Reversible vs Irreversible reactions

[Remi Lathrop 1G]

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]

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]

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.