018 12:26 am

It's also important to remember that $\Delta S$ is not entropy of a system, but the change in entropy as a result of that reaction. So if a reaction happens in multiple steps, and each of those steps causes a change in entropy, adding all those changes together gives you the total change as a result of the reaction.

Christopher Anisi 2K
Posts: 30
Joined: Fri Sep 28, 2018 12:21 am

Adding entropies is possible due to the fact that entropy is a state function and therefore is additive.

Brian Kwak 1D
Posts: 88
Joined: Fri Sep 28, 2018 12:17 am