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Why is Delta S purely dependent on gases?

  1. Feb 17, 2015 #1
    My teacher constantly stresses that 10 moles of a gas going to 11 moles of a gas will ALWAYS cause an increase in entropy (ΔS>0). But why? For example (I know that O2 shouldn't technically be a liquid, but just as an example):
    2H2O(g)⇔H2O(g)+(3/2)O2(l)+2H2
    It seems iffy to me. We go from 2mol(g)↔1mol(g), BUT we also have 2moles⇔(7/2)moles. Is the moles of gas rule just a "it usually works" kind of rule, or is it a tried-and-true physics fact?
    Thanks so much! :smile:
     
  2. jcsd
  3. Feb 17, 2015 #2

    Borek

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    Staff: Mentor

    10 moles of a gas don't change into 11 moles of a gas, that's mass conservation at wok. Apparently you think about some process in which number of moles of gas changes - but what process it is, and why does the number of moles of gas change? What are other things involved?

    Or perhaps you just misunderstand the idea that entropy is an extensive property?
     
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