1. Liquid - Liquid Phase Diagrams
Liquid - liquid phase diagrams occur at pressures and temperatures where only liquid phases are present and where for certain values of pressure, temperature, and composition, the liquids are immiscible.
Consider the liquid - liquid phase diagram for nitrobenzene and normal hexane at atm and temperatures less than 300 K:
Temperature (K) w2 w1 1
wNitrobenzene w3
1 phase
2 phases T1
295 K
upper consulate point
The upper consulate temperature is the temperature at which thermal motion overcomes the tendency for phase separation at all compositions.
What is the upper consulate temperature in the nitrobenzene and n-hexane phase diagram?
45.1

2. Suppose we add nitrobenzene to pure n-hexane isothermally at T1
Suppose we add nitrobenzene to pure n-hexane isothermally at T1. At first the nitrobenzene dissolves in the n-hexane giving a solution (Is this a single phase?).
Eventually we reach a point where the n-hexane rich solution is saturated in nitrobenzene and a 2nd nitrobenzene rich phase saturated in n-hexane appears. At what composition does this happen? What are the compositions of the two phases that are in equilibrium?
As more nitrobenzene is added to the solution, the amount of the nitrobenzene rich phase that is saturated with n-hexane grows relative to the amount of the n-hexane rich phase that is saturated in nitrobenzene. At any point in the 2-phase region the relative amounts of the two phases that are in equilibrium is given by the lever law, e.g. at overall composition w2 the relative masses in the two phases are related by:
mw1 (w2 - w1) = mw3 (w3 - w2)
If 72.0 grams of nitrobenzene were added to grams of n-hexane in reaching w2 and if w1 = and w3 = 0.800, how many grams of hexane are present in the nitrobenzene rich phase?
When enough nitrobenzene has been added that the overall composition is w3, then the hexane that is present is completely soluble in the nitrobenzene present and the system reverts to one phase.
45.2

3. The liquid - liquid phase diagram for the binary nicotine and water system is shown below:1
Xnicotine
Temperature ( oC ) X2 X1 X3 T1
210 oC
61 oC T2 T3
nicotine / water solution
nicotine saturated water rich phase in equilibrium with a water saturated nicotine rich phase T4
lower consulate temperature
We will follow the changes that occur as we cool a nicotine water solution of composition X2 from some temperature above the upper consulate temperature of 210 oC.
At temperatures greater than T1, thermal motion prevents separation into two phases and the nicotine and water are miscible.
When T1 is reached, thermal motion is no longer capable of preventing phase separation and a water saturated nicotine rich phase just begins to form and is in equilibrium with the predominant nicotine saturated water rich phase.
45.3

4. As the system is further cooled more of the water saturated nicotine rich phase forms at the expense of the nicotine saturated water rich phase. In the two phase region the relative amounts of the phases present are again given by the lever law, e.g. at T2 we have:
nX1 (X2 - X1) = nX3 (X3 - X2)
Cooling the system also favors the formation of a weak nicotine/water complex resulting in nicotine redissolving in the water rich phase. What must this imply about the relative amounts of the water and nicotine rich phases?
At temperatures below T3, this effect begins to dominate the lower temperature induced phase separation and the phase boundaries begin to approach each other.
Ultimately the formation of this weak complex will result in the phase boundaries meeting at a lower consulate temperature below which the two components are completely miscible in each other at all compositions. What is the lower consulate temperature for the nicotine water system?
By the time the temperature reaches T4 the formation of this weak nicotine/water complex results in all of the nicotine redissolving in the water and the water saturated nicotine rich phase disappears.
45.4

5. A mixture of 40. 0 grams of component A and 60
A mixture of 40.0 grams of component A and 60.0 grams of component B is brought into equilibrium at 115 oC. At this temperature and overall composition, 62.5 grams of a phase rich in B separates from a phase which is 70.0% by weight rich in A.
Temperature ( oC )
100 wA
115 oC
What is the weight percent composition of the phase rich in B that is in equilibrium with the A rich phase?
45.5