Entropy generation in chemical reactions

  1. 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!
     
  2. jcsd
  3. DrDu

    DrDu 4,460
    Science Advisor

    1. No, you can imagine that dQ=Sgen/T
    2. What happens if you mix salt and ice in a thermo jar?
     
  4. 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?
     
  5. DrDu

    DrDu 4,460
    Science Advisor

    Yes, and gets cooler!
     
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