Entropy generation in chemical reactions

[Urmi Roy]

So from the first law for a closed system,

dU=dQ-dW=dQ-PdV

From the second law,

dS=dQ/T + Sgenerated (i.e. the entropy generated)

Putting expression of dQ from second law into first law,
dU=T*dS-T*Sgen-PdV

If s and v are constant,
dU= -T*Sgen>0

Hence dU<0
This is a derivation that was given in class

My questions are as follows:
1. When we say s is constant, does it mean there is not heat flow into/out of the system?

2.If dU<0, does the temperature decrease? I find it hard to understand how there can be a positive Sgen and decrease in temperature/ internal energy simultaneously!

 

[DrDu]

My questions are as follows:
1. When we say s is constant, does it mean there is not heat flow into/out of the system?

2.If dU<0, does the temperature decrease? I find it hard to understand how there can be a positive Sgen and decrease in temperature/ internal energy simultaneously!

1. No, you can imagine that dQ=Sgen/T
2. What happens if you mix salt and ice in a thermo jar?

 

[Urmi Roy]

About your Q2...not too sure, but I know that having a salt in water lowers the freezing point of water...So the ice melts back?

 

[DrDu]

Yes, and gets cooler!