alcohols

Question 1

what is the general formula for an alcohol? (1)

Answer 1

CnH2n+1OH

Question 2

what is the prefix or suffix given for alcohols? (2)

Answer 2

prefix; -ol
suffix; (given when there are other functional groups present) -hydroxy

Question 3

what is the main ingredient in antifreeze? (1)

Answer 3

ethane-1,2 -diol

Question 4

what is the shape of an alcohol? (1)

Answer 4

in alcohols, the oxygen has two bonding pairs of electrons and two lone pairs. The C-O-H angle is about 105 degrees because the 1o9.5 degree angle of the perfect tetrahedron is ‘squeezed down’ by the presence of the lone pairs.

Question 5

how are alcohols classified? (3)

Answer 5

alcohols are classified as primary (1 degree), secondary (2nd degree) and tertiary (3rd degree)

Question 6

what is a primary alcohol? (1)

Answer 6

in a primary alcohol the carbon with the -OH has one R group (and therefore 2 hydrogen atoms)

Question 7

what is a secondary alcohol? (1)

Answer 7

in a secondary alcohol, the -OH group is attached to a carbon with two R groups (and therefore one hydrogen atom)

Question 8

what is a tertiary alcohol? (1)

Answer 8

tertiary alcohols have three R groups attached to the carbon that is bonded to the -OH (has no hydrogen atoms)

Question 9

what are the physical properties of an alcohol? (3)

Answer 9

-high melting and boiling points
-alcohols with short hydrocarbon chains are soluble in water
-alcohols with long hydrocarbon chains are insoluble in water

Question 10

why do alcohols have high melting and boiling points? (1)

Answer 10

the -OH group in alcohols means that hydrogen bonding occurs between the molecules.

Question 11

why are short alcohol chains soluble in water? (1)

Answer 11

the -OH group in alcohols can hydrogen bond to water molecules, but the non-polar chain cannot - the hydrogen bonding predominates

Question 12

why are long alcohol chains insoluble in water? (1)

Answer 12

the non-polar hydrocarbon chain dominates and the alcohols become insoluble in water

Question 13

what is the formula for ethanol? (1)

Answer 13

C2H5OH

Question 14

how is ethanol made industrially? (1)

Answer 14

by reacting ethene (made by cracking crude oil) with steam.

Question 15

what catalyst is used in the production of ethanol? (1)

Answer 15

phosphoric acid

Question 16

what are the two ways in which ethanol can be made? (2)

Answer 16

-cracking and hydration
-fermentation and distillation

Question 17

what is meant hydration? (1)

Answer 17

water is added across the double bond.

Question 18

what happens during fermentation? (1)

Answer 18

carbohydrates from plants are broken down into sugars and then converted into ethanol by the action of enzymes from yeast.

Question 19

what do the carbohydrates used in fermentation come from? (1)

Answer 19

the carbohydrates come from crops such as sugar cane and sugar beet

Question 19

what is the key step in the breakdown of sugar? (1)

Answer 19

a process called anaerobic respiration

Question 20

what is the rate of anaerobic respiration in the breakdown of sugars effected by? (1)

Answer 20

temperature, its is slow temperatures but the enzymes are made ineffective if the temperature is too high.

Question 21

what is the best temperature for fermentation of ethanol? (1)

Answer 21

a compromise temperature of 35 degrees is used

Question 22

why does fermentation occur in a sealed vessel? (1)

Answer 22

air is kept out of the fermentation vessels to prevent oxidation of ethanol to ethanoic acid (the acid in vinegar)

Question 23

what are the differences between the two methods of ethanol production? (4)

Answer 23

rate of reaction;
-C/H: Fast
-F/D: Slow

type of process;
-C/H: Continuous
-F/D: Batch

purity;
-C/H: Essentially pure
-F/D: aqueous solution of ethanol is produced

Question 24

what is produced in the complete combustion of alcohols? (1)

Answer 24

alcohols burn completely to carbon dioxide and water if there is enough oxygen available.

Question 25

what is produced in the incomplete combustion of alcohols? (1)

Answer 25

carbon dioxide or carbon is produced

Question 26

what is an elimination reaction? (1)

Answer 26

elimination reactions are ones in which a small molecule leaves the parent molecule, in the case of alcohols this molecule is always water. (are always dehydrations)

Question 27

how can alcohols be dehydrated? (2)

Answer 27

alcohols can be dehydrated with excess hot concentrated sulfuric acid or by passing their vapours over heated aluminium oxide.

Question 28

what is produced from the dehydration of an alcohol? (1)

Answer 28

an alkene is produced

Question 29

what is combustion usually? (1)

Answer 29

combustion is usually complete oxidation

Question 30

what are primary alcohols normally oxidised to? (1)

Answer 30

primary alcohols are oxidised to aldehydes, aldehydes can be further oxidised to carboxylic acids

Question 31

what are secondary alcohols oxidised to? (1)

Answer 31

secondary alcohols are oxidised to ketones, ketones are not oxidised further.

Question 32

what are tertiary alcohols oxidised to? (2)

Answer 32

tertiary alcohols are not easily oxidised because oxidation would need a C-C bond rather than a C-H bond

Question 33

what experiment can be used to oxidise aldehydes and ketones? (1)

Answer 33

a solution of potassium dichromate, acidified with dilute sulfuric acid (oxidising agent)

Question 34

what are the similarities between aldehydes and ketones? (1)

Answer 34

both have the C=O group (carbonyl group)
-aldehydes; at the end of the chain
-ketones; the body of the chain

Question 35

what are the two tests which can distinguish a ketone and aldehyde? (2)

Answer 35

-the tollens test
-the fehlings test

Question 36

what is the tollens test? (2)

Answer 36

its a solution of silver nitrate in aqueous ammonia, it oxidises aldehydes but has no effect on ketones. A deposit of metallic silver is formed on the inside of the test tube

Question 37

what is the fehlings test? (2)

Answer 37

the blue solution gradually changes to a brick red precipitate of copper oxide.