EXERCISE 5 - RECRYSTALLIZATION AND SUBLIMATION

recrystallization

also known as fractional crystallization

re-formation of the crystals upon cooling

involves the dissolution of the solid in an appropriate solvent at an elevated temperature and the subsequent ____________________________, so that any impurities remain in solution

solids

almost all ____________ are more soluble in a hot than in a cold solvent

solution crystallization

_______________ takes advantage of the fact that almost all (question no. 3 answer) are more soluble in a hot than in a cold solvent

recrystallization process step 1

Selection of an appropriate solvent

recrystallization process step 2

Dissolution of the solid to be purified in the solvent

recrystallization process step 3

Decoloration with activated charcoal

recrystallization process step 4

Filtration of the hot solution to remove insoluble impurities

recrystallization process step 5

Formation of the crystals as it cools

recrystallization process step 6

Isolation of the purified crystals by filtration

recrystallization process step 7

Drying the crystals

selection of an appropriate solvent

most critical step in the (recrystallization) process

good solvent

solvent that will dissolve the solute in hot solution but not in cold solution

poor solvent

solvent that will dissolve the solute in cold solution but not in hot solution

organic compounds

most ____________________ dissolves in hot solvents than in cold ones

dissolution of the solid to be purified in the solvent

step in the recrystallization process where we add enough solvent to dissolve the solute to get a saturated solution at the boiling point of the solvent

too much solvent is used, supersaturated solution

common errors in dissolution of the solid to be purified in the solvent

decolorization with activated charcoal

step in the recrystallization process that removes high molecular weight impurities

high molecular weight

___________________ impurities are often colored and relatively less soluble

activated charcoal

high degree of microporosity; has large surface area

filtration of the hot solution to remove insoluble impurities

step done in the recrystallization process because sometimes the sample is contaminated with small quantity of insoluble impurity that will never dissolve

premature crystallization

it is important to filter the solution while hot to prevent __________________ during filtration

short-stemmed/stemless glass

a ________ or ________ funnel should be used to minimize crystallization in the funnel

fluted filter paper

using ___________ will minimize crystallization on the filter

Cooling

reduce the temperature
to induce crystal
formation

not be disturbed

generally the solution should ___________ as it cools, since this also leads to production of small crystals

Seeding

crystal of the original solid is added to the solution to induce crystallization, which may then be quite rapid

Scratching

a glass rod is to rub the inside surface of the crystallization vessel at or just above the air/solution interface

sharp edges

microscopic scratches provide ___________ where crystals may grow

Stirring and Agitation

induce bumping of crystals which causes to reformation of more crystals

airborne contaminants

during cooling and crystallization, the solution should be protected from ___________

production of small crystals

solution should not be disturbed as it cools, since this also leads to ______________

ice bath

cool the solution in an ___________ to hasten crystal formation

low

crystals easily form at a _______ temperature

small crystals

be careful not to use too low temperature as it may form ______________

oiling out

type of separation where the solute will separate from solution as an oil rather than
a solid

undesirable

oiling out is (desirable/undesirable) for purification of solutes because the oils usually contain significant amounts of impurities

pure, cold solvent

crystals normally are washed with a small amount of

vacuum filtration

the solid product is isolated by ______________ using a Büchner or Hirsch funnel and a clean, dry filter flask

vacuum filtration, gravitational
filtration

2 ways to isolate the solid product in isolation of purified crystals by filtration step

air or oven drying

removing the last traces of solvent from the crystalline product may be accomplished by ____________

below

in oven drying, the temperature of the oven must be __________ the melting point of the product

(weight of crystals recovered/weight of the original sample) x 100

% recovery formula

melting point determination

the purity of the crystals is usually assessed using ______________

decreases

presence of the impurities _____________ the melting point of the product

Silver Nitrate Test

__________ test is employed
to detect any sodium chloride
impurity left on the purified
crystals

white precipitate, turbid solution

____________ or ____________ indicates presence
of NaCl

sublimation

defined as the direct conversion of a substance from its solid phase to its gas phase without passing through the liquid phase

equilibrium between the solid and gas phases

sublimation entirely relies on the ___________________ rather than
solid and liquid phases

energy builds up

in sublimation, enough _________________ in the molecules during heating and then escapes into
gas phase

outer or lower vessel

this is where the impure sample is placed in a sublimation setup

inner or upper vessel

this is where the pure solid is formed again in a sublimation setup

cold

the upper vessel must be ______ for the pure solids to form on it

light and feathery crystals, round and scaly crystals, needle-like crystals, blocks of crystals

appearance/forms of crystals (4)

sublimate

the crystals, deposit, or material obtained when a substance is heated

sublimate

has high vapor pressure, low melting point, and more volatile

residue

the solid particle that remained on the dish

residue

has low vapor pressure, high melting point, and less or non-volatile

volatility

as a purification process, it also depends on
_________

residue

less volatile or non-volatile impurities remain as a ________

similar vapor pressures

sublimation cannot be used for separating compounds that
have __________________

barium chloride test

__________________ was employed to differentiate the organic substance from the inorganic residue

white precipitate

BaCl2 will react with Na2SO4 producing _______________ precipitate.

appearance of benzoic acid

colorless crystalline solid

chemical formula of benzoic acid

C7H6O2 or C6H5COOH

molecular weight of benzoic acid

122.12 g/mol

melting point of benzoic acid

122-123°C

boiling point of benzoic acid

249°C

use of benzoic acid

antifungal and preservative

synonym of benzoic acid

Carboxybenzene, Benzene carboxylic acid

density of benzoic acid

1.32 g/cm3

toxicity of benzoic acid

toxic to the lungs & nervous system

salicylic acid appearance

colorless needle- like crystals

Chemical formula of salicylic acid

C7H6O3

Molecular Weight of salicylic acid

138.12 g/mol

melting point of salicylic acid

158°C

boiling point of salicylic acid

211°C

use of salicylic acid

keratolytic, used to treat acne, seborrhea, corn, calluses and warts

Synonym of salicylic acid

2-hydroxybenzoic acid

density of salicylic acid

1.44 g/cm3

vapor pressure of salicylic acid

8.2 × 10-5 mmHg at 25°C

toxicity of salicylic acid

salicylism