Lab 3: Recrystallization and Melting Point

Question 1

What is true about melting point determination?

Answer 1

• The melting point is a physical measure of the forces holding the material into its crystal lattice
• The upper limit of the melting temperature range occurs when the last remaining crystal melts
• The lower limit of the melting temperature range occurs when the first crystal starts to melt
• Pure substances typically have a melting range of less than 2C

Question 2

Why is it important to allow the Mel-Temp to cool 15C before placing the second capillary tube in, and to set the voltage to 1-2C temp increase per min?

Answer 2

A rapid temperature increase will give a melting point range that is too high.

Question 3

How wide is the typical melting point range for a pure compound?

Answer 3

0.5C - 1.5C

Question 4

What is eutectic composition?

Answer 4

The composition of a mixture of two components that generates the lowest possible melting point.

Question 5

What are the steps to perform an accurate melting point determination?

Answer 5

1. Press the open end of a capillary tube into the solid sample
2. Invert the tube and tap gently to move the solid to the bottom of the capillary
3. Place the tube with sample into the melting point apparatus, and heat at 10C / min
4. Observe the material through the complete melting process to get a rough estimate of the melting range
5. Turn off and allow to cool by 15 degrees
6. Replace the existing tube with a new sample
7. Heat at 2 degrees per minute through the expected range.

Question 6

Which type of molecular interaction determines a molecule’s melting range?

Answer 6

Hydrogen bonding
dipole-dipole
van der waals

Question 7

How would having too much sample in the melting point tube affect a melting point measurement?

Answer 7

The melting point range would be broader than expected

Question 8

True or false: recrystallization is the process of dissolving a solid material into a suitable solvent or mixture of solvents and allowing the material to crystallize from this solution.

Answer 8

True

Question 9

True or false: for recrystallization, rapid cooling gives the best crystals

Answer 9

flase

Question 10

When is recrystallization not a suitable method to purify an organic material?

Answer 10

When the organic material is liquid instead of a solid

Question 11

while performing a recrystallization, a chemist notices that a small amount of the sample will not dissolve, even after the recrystallization solution has been boiling for some time. What should they do?

Answer 11

Perform a gravity filtration with a stemless funnel

Question 12

What are some things to know about recrystallization?

Answer 12

• the solvent should not chemically react with the compound
• the solvent should dissolve the compound while hot
• the solvent should not dissolve the compound when cold
• the solvent should either dissolve the impurities at all temps or not dissolve the impurities at all

Question 13

During the recrystallization of a solid, the sample is dissolved in hot solvent then filtered while hot. The filtrate is allowed to cool and the mixture is filtered a second time. What will result if the filtrate solution is cooled too fast?

Answer 13

small impure crystals will form

Question 14

While performing a recrystallization, a chemist accidentally uses twice the amount of the recrystallizing solvent, water, than what is optimal for this experiment. What most likely happened because of this?

Answer 14

The yield of recrystallized solute will be lower

Question 15

During the recrystallization of an impure solid sample of acetanilide, the sample is dissolved in hot water then filtered while hot. The filtrate is allowed to cool and the mixture is filtered a second time. What is a way to increase the percent recovery of the acetamide solution?

Answer 15

After the first filtration, allow the solution to cool slowly, then place the solution in an ice-water bath for about 10 minutes