Experiment#3: Recrystallization

Question 1

RECRYSTALLIZATION
Enumerate the 4 requirements a solvent must have.

Answer 1

• Low BP, so oiling out doesn’t occur.
• Volatile, so the solvent can be vaporized from the sample.
• Chemically Inert - solvent must not decompose or oxidize the substance.
• Unreactive with the solute.

Question 2

The deposition of crystals from a solution.

Answer 2

Crystallization

Question 3

Common method used to purify organic compounds that are solid at room temperature.

Answer 3

Crystallization

Question 4

CRYSTALLIZATION
During crystallization, why is it that molecules have the tendency to be attached to a growing crystal composed with the same type of molecules?

Answer 4

Because they fit better in the molecule’s crystal lattice.

Question 5

A fixed and rigid arrangement of atoms, molecules or ions.

Answer 5

Lattice Structure

Question 6

LATTICE STRUCTURE
In each unit cell, there are 8 spheres at each end of a three-dimensional cube that represent an atom, ion, or molecule. What are these spheres called?

Answer 6

Lattice Point

Question 7

LATTICE STRUCTURE
Repeating unit cells form the ______ ______ of a crystalline solid.

Answer 7

Lattice Structure

Question 8

RECRYSTALLIZATION
What 2 factors does Recrystallization take advantage of being that it is a common method of purifying organic substances from the traces of foreign substances?

Answer 8

1) The difference in solubility at different temperatures and,
2) The tendency of a growing crystal to exclude anything that does not fit into its lattice.

Question 9

The maximum amount of solute that dissolves in a given volume of solvent at a given temperature.

Answer 9

Solubility

Question 10

RIGHT or WRONG | SOLUBILITY
NaCl - 100 mL water/35.9 g at 25 °C

Is this unit of solubility correct?

Answer 10

WRONG. The unit for solubility is grams of solute/mL of solvent meaning if we were to correct this, it would be:

NaCl - 35.9g/100 mL water at 25 °C

Question 11

SOLUBILITY
The one to be dissolved.

Answer 11

Solute

Question 12

SOLUBILITY
The one used to dissolve.

Answer 12

Solvent

Question 13

SOLUBILITY
What is the unit for solubility?

Answer 13

grams of solute/mL of solvent

Question 14

A solvent that shows the desired solubility behavior for the substance to be crystallized.

Answer 14

Recrystallizing Solvent

Question 15

RECRYSTALLIZING SOLVENT
What are the 3 requirements in picking a solvent for the experiment?

Answer 15

• INSOLUBLE at Room Temperature
• SOLUBLE when Heated
• INSOLUBLE/CRYSTALLIZES when Cooled

Question 16

TRUE OR FALSE | RECRYSTALLIZATION
Unwanted impurities should be soluble at room temperature or insoluble during heating.

Answer 16

TRUE

Question 17

PROCESS OF RECRYSTALLIZATION
If there are colored impurities, what is added to the hot solution?

Answer 17

Activated Carbon

Question 18

PROCESS OF RECRYSTALLIZATION
If crystallization does not happen, what 3 things can be done in order to promote crystal growth?

Answer 18

1) Scratching the inside surface of the glass vessel
2) Adding in a ”seed” crystal of the compound
3) Dipping a glass rod in the solution, aerating it to dry off the solvent before returning the rod to the solution

Question 19

PROCESS OF RECRYSTALLIZATION
Enumerate the first 3 steps of Recrystallization.

Answer 19

1) Selection of Solvent
2) Dissolution of crystal in the solvent
3) Filtration of the hot mixture

Question 20

PROCESS OF RECRYSTALLIZATION
Enumerate the last 2 steps of Recrystallization.

Answer 20

4) Slow cooling to allow crystallization
5) Removal of soluble impurities

Question 21

PROCESS OF RECRYSTALLIZATION
Performed to separate insoluble impurities from the hot solution of the compound.

Answer 21

Hot Gravity Filtration

Question 22

PROCESS OF RECRYSTALLIZATION
Why is it a requirement for Hot Gravity Filtration to be done as rapidly as possible?

Answer 22

To avoid premature recrystallization on the funnel.

Question 23

PROCESS OF RECRYSTALLIZATION
Used to maximize the flow rate during the filtration because of its larger surface area.

Answer 23

Fluted Filter Paper

Question 24

PROCESS OF RECRYSTALLIZATION
Used to minimize clogging during filtration.

Answer 24

Short-stemmed Funnel

Question 25

**PROCESS OF RECRYSTALLIZATION** This employs a **vacuum** to **hasten the passage of filtrate** through the filter paper.

Answer 25

Suction Filtration

Question 26

**PROCESS OF RECRYSTALLIZATION** What is Suction Filtration also known as?

Answer 26

Vacuum Filtration

Question 27

**PROCESS OF RECRYSTALLIZATION** The name of the porous funnel used in Suction Filtration.

Answer 27

Buchner Funnel

Question 28

**PROCESS OF RECRYSTALLIZATION** The name of the flask used in Suction Filtration.

Answer 28

Suction Flask

Question 29

**PROCESS OF RECRYSTALLIZATION** What are the **4 different solvents** to choose from in the experiment?

Answer 29

- Water - Ethanol - Benzene - Ethyl Acetate

Question 30

**PROCESS OF RECRYSTALLIZATION** What compound do we use in the Recrystallization experiment?

Answer 30

Acetanilide

Question 31

**PROCESS OF RECRYSTALLIZATION** Which of the 4 aforementioned solvents do we eventually end up using in the experiment?

Answer 31

Water, since the *acetanilide* - once dissolved - is **insoluble** in **room temperature**, **soluble** when heated, and **insoluble/crystallizes** when cooled.

Question 32

**TRUE or FALSE | PROCESS OF RECRYSTALLIZATION** Adding **activated carbon** while the solution is boiling will cause it to boil over.

Answer 32

**TRUE**

Question 33

**PROCESS OF RECRYSTALLIZATION** What do we call a substance ***after*** it has been filtered?

Answer 33

Filtrate

Question 34

**PROCESS OF RECRYSTALLIZATION** In Suction Filtration, what a) ends up ***inside*** the suction flask and what b) ends up left behind on top of the Buchner funnel?

Answer 34

A) Filtrate (*Solvent and Impurities*) B) Recovered Acetanilide

Question 35

**PROCESS OF RECRYSTALLIZATION** If the initial mass of impure acetanilide is **3.506 g** and the mass of the recrystallized acetanilide is **1.006 g**, what is the percent recovery?

Answer 35

**28.69%** [% Recovery = (***mass of recrystallized solid***/***mass of crude sample***)x100] = (1.006/3.506)x100 = **28.69%**