University College - Chemistry Chem 112A Lecture Notes - Lecture 39: Ideal Solution, Boiling Point, Phase Rule
13 Dec 2016
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Department
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25 April 2016
Lecture 39: Temperature/Phase Diagrams
Equation of the Day: F = 4 – P (for two component mixtures)
I. Temperature Phase Diagrams
A. Basics
1. Ex: Benzene (bp = 110°C), Toluene (bp = 80°C) form an ideal mixture
a. Assuming 1 atm external pressure, we plot temperature vs. Xbenzene on the x-axis from 0.0
to 1.0
b. The x-axis here represents the total composition of liquid AND vapor (since it is in a
closed system)
c. The upper line is the condensation point curve
d. The lower line is the boiling point curve
e. In between is the two phase or “boiling region”
f. If we draw a horizontal tie line across (for a given temperature), hitting the boiling point
curve and dropping down gives the liquid mole fraction, hitting the condensation point
curve and dropping down gives the vapor mole fraction
g. Remember, the more volatile liquid is enriched in the vapor phase
2. Bullet Points on Temperature/Phase Diagrams
a. The lower line (boiling point line) represents the boiling point curve for various liquid
mixtures
b. The mixture continues to boil throughout the two phase region
c. A horizontal tie line gives the composition of both the liquid (lower line intersection) and
the vapor (upper line intersection)
3. Revised phase rule
a. F = C + 2 – P
b. When C=1, we have 1 component, F (degrees of freedom) = 3 – P (where P is phases
present)
c. When C=2, F = 4 – P
d. However, for temperature phase diagrams, if Pext is constant, then F = 3 – P (since we
remove pressure as a degree of freedom)
• Therefore, if P = 1, F = 2
• If P = 2, F = 1 (picking a temperature has a fixed composition if in the two phase
region)
4. Fractional Distillation
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