Lesson 3.3. Recrystallization and Melting Points

pure compound

homogeneous sample consisting only of molecules having the same structure

rarely pure

A compound formed in a chemical reaction or extracted from some natural source is ____________ when initially isolated.

Recrystallization

• involves dissolving the solid in an appropriate solvent at an elevated temperature and allowing the crystals to re-form on cooling, so that any impurities remain in solution

• uses solvents and heating

Sublimation

• alternative approach that involves melting the solid in the absence of solvent and then allowing the crystals to reform so that impurities are left in the melt

• direct method

Deposition

gaseous vapor will turn solid

high

Pure compounds have ___________ melting point

Selection

TECHNIQUES in RECRYSTALLIZATION

1. _____________ of an appropriate solvent.

a. The desired compound should be reasonably _______ in the hot solvent.

b. It should be ___________________________ in the cold solvent.

c. Often the reference temperature for determination of the solubility in “cold” solvent is taken to be ____________________.

2. _____________ of the solid to be purified in the solvent near or at its boiling point.

3. _____________ with an activated charcoal, if necessary, to remove colored impurities and filtration of the HOT solution to remove insoluble impurities and excess activated charcoal.

4. __________ of crystalline solid from the solution as it cools.

5. __________ of the purified solid by filtration.

6. __________ the crystals.

1 = ?

soluble (1g/20 mL)

TECHNIQUES in RECRYSTALLIZATION

1. _____________ of an appropriate solvent.

a. The desired compound should be reasonably _______ in the hot solvent.

b. It should be ___________________________ in the cold solvent.

c. Often the reference temperature for determination of the solubility in “cold” solvent is taken to be ____________________.

2. _____________ of the solid to be purified in the solvent near or at its boiling point.

3. _____________ with an activated charcoal, if necessary, to remove colored impurities and filtration of the HOT solution to remove insoluble impurities and excess activated charcoal.

4. __________ of crystalline solid from the solution as it cools.

5. __________ of the purified solid by filtration.

6. __________ the crystals.

1.a. = ?

insoluble or nearly insoluble

TECHNIQUES in RECRYSTALLIZATION

1. _____________ of an appropriate solvent.

a. The desired compound should be reasonably _______ in the hot solvent.

b. It should be ___________________________ in the cold solvent.

c. Often the reference temperature for determination of the solubility in “cold” solvent is taken to be ____________________.

2. _____________ of the solid to be purified in the solvent near or at its boiling point.

3. _____________ with an activated charcoal, if necessary, to remove colored impurities and filtration of the HOT solution to remove insoluble impurities and excess activated charcoal.

4. __________ of crystalline solid from the solution as it cools.

5. __________ of the purified solid by filtration.

6. __________ the crystals.

1.b. = ?

room temperature

TECHNIQUES in RECRYSTALLIZATION

1. _____________ of an appropriate solvent.

a. The desired compound should be reasonably _______ in the hot solvent.

b. It should be ___________________________ in the cold solvent.

c. Often the reference temperature for determination of the solubility in “cold” solvent is taken to be ____________________.

2. _____________ of the solid to be purified in the solvent near or at its boiling point.

3. _____________ with an activated charcoal, if necessary, to remove colored impurities and filtration of the HOT solution to remove insoluble impurities and excess activated charcoal.

4. __________ of crystalline solid from the solution as it cools.

5. __________ of the purified solid by filtration.

6. __________ the crystals.

1.c. = ?

Dissolution

TECHNIQUES in RECRYSTALLIZATION

1. _____________ of an appropriate solvent.

a. The desired compound should be reasonably _______ in the hot solvent.

b. It should be ___________________________ in the cold solvent.

c. Often the reference temperature for determination of the solubility in “cold” solvent is taken to be ____________________.

2. _____________ of the solid to be purified in the solvent near or at its boiling point.

3. _____________ with an activated charcoal, if necessary, to remove colored impurities and filtration of the HOT solution to remove insoluble impurities and excess activated charcoal.

4. __________ of crystalline solid from the solution as it cools.

5. __________ of the purified solid by filtration.

6. __________ the crystals.

2 = ?

Decoloration

TECHNIQUES in RECRYSTALLIZATION

1. _____________ of an appropriate solvent.

a. The desired compound should be reasonably _______ in the hot solvent.

b. It should be ___________________________ in the cold solvent.

c. Often the reference temperature for determination of the solubility in “cold” solvent is taken to be ____________________.

2. _____________ of the solid to be purified in the solvent near or at its boiling point.

3. _____________ with an activated charcoal, if necessary, to remove colored impurities and filtration of the HOT solution to remove insoluble impurities and excess activated charcoal.

4. __________ of crystalline solid from the solution as it cools.

5. __________ of the purified solid by filtration.

6. __________ the crystals.

3 = ?

Formation

TECHNIQUES in RECRYSTALLIZATION

1. _____________ of an appropriate solvent.

a. The desired compound should be reasonably _______ in the hot solvent.

b. It should be ___________________________ in the cold solvent.

c. Often the reference temperature for determination of the solubility in “cold” solvent is taken to be ____________________.

2. _____________ of the solid to be purified in the solvent near or at its boiling point.

3. _____________ with an activated charcoal, if necessary, to remove colored impurities and filtration of the HOT solution to remove insoluble impurities and excess activated charcoal.

4. __________ of crystalline solid from the solution as it cools.

5. __________ of the purified solid by filtration.

6. __________ the crystals.

4 = ?

Isolation

TECHNIQUES in RECRYSTALLIZATION

1. _____________ of an appropriate solvent.

a. The desired compound should be reasonably _______ in the hot solvent.

b. It should be ___________________________ in the cold solvent.

c. Often the reference temperature for determination of the solubility in “cold” solvent is taken to be ____________________.

2. _____________ of the solid to be purified in the solvent near or at its boiling point.

3. _____________ with an activated charcoal, if necessary, to remove colored impurities and filtration of the HOT solution to remove insoluble impurities and excess activated charcoal.

4. __________ of crystalline solid from the solution as it cools.

5. __________ of the purified solid by filtration.

6. __________ the crystals.

5 = ?

Drying

TECHNIQUES in RECRYSTALLIZATION

1. _____________ of an appropriate solvent.

a. The desired compound should be reasonably _______ in the hot solvent.

b. It should be ___________________________ in the cold solvent.

c. Often the reference temperature for determination of the solubility in “cold” solvent is taken to be ____________________.

2. _____________ of the solid to be purified in the solvent near or at its boiling point.

3. _____________ with an activated charcoal, if necessary, to remove colored impurities and filtration of the HOT solution to remove insoluble impurities and excess activated charcoal.

4. __________ of crystalline solid from the solution as it cools.

5. __________ of the purified solid by filtration.

6. __________ the crystals.

6 = ?

Differential Scanning Calorimetry

Apparatus used to determine the specific melting point of solids

capillary tube

TECHNIQUES in MELTING POINT DETERMINATION:

1. Suspend/Fill 1 mm of a the sample in a _________________, closed on one end by heating it in using an alcohol lamp/bunsen burner.

2. Using a ______________, attach the capillary tube to a thermometer.

3. Submerge the thermometer in the ____________________.

4. Observe and determine the ___________________ when the solid melts.

a. Take note of 2 temperatures: T1 = temp when the solid begins to melt and T2 = when the solid is fully melted. Then, take the average.

1 = ?

rubber band

TECHNIQUES in MELTING POINT DETERMINATION:

1. Suspend/Fill 1 mm of a the sample in a _________________, closed on one end by heating it in using an alcohol lamp/bunsen burner.

2. Using a ______________, attach the capillary tube to a thermometer.

3. Submerge the thermometer in the ____________________.

4. Observe and determine the ___________________ when the solid melts.

a. Take note of 2 temperatures: T1 = temp when the solid begins to melt and T2 = when the solid is fully melted. Then, take the average.

2 = ?

oil (cooking oil) bath

TECHNIQUES in MELTING POINT DETERMINATION:

1. Suspend/Fill 1 mm of a the sample in a _________________, closed on one end by heating it in using an alcohol lamp/bunsen burner.

2. Using a ______________, attach the capillary tube to a thermometer.

3. Submerge the thermometer in the ____________________.

4. Observe and determine the ___________________ when the solid melts.

a. Take note of 2 temperatures: T1 = temp when the solid begins to melt and T2 = when the solid is fully melted. Then, take the average.

3 = ?

temperature

TECHNIQUES in MELTING POINT DETERMINATION:

1. Suspend/Fill 1 mm of a sample in a _________________, closed on one end by heating it in using an alcohol lamp/bunsen burner.

2. Using a ______________, attach the capillary tube to a thermometer.

3. Submerge the thermometer in the ____________________.

4. Observe and determine the ___________________ when the solid melts.

a. Take note of 2 temperatures: T1 = temp when the solid begins to melt and T2 = when the solid is fully melted. Then, take the average (T1+T2/2).

4 = ?