Cracking and alkenes

Question 1

What is cracking and why do we do it?

Answer 1

• Cracking is the process of breaking down longer chain hydrocarbons into smaller, more useful alkanes and alkenes
• It is done because shorter chain hydrocarbons are more volatile and flammable, making them better fuelds. Longer chain hydrocarbons are more viscous (thick) and less volatile so they are less useful. We use cracking to make longer chain hydrocarbons more useful

Question 2

What type of reaction is cracking?

Answer 2

• Cracking is a thermal decomposition reaction (breaking down by heat)
• This means cracking is endothermic

Question 3

What are the two methods of cracking?

Answer 3

• Catalytic cracking
• Steam cracking

Question 4

Explain the process of catalytic cracking

Answer 4

• Hydrocarbon is vaporised (heated to a gas)
• It is passed over hot, powdered aluminium oxide (catalyst)
• This breaks down the long chain hydrocarbon into a short chain alkane and alkene

Question 5

Explain the process of steam cracking

Answer 5

• Hydrocarbon is vaporised (heated to a gas)
• It is then miced with steam and heated to a high temperature
• This breaks down the long chain hydrocarbon into a short chain alkane and alkene

Question 6

What is an akene and its properties?

Answer 6

• Homologous series (similair properties and reactions)
• Have double bonds - they are saturated
• More reactive than alkanes - can react with bromine to turn it coulourless
• Can be added together to make polymer (as the double bond can open up - alkanes can’t)

Question 7

What is the general formula of an alkene?

Answer 7

• CnH2n

Question 8

How do we differentiate between an alkene and an alkane (test for alkene)?

Answer 8

• Mix with bromine water
• As alkene is more reactive, it will turn the bromine water colourless, alkanes won’t