1. What two effects do impurities have on the melting point of an organic compound?
Soluble impurities affect the melting point of a solid in the following manner.
a. The presence of an impurity in the molten compound reduces its vapor pressure thus lowering the melting point of the compound.
b. Broaden the melting point range.
2. For what tow purpose are melting points routinely used
a. To determine the identity of an organic solid.
b. To determine the purity of an organic solid.
3. What effects on the measured melting point would you expect in each of the following cases?
a) Presence of pieces of glass in the sample
b) Presence of solvent within the crystals
Melting point depression, Melting point range broaden.
c) Using too large sample when determining the melting point
Too much material will broaden range &may increase melting point
1. Define the following
A. Melting point
B. Intermolecular forces
C. Hydrogen bonding
2. Arrange the following IMF according to there strength
Dipole-Dipole interaction forces, Hydrogen bonding, Van der Waals forces
3. How does the atmospheric pressure affect the melting point?
4. What is the purpose of determining the melting point in this lab?
5. Why is this method not used for finding the melting points of inorganic ionic compounds?
1. What is the purpose of recrystallization?
It is the primary method of purifying crude solid compounds.
2. What is the recrystallization method based on?
It is a process based on the differences in solubilities of solids in a given solvent system.
3. How can you increase the solubility of a compound?
You can increase it with added heat.
4. How do you get rid of insoluble impurities?
They are removed through the method of hot filteration.
5. How do you separate the purified solid crystals from the surrounding liq.(called Mother Liquor)?
Suction or Vacuum Filteration.
6. What should you do if the single solvent does not fulfill all the four requirements?
You should create a "Solvent Pair" where a second solvent miscible with the first solvent is added.
7. How should the "solvent pair" work?
One solvent should dissolve the solute well while the other should dissolve the solute poorly.
8. When must a Hot Filtration be done during a recrystallization?
There are insoluble impurities remaining in the solution and there are colored, soluble impurities which necessitates charcoal.
9. Why are the apparatus used in the Hot filtration process heated before use?
To prevent pre-mature precipitation of the solid on the funnel or in the receiver.
10. What is Charcoal used for?
They are used to remove colored, soluble impurities
11. How does Charcoal work?
The colored substances attach to the activated surfaces of the Charcoal by adsorption.
12. At what temp. are the best crystals obtained?
At room temperature
13. What is a disadvantage of using Ice temp. for recrystillization?
If the flask is cooled in an ice bath crystallization time is shortened but crystal size may be smaller; higher yield.
14. What are some other methods to initiate crystal growth?
Scratching, seeding, and reducing the volume of the solvent through evaporation or adding and incompatible solvent to the sys.
Answers for Recrystallization Experiment’s questions
1. Why is it recommended to use no more than the minimum amount of solvent than is necessary to dissolve the solid being recrystallized?
A minimum amount of solvent will minimize the amount of desired compound that remains dissolved in the solvent and must be thrown away.
2. Why should the hot filtrate be allowed to cool slowly as the desired product...
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