Ch 12 - Separation and Purification

Question 1

What is extraction?

Answer 1

combines 2 immiscible liquids, one of which easily dissolves the compound of interest
- carried out in a separatory funnel (one phase collected and the solvent is then evaporated)

Question 2

What is the difference between the aqueous and organic phase in an extraction?

Answer 2

• polar (water) layer is called the aqueous phase and dissolves compounds with hydrogen bonding or polarity
• nonpolar layer is called the organic phase and dissolves nonpolar compounds

Question 3

What can be used to increase solubility in extraction?

Answer 3

acid-base properties

Question 4

What is wash?

Answer 4

a reverse of extraction, in which a small amount of solute that dissolves impurities is run over the compound of interest

Question 5

What is filtration?

Answer 5

isolates a solid (residue) from a liquid (filtrate)

Question 6

What is the difference between gravity and vacuum filtration?

Answer 6

• gravity: used when the product of interest is in the filtrate; hot solvent is used to maintain solubility
• vacuum: used when the produce of interest is the solid; a vacuum is connected to the flask to pull the solvent through more quickly

Question 7

What happens in recrystallization?

Answer 7

• the product is dissolved in a minimum amount of hot solvent
• if the impurities are more soluble the crystals will reform while the flak cools, excluding the impurities

Question 8

What is distillation?

Answer 8

separates liquids according to different boiling points; the liquid with the lowest boiling point vaporizes first and is collected as the distillate

Question 9

What is simple distillation?

Answer 9

can be used if the boiling points are under 150 C and are at least 25 C apart

Question 10

What is vacuum distillation?

Answer 10

should be used if the boiling points are over 150 C to prevent degradation of the product

Question 11

What is fractional distillation?

Answer 11

should be used if the boiling points are less than 25 C apart because allows more refined separation of liquids by boiling point

Question 12

What do all forms of chromatography use?

Answer 12

2 phases to separate compounds based on physical and chemical properties (based on how strongly they adhere to the solid, or stationary, phase)

Question 13

What is the difference between the stationary and mobile phase of chromatography?

Answer 13

• stationary (adsorbent) usually a polar solid
• mobile runs through the stationary phase and usually a liquid or gas (this elutes the sample through the stationary phase)

Question 14

What happens to compounds with higher affinity for the stationary vs mobile phase in chromatography?

Answer 14

• compounds with higher affinity for the stationary phase have smaller retardation factors and take longer to pass through, if at all
• compounds with higher affinity for the mobile phase elute through more quickly

Question 15

How are compound separated in chromatography?

Answer 15

partitioning

Question 16

How are thin layer and paper chromatography used to identify a sample?

Answer 16

• stationary phase a polar material (silica, alumina, paper)
• mobile phase a nonpolar solvent, which climbs the card through capillary action
• the card is spotted and developed; Rf values can be calculated and compared to reference values

Question 17

What is reverse phase chromatography?

Answer 17

uses a nonpolar card with a polar solvent

Question 18

What is column chromatography?

Answer 18

utilizes polarity, size, or affinity to separate compounds based on their physical and chemical properties
- the less polar compounds travels most rapidly

Question 19

What are the stationary and mobile phase of column chromatography?

Answer 19

• stationary is a column containing silica or alumina beads

- mobile is a nonpolar solvent, which travels through the column by gravity

Question 20

What is ion exchange chromatography?

Answer 20

the beads are coated with charged substances to bind compounds with opposite charges

Question 21

What is size exclusion chromatography?

Answer 21

the beads have small pores which trap smaller compounds and allow larger compounds to travel through faster

Question 22

What is affinity chromatography?

Answer 22

the column is made to have high affinity for a compound by coating the beads with a receptor or antibody to the compound

Question 23

What is gas chromatography and its phases?

Answer 23

• separates vaporizable compounds according to how well they adhere to the adsorbent in the column
• stationary is a coil of crushed metal or a polymer
• mobile is a nonreactive gas

Question 24

What is mass spectrometry?

Answer 24

ionizes and fragments molecules and passes these fragments through a magnetic field to determine molecular weight or structure

Question 25

What is high performance liquid chromatography (HPLC)?

Answer 25

• similar to column chromatography but uses sophisticated computer-mediated solvent and temperature gradients
• used if the sample size is small or if forces such as capillary action will affect results
• formally called high pressure liquid chromatography

Question 26

What are the 3 intermolecular forces that affect solubility and how does extraction depend on them?

Answer 26

• H bonding: compounds that can do this, such as alcohols and acids, will move most easily into the aqueous layer
• dipole-dipole interactions: these compounds are less likely to move into the aqueous layer
• van der Waals (London): with only these interactions, compounds are least likely to move into the aqueous layer

Question 27

What must be true about the 2 solvents used for an extraction to work?

Answer 27

• the 2 solvents must be immiscible and must have different polarity or acid-base properties that allow a compound of interest to dissolve more easily in one than the other

Question 28

When doing an extraction, would it be better to do 3 extractions with 10 mL of solvent or one extraction with 30 mL?

Answer 28

• better to do 3 washess because more of the compound of interest would be extracted with multiple sequential extractions than one large one

Question 29

Would acid dissolve better in aqueous acid or base and why?

Answer 29

• acid dissolves better in aqueous base because it will dissociate to form the conjugate base and being more highly charged, will become more soluble
• like dissolves like applies to polarity, acids and bases dissolve more easily in solutions with opposite acid-base characteristics

Question 30

How is retardation factor (Rf) calculated?

Answer 30

Rf = distance spot moved/distance solvent front moved