DSE Expt
Fractional Distillation and Ideal Solution (Micro-scale)
(Adapt as appropriate, the vapour pressure part is not included in the syllabus))
Purpose
1. To plot the boiling point- composition diagram for the methanol/water ideal solution
2. To carry out micro-scale fractional distillation of commercial methylated spirit
Introduction and theory
Due to similar intermolecular forces between solvent/solvent, solute/solute and
solute/solvent molecules, vapour pressure of an ideal solution varies linearly with
composition (Fig. 1). Normally a more volatile component (with a less b.pt.) has a higher
vapour pressure. When boiling of an ideal solution occurs, temperature of the liquid and the
vapour right above it are not the same. The vapour should have a lower temperature because
it has more volatile component. Hence the graph is not a straight line but with a lower liquid
curve and an upper vapour cure (Fig. 2 and 3).
Boiling
point
Vapour
pressure
liquid
X
Y
Percentage composition
100%
0%
Fig.1
Vapour/composition diagram
Percentage composition
X
Fig.2
Y
100%
0%
Boiling point/composition diagram
Upon simple distillation, vapour formed from an ideal solution has a composition rich in the
more volatile component. When this vapour is condensed and distilled again, the subsequent
vapour will be even richer in the more volatile component, as shown in Fig. 3. Repeated
evaporation and condensation can completely separate the two components. This is the basis
of fractional distillation.
Boiling
point
vapour
2nd distillation
X
0%
Fig.3
1st distillation
liquid
Percentage composition
Y
100%
Complete boiling point/composition diagram
Expt
Fractional Distillation
Page 2
The ideal solution chosen for the experiment is a methanol/water mixture.
Safety
Wear safety spectacles and avoid skin contact with the chemicals. Use the
minimum quantity of materials required. Dispose of chemical waste and excess
materials according to your teacher’s instruction.
EYE PROTECTION
MUST BE WORN
Materials and Apparatus
Chemicals and consumables
Qty
Equipment
Qty
Commercial methylated violet spirit
10 cm3
Micro-fractional distillation set
1
Methanol
10 cm3
DMM with thermistor
1
Test tube
4
Test tube rack
1
Beaker 250 cm3
1
Plastic dropper
4
Wash bottle with distilled water
1
Anti-bumping granules
1g
Experimental procedures
(A) Boiling point vs composition diagram of a methanol/water mixture
1.
Using a water bath, a small test tube and some anti-bumping granules, the boiling point
of 20 drops each of methanol and water are determined
2.
Using new small test tube, add drops of water and methanol according to the following
scheme:
Small test tube
1
2
3
4
5
6
7
8
9
No. of drops of methanol
2
4
6
8
10
12
14
16
18
18
16
14
12
10
8
6
4
2
No. of drops of water
3. Determine the boiling point of the contents in each small test tube
4. Plot a graph of boiling point vs percentage composition of methanol/water. It should
look like Fig. 2, if you take methanol as X and water as Y.
(B) Micro-scale fraction distillation of commercial methylated spirit
1.
Determine the boiling point of the given sample of blue methylated spirit.
2.
Set up the micro fractionating column as shown in Fig. 4. Place about 5 cm3 of the
methylated spirit into the small conical flask and add some anti-bumping granules.
Expt
Fractional Distillation
Page 3
Pasteur
pipette
Glass wool
small bits of
glass rod
Methylated spirit
Anti-bumping
granules
Fig. 4
Micro-scale fractionating column
Fig. 5
Micro-scale fractionating column
Fig. 7
DMM/thermistor temperature
measurement
Fig. 6
Fig. 8
Column/flask arrangement
Repeated evaporation/condensation
inside the column/
Expt
Fractional Distillation
Page 4
Place the setup in a water or sand bath. Wait for boiling and allow time for attaining thermal
equilibrium. Determine the boiling point of the liquid collected at the top part of the micro
column.
Results
(A) Boiling point vs composition diagram of a methanol/water mixture
Boiling point of methanol = ____________oC
Boiling point of water = _______________ oC
Small test tube
Percentage
composition
1
2
3
Methanol
Water
Boiling point /oC
Plot of boiling point vs percentage composition:
4
5
6
7
8
9
Expt
Fractional Distillation
Page 5
(B) Micro-scale fractional distillation of commercial methylated spirit
Boiling point of the given sample of methylated spirit = ____________ oC
Boiling point of the fraction at the top of the column = ______________ oC
Discussion
1. Why does the boiling point/composition graph of an ideal solution not shown as a
straight line?
_______________________________________________________________________
_______________________________________________________________________
2. What do you think is the relationship between the length of a fractionating column and
efficiency of separation of components?
_______________________________________________________________________
3. What will be left in the flask if the condensed liquid at the top of the column is
continuously removed?
_______________________________________________________________________
4. Name one more industrial use of fractional distillation apart from crude oil refinery.
_______________________________________________________________________
Web search
Connect to the internet and search for information for the following topic:
Latent heat of vaporization
(i)
Why does the temperature at boiling remain constant?
____________________________________________________________________
_____________________________________________________________________
(ii)
Describe briefly the function of excess heat removal from human bodies in terms of
vaporization.
____________________________________________________________________
____________________________________________________________________
____________________________________________________________________
Conclusion
_________________________________________________________________________
_________________________________________________________________________