Quiz 2 - Crystallization/Recrystallization/Extractions

Question 1

extraction

Answer 1

technique used to separate a mixture into its individual components via differences in polarity and solubility

Question 2

polarity

Answer 2

Polarity: distribution of partial charges or electron density throughout a molecule

Question 3

solubility

Answer 3

• the extent to which a solute will dissolve in a solvent
• “Like dissolves like” rule – relates solubility and polarity
• Like polarities solutes will dissolve in like polarity solvents

Question 4

miscibility

Answer 4

refers to the interaction between 2 liquids

Question 5

miscible

Answer 5

similar polarities will appear as one phase when mixed

Question 6

immiscible

Answer 6

dissimilar polarities will generate 2 layers when mixed
Identify layers using density

Question 7

typical densities

Answer 7

Halogenated organics > water > non-halogenated organics

***typically aqueous solution is on the bottom BUT cannot assume

Question 8

solid-liquid extraction

Answer 8

• Separate a mixture of solids by adding a solvent that will dissolve desired solids and leave behind insoluble solids
• Filtration is done to finalize separation

Ex/ mixture of sand and salt
Salt will be dissolved by water through filter paper and sand will remain on top of filter paper

Question 9

liquid-liquid extraction

Answer 9

• Separate two solutes by using 2 immiscible solvents with opposing polarity
• using a separatory funnel (i.e. our week 3 experiment)
• drain out solution to get desired layer/solution

Question 10

chemically active extraction

Answer 10

• Separate 2 solutes with similar polarity
  *Liquid-liquid extraction will not be effective.
• Must make compound prefer water for a solvent by making it more polar via acids/bases protonation/deprotonation
• The charged ion will stay in the aqueous layer and the less polar molecule will separate in the organic layer

Question 11

Kd

Answer 11

• distribution coefficient that provides a numerical representation of the solubility equilibrium of a given solute in a given solvent system

Question 12

Kd equation

Answer 12

• Kd = solute organic / solute aqueous
• Kd > 1 → solute prefers organic layer
• Kd < 1 → solute prefers aqueous layer
• Kd = 1 → solute is distributed equally between both layers

Question 13

most effective extractions?

Answer 13

• Multiple smaller extractions are more effective than one large extraction

Question 14

overall chemical reaction of week 3 extraction

Answer 14

• mixture in methyl orange is filtered and heated
• insoluble cellulose filtered off
• soluble benzoic acid and methyl orange in organic layer
• NaOH makes benzoic acid negative into benzoate ion
• Benzoate ion is separated into aqueous layer and methyl orange stays in organic layer
• HCl added to reform benzoic acid (impure solid with some MO)

Question 15

Crystallization

Answer 15

• formation of new, insoluble product by chemical reaction
• This product then precipitates out of the reaction solution as a solid containing many trapped impurities

Question 16

Recrystallization

Answer 16

• does not involve any chemical reaction
• The crude product is dissolved into solution; conditions are changed that allow crystals to reform
  **produces a more pure final product
  **can be used as a method to remove impurities

Question 17

where do impurities come from?

Answer 17

1. side products of chemical reactions
2. Unreacted starting material
3. Contaminants

Question 18

crystal formation

Answer 18

• When crystals are being formed → the same individual molecules will fit in a specific crystal lattice structure
• Equilibrium process that produces very pure material
• Form quickly ⇒ impurities can become trapped within the crystal lattice resulting in an impure solid
• Form slowly/controlled ⇒ impurities are excluded from the crystal lattice resulting in a pure solid

Question 19

Extraction of Benzoic Acid Activity

Answer 19

• Extract containing benzoate will contain impurity of methyl orange
• Benzoate ion is soluble in water
• Adding HCl will reform benzoic acid from benzoate (insoluble in water)
• Crystals form too rapidly trapping impurities within growing crystal lattice
  ***Benzoic acid crystals with methyl orange tapped inside (crude mixture)

Question 20

recrystallization

Answer 20

• purification method of non-volatile solids
• General technique involves dissolving the solid crystals in a hot solvent and then cooling slowly so pure crystals form
• then filter pure crystals off leaving impurities in solution

Question 21

When can you use recrystallization ?

Answer 21

• Desired compound is more concentrated than the impurity in the solid material
• Impurity is easily dissolved or not dissolved at all
• Will not be incorporated into the crystal so long as crystal formation is slow
• If the impurities are not dissolved, they can be filtered off

Question 22

3 stages of solubility

Answer 22

1. collision
2. dissociation
3. solvation

Question 23

collision

Answer 23

when a solvent (S) molecule collides with the solid crystal surface

Question 24

Dissociation

Answer 24

• when collision results in the breakaway of a molecule from the solids

***Boiling solvents results in collisions with sufficient energy for dissociation.

Question 25

solvation

Answer 25

when the solute molecules are “in solution”. The solute molecules are solvated by the solvent

Question 26

Choosing an ideal recrystallization solvent →

Answer 26

1. High temperature coefficient: Solid is insoluble in the solvent at low temp but very soluble at a high temp
2. The solvents boiling point needs to be less than the melting point of the solid (dissolve the solid, not melt it/oil out)
3. The solid and solvent must be unreactive
4. The solvent needs to dissolve impurities easily/not at all
5. The solvent should be volatile; Solvent will easily evaporate from the surface of the reformed crystals

Question 27

Charcoal purpose

Answer 27

• be used to remove colored impurities
• Color indicates conjugation
• Charcoal binds to methyl orange (pink color)
• Charcoal is insoluble in H2O; Charcoal is filtered off to remove methyl orange

Question 28

hot gravity filtration

Answer 28

• Removes insoluble materials from the solution while the desired solid remains dissolved in the hot solution
• Must remain hot so that crystals do not begin to form

Question 29

nucleation

Answer 29

Methods of inducing crystal formation
- seeding: addition of a small crystal of the material to provide a surface for crystal formation to adhere to

• scratching: scratch bottom of flash with glass rod

Question 30

melting point

Answer 30

• Used to test the purity of crystals
• Melting point is a range (temp at beginning to temp at end)

Question 31

melting point range meaning

Answer 31

(Temp @ melting beginning - temp @ melting end)

• More pure solid ⇒ high melting point, narrower range
• More impurities ⇒ low melting point, wider range
  — Bc the lack of homogeneity of the sample allows for different melting points of the molecules