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Homework 9.5

Posted: Mon Jan 29, 2018 3:02 pm
by Sabah Islam 1G
For question 9.5, why are we supposed to use the equation delta S=-q/T for the entropy at 800 K and the equation delta S=q/T for the entropy at 200 K? Why is there a negative in one equation and no negative in another?

Re: Homework 9.5

Posted: Mon Jan 29, 2018 3:08 pm
by Qining Jin 1F
We use S=-q/T for the entropy at 800 K because heat released, and we use S=q/T for the entropy at 200 K because heat is absorbed by the reservoir

Re: Homework 9.5

Posted: Mon Jan 29, 2018 10:24 pm
by Ramya Lakkaraju 1B
Whenever the sign of something is not immediately obvious think about whether the system or the surroundings are gaining or losing heat; losing heat is always signified by a negative sign.

Re: Homework 9.5

Posted: Tue Jan 30, 2018 12:20 am
by Brigitte Phung 1F
When it comes to these problems, it helps to look at the direction of heat transfer from each of the objects/systems involved! When heat is being released by an object, q is negative with respect to that object, whereas when heat is being absorbed by another object, q is positive.

Re: Homework 9.5

Posted: Tue Jan 30, 2018 12:51 am
by MCracchiolo 1C
It goes along with the same concept of q(system)=-q(surroundings) where the heat released by one system must be absorbed by something else and vice versa

Re: Homework 9.5

Posted: Tue Jan 30, 2018 11:41 am
by Sarah 2F
You can see that the system is losing heat so you add a negative to the reaction!

Re: Homework 9.5

Posted: Wed Jan 31, 2018 5:21 pm
by Ammarah 2H
Is S final - S initial a correct approach for these types of problems? Because you still get the right answer, I'm just not sure if it works for every problem.

Re: Homework 9.5

Posted: Wed Jan 31, 2018 6:09 pm
by Swetha Sundaram 1E
I'm pretty sure Sfinal -Sinitial works to solve most problems.

Re: Homework 9.5

Posted: Thu Feb 01, 2018 7:53 pm
by Varsha Swamy 2J
I think Sfinal - Sinitial works in general because that is the definition of delta S. In this case, I think it can be explained by the concept above; because the 800. K reservoir is losing heat, so its entropy when you calculate overall entropy change should be negative.

Re: Homework 9.5

Posted: Sat Feb 03, 2018 2:18 pm
by Shreya Ramineni 2L
Its based on system vs. surroundings and where the heat is transferring. Because heat goes from hot to cold, the 800 K part is losing and has a negative sign while the 200 K is gaining and has a positive sign.

Re: Homework 9.5

Posted: Sat Feb 03, 2018 6:26 pm
by Dylan Mai 1D
what are the units for delta S? sometimes it is per mol and sometimes it is not. I assume this depends on the question?

Re: Homework 9.5  [ENDORSED]

Posted: Mon Feb 05, 2018 2:08 pm
by Chem_Mod
The units for entropy are Joules per Kelvin, but molar entropies can appear as Joules per mole Kelvin