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How can standard enthalpy values be negative?

  1. Mar 19, 2015 #1
    Certain aqueous ions have negative values for SO, such as Ca2+ with -55.2 J/(K*mol); how is this possible when dissolution is usually an increase in entropy?

    Also, all pure elements seem to have positive standard entropy value; why is this the case?

    Thank you.
     
  2. jcsd
  3. Mar 19, 2015 #2
    Enthalpy or entropy?

    Aceix..
     
  4. Mar 19, 2015 #3
    Oh jeez, I'm so sorry. I meant to say entropy. My bad.
     
  5. Mar 19, 2015 #4

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    Start dropping solvent dipoles into an electrical potential well. There's more going on than just entropy of mixing.
     
  6. Mar 19, 2015 #5
    So is the dipole interaction between the solute and the solvent great enough that dissolution will result in an overall decrease in entropy despite the entropy of mixing?
     
  7. Mar 19, 2015 #6

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    Can the charge-dipole interaction be that large? Yes. Is it always? No. That's what keeps measurement labs busy.
     
  8. Mar 19, 2015 #7
    Makes sense. Thank you.
    Also, could you answer my other question about elements always having a positive standard enthalpy?
     
  9. Mar 19, 2015 #8

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    Enthalpy? Again, you mean "entropy?" It's the integral of Cp/T from absolute zero to 298 K (or whatever T for the table).
     
  10. Mar 19, 2015 #9
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