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Why is the salt solubility curve flat?

  1. Dec 20, 2012 #1
    I know most salts' have increased solubility in 100g of water with an increase in temperature, a few have an inverse relationship, but why does NaCl flatline regardless of temperature? Like is there a mechanism that explains this phenomenon? Thanks in advance.
     
  2. jcsd
  3. Dec 20, 2012 #2
    Is it completely flat? Not just mostly flat?

    My guess is that the enthalpy change of dissociation is sufficient that temperature has a negligible effect.

    I know that Sodium and Chlorine are strong acids/bases respectively, and we were taught that strong acids/bases dissociate completely.
     
  4. Dec 21, 2012 #3

    Borek

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    Staff: Mentor

    It is not perfectly flat.

    Note that if some salts have solubility that increases with temperature and some have solubility that decreases with temperature, there is nothing strange with some salts being in between - and having temperature coefficient of solubility close to zero. NaCl happens to be one of these salts.
     
  5. Dec 22, 2012 #4

    morrobay

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    Gold Member

    The solubility curve is related to the molar enthalpy of ion formation in solution,
    NaCl(s) --> Na+ + Cl- (aq)
    ΔH (298K) = 3.9 kJ/mole
    Which is a two part process : Lattice enthalpy, breakdown of crystal lattice and ionic enthalpy of hydration.
    The first is endothermic the second exothermic. So with Le Chatelier principle a temperature
    increase favors the first process and is against the second process.
    In the case of NaCl these two parts of the process cancel each other out with increasing temperature.
    Entropy is more important than temperature in free energy.
    Δ G = ΔH - TΔS , - 8.94 kJ/mole = 3.9 kJ/mole - 12.84 kJ/mole
     
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