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Chem Lab > Crystallization: Videos and Manual > Flashcards

Crystallization: Videos and Manual Flashcards

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1
Q

Melting point is used to establish purity, while crystallization is used to

A

purify

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2
Q

crystallizatio involves the removal of ____ impurities and the removal of ____ impurities

A

insoluble
soluble

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3
Q

Describe the differences between crystallization and precipitation

A

crystallization: slow and selective, results in PURE material

precipitation: solid “crashes” out of solution in chemical reaction. Process is rapid and nonselective, resulting in IMPURE material.

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4
Q

In this crystallization experiment, you will accomplish two tasks which are:

A
  1. purify a ‘dirty’ unknown cmpound via crystallization from water. The unknown could be benzoic acid, salicylic acid, or sorbic acid
  2. identify the unknown by measuring the melting point and doing the mixed melting point analysis
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5
Q

Why is the melting point for salicylic acid higher than sorbic acid? Look at the structure for the two acids

A

Salicylic acid has an extra hydroxyl group, meaning more hydrogen bonds and more energy needed to break those bonds

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6
Q

What is the first problem in performing a crystallization?

A

selecting a solvent in which the material to be crystallized shows the desired solubility behavior

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7
Q

There are three general solubility behaviors. Name them and label the behavior ideal for recrystallization

A
  1. cmpd has HIGH solubility in BOTH hot and cold solvent
  2. cmpd has LOW solubility in BOTH how and cold solvent
  3. cmpd has HIGH solubility in HOT solvent but LOW solubility in COLD solvent
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8
Q

Why is a cmpd that has HIGH solubility in HOT solvent but LOW solubility in COLD solvent ideal for recrystallization?

A

If it dissolves in both a hot and cold solvent, would not be possible to reclaim the solid.
However, if it has a high solubility in cold because it is possible to reclaim it as the hot liquid cools down.

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9
Q

True or false: an ideal recrystallization solvent should dissolve all of the compound when the solvent is HOT (boiling)

A

True

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10
Q

True or false: an ideal recrystallization solvent should dissolve the maximum amount of the compound when the solvent is at room temperature

A

FALSE: it should be the minimum amount

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11
Q

Why should a solvent have a lower boiling point than the melting point of a compound?

A

Solid would begin to melt and there would be two liquids suspended in the other

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12
Q

True or false: an ideal recrystallization solvent should have a different solubility for the compound and impurities

A

TRUE

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13
Q

Please explain:
benzyl alcohol (bp 205 C) was selected by a student as the solvent to crystallize fluorenol (mp 153-154) because the solubility characteristics of this solvent are appropriate. However, this solvent is not a good choice.

A

The solvent has a higher boiling point than the solid, so fluorenol would melt before the benzyl alcohol begins to boil

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14
Q

Why can a base not be used to purify an acid?

A

They would react with each other

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15
Q

To minimize the loss of material in the mother liquor, it is desirable to dissolve the solid in minimum amount of _____ solvent

A

near-boiling

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16
Q

When given the initial sample in this experiment, would you expect a small melting point range?

A

No it is an impure sample, so it should have a broad range

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17
Q

When is it necessary to use hot filtration?

A

if insoluble material remains in hot solution or if decolorising charcoal has been used

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18
Q

What is hot filtration?

A

method is the most effective way to remove insoluble impurities

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19
Q

Describe process of hot filtration

A

First add small amount of hot solvent to wet fluted paper
then pour hot solution to the flask through the funnel quickly
rinse filter paper with small amount of hot solvent to prevent crystallization

20
Q

How will you know if you have trace pigments?

A

Solid will be muddy or have some discoloration, rather than pure white color

21
Q

True or false: the more slowly a solution is allowed to cool, the worse purity of the crystals obtaiend

A

FALSE: slow cooling allows for better crystals

22
Q

How will insoluble impurities be separated or removed from the mix?

A

Gravity filtration

23
Q

During recrystallization, the dissolved compound was treated w activated carbon, why?
a. improve recovery/yield of desired product
b. increase solubility of dissolved compound at higher temperature
c. remove colored impurities
d. all of the above

A

C

24
Q

How might one induce crystallization?

A
  1. scratching inside surface of flask vigorously with glass rod
  2. adding small amount of original material “seeding”
25
Q

Define solubility

A

the maximum quantity of the substance, expressed in grams, that will dissolv ein a certain solvent, expressed in grams or mL at a specific temperature

26
Q

Define a solution

A

homogeneous mixture of two or more substances of ions or molecules

27
Q

Define solute

A

part of a solution being dissolved

28
Q

Define solvent

A

part of a solution that dissolves solute

29
Q

Define a saturated solution

A

a solution that contains maximum quantity of solute that dissolves at that temperature

30
Q

Define unsaturaated solution

A

a solution that contains less than the maximum amount of solute that can dissolve at a particularl temperature

31
Q

Define supersaturated solution

A

a solution that contains more solute than a saturated solution

32
Q

How might a supersaturated solution form?

A

heating the solution up to dissolve more solute at that higher temperature and then letting the solution cool
once cooled, agitation causes crystals to precipitate (separate from solution) out of super saturated solution

33
Q

What are solubility characteristics of a good recrystallization solvent?

A

High solubility at high temperature, low solubility at low temperature

34
Q
A
35
Q

true or false: an unsaturated solution has a higher concentration than saturated

A

false.

36
Q

Why are solutes more soluble in a hot solvent than a cool solvent?

A

Increasing temp causes more solvent particles to bump into the solute, resulting in solute particles breaking loose and dissolving faster

37
Q

The solubility of benzoic acid in water is 7g/mL at 95 C and 0.5g/100mL at 5 C. You are given 21g of benzoic acid. How much water would you use to recrystallize the sample

A

7g BA/100mL-21g BA/X
X=300mL

38
Q

Why does adding halogens (F, Cl, Br, I) to an organic molecule not alter its polarity significantly?

A

Not a big change in dipole movement

39
Q

As carbon chain length increases, compounds withing the same family will decrease in polarity. Why?

A

Less polar and less soluble as carbon chain gets longer
Ex. CH3OH is more polar than CH3CH2CH2CH2OH

40
Q

When H-bonding can be formed between solute and solvent, solubility _____

A

increases

41
Q

The most fundamental property of a good solvent for crystallization of a solid is that the hot solvent dissolves the
substance _____, while the cold solvent dissolve it ______

A

The most fundamental property of a good solvent for crystallization of a solid is that the hot solvent dissolves the
substance readily, while the cold solvent dissolve it sparingly.

42
Q

True or False: A single solvent is always best for reproducibility

A

True

43
Q

If one of the components is poorly soluble in the chosen solvent, one could

A

could go on adding the hot solvent for
a long time before the mixture is dissolved completely.

44
Q

What might happen if you use too much solvent?

A

the desired compound will not come out of solution when you cool it

45
Q

What occurs during crystallization using a solvent pair?

A

Dissolve the compound in a warm volume of the more soluble solvent
The second less soluble solvent is added slowly until a slight turbidity (cloudiness)
appears.
The solution is reheated and/or a drop of the first solvent is added to obtain a clear solution.
The solution
is then slowly cooled to obtain the crystallized compound.

Chem Lab (3 decks)

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