Homework 9.5  [ENDORSED]


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Sabah Islam 1G
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Homework 9.5

Postby Sabah Islam 1G » Mon Jan 29, 2018 3:02 pm

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?

Qining Jin 1F
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Re: Homework 9.5

Postby Qining Jin 1F » Mon Jan 29, 2018 3:08 pm

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

Ramya Lakkaraju 1B
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Re: Homework 9.5

Postby Ramya Lakkaraju 1B » Mon Jan 29, 2018 10:24 pm

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.

Brigitte Phung 1F
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Re: Homework 9.5

Postby Brigitte Phung 1F » Tue Jan 30, 2018 12:20 am

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.

MCracchiolo 1C
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Re: Homework 9.5

Postby MCracchiolo 1C » Tue Jan 30, 2018 12:51 am

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

Sarah 2F
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Re: Homework 9.5

Postby Sarah 2F » Tue Jan 30, 2018 11:41 am

You can see that the system is losing heat so you add a negative to the reaction!

Ammarah 2H
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Re: Homework 9.5

Postby Ammarah 2H » Wed Jan 31, 2018 5:21 pm

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.

Swetha Sundaram 1E
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Re: Homework 9.5

Postby Swetha Sundaram 1E » Wed Jan 31, 2018 6:09 pm

I'm pretty sure Sfinal -Sinitial works to solve most problems.

Varsha Swamy 2J
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Re: Homework 9.5

Postby Varsha Swamy 2J » Thu Feb 01, 2018 7:53 pm

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.

Shreya Ramineni 2L
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Re: Homework 9.5

Postby Shreya Ramineni 2L » Sat Feb 03, 2018 2:18 pm

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.

Dylan Mai 1D
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Re: Homework 9.5

Postby Dylan Mai 1D » Sat Feb 03, 2018 6:26 pm

what are the units for delta S? sometimes it is per mol and sometimes it is not. I assume this depends on the question?

Chem_Mod
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Re: Homework 9.5  [ENDORSED]

Postby Chem_Mod » Mon Feb 05, 2018 2:08 pm

The units for entropy are Joules per Kelvin, but molar entropies can appear as Joules per mole Kelvin


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