exp 2

Question 1

what is RECRYSTALLIZATION?

Answer 1

process used to purify a solid compound from traces of foreign substances after isolated from a mixture

Question 2

The process of Recrystallization takes advantage of….? (2)

Answer 2

1. the effect of temperature on solubility
2. tendency of a growing crystal to exclude anything that does not fit into its LATTICE

Question 3

Criteria for choosing a SOLVENT for crystallizing a solute

Answer 3

solute compound CAN EASILY DISSOLVE when hot

and CRYSTALLIZES upon cooling

Question 4

5 steps in PURIFICATION by RECRYSTALLIZATION

Answer 4

1. selection of solvent
2. dissolution of crystals in minimum amount (recrystallization solvent) at higher temperature
   **if colored impurities are present, add decolorizing carbon to hot solution
3. filter hot mixture (hot gravity filtration)
4. slow cool to crystallize.
   **if nothing happens: (1) scratch inside surface of glass vessel ; (2) add “seed” crystal of compound
5. remove soluble impurities from mixture
   *suction filtration
   *wash with cold solvent

Question 5

3 important techniques in recrystallization

Answer 5

1. Hot Gravity Filtration (during recrystallization 1st)
2. Suction Filtration (during recrystallization 2nd)
3. Melting Point Determination (after recrystallization)

Question 6

Should HOT GRAVITY FILTRATION be done fast or slow?

Answer 6

fast - to avoid undesirable premature recrystallization

Question 7

2 main tools used for HOT GRAVITY FILTRATION

and why?

Answer 7

1. Short-stemmed funnel (minimize clogging)
2. Fluted filter paper (maximize filtration due to large surface area)

Question 8

When is SUCTION FILTRATION used?

Answer 8

Filtration of impurities but mixture should not be “hot/heated”

Question 9

Instead of GRAVITY, suction filtration uses what?

Answer 9

Vacuum

Question 10

2 main tools used for SUCTION Filtration

Answer 10

1. Buchner Funnel
2. Circular filter paper

Question 11

When will the melting point be determined?

Answer 11

When all solvent has been removed from the RECRYSTALLIZATION process

Question 12

MEANING:

wide melting point range
narrow melting point range

Answer 12

wide: impure
narrow: pure

Question 13

Impurities causes melting to occur at a ____ temperature

Answer 13

lower

Question 14

2 main SPECIAL EQUIPMENT to perform melting point analysis

AND their main required tool

Answer 14

1. Fischer-Johns hot stage apparatus (requires microscope cover glasses)
2. Digital melting point apparatus (requires capillary tubes)