system v surroundings entropy

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inlovewithchemistry
Posts: 104
Joined: Fri Sep 28, 2018 12:19 am

system v surroundings entropy

Postby inlovewithchemistry » Tue Feb 12, 2019 1:36 am

I know we use deltaH/T to calculate the delta S of both system and surroundings. I also know that Ssys=-Ssurr. Does it matter whether the system or surroundings side is negative? It seems to me like it shouldn't make a difference if the system is reversible, but the book always make Ssurr negative over Ssys. Thank you!

Camille Marangi 2E
Posts: 60
Joined: Fri Sep 28, 2018 12:26 am

Re: system v surroundings entropy

Postby Camille Marangi 2E » Tue Feb 12, 2019 11:52 am

Generally we are more interested in the system specifically gaining entropy or losing entropy. It just so happens that the majority of the time, the system tends to gain entropy and is therefore positive, meaning the surroundings must lose entropy and therefore the Ssurr must be a negative value. So, the sign is just meant to show what gained/lost entropy-that being said, should the system lose entropy, then the surroundings would be positive as a result of that.

michelle
Posts: 62
Joined: Fri Sep 28, 2018 12:23 am

Re: system v surroundings entropy

Postby michelle » Fri Feb 15, 2019 4:22 pm

I think the book wrote that way because we usually can find out S(system) so that we relate that to S(surroudings).

inlovewithchemistry
Posts: 104
Joined: Fri Sep 28, 2018 12:19 am

Re: system v surroundings entropy

Postby inlovewithchemistry » Tue Feb 19, 2019 11:14 pm

I now understand that the book wrote it like this to help us understand that the energy released from the surroundings is absorbed by the system. It was hard for me to grasp this concept, but I thinking about it in relation to the first law of thermodynamics was helpful.

Alysa Rallistan 2G
Posts: 32
Joined: Thu Jan 17, 2019 12:16 am

Re: system v surroundings entropy

Postby Alysa Rallistan 2G » Thu Mar 07, 2019 12:26 am

Generally, when we consider a reaction and the system it occurs in the surroundings are the immediate surroundings so when calculating entropy for them we usually keep the system positive because the reaction is what we observe the exchange of entropy to the surroundings are negative because it is the opposite of what the system has lost; similar thought process as enthalpy too


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