Recrystallization

Question 1

What are the ideal characteristics for the solvent with respect to solute?

Answer 1

1. ) Dissolves the solute completely when a solvent is hot (at the boiling point of the solvent)
   2) Precipitates the solute out when the solvent is cold.
2. ) Does not react with the solute.

Question 2

What are the advantages if the solute and solvent are too similar in polarities?

Answer 2

• Advantages: Solute will definitely dissolve when hot
• Disadvantage: Solute will dissolve when cold (poor recovery).

Question 3

What are the advantages if the solute and solvent have very different polarities?

Answer 3

• Advantage: Solute will definitely precipitate when cold.
• Disadvantage: Solute may not completely dissolve when hot which does not allow for impurities to be removed.

Question 4

What should the polarities of the solute and solvent be?

Answer 4

• They should have a balance in polarity.

Question 5

What are the ideal characteristics for the solvent with respect to impurities?

Answer 5

1. ) Does not dissolve the impurities at all.
   - So impurities can be filtered off from the hot solution
   - The solvent must be able to dissolve the solute but not the impurities when hot.

OR

2. ) Dissolve impurities very well (even when the solvent is cold)
   - So impurities along with the cold solvent can be removed from crystals via filtration.
   - The solvent must precipitate solute and keep impurities in solution when cold.

If the solvent does neither of these than the solvent is suspect.

Some other characteristics include the solvent being nonflammable, inexpensive and very volatile so it can be removed from the crystals.

Question 7

How much solvent should be used when dissolving the solute in it and why?

Answer 7

• Dissolve the solute in the minimum amount of solvent to make sure the solution is saturated.

Question 8

What should you do before adding drops to ensure only the minimum amount of solvent is used and why?

Answer 8

• Allow sufficient time for dissolving to occur to keep the solution as saturated as possible.

Question 9

Why is it best that the solid is crushed to powder?

Answer 9

• The smaller surface makes the solid easier to dissolve.

Question 10

If adding solvent fails to dissolve any more solid, it is likely that _______. How can they be removed?

Answer 10

• Insoluble impurities are present.
• They can be removed in the filtering suspended solids step (Step 4).

Do not heat the crystal solution too strongly at this step because the solvent will boil away bringing the impure solid out of the solution.

Question 11

What characteristics should solvent pairs have? Why are solvent pairs used?

Answer 11

1. ) Should be miscible in one another.
2. ) One solvent has to be a poorer solvent for the solute than the other.
3. ) Used when no one solvent has ideal properties with respect to a solute.

Question 12

What should you do if you add too much solvent? What happens if you do not do this?

Answer 12

• You should evaporate the solvent to the point of saturation.
• If you do not do this then you will lose stuff.

Question 13

When using solvent pairs, what happens when you add the 2nd solvent to the first? What is observed?

Answer 13

• This results in a solvent mixture where the solute is less soluble.
• You would observe cloudiness (indicates initial crystal formation).

Question 14

What are the danger and remedy if the 2nd solvent has a lower boiling point than the first in a mixture?

Answer 14

• Danger: This can cause sudden vigorous boiling of the mixture and the hot solvent might spew from the apparatus.
• Remedy: Lower the temperature of the solution slightly before adding the 2nd solvent.

Question 15

What happens if you add too much of the first solvent?

Answer 15

• You will need to use an extremely large amount of 2nd solvent to start crystallization.

Question 16

When crystallizing the solute, why should you remove the boiling stick? Why would this easlily happen? What is the problem if it does?

Answer 16

• So crystals do not form on the boiling stick.
• easily would easily happen due to heterogeneous nucleation.
• This is a problem because you would lose some of your crystals (affects % recovery). You would need to scrape off the crystals.

Question 17

Why should you cool the solution slowly?

Answer 17

• To keep the solution saturated.
• If cooled too fast, solid will come “crashing out” of the solution as powder, precipitating impurities along with it.
• Does not affect % recovery but affects purity.

Question 19

How do you ensure slow cooling?

Answer 19

• You need to insulate the test tube with paper towels.

Question 20

Why should you not disturb or move crystallizing solution?

Answer 20

• Because if disturbed, too many nuclei for crystallizing form leading to small crystals that come out of solution along with impurities.
• Does not affect % recovery but affects purity.

Question 21

The colder the solution _____

What would be the limitation?

Answer 21

• The better the recovery of crystals.
• Limitation: If the solvent would freeze when the solution is at a cold temperature.

Question 22

Why do you lose crystals at the washing and collecting stages? What does this effect?

Answer 22

• Washing: Some of the crystals will dissolve.
• Collecting: Some of the crystals adhere to the filter paper.
• This effects % recovery.

Question 23

Why is drying necessary?

Answer 23

• Wet crystals have a lower melting point which affects purity.

Question 24

What happens if you discover that your % recovery is greater than 100%?

Answer 24

• Your crystals are still wet.

Question 25

How could you have a lower % recovery if your crystals are wet?

Answer 25

• Some of the wet crystals are more likely to stick to the filter paper.

Question 26

With what apparatus do you dry the crystals with and what happens if you remove crystals too early? What does this impact?

Answer 26

• If removed too early, tiny paper particles will contaminate the crystals when they are scraped off the filter paper. Affects purity

OR

2.) Crystals remain on the filter paper. Affects % recovery

Question 27

What is another source of error for low percent recovery?

Answer 27

• Materials left behind after each transfer (in test tube, on spatula, on weighing paper.

Question 28

What is the % recovery formula?

Answer 28

Mass of recrystallized solid/mass of original solid x 100%

Question 29

Why is crystallization used? What is an example in the human body?

Answer 29

• To purify solids.
• An example in the human body would be kidney stones.
• Calcium oxalate is the most common to form into crystals. It is soluble in urine and blood.
• Kidney stones form when the saturation level goes up when urine levels are low, a high concentration of stone promoters.
• Nucleation of calcium phosphate (heterogeneous nucleation) presents a solid surface on which seed crystals from and eventually grow.

Question 31

The solubility of the desired solute have to be ideally different than the solubility of the impurity in a particular solvent? What happens if it is not?

Answer 31

• If not, then the concentration of the impurity must be significantly low to not precipitate along with the desired solute.

Question 32

Name the desirable and undesirable qualities of water, ethanol, acetone, and ethyl acetone

Answer 32

1. ) Water:
   - Desirable: Nonflammable, inexpensive, nontoxic, and will dissolve a large variety of polar organic molecules.
   - Undesirable: Its high boiling point and high heat of vaporization make it difficult to remove from crystals.
2. ) Ethanol:
   - Desirable: High boiling point makes it a better solvent for less polar molecules than methanol. It readily evaporates from crystals.
   - Undesirable: