Investigation of the variation of boiling point with composition for different mixture of two miscible liquids

2.  Variation of boiling point

with composition using

trichloromethane/ethyl ethanoate mixtures

1. The flask was heated gently with a small Bunsen flame until gentle boiling was observed.
2. When the reading on the thermometer was observed to become steady, the boiling point was recorded in TABLE 1.
3. A beaker of tap water was prepared to cool down the liquid in the flask.
4. For safety and also the liquid inside the flask was highly flammable, the Bunsen flame was turned off and the flask was removed to a beaker a tap water to be cool down to a certain level around 35℃
5. 2 ㎝3 of ethyl ethanoate was added to the content of the flask.
6. The flask was swung to make sure two liquids were mixed up.
7. The mixture was then reheated until boiling gently again.
8. Further additions of 3 ㎝3  and  5 ㎝3  of ethyl ethanaote was added into the mixture and boiling point of each mixture was recorded down in the TABLE 1.
9. After that, the flask was emptied out and it was kept away from any naked fire while at the same time it was allowed to dry as the mixture was volatile and flammable.
10. Adding in succession 2 ㎝3 and 3㎝3 and 5 ㎝3 of trichloromethane was repeated and results were recorded in the TABLE 1.

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3.  Variation of boiling point

with composition using

hexane/propanone mixtures

1. The procedures in Part A were repeated for the solution of hexane.
2. The flask was heated gently with a small Bunsen flame until gentle boiling was observed.
3. When the reading on the thermometer was observed to become steady, the boiling point was recorded in TABLE 1.
4. A beaker of tap water was prepared to cool down the liquid in the flask.
5. For safety and also the liquid inside the flask was highly flammable, the Bunsen flame was turned off and the flask was removed to a beaker a tap water to be cool down to a certain level around 35℃
6. 2 ㎝3 of propanone was added to the content of the flask.
7. The flask was swung to make sure two liquids were mixed up.
8. The mixture was then reheated until boiling gently again.
9. Further additions of 3 ㎝3  and  5 ㎝3  of propanone was added into the mixture and boiling point of each mixture was recorded down in the TABLE 1.
10. After that, the flask was emptied out and it was kept away from any naked fire while at the same time it was allowed to dry as the mixture was volatile and flammable.
11. Adding in succession 2 ㎝3 and 3㎝3 and 5 ㎝3 of hexane was repeated and results were recorded in the TABLE 1.

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Apparatus:

Chemicals:

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Result:

TABLE 1

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TABLE 2

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Precautions:

      Trichloromethane is known as harmful.  The vapour of this liquid irritates the eyes, skin and lungs. It depresses the action of the central nervous system and may cause damage to the liver.  Thus, care is needed to handle this chemical during the experiment.

      Since the chemicals used in this experiment are easily vaporize, the volume of substances added may be affected so the transfer process and the experiment should be done as fast as possible.

      Either trichloromethane – ethyl ethanoate mixture or hexane – propanone mixture should be stirred gently and extremely carefully with the thermometer in the pear shaped flask.

      Ethyl ethanoate, propanone and hexane are flammable.  It should be ensure that there is no fire around.

      Moreover, care has to be exercised while using ethyl ethanoate as it is highly flammable and its vapour may irritate the eyes and respiratory.

Propanone and trichloromethane should be disposed of separately as the two would react exothermically, which may lead to an explosion

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Discussion:

      From Raoult’s Law, it is known that in the ideal solution, the vapour pressure of each component is equal to the product of its mole fraction and its vapour pressure when pure.  For ideal mixture, neither the liquid curve nor the vapour curve is linear.  Moreover, the boiling point – composition graph for ideal solution should have not have any max point or min point between the curve.  By thess characteristics, it can be known that both liquid mixtures are not ideal mixtures.

      However, for the mixture of ethyl ethanoate and trichloromethane, the liquid curve shows the characteristic of negative deviation from Raoult’s Law.  As the mixture of liquids have dissimilar structures and have marked deviations from the linear boiling point - composition relationship.  There has a maximum boiling point also.  Furthermore, since the intermolecular force of attraction lower the vapour pressure, it is also a reason why the system shows a negative deviation from Raoult’s Law.

      In this case due to the formation of hydrogen bonds, the force of attraction between the molecules of the mixture is greater than the ethyl ethanoate molecules.  This increases in the intermolecular attraction between the ethyl ethanoate molecules decreases its tendency to escape from the liquid to vapour phase.  The vapour pressure therefore decreases and the boiling point increases.  As hydrogen bonds are being formed in this case, a rise in the temperature is observed when two liquids are mixed.

      Besides, for the mixture of hexane and propanone, the liquid curve shows the characteristic of positive deviation from Raoult’s Law.  As the mixture of the liquids have dissimilar structures and have marked deviations from the linear boiling point - composition relationship.  There has a minimum boiling point also.  Furthermore, since the interaction is weaker after the mixing of the two miscible liquids.  The tendency for the molecules to escape is higher than from the pure liquids.  The enthalpy change is known to be endothermic as the energy need for the breaking interaction between molecules in pure solution is greater than the energy need for the formation of new interaction between two kinds of molecules in mixture.

      In such case, the molecules are held together by hydrogen bonds.  On adding hexane to propanone, the hexane molecules interpose between the molecules of propanone and break the hydrogen bonds thereby reducing the intermolecular attraction between propanone molecules.  This increases the tendency of the propanone molecules to escape more readily from the liquid to vapour phase.  The vapour phase therefore increases and the boiling point decreases.

The escape tendency of molecules in the mixture is higher than expected from the Raoult's Law.  Therefore, this mixture has a positive deviation compared with ideal solution.

TABLE 1

TABLE 2

Reference:

2. SAT II Chemistry – Barron
3. Dictionary of Chemistry – Andrew Hunt

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