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    Understanding Vapour Pressure

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    Which would you expect to exhibit the highest vapor pressure at room temperature?
    - Helium(Delta H fusion 0.02, Delta H vaporization 0.10)
    - Benzene(Delta H fusion 9.95, Delta H vaporization 30.8)
    - Water(Delta H fusion 6.01, Delta H vaporization 40.67)
    - Mercury(Delta H fusion 2.33, Delta H vaporization 56.9).

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    The transition at constant pressure for a physical change or chemical reaction depends on the combination of two energy factors. The combination of these two energy factors, when the pressure surrounding the entire system under transition remains constant, is called the Gibbs Free Energy. The combination of these two energy factors for a transition at constant volume around the system is called the Helmholtz Free Energy, and the factors in this case are different. In the case of a phase transition, solid to liquid, liquid to gas, etc., the pressure over the phases is constant so the interesting combination is the Gibbs Free Energy; which is the sum of the enthalpy change and the heat associated with the entropy change (-TdeltaS). The change in entropy in transition from a condensed state to a more random state is positive. The temperature in Kelvins is positive. So the heat associated with an increase in disorder is negative and exothermic. The positive value of deltaS means the molecular associations have increased in randomness in going from one state to the next as we see in looking at the structure of a solid with a fairly specific lattice, to a liquid with various electronic bonds still holding a pretty interesting structure, to a gas with very little structure, to a ...

    Solution Summary

    This solution provides a very detailed description of this specific chemistry-based topic and goes through the vapour pressure for each of the elements and gases which are part of this question. This solution will act as a great study tool and increase a student's comprehension of this topic.