Alcohols

Question 1

What is the general formula of alcohols?

Answer 1

CnH2n+1OH

Question 2

How do you classify alcohols?

Answer 2

primary/secondary/tertiary depending on how many carbon atoms are directly attached to the carbon with the -OH bonded to it

Question 3

What is fermentation used for?

Answer 3

making ethanol

Question 4

What happens in the fermentation process?

Answer 4

an aqueous solution of plant sugar is converted into ethanol and carbon dioxide by living yeast cells

Question 5

What is the equation and conditions for fermentation?

Answer 5

C6H12O6 —> 2C2H5OH + 2CO2

35C in aqueous solution with yeast under anaerobic conditions

Question 6

Why does fermentation need to be carried out in anaerobic conditions?

Answer 6

the presence of oxygen would cause the ethanol to oxidise to ethanoic acid (vinegar)

Question 7

Why can fermentation not be carried out at high temperatures?

Answer 7

because the yeast cannot survive and the enzyme it contains will be denatured

Question 8

Why can fermentation not be carried out at lower temperatures?

Answer 8

the enzymes in yeast are not activated and the rate of reaction is too slow

Question 9

What type of process is fermentation and what does it produce?

Answer 9

a batch process and produces a mixture of compounds

Question 10

What happens in a batch process?

Answer 10

the reactants are mixed together and once the reaction is over the product is removed and the process repeats

Question 11

How is the ethanol produced separated?

Answer 11

by fractional distillation of fermented products

Question 12

What is fermentation used for?

Answer 12

make alcoholic drinks/bread/biofuel

Question 13

Compare direct hydration with fermentation for making alcohols in industry

Answer 13

direct hydration-
-continuous process- cheap on manpower, expensive equipment and high energy use
-fast reaction
-pure product
-high yield
-uses ethene from crude oil which is a finite resource

fermentation-
-batch process- expensive on manpower, cheap equipment and low energy use
-slow reaction
-impure product
-low yield
-uses sugars which is a renewable resource

Question 14

What is biofuel?

Answer 14

fuels made from plants or biomass

Question 15

Why are biofuels classified as renewable?

Answer 15

the plants can be grown again quickly

Question 16

What is a carbon neutral activity?

Answer 16

an activity that has no net annual carbon/greenhouse gas emissions to the atmosphere

Question 17

Why is ethanol biofuel considered to be carbon neutral?

Answer 17

the amount of carbon dioxide made during ethanol combustion + the amount of carbon dioxide released by ethanol formation = amount of carbon dioxide removed from the atmosphere during photosynthesis

PHOTOSYNTHESIS-
6CO2 + 6H20 —-> C6H12O6 +6O2
6 moles of CO2 removed per 1 molecule of sugar

FERMENTATION-
C6H12O6 —-> 2C2H5OH +2CO2

COMBUSTION-
2C2H5OH + 6O2 —->4CO2 +6H2O
6 moles of of CO2 released during the fermentation and combustion of ethanol

Question 18

Why might some people disagree with ethanol biofuel being carbon neutral?

Answer 18

the fermentation production and processing steps require energy, and this energy may involve the release of additional carbon dioxide (such as fractional distillation)

crops being grown for fermentation may be contributing to deforestation in order for more space, which releases the tree’s natural store of CO2

Question 19

What is an elimination reaction?

Answer 19

where a small group of atoms breaks away from a larger molecule, not being replaced by anything else

Question 20

What alcohols can undergo elimination reactions?

Answer 20

primary and secondary alcohols- NOT tertiary
as long as there is a carbon attached to the C-OH that also has a H bonded then it can

Question 21

What do elimination reactions of alcohols produce?

Answer 21

alkenes and water

Question 22

When do alcohols undergo elimination?

Answer 22

when they are heated with a suitable acid catalyst (such as concentrated phosphoric acid H3PO4 or concentrated sulphuric acid H2SO4

Question 23

Why is it called a dehydration reaction?

Answer 23

water molecule is eliminated in the reaction

Question 24

Draw the full exam mechanism for the dehydration reaction of ethanol

Answer 24

google to check is right but- lone pair on the oxygen in the OH attacks the H+, which makes it O+, so the covalent bond breaks between C-O leaving water molecule and a C+, so the H on the other C-H forms a C=C bond and a H+

Question 25

When can there be more than one product in dehydration?

Answer 25

when its a secondary alcohol

Question 26

Describe the type of reaction (not the mechanism) that dehydration is and it’s similarities to direct hydration

Answer 26

both are reversible reactions

alcohol ⇌ alkene + water (heat/acid catalyst)

alkene + steam ⇌ alcohol (300C/phosphoric acid/65000kPa)

dehydration is elimination
direct hydration is electrophilic addition

Question 27

How can dehydration reactions be involved in addition polymerisation?

Answer 27

instead of doing fractional distillation of crude oil to make alkanes, then cracking them to get alkenes to be used in addition polymerisation

you can do dehydration of alcohols to make alkenes directly