Module 6.2: Liquid Mixtures

I Raoult's Law: the vapor pressure of a liquid is proportional to its mole fraction

$\Rho_{A} = \Chi_{A} \Rho_{A}^{*}$ (Eq. 102)

1. A limiting law, Raoult's Law becomes increasingly valid as the concentration of one component approach zero

   1. assumes that there is no heat released from the mixing of the two components; all intermolecular interactions are equally strong

2. $\Rho_{A}^{*}$ is the vapor pressure of the pure liquid solvent (when $\Chi_{A} = 1$)

3. Provided Raoult's law is obeyed, a solute will always lower the vapor pressure of a solvent

II The phase diagram for binary liquid mixtures have many variables (temperature, pressure, composition), and conditions are set to avoid three-dimensional diagrams:

1. Both components are miscible in all proportions (avoids complex case scenarios)

2. The pressure is fixed at 1 atm (sets pressure constant, removes one variable)

3. One the liquid and vapor phases are considered

III The vapor pressure of the mixture is dependent on composition; the vapor will contain a higher amount of the more volatile component than the liquid (Raoult's Law and partial pressures)

1. As the vapor pressure changes with composition, so does the boiling point

   1. when vapor pressure equals external pressure, the substance boils

IV distillation: the separation of two components with different boiling points

1. by boiling the mixture and condensing the vapor and repeating this process, the distillate becomes richer in the component with the higher vapor pressure

   1. Refer to Figure 75 where as you distill the binary mixture of benzene and toluene, the condensed vapor (distillate) becomes richer and richer in benzene and the boiling point of the distillate decreases

   2. Benzene and toluene differ by one methyl (-CH3) group and are miscible at almost all compositions

V Azeotropes: binary mixtures that deviates from Raoult's Law

1. all deviations result from $\Delta \textrm{H}_{mix}$, or the enthalpy of mixing

   1. if interactions between two components are relatively favorable (stronger than interactions between each type of molecule separately): $\Delta \textrm{H}_{mix} \lt 0$, and results in a positive deviation

   2. if interactions between two components are relatively unfavorable (weaker than interactions between each type of molecule separately): $\Delta \textrm{H}_{mix} \gt 0$, and results in a negative deviation

2. these deviations make perfect distillations impossible (Ref. Figure 76)