Cracking

Question 1

cracking

Answer 1

a process carried out on fractions in oil refineries after fractional distillation

larger alkanes are broken down into smaller, more useful alkanes and alkenes

Question 2

Describe the process of cracking

Answer 2

• crude oil is heated and evaporated
• vapour passes over a catalyst containing aluminium oxide
• heated to about 650C to speed up the reaction
• molecules are broken down

Question 3

need for cracking

Answer 3

-long, saturated molecules are not very useful

• short molecules are in higher demand than their production
• long molecules are in lower demand than production
• short molecules are in higher demand than long molecules
• shorter alkanes are useful as fuels
• alkenes are used to make polymers

Question 4

products of cracking

Answer 4

alkanes - useful as fuels
(shorter chains)

alkenes - used to make polymers
(contain a double bond between at least two carbon atoms)

Question 5

alkenes

Answer 5

contain a double bond between at least 2 carbon atoms

used to make polymers - polyester, nylon

unsaturated (with at least 1 double bond)

Question 6

(un)saturated molecule

Answer 6

a saturated molecule contains only single bonds

an unsaturated molecule has at least one double bond

Question 7

Give an example of what C8H18 could be broken down into

Answer 7

C8H18 -> C6H14 + C2H4

etc

Question 8

How can you tell if a molecule is an alkane or an alkene?

Answer 8

due to the arrangement of atoms:

an alkane always has an amount of hydrogen atoms equal to double the carbon atoms + 2

(e.g. C6H14)

an alkene has double the amount of hydrogen atoms compared to carbon atoms

Question 9

Describe the setup for cracking paraffin in the lab

Answer 9

• paraffin is absorbed by mineral wool and placed in the end of a test tube
• broken porous pot is placed in the tube as a catalyst
• a bung and delivery tube cause the product (ethene gas) to build up in an upturned test tube
• the test tube is placed in a container of water

Question 10

Describe the process for cracking liquid paraffin in the lab

Answer 10

• the porous pot catalyst is heated strongly
• liquid paraffin is heated and evaporated
• paraffin vapour passes over the hot porous pot and the hydrocarbon molecules break down
• one of the products is ethene, which is a gas and collects in the other tube

Question 11

List all the equipment needed for cracking liquid paraffin

Answer 11

• liquid paraffin
• mineral wool
• broken porous pot
• test tubes
• bung
• delivery tube
• container with water

-method of heating

Question 12

Use the information below to explain how cracking helps to balance supply with demand.

Supply/demand of some of the fractions obtained from crude oil as a percentage of all the fractions:

Petrol:
Supply = 10%
Demand = 30%

Bitumen and fuel oil:
Supply = 40%
Demand = 20%

Answer 12

some larger alkanes (bitumen) are in greater supply than their demand

cracking converts these alkanes into smaller hydrocarbons (petrol) which are in greater demand than can be supplied by fractional distillation alone

Question 13

a) Give the formula of the substance needed to balance this cracking reaction equation:

C10H22 -> C8H18 + ?

b) State a use for the hydrocarbon in a) other than as a fuel

Answer 13

a) C2H4

b) can be used to make polymers