# Entropy generation in chemical reactions

by Urmi Roy
Tags: chemical, entropy, generation, reactions
 P: 689 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!
P: 3,281
 Quote by Urmi Roy 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?
 P: 689 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?