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AQA A Level Chemistry > 3.5 Alcohols > Flashcards

3.5 Alcohols Flashcards

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1
Q

Alcohols have _____ melting and boiling points than alkanes of similar relative molecular mass

A

higher

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2
Q

Why do alcohols have higher melting and boiling points than alkanes?

A

∵ -OH group in alcohol means hydrogen bonding occurs between molecules

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3
Q

-OH group of alcohols can…

A

hydrogen bond to water molecules

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4
Q

….cannot bond to water molecules

A

Non-polar hydrocarbon chain

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5
Q

Why are alcohols with short hydrocarbon chains are soluble in water?

A

∵ of hydrogen bonding predominates

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6
Q

Why are alcohols with longer hydrocarbon chains are insoluble in water?

A

Non-polar hydrocarbon chain dominates

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7
Q

What is the benefit of making alkenes from a dehydration reaction?

A

Allows you to make alkenes from renewable resources i.e. without crude oil

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8
Q

Elimination Reaction

Name the condition and product when an alcohol is used as a reactant

A

Product: Alkene

  • Al2O3 Catalyst
  • 450 K
  • Concentrated, hot H2SO4
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9
Q

Ethanol + H2SO4

Name the mechanism

A

Elimination

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10
Q

Ethanol + H2SO4

Mechanism

A

H2SO4 - acts as a catalyst

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11
Q

How does distillation work?

A

Uses the fact that chemicals have different boiling points & separates them out to get a single purified substance

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12
Q

Name 2 ways ethanol can be produced

A
  • Hydration
  • Fermenation
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13
Q

Describe hydration (4)

A
  1. Hydration of ethene using steam to produce ethanol
  2. Acid catalyst
  3. High temperature + high pressure
  4. H2O added across double bond
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14
Q

Hydration

Name 3 conditions for producing ethanol

A
  • 300°C
  • Pressure of 60 atm
  • Solid phosphoric(V) acid catalyst
    • H3PO4
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15
Q

Describe fermenation

A
  • Fermentation (exothermic reaction) is carried out by yeast in anaerobic respiration
  • Yeast produces an enzyme that converts sugars into ethanol and carbon dioxide
  • Ethanol formed is separated by fractional distillation
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16
Q

What is an advantage of using fermentation to make alcohol?

A

Low tech process (uses cheap equipment & renewable resources)

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17
Q

What is a disadvantage of using fermentation to make alcohol?

A

Fractional distillation is needed for purification = expensive & time consuming

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18
Q

State the equation for the hydration of ethene (include state symbols)

A

C2H4(g) + H2O(g) ⇌ C2H5OH(g)

19
Q

State the equation for fermentation (include state symbols)

A
20
Q

Name 2 ways alcohols can be oxidised

A
  1. Burn them
  2. Use oxidising agent acidified potassium dichromate(VI), K2Cr2O7 to mildly oxidise them
21
Q

Define Oxidation

A
  • Loss of electrons
  • Gain of oxygen
  • (Loss of hydrogen)
22
Q

Define Reduction

A
  • Gain of electrons
  • Loss of oxygen
  • (Gain of hydrogen)
23
Q

Define oxidising agent

A

Chemical that accepts electrons and gets reduced

(supplies the oxygen or removes the hydrogen)

24
Q

Define reducing agent

A

Chemical that donates electrons and gets oxidised

(removes the oxygen or supplies the hydrogen)

25
Q

What are primary alcohols oxidised to?

A

Aldehydes and then to carboxylic acids

26
Q

Draw the a primary alcohol oxidising into an aldehyde & then draw an aldehyde oxidising into a carboxylic acid

A
27
Q

Draw the a primary alcohol oxidising into a carboxylic acid

A

+ H2O

28
Q

Describe how you would oxidise an alcohol to just get an aldehyde

A
  1. Gently heating alcohol with potassium dichromate(VI) & sulfuric acid in test tube = aldehyde
  2. To get just aldehyde, distill it off immediately
29
Q

Describe how you would oxidise an alcohol to just get a carboxylic acid

A

Alcohol has to be vigorously oxidised:

alcohol is mixed with excess oxidising agent & heated under reflux

30
Q

Explain how heating under reflux ensures the complete oxidation of an alcohol to a carboxylic acid (2)

A
  • Any escaped vapour is condensed back into liquid
  • So any unreacted aldehyde can be oxidised/react further
31
Q

What are secondary alcohols oxidised into?

A

Ketones

32
Q

Describe how you would oxidise a secondary alcohol to a ketone

A

Reflux a secondary alcohol with acidified potassium dichromate(VI)

33
Q

Draw the a secondary alcohol oxidising into a ketone

A
34
Q

State what colour changes occur when potassium dichromate(VI) is added to a primary, secondary and tertiary alcohol

A

Primary and secondary alcohol:
Orange → Green

Tertiary alcohol:

No colour change - remains orange

35
Q

Explain why potassium dichromate(VI) turns from orange to green

A

Orange dichromate(VI) ion, Cr2O72-, is reduced to green chromium(III) ion, Cr3+

36
Q

Can tertiary alcohols be oxidised by reacting them with potassium dichromate(VI)?

A

NO

37
Q

Why can’t tertiary alcohols be oxidised by reacting them with potassium dichromate(VI)?

A

∵ of lack of hydrogen atoms on carbon atom which the hydroxyl group is attached to

38
Q

What is the only way to oxidise tertiary alcohols?

A

By burning them

39
Q

List alcohol A, ketone B and alkene C in order of increasing boiling point

A

CBA

Alkene, ketone, alcohol

40
Q

Explain why butanal has a lower boiling point than both butan-1-ol and butanoic acid (3)

A
  • Dipole-dipole forces between butanal molecules
  • Hydrogen bonds in both other molecules
  • Dipole-dipole forces are weaker than hydrogen bonds, require less energy to break
41
Q

Ethanol can be oxidised to ethanal. Describe this reaction. (3)

A
  • Oxidising agent
  • 2 hydrogen atoms removed
  • Forming water
42
Q

What is meant by a primary alcohol? (1)

A

Functional group (OH) at the end of the chain

43
Q

Describe what happens when an alcohol is heated under reflux (3)

A
  1. Mixture of liquids heated to boiling point for prolonged time
  2. Vapour is formed which escapes from liquid mixture & is changed back into liquid + returned to liquid mixture
  3. Any alcohol and aldehyde that initally evaporates can then oxidised

AQA A Level Chemistry (36 decks)

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