Reversible vs Irreversible reactions

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Remi Lathrop 1G
Posts: 51
Joined: Fri Sep 29, 2017 7:07 am

Reversible vs Irreversible reactions

Postby Remi Lathrop 1G » Mon Feb 05, 2018 9:22 pm

Can someone please explain the difference between reversible and irreversible reactions in terms of equations? How do I identify each system and how do we know which equations to use for each?

For example problem 47 from chapter 9 (9.47) asks us to determine the entropy for an expanding gas by a) isothermal, reversible expansion and then b) isothermal, irreversible free expansion and I'm having a hard time understanding the difference between the two.

Grace Han 2K
Posts: 33
Joined: Tue Nov 15, 2016 3:00 am

Re: Reversible vs Irreversible reactions

Postby Grace Han 2K » Mon Feb 05, 2018 10:10 pm

I also had a question regarding reversible vs irreversible reactions.

For chapter 19, Self test 9.17a & b, the question gives us both change in volume and change in pressure, but when solving it why do we only use the change in volume and not the pressure?

Grace Han 2K
Posts: 33
Joined: Tue Nov 15, 2016 3:00 am

Re: Reversible vs Irreversible reactions

Postby Grace Han 2K » Mon Feb 05, 2018 10:20 pm

For reversible, you can assume that the total change in entropy should equal to zero.

For example, change in entropy for the system could be calculated using nRln(V2/V1).
Change in entropy for the surroundings could be calculated by:
deltaU=q+w
deltaU is equal to zero for an isothermal expansion for an ideal gas, and therefore q= -w
qSURR=-q=w
w= -nRTln(V2/V1)
change in entropy for surrounding= qSURR/T= -nRTln(V2/V1)/T= -nRln(V2/V1)= -deltaS
Therefore, change in entropy TOTAL=0

For irreversible, you can assume that total change in entropy is positive.
There is no work done in free expansion, so w=0
then 0=q+0, q=o
deltaSSurr= qSURR/T= 0/T= 0

Change in entropy of system could be calculated the same was as the reversible.

Then Change in total entropy should yield a positive number, which means it is a spontaneous (favorable) rxn.

Page 346 in the textbook should give you step-by-step instructions.


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