4.7.1.4 Cracking and alkenes

Question 1

Why do we crack hydrocarbons?

Answer 1

• To break down (crack) the longer chain length hydrocarbon to produce smaller more useful molecules

Question 2

What reaction is cracking?

Answer 2

• A thermal (heating) decomposition (break down) reaction

Question 3

What two ways of cracking are there?

Answer 3

• Catalytic cracking
• Steam cracking

Question 4

What are the conditions used for catalytic cracking?

Answer 4

1. Heat the long chain hydrocarbons and vaporise them (turn into a gas)
2. Hot powdered aluminium oxide (catalyst) ready
3. Pass the hydrocarbon vapour over the powder
4. As long hydrocarbon has come into contact with the catalyst it splits apart into two smaller hydrocarbons

Question 5

What are the conditions used for steam cracking?

Answer 5

1. Heat the long chain hydrocarbons and vaporise them (turn into a gas)
2. Mix the vaporised hydrocarbon with steam
3. Heat to a very high temperature and hydrocarbon splits into two smaller ones

Question 6

What is the general formula for the cracking of long hydrocarbons?

Answer 6

• Long chain alkane = short chain alkane + alkene

Question 7

Why does cracking produce an alkane and an alkene?

Answer 7

• The number of hydrogens and carbons on each side has to be the same
• Not enough hydrogens for both products to be saturated and have only single bonds
• Therefore one product has a double bond making it an alkene

Question 8

Instead of being a waste product what can an alkene be used for?

Answer 8

• Produce polymers
• Starting materials for producing many other chemicals

Question 9

Decane C10H22 can be cracked into ethene C2H4 and one other hydrocarbon write a balanced equation for this reaction.

Answer 9

C10H22 = C8H18 + C2H4

Question 10

What is the only structural difference between alkanes and alkenes?

Answer 10

• Alkanes are single bonds only whereas alkenes have double bonds

Question 11

What is the differences between alkanes and alkenes?

Answer 11

• Alkanes are saturated, only have single bonds
• Alkenes are unsaturated and have double bonds
  
  • (more reactive than alkanes)
  • (can be added together to form
    polymers (the double bond can break
    to form two or more bonds))

Question 12

What is the test for alkanes?

Answer 12

• Mix bromine water (a brown liquid) with a solution of alkenes the mixture will turn colourless

Question 13

Give examples to illustrate the usefulness of cracking.

Answer 13

• The smaller hydrocarbons formed by cracking are used as fuels (which is in high demand), and the alkenes are used to make polymers in the manufacturing of plastics.

Question 14

Explain how the modern life depends on the uses of hydrocarbons.

Answer 14

• Hydrocarbons is used for fuels, electrical power generation, and heating.
• The chemical, petrochemical, plastics and rubber industries are also dependent upon hydrocarbons