Experiment 2: Purification of Organic Compounds (compiled)

Question 1

What is Experiment 2 entitled:

Answer 1

Purification of Organic Compounds

Question 2

A1. What is purification by Recrystallization?

Answer 2

It is to dissolve an impurified substance in a solvent and again put in a crystalline form under conditions that ensure greater purity.

Question 3

A2. In Recrystallization, what must a solvent do?

Answer 3

It must dissolve the impurity B more freely than compound A

It must show a high solubility for Compound A when hot, but only a moderate solubility for Compound A when cold

Question 4

A3. What will happen to Compound A after impurity B is removed?

Answer 4

It will precipitate/crystallize in a supersaturated solution

Question 5

A4.Recrystallization experiment procedure

Answer 5

Materials

1. C6H5NHCOCH3 or acetanilide impurified by NaCl (Sodium chloride)
2. Tap water*
3. C32H22N6Na2O6S2 (congo red)
4. Animal charcoal

Procedure

1. Place 5 grams of impure acetanilide ( 90% C6H5NHCOCH3 + 10% NaCl), 150mL tap water and 1-2 drops of C22H22N6Na2O6S2 in a beaker.
2. Heat and stir until C6H5NHCOCH3 dissolves, then add 1 gram of animal charcoal.
3. Filter solution; filtrate must be colorless.
4. Collect equal amounts of filtrate in two separate containers. Cover one with a watch glass then set aside; the other placed in ice water.
5. After, combine crystals, filter and wash.
6. Remove crystals spread apart on watch glass to dry. Cover with filter paper.
7. Weigh, then calculate percentage yield.

Question 6

A5. Percentage Yield formula:

Answer 6

(Actual yield / theoretical yield) * 100

Theoretical yield: 5 grams x .90%

Question 7

A6. Why are room-temperature crystals larger than ice-water crystals?

Answer 7

Warmer temperatures produce more singular and larger crystals because they form slowly as the molecules are further apart. The dissolved crystals weren’t rushed to form, so the impurities were not forced into the crystal pattern

Solutions in colder temperatures contract and force the minerals closer together, creating bonds while rapidly trapping impurities in their structure.

Question 8

B1. What is Sublimation?

Answer 8

It is a process that occurs when a heated substance passes directly from solid to gas, and when a cooled substance passes from vapor/gas to solid without appearing at liquid state

Question 9

B2. Why does sublimation happen to substances with non-volatile impurities?

Answer 9

This is because of the presence of high-temperature vapor pressure of a substance below the substance’s melting point

Question 10

B3. Sublimation procedure:

Answer 10

Materials

1. C6H5COOH or Benzoic acid impurified by NaCl (Sodium chloride)

Procedure

1. Place 1 gram of C6H5COOH contaminated with NaCl in a clean and dry 400 mL beaker.
2. Cover it by placing an Erlenmeyer flask on top, half-filled with tap water.
3. Heat slowly. Then turn off
4. After, remove Erlenmeyer flask and scrape off formed crystals.
5. Transfer sublimate crystals in a dry test tube.

Question 11

B4. Why was the C6H5COOH considered impure?

Answer 11

It was contaminated by NaCl Sodium chloride)

Question 12

B5. What happened to impure C6H5COOH when it was heated?

Answer 12

It vaporized into a gaseous phase

Question 13

B6. What happened to C6H5COOH when the burner was turned off?

Answer 13

It was cooled and transitioned into a solid phase by crystallizing

Question 14

C1. What is Distillation?

Answer 14

A process wherein heat converts liquid to vapor, and vapor is condensed back to liquid distillate through cooling

Question 15

C2. What happens temperature-wise when a pure liquid is distilled?

Answer 15

The constant temperature at which the liquid is distilled is over the boiling temperature

Question 16

C3. What will happen if volatile impurities are present?

Answer 16

They will vaporize when the mixture is boiled

Question 17

C4. The vapor pressure of pure substance and impurity will _____ at any given temperature.

Therefore, pure substance and impurity will distill at _____ rates

Answer 17

Seldom be the same / not always similar;

Unequal

Question 18

C5. Why will boiling points not remain the same during distillation?

Answer 18

Because the liquid’s composition will be altered before and after

Question 19

C6. Index of purity in distillation

Answer 19

Boiling point range

Question 20

C7. Distillation test procedure:

Answer 20

Materials

1. C2H5OH or Colored alcoholic solution

Procedure

1. Prepare a simple distillation setup
2. Pour 120 cc colored alcoholic solution to the distilling flask and heat.
3. Note thermometer reading at first drop.
4. Record temperatures for every 5 cc (15 drops).
5. Test flammability of distillate by igniting 5 cc of it on a watch glass/evaporating dish and igniting it. Record length of time it burns.

Question 21

C8. In a distillation setup, why should you not fasten the receiver to the condenser with a tightly-fitting cork?

Answer 21

With a tightly-fitting cork, the pressure inside the condenser will not be released and will lead to an explosion

Question 22

C9. Why should a distilling flask not be filled more than half full?

Answer 22

A full flask may lead to:

• Liquid overboiling into condenser without vaporizing
• Lack of space for vapors to rise