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General Chemistry lab > Chem 14BL wk 9-12 > Flashcards

Chem 14BL wk 9-12 Flashcards

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1
Q

Definition of recrystallization

A

a process in which a crystalline material (solute) dissolves in a hot solvent, then returns to a solid again by crystallizing in the cooled solvent

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2
Q

What is the purpose of recrystallization?

A

purify impure/crude products to remove any impurities that may have been left in synthesized organic product

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3
Q

General procedure of recrystallization (4 steps)

A
  1. Dissolve the impure product in a minimum amount of a suitable hot solvent
  2. filtering the hot solution to remove, any insoluble impurities
  3. Let the solution cool
  4. Filter the purified product
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4
Q

Four properties of a good solvent for recrystallization

A
  1. Dissolve a large quantity of the product while hot
  2. NOT dissolve the product while cold (guarantees max yield)
  3. Dissolve the impurities even at low temperatures (impurities should not crystallize)
  4. NOT react in any way with the product
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5
Q

Assuming a good solvent is chosen for recrystallization, the product (or solute) should have a MAX solubility in the hot solvent and a MIN solubility in the cold solvent. Why?

A

Max solubility in hot solvent allows impure solid to dissolve; Minimum solubility in cold solvent allows product to recrystallize

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6
Q

Two requirements of a mixed solvent system

A
  1. made up of two mutually MIXABLE solvents

2. One of the solvents should dissolve the product MORE READILY than the other solvent

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7
Q

Procedures for a mixed solvent system

A
  1. Dissolve the product in better solvent at high temperature
  2. Add the poorer solvent dropwise until the solution turns cloudy
  3. Heat the solution to redissolve all the crystals. Add a few drops of the better solvent if necessary to force the crystals back into the solution
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8
Q

The solvent chosen for recrystallization should have?

A

high solubility at all temperatures for the impurities and the crude product should have high solubility at high temperatures and low solubility at low temperatures

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9
Q

Slow Recrystallization. Advantages? Disadvantages?

A

Advantages: maximizes yield in good quality; gives larger, purer crystals
Disadvantages: time consuming; may gain impurities if solution is left out for too long

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10
Q

Fast Recrystallization. Advantages? Disadvantages?

A

Advantages: fast crystals form
Disadvantages: not good yield or quality

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11
Q

Sometimes the dissolved compound fails to crystallize from the solution on cooling. What could have caused this, and how can it be resolved?

A

Problem: Compound was dissolved in too much hot solvent
Solution: Reheat solution to boiling to boil off some of the solvent and then allow solution to cool to room temperature again to effect crystallization

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12
Q

Why might we not repeat the recrystallization process multiple times?

A

Lose some impurities, increasing purity BUT also sacrificed some of the desired product, decreasing product yield

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13
Q

How do you verify the purity of a product?

A
  1. Volumetric Analysis (Titration)
  2. Chromatography
  3. Compare melting point of the product with the literature value
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14
Q

How do you verify the purity of a product?

A
  1. Volumetric Analysis (Titration)
  2. Chromatography
  3. Compare melting point of the product with the literature value
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15
Q

Definition of solubility

A

property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent

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16
Q

Solubility depends on three things

A

concentration
polarity
temperature

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17
Q

What is “crude product”

A

the product initially isolated from the reaction mixture, prior to a purification step

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18
Q

% yield equation

A

100% x [(moles of product recovered)/ (max theoretical moles of product)]

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19
Q

% corrected yield equation

A

100% x [(moles of product recovered + moles of product left in solution)/(max theoretical moles of product)]

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20
Q

Crude yields, less than 100%

A

Reaction probably resulted in a low yield and/or problem with isolation of crude product resulted in low mass recovery

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21
Q

crude yields, equal to 100%

A

reaction probably resulted in a high yield; should expect a high final yield of purified product (assuming an efficient purification step)

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22
Q

crude yields, greater than 100%

A

major contamination of the product, which can be caused by the solvent or some reaction byproduct

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23
Q

Definition of solubility

A

property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent

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24
Q

Solubility depends on three things

A

concentration
polarity
temperature

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25
Q

What is “crude product”

A

the product initially isolated from the reaction mixture, prior to a purification step

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26
Q

% yield equation

A

100% x [(moles of product recovered)/ (max theoretical moles of product)]

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27
Q

% corrected yield equation

A

100% x [(moles of product recovered + moles of product left in solution)/(max theoretical moles of product)]

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28
Q

Crude yields, less than 100%

A

Reaction probably resulted in a low yield and/or problem with isolation of crude product resulted in low mass recovery

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29
Q

crude yields, equal to 100%

A

reaction probably resulted in a high yield; should expect a high final yield of purified product (assuming an efficient purification step)

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30
Q

crude yields, greater than 100%

A

major contamination of the product, which can be caused by the solvent or some reaction byproduct

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31
Q

% purity equation

A

100% x [(actual moles of acid)/ (theoretical moles of aspirin in sample)]

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32
Q

Definition of solubility

A

property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent

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33
Q

Solubility depends on three things

A

concentration
polarity
temperature

34
Q

What is “crude product”

A

the product initially isolated from the reaction mixture, prior to a purification step

35
Q

% yield equation

A

100% x [(moles of product recovered)/ (max theoretical moles of product)]

36
Q

% corrected yield equation

A

100% x [(moles of product recovered + moles of product left in solution)/(max theoretical moles of product)]

37
Q

Crude yields, less than 100%

A

Reaction probably resulted in a low yield and/or problem with isolation of crude product resulted in low mass recovery

38
Q

crude yields, equal to 100%

A

reaction probably resulted in a high yield; should expect a high final yield of purified product (assuming an efficient purification step)

39
Q

crude yields, greater than 100%

A

major contamination of the product, which can be caused by the solvent or some reaction byproduct

40
Q

% purity equation

A

100% x [(actual moles of acid)/ (theoretical moles of aspirin in sample)]

41
Q

Definition of solubility

A

property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent

42
Q

Solubility depends on three things

A

concentration
polarity
temperature

43
Q

What is “crude product”

A

the product initially isolated from the reaction mixture, prior to a purification step

44
Q

% yield equation

A

100% x [(moles of product recovered)/ (max theoretical moles of product)]

45
Q

% corrected yield equation

A

100% x [(moles of product recovered + moles of product left in solution)/(max theoretical moles of product)]

46
Q

Crude yields, less than 100%

A

Reaction probably resulted in a low yield and/or problem with isolation of crude product resulted in low mass recovery

47
Q

crude yields, equal to 100%

A

reaction probably resulted in a high yield; should expect a high final yield of purified product (assuming an efficient purification step)

48
Q

crude yields, greater than 100%

A

major contamination of the product, which can be caused by the solvent or some reaction byproduct

49
Q

% purity equation

A

100% x [(actual moles of acid)/ (theoretical moles of aspirin in sample)]

50
Q

Definition of solubility

A

property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent

51
Q

Solubility depends on three things

A

concentration
polarity
temperature

52
Q

What is “crude product”

A

the product initially isolated from the reaction mixture, prior to a purification step

53
Q

% yield equation

A

100% x [(moles of product recovered)/ (max theoretical moles of product)]

54
Q

% corrected yield equation

A

100% x [(moles of product recovered + moles of product left in solution)/(max theoretical moles of product)]

55
Q

Crude yields, less than 100%

A

Reaction probably resulted in a low yield and/or problem with isolation of crude product resulted in low mass recovery

56
Q

crude yields, equal to 100%

A

reaction probably resulted in a high yield; should expect a high final yield of purified product (assuming an efficient purification step)

57
Q

crude yields, greater than 100%

A

major contamination of the product, which can be caused by the solvent or some reaction byproduct

58
Q

% purity equation

A

100% x [(actual moles of acid)/ (theoretical moles of aspirin in sample)]

59
Q

melting point

A

temperature at which both solid and liquid have the same vapor pressure

60
Q

what is vapor pressure?

A

the pressure of the gas exerted when the solid or liquid phase is in equilibrium with the gas phase in a closed container

61
Q

what affects vapor pressure?

A

temperature and type of molecule

62
Q

in general, do solids or liquids have a higher vapor pressure?

A

liquids

63
Q

What is the purpose of melting point analysis? what is the dilemma?

A
  1. It helps confirm identity of a compound
    dilemma: many organic compounds have similar melting points
  2. Indicates purity of a compound (based on melting point range)
    pure substances have a narrow melting point range
    impure substances have a wide mpr
64
Q

Effects of impurities on melting point
(assume a presence of a small amount of impurities)

what type of impurities are we referring to?

what is nonvolatile?

what happens to the vapor pressure when impurities are present?

A
  1. usually produce a significant depression (decrease) in melting point (aka melting point depression)
  2. Increase melting point range

nonvolatile soluble impurities

Does not vaporize easily

decrease in overall vapor pressure of system

65
Q

which molecule, ibuprofen or aspirin, has a higher melting point?

A

aspirin has a higher melting point due to the greater number of intermolecular forces

66
Q

what happens to snow when you add the salt NaCl? why?

A

snow melts when you add NaCl. NaCl disrupts the intermolecular forces of water. NaCl acts as an impurity by decreasing the overall vapor pressure of the system and as a result lowers the melting point

67
Q

Define eutectic point

A

eutectic point is defined as the lowest temperature at which a liquid mixture can co-exist in equilibrium with the solid mixture (lowest melting point of a mixture)

*just because a substance has a sharp melting range does not mean it is a pure substance

68
Q

partial pressure

A

the pressure that the gas would exert if it occupied the container alone

69
Q

vapor pressure

A

the pressure of the gas exerted when the solid or liquid phase is in equilibirum with the gas phase in a closed container

70
Q

Dalton’s Law of Partial Pressures

A

the total pressure of the liquid solution is equal to the sum of the partial pressure of the components

71
Q

Raoult’s Law

A

the vapor pressure of a liquid solution of a nonvolatile solute is directly proportional to the mole fraction of the solvent of the solution

72
Q

Kinetics

A

study of rates and mechanisms of chemical reactions to help us understand how chemical reactions occur and how fast they proceeed

73
Q

factors that affect the rate of chemical reactions

A
  1. concentration of reactants
  2. temperature
  3. catalysts (enzyme)
74
Q

Instantaneous rate

A

the slope of the tangent of a graph of concentration against time

75
Q

rate law

A

an equation expressing the instantaneous reaction rate in terms of concentrations, at any instant, of the substances taking part in the reation

*the rate law for a reaction is determined experimentally and cannot be inferred from the chemical equation for the reaction

76
Q

Rate constant (k)

factors that affects the rate constant

A

constant of proportionality in a rate law

  1. temperature
  2. catalysts (enzyme)
77
Q

Integrated rate law

A

gives the concentration of reactants or products at any time after the start of the reaction

78
Q

reaction mechanisms

A

a sequence of elementary reactions describing the changes that we believe takes place as reactants are transformed into products

79
Q

how do you determine the rate determining step?

A

the slow step is the rate determining step

80
Q

why do you not want an intermediate in your rate law?

A

an intermediate is highly reactive. therefore, it is very difficult to measure an intermediate in the lab

81
Q

pseudo rate law.

assumption?

A

the concentration of a species is in excess. Thus, since the concentration of the species is in excess, the concentration remains constant and barely changes as the reaction proceeds

General Chemistry lab (3 decks)

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