TITLE: Phase Diagram of Two Component System
OBJECTIVES:
OBJECTIVES:
- To measure the miscibility temperatures of water-phenol mixtures of known composition.
- To determine the graphs of phenol composition temperature and the critical solution temperatures.
- To understand the effect of presence of a third component on the water-phenol critical point.
INTRODUCTION:
Phenol and water are partially miscible liquids, but here phenol is not really liquid, it is considered as liquid since the addition of first part of water reduces the solid’s melting point under room temperature to produce a liquid-liquid system. It forms a two component systems containing liquid phases.
Phenol, also known as carbolic acid, hydroxybenzene and phenyl alcohol, is produced at the rate of millions of tons per year, mostly from isopropyl benzene. Phenol is a starting material in the manufacture of plastics and drugs. It was used as an antiseptic in the past. However, phenol is poisonous. The phenol-water mixtures used in this experiment are concentrated and dangerous by contact or ingestion.
Aqueous phenol solutions are used pharmaceutically. At low and high percentages of phenol, water and phenol mix completely, forming a single liquid phase. However, at intermediate compositions, mixtures of phenol and water will separate into two liquid phases.
Above the critical temperature, phenol and water are completely miscible. At any temperature below certain critical solution temperature, the compositions for two liquid phases in equilibrium are constant and are not affected by the relative amount of these two phases. These phases are termed conjugate phases. The relative amounts of the two phases vary. The miscibility between two partial miscible liquids is affected by the existence of third component. All systems prepared on a tie line, at equilibrium will separate into phases of constant composition.APPARATUS :
Boiling tube, measuring cylinder, water bath, dropper, pipette, thermometer, parafilm sealed, test tube holder
MATERIALS :
Phenol solution, distilled water
 |
| Phenol solution |
 |
| Distilled water |
PROCEDURE :
- 8 boiling tubes are prepared and labelled with A, B, C, D, E, F, G and H.
- Each of the boiling tube is filled with 20ml of phenol and water to produce a phenol concentration scale between 8% and 80%. Boiling tubeABCDEFGHPercentage of Phenol (%)811253550637580
- A thermometer is placed in boiling tube A and is fixed in place using parafilm sealed.
- Then, boiling tube A is heated in a water bath and is swirled continuously.
- The temperature for boiling tube A at which the turbid liquid becomes clear is observed and recorded.
- Then, boiling tube A is removed from the water bath and is let to cool until the liquid to become turbid and two layers are separated. The temperature is recorded.
- The average temperature of boiling tube A at which two phases are no longer seen or at which two phases exist is determined.
- Steps 3 to 7 are repeated using boiling tube B, C, D, E, F, G and H.
RESULT :
Boiling tube
|
Percentage of phenol(%)
|
Volume of phenol
(ml)
|
Volume of water
(ml)
|
Temperature (̊C)
|
Average temperature(̊C)
| |
Single phase
|
Two phase
| |||||
A
|
8
|
1.6
|
18.4
|
52
|
45
|
48.5
|
B
|
11
|
2.2
|
17.8
|
62
|
50
|
56
|
C
|
25
|
5.0
|
15.0
|
64
|
55
|
59.5
|
D
|
35
|
7.0
|
13.0
|
66
|
58
|
62.0
|
E
|
50
|
10.0
|
10.0
|
74
|
65
|
69.5
|
F
|
63
|
12.6
|
7.4
|
77
|
68
|
72.5
|
G
|
75
|
15.0
|
5.0
|
75
|
65
|
70
|
H
|
80
|
16.0
|
4.0
|
70
|
60
|
65
|
QUESTION :
- Discuss the diagrams with reference to the phase rule.
The phase rule is a
useful device to relate the effect of least number of independent variables
such as temperature, pressure and concentration upon the various phases such as
solid, liquid and gaseous that can exist in an equilibrium system containing a
given number of components.
The phase rule is
expressed as :
F = C – P + 2
Where F is the number
of degrees of freedom in the system, C is the number of components and P is the
number of phases present.
Below the graph, the
two component system, which are phenol and water exists as two phase, so the
degrees of freedom is
F = C – P +2
= 2 – 2 + 2
= 2
This shows that the
system has 2 independent variables which is temperature and concentration.
Above the graph, the
two component system, which are phenol and water exists as one phase, so the
degrees of freedom is
F = C – P + 2
= 2 – 1 + 3
= 3
This shows that the
system has 3 independent variables which are temperature, concentration and
pressure.
2. Explain
the effect of adding foreign substances and show the importance of this effect
in pharmacy.
Addition of foreign
substances such as salt can affect the critical temperature and the phase
separation. The addition of salt will reduce the miscibility of the phenol and
water which cause the phase separation. The water molecules will associate with
the salt ions and hydrating them. So, the simple ion will lower the tendency of
the water to solvate the phenol. Addition of the salt will increase the
critical temperature of phenol on phenol rich side of the coexistence of the
curve. If the foreign substances are soluble in both liquid, it is called as
blending. For example, when succinic acid is added to the water-phenol mixture
, succinic acid is soluble or completely miscible in each water and phenol
therefore it causes a blending of the liquids making the mixture one phase. The
purity of the substances can be determined by the solubility of the substance.
DISCUSSION:
Miscibility is the property of
substances to mix in all proportions, forming a homogeneous solution.
The term is most often applied to liquids, but applies also to solids and
gases. Water and ethanol, for example, are miscible because they
mix in all proportions. By contrast, substances are said to be immiscible if a
significant proportion does not form a solution. Otherwise, the substances
are considered miscible. For example, phenol is significantly soluble
in water, but these two solvents are not miscible because they are not soluble
in all proportions.
A liquid is said to be miscible if it
dissolves completely in another liquid and is difficult to separate like
alcohol is miscible in water. An immiscible liquid is one which does not
dissolve but forms a layer over another liquid and can be separated easily like
oil is immiscible in water. If we were to pour a little of an immiscible liquid
into a test tube containing water we would see that it forms a thin layer above
the water. In organic compounds, the weight
percent of hydrocarbon chain often determines the compound's
miscibility with water. For example, among
the alcohols, ethanol has two carbon atoms and is
miscible with water, while octanol with eight carbons is not.
Octanol's immiscibility leads it to be used as a standard for partition
equilibria. This is also the case with lipids; the very long carbon chains
of lipids cause them almost always to be immiscible with water.
Simple aldehydes and ketones tend to be miscible with
water, because a hydrogen bond can form between the hydrogen atom of
a water molecule and the ‘lone pair’ of electrons on
the carbonyl oxygen atom. Like any other solubility phenomenon,
miscibility depends on the forces of attraction between the molecules of the
different liquids. The basic rule is liquids with similar molecular structures,
in particular similar polarity, will likely dissolve in each other. Polarity
means the extent to which partial positive and negative charges
appear on a molecule, because of the type and arrangement of its component
atoms. Both water and ethyl alcohol have very polar hydroxyl groups (-OH) on
their molecules, and therefore both undergo the strong intermolecular
attraction known as "hydrogen bonding." Phenol, on the other hand, is
not miscible with water at certain proportion though its molecular structure
contains a polar groups, -OH, that would be attracted to the water molecules,
it also contain a Hexene ring in which electrons are delocalized in it which
make the phenol polarity to be reduced.
But in this experiment, we are able to view the effects of temperature(T) on
the miscibility of different liquids( in this experiment are water and phenol).
The experiment was conducted until the turbid liquid had turned into a clear
liquid. This is because when the 2 solutions are completely miscible, it will
appear as one liquid, which in this case, a clear liquid. In this experiment,
different proportion of phenol and water were used to see which of those
proportion lead to a miscible or immiscible liquid mixture in the test tube.
Some of these proportion causes the mixture to be immiscible but those mixture
was able to be changed into a miscible mixture when they are heated to a
certain temperature(T).
Using
the second law of Thermodynamics, we can explain why the mixture of water and
phenol that are immiscible at certain proportion at room temperature and
pressure tend to become more miscible when the temperature rises. Heating a
mixture of liquids make it easier for the molecule of the liquids to move
between miscible and immiscible state. The Second Law predicts that they will
shift to the more disordered, more highly dispersed, and therefore, more
probably miscible state. But different proportion of mixture of phenol and
water required the solution to be heated to different temperature in order to
make them miscible.
From the graph above, may know that all mixture of water and phenol under the
graph are immiscible and all the mixture of the water and phenol above the
graph are miscible(with respect to each solutions’ proportion). Moving along
the graph(left to right), the concentration of phenol increases. This means
that it is moving from a water-rich phase to the phenol-rich phase. The average
temperature(T) was lowest(48.5ºC) when the percentage of phenol is 8% and the
highest average temperature(T) was when the concentration of phenol was 63%
which is at 72.5ºC. All proportional combination of water and phenol above
72.5ºC of temperature are completely miscible and yield a one-phase liquid
system.
But the results obtain from this experiment are not exactly spot
on to the theoretical values. This may due to several errors were done while
conducting the experiment. An example of these errors are such as not sealing
properly the test tube which contain the mixture of both solutions(water and
phenol). There is also the over heating of the solutions which leads to the
reading recorded(temperature) was way above the actual temperature of which the
solution become miscible. Plus, there is also the possibility of not immersing
the test tube completely in the water bath(the line of the solution in the test
tube are above the water bath), which causes the inefficient heat transfer.
Last but not least, during the heating process in the water bath, the solution
was not swirled in order to distribute the heat evenly, thus, the heat
distribution was not efficient which causes the results(temperature) obtained
to be off from the actual value.
CONCLUSION:
When the two-component system’s (in immiscible state of
known proportion) temperature rises to its miscible temperature, the liquids
will appear as a single phase. Thus from this experiment, the miscible
temperature for each composition of phenol and water was able to be determined.
They are 52°C for 8% of phenol, 62°C for 11%, 64°C for 25%, 66°C for 35%, 74°C
for 50%, 77°C for 63%, 75°C for 75% and 70°C for 80% of phenol.
The graph of phenol’s concentration against the average
temperature was plotted and the critical temperature was determined, which is
72.5°C.
REFERENCES :
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