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What factors determine volatility?

  1. Nov 24, 2015 #1
    Is the boiling point of a pure component the only factor used to determine which component is more volatile in a binary mixture. Take, for example, the case of isopropanol and cyclohexane.

    Isopropanol has a bp of 82.6 C, while cyclohexane's is 81.

    The two are are very close in value; however cyclohexane's is lower so does that automatically make it more volatile?
  2. jcsd
  3. Nov 24, 2015 #2
    Boiling point is more of a proxy for the substance's vapor pressure. The component with the higher vapor pressure is the more volatile species. Check out the different ways to model vapor pressure for more insight.
    In regards to your example,

    As a component's vapor pressure rises, its boiling point decreases. Because cyclohexane has the lower boiling point, it has the highest volatility.
  4. Dec 12, 2015 #3
    dp/dt = L/(t*deltaV)
    change in specific pressure over change in temperature is equal to the latent heat of vaporization divided by the temperature times the change in volume associated with the phase change.
    dp = change in specific pressure
    dt = change in temperature
    L = latent heat of vaporizaton = T*(S_g - S_L) = energy required to change a molecule from liquid to gas
    t = temperature
    deltaV = change in specific volume for one molecule from liquid to gas.

  5. Dec 12, 2015 #4
    Always check for azeotropes. Isopropanol and cyclohexane form an azeotrope at 67wt% cyclohexane with a boiling point of 68.6oC. The volatilities are reversed on either side of the azeotrope.
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