Separation and Purification

Question 1

Separation relies on differences in which physical or chemical properties

Answer 1

Melting point, boiling point, solubility, polarity and acidity/basicity

Question 2

What does filtration do

Answer 2

Separates two phases, one passes through filter and other does not

Question 3

What is filter

Answer 3

A porous material such as paper, sintered glass or metal, porous polymer

Question 4

How does filtration work

Answer 4

Pressure difference forces the filtrate through

Question 5

Describe gravity filtration

Answer 5

Paper filter, for collecting filtrate

Question 6

Describe Buchner filtration

Answer 6

Paper or glass filer, vaccum applied. For collecting solid

Question 7

What does crystallisation require

Answer 7

Difference in solubility between desired compound and impurities

Question 8

Describe the procedure of crystallisation

Answer 8

Find a solvent that dissolves compound and when hot but not cold. Add a minimum quantity of hot solvent to dissolve. Allow to cool. Wait for crystals. Filter

Question 9

What does evaporation rely on

Answer 9

A large difference in boiling point.

Question 10

Describe evaporation

Answer 10

One component is essentially non-volatile. It is usually for the removal of a solvent. Accelerated by heat and/or vacuum

Question 11

What does distillation rely on

Answer 11

Boiling point differences

Question 12

Describe distillation

Answer 12

Liquid is in equilibrium with vapour. The more volatile substance has a higher vapour pressure. Liquid boils when vapour pressure is equal to atmospheric pressure, Vapour becomes enriched in more volatile compound. Cool the vapour to condense it

Question 13

What is simple distillation apparatus used for

Answer 13

Used for compounds with large differences in boiling point (at least 25 degrees).

Question 14

What is fractional distillation used for

Answer 14

Liquids with similar boiling points

Question 15

What does fractional distillation require

Answer 15

A fractionating column

Question 16

Describe sublimation

Answer 16

Some solids do not melt and convert straight to gas (sublime). This allows easy purification from non-volatile material

Question 17

What does liquid-liquid extraction rely on

Answer 17

Differences in solubility between two solvents

Question 18

What does liquid-liquid extraction require

Answer 18

Usually water/ organic solvent. Solvent must be immiscible with H2O. Solvent should be easy to evaporate (so low bp)

Question 19

What are the solubility trends for hydrocarbons in water

Answer 19

Decrease solubilty

Question 20

What are the solubility trends for polar groups in water

Answer 20

Increase solubility

Question 21

What are the solubility trends for hydrogen bonding groups in water

Answer 21

Increase solubility

Question 22

What are the solubility trends for charges in water

Answer 22

Often increase solubility

Question 23

What do you use acid/base chemistry for

Answer 23

To control solubility (salt formation increases aqueous solubility)

Question 24

What does chromatography rely on

Answer 24

Equilibrium of a substance between two phases. Stationary phase- immobilise solid/ liquid (in column or as 2 dimensional sheet). Mobile phase (elution)- a flowing liquid or gas (passes through column or up the sheet)

Question 25

What is elution

Answer 25

The motion of molecules along the column (molecules with higher affinity for stationary phase elute more slowly, increasing affinity for mobile phase speeds up elution).

Question 26

Why does relative affinity for mobile vs stationary phase depend on molecular structure

Answer 26

Hyydrogen bonding, polarity, van der Waals. Different compounds move at different speeds- emerge from the column at different times

Question 27

Describe planar chromatography

Answer 27

Paper stationary phase- cellulose stationary phase, water or water + organic solvent mobilise phase, useful for sugars, plants pigments and amino acids. 
Thin layer (TLC)- silica gel or alumina particles coated on plastic/ glass/ aluminium sheets, organic solvent mobile phase, widely used by organic chemists

Question 28

Describe planar chromatography

Answer 28

Apply spots of mixture to base-line. Stand in solvent to elute. Mark solvent front- calculate Rf vales (Rf= distance moved by compound/ distance moved by solvent)

Question 29

Describe planar chromatography

Answer 29

Coloured compounds visualised directly. Visualise colourless compounds by UV light or staining.

Question 30

How do you visualise colourless compounds by UV light in planar chromatography

Answer 30

Compounds may fluoresce (bright spots). Stationary phase often has fluorescent modifier added (dark spots on fluorescent green background)

Question 31

How do you visualise colourless compounds by staining in planar chromatography

Answer 31

Best stain depends on compounds being analysed. Iodine- stains many compounds brown. KMnO4- charring/ brown staining. Ninhydrin- staines amines blue

Question 32

Describe column chromatography

Answer 32

Gravity/ flash chromatography. For mg-grams. Glass column packed with silica gel/ alumina. Organic solvent mobilise phase flows through. Can be automated

Question 33

Describe how organic solvent mobile phase flows through

Answer 33

Flow rate increased using gas pressure (“flash”).
Collect fractions in separate tubes.
Check purity by TLC

Question 34

Describe how gravity/ flash chromatography can by automated

Answer 34

Disposable silica cartridges.
Automatic fraction collector.
UV monitoring.

Question 35

What is HPLC

Answer 35

High performance (or pressure) liquid chromatography

Question 36

Describe HPLC

Answer 36

From analytical scale (100 bar, stainless steel column and tubing). Detector (UV, refractive index, fluorescence, mass spectrometer etc.). Optional autosampler, fraction collector

Question 37

Describe gas chromotography

Answer 37

Mobile phase is inert gas (Ar, He, N2). Two types of column- packed and open tubular capillary. Contained in an oven. Many stationary phase materials