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CHE 231 > Final Exam > Flashcards

Final Exam Flashcards

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1
Q

What is recrystallization?

A

One of the most common purification techniques for solids.

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2
Q

What is the process of recrystallization?

A

An impure solid is dissolved in hot solvent then recovered back as a purified material by cooling the solution.

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3
Q

What must remain soluble in the solvent at room temperature during recrystallization?

A

Impurities.

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4
Q

What should be chosen as the solvent for recrystallization?

A

A solvent such that impurities are more soluble than the substance being recrystallized at room temperature.

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5
Q

When is recrystallization typically used?

A

When the amount of impurities is not too large (usually <10%).

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6
Q

What is the key consideration regarding solvent in recrystallization?

A

Minimum—too much solvent will reduce recovery.

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7
Q

Outline the steps of the recrystallization experiment.

A

Select appropriate solvent, dissolve solid near B.P. of solvent, filter hot solution, crystallize solids by cooling, separate crystals, wash and dry crystals.

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8
Q

What compound is used as an example for purification in recrystallization?

A

Aspirin.

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9
Q

What is a criterion for the solute in recrystallization?

A

Solute should be soluble in the solvent at high temperature and insoluble at room temperature.

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10
Q

What is the right solvent for recrystallization of aspirin?

A

Chloroform.

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11
Q

What should the boiling point (B.P.) of the solvent be compared to the melting point (M.P.) of the solute?

A

B.P. of solvent < M.P. of solute.

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12
Q

What should the solvent not do during recrystallization?

A

React with the solute.

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13
Q

What does the solubility guideline ‘Like dissolves like’ imply?

A

Any substance is soluble in another substance only if their chemical nature is similar.

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14
Q

What are C5 guidelines in solubility?

A

Organic compounds with less than 5 carbons and a polar function are water soluble.

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15
Q

How does branching affect solubility?

A

Branched chain compounds are more soluble than corresponding straight chain compounds.

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16
Q

What types of molecules are considered water soluble according to solubility guidelines?

A

Small molecules (5 carbons or less) with at least one polar functional group and larger molecules with more than one polar functional group.

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17
Q

Why are salts (ionic) water soluble?

A

Due to ion-dipole interactions.

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18
Q

What does melting point measurement indicate?

A

It can provide information about purity.

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19
Q

What is the melting point range for a pure compound?

A

Very narrow and reproducible range of temperatures.

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20
Q

What type of waste is acetone classified as?

A

Organic liquid hazardous chemical waste.

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21
Q

What is fractional distillation used for?

A

Separating a mixture of liquids.

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22
Q

On what basis does fractional distillation separate components?

A

Based upon the differences in their boiling points (B.P.) and vapor pressure.

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23
Q

What is the relationship between boiling point and vapor pressure?

A

B.P. and vapor pressure are inversely proportional.

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24
Q

Provide examples of boiling points for hexane and toluene.

A

Hexane: 69 degrees C; Toluene: 111 degrees C.

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25
What is the preferred difference in boiling points for effective distillation?
Preferably between 30-40 degrees Celsius.
26
What occurs during the process of distillation?
A liquid is heated to its boiling point, and the vapors generated are condensed by cooling and collected in a receiving vessel or return to the boiling mixture.
27
What does a theoretical plate represent in distillation?
One vaporization/condensation cycle.
28
How many theoretical plates does a distillation have if two cycles occur?
Two theoretical plates.
29
What are the types of fractional distillation?
* Macro-scale simple distillation * Fractional distillation
30
What is the purpose of the added column in fractional distillation?
It provides a larger surface area for multiple simple distillations.
31
In a micro scale experiment using a Hickman still, how many thermometers are used?
Only one thermometer.
32
What is used instead of a sand bath in micro scale experiments?
Al block.
33
Why can't a complete separation of hexane and toluene be achieved in one cycle using a Hickman still?
Because the Hickman still has a short column.
34
What must be done to the fractions produced in the first distillation?
They must be redistilled to reproduce more pure portions of hexane and toluene.
35
What type of distillation is referred to when two rounds of distillation occur?
Fractional distillation.
36
What is the purpose of gas chromatography?
Used for separation of multicomponent mixtures.
37
What is the principle of chromatography based on?
Separation is based on partitioning of a mixture between a stationary and mobile phase.
38
What is the stationary phase in gas chromatography?
A non-moving component, typically a solid or a liquid coated on a solid support, that is used to separate the compounds in a sample
39
What is the mobile phase in gas chromatography?
An inert carrier gas, such as helium.
40
What happens as the mobile phase progresses in gas chromatography?
It carries the components of an injected sample.
41
How do components that interact strongly with the stationary phase behave?
They are retained longer.
42
What is retention time in the context of chromatography?
The amount of time a component interacts with the stationary phase.
43
Which component has a lower retention time: hexane or toluene?
Hexane.
44
What does the area under the peak in a gas chromatogram represent?
It is proportional to the amount of each component in the sample.
45
During the first distillation, how should the liquid mixture be separated?
Into three fractions.
46
What will the first fraction mostly contain after distillation?
Mostly hexane (lower B.P.).
47
What will the second fraction be a mixture of?
A mixture of hexane and toluene.
48
What will the third fraction mostly contain?
Mostly toluene.
49
How is % recovery calculated?
% recovery = (amount recovered) / (amount original) * 100
50
What is the main purpose of Experiment #3?
Reduction of ketones using metal hydride to synthesize cyclohexanol
51
What is the role of metal hydrides in reduction reactions?
They act as reducing agents, getting oxidized in the process
52
What is a powerful reducing agent mentioned in the text?
LiAlH4
53
What types of functional groups can LiAlH4 reduce?
* Aldehydes * Ketones * Esters * Lactones * Carboxylic acids
54
What is a significant safety concern when using LiAlH4?
It reacts violently with protic solvents, generating hydrogen gas
55
What is sodium borohydride (NaBH4) known for?
It is a selective, mild reducing agent that reduces only aldehydes or ketones
56
How many hydride ions can one mole of sodium borohydride provide?
Four hydride ions
57
What is the reaction mechanism for the reduction involving sodium borohydride?
Nucleophilic addition to carbonyl
58
In the reduction mechanism, what acts as the nucleophile?
Electron-rich hydride ion
59
What is the role of methanol in the reaction with sodium borohydride?
It acts as a protic solvent providing hydrogen attached to oxygen
60
What is the principle behind solvent extraction?
Separates compounds based on differing solubility behaviors
61
What are the two types of solvent extraction mentioned?
* Solid-liquid extraction * Liquid-liquid extraction
62
What is required for liquid-liquid extraction to occur?
Two immiscible liquids
63
How is the solubility of a compound expressed mathematically?
Kp = [XSolvent 2] / [Xsolvent1]
64
What is the % yield formula?
(experimental yield/theoretical yield) * 100
65
What two components are separated using column chromatography in Experiment #4?
* Ferrocene * Acetylferrocene
66
What is the Huckel rule for aromaticity?
A molecule is Huckel aromatic when it is cyclic, planar, conjugated, and contains pi electrons where n = 0, 1, 2... **4n + 2** pi bonds
67
What is the stationary phase in column chromatography?
Solid - alumina
68
What is the mobile phase in column chromatography?
Liquid - organic solvents
69
Which solvent is used to elute non-polar ferrocene?
Hexane
70
Which solvent is used to elute polar acetylferrocene?
Methylene chloride
71
What factors determine the separation of components in column chromatography?
Relative attraction towards stationary phase and polarity of the solvents
72
What is the stationary phase in Thin Layer Chromatography (TLC)?
Solid - silica gel
73
What does Rf (retention factor) represent?
Distance traveled by the solute / Distance traveled by the solvent front
74
What is the purpose of using an ultra-violet lamp in TLC?
To visualize compounds that enhance or quench fluorescence
75
What types of compounds are typically visible under UV light in the experiment?
Aromatic compounds
76
What defines an acid according to Bronsted definition?
Proton donor
77
What defines a base according to Bronsted definition?
Proton acceptor
78
What is the relationship between a substance's solubility and its chemical nature?
A substance is soluble in another only if their chemical nature is similar
79
What are examples of water-soluble compounds?
* Small molecules (5 carbons or less) with at least one polar functional group * Larger molecules with more than one polar functional group * Salts (ionic) due to ion-dipole interactions
80
What types of compounds are typically water-insoluble?
* Molecules without polar functional groups * Monofunctional compounds with more than 5 carbon atoms * Compounds like methylene chloride, chloroform, and carbon tetrachloride

CHE 231 (1 decks)

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