3.13 Cracking & Alkenne

Question 1

What is a functional group?

Answer 1

A group of atoms within a molecule that determine how a molecule reacts. All members of a particular homologous series will contain the same functional group. The functional groups are responsible for the differences in chemical reacts between the different homologous series.

Question 2

Do alkanes have a functional group and what does this effect and how do alkenes and alcohols differ from this?

Answer 2

Alkanes don’t have a functional group so only easily take part in combustion reactions. Alkenes have a C=C double bond functional group and alcohols have an -OH functional group. These functional groups mean that alkenes and alcohols can take part in a greater range of reactions.

Question 3

What are alkenes? (5)

Answer 3

• Alkenes are a homologous series of hydrocarbons.
• The C=C double bond in alkenes means that they have two fewer hydrogens compared with alkanes containing the same number of carbon atoms.
• The C=C double bond can open up to make a single bond, allowing the two carbon atoms to bond with other atoms. This allows them to make polymers as they can bond to the adjacent molecules. Also, this makes alkenes reactive - far more reactive than alkanes.
• Alkenes have twice as many hydrogen atoms as carbon.
• The first four alkenes are ethene, propene, butene, and pentene.

Question 4

What is the formula for alkenes?

Answer 4

C2 H2n

Question 5

What is cracking?

Answer 5

Cracking is the process in which larger chain hydrocarbons are split into smaller, more useful hydrocarbons. A thermal decomposition reaction: breaking down molecules by heating them.

Question 6

What are the two methods of cracking?

Answer 6

• Catalytic cracking
• Steam cracking

Question 7

Method of catalytic cracking

Answer 7

• Heat the long chain hydrocarbons and vaporise them (turn them into a gas).
• Pass the hydrocarbon vapour over the hot powdered aluminium oxide, and as the long hydrocarbon comes into contact with the catalyst, it splits apart into two smaller alkanes and an alkene.

Question 8

Method of steam crackling

Answer 8

• Heat the long chain hydrocarbons and vaporise them (turn them into a gas).
• Mix the gas with steam and heat them with very high temperature. This causes the long hydrocarbon chains to split apart into shorter ones.

Question 9

How do you write an equation for the cracking of an alkane?

Answer 9

Longer chain alkane -> shorter alkane + alkene

Question 10

What can the alkenes (waste products) in the cracking of alkanes be used for?

Answer 10

They can be used to produce polymers and can also be used as starting materials for producing many other chemicals.

Question 11

What will happen if bromine water is added to a solution of alkenes?

Answer 11

They will de-colourise the bromine water from orange to colourless.

Question 12

How is steam cracking different to catalytic cracking?

Answer 12

Steam cracking is different because there is no catalyst involved. Instead the vaporised long chain alkane is mixed with steam at very high temperatures.

Question 13

Why does cracking produce an alkene as well as an alkane?

Answer 13

When a long chain alkane is cracked, there aren’t enough hydrogen atoms to make two alkanes. Hence, cracking will always produce one alkane and one alkene.

Question 14

What is another way to express the presence of the double bond between two carbon atoms in alkenes?

Answer 14

By saying that alkenes are unsaturated.

Question 15

What test is used to distinguish between alkanes and alkenes?

Answer 15

The bromine water test

Question 16

Bromine test (2)

Answer 16

• Bromine water (just bromine dissolved in water) by itself is an orange colour. But when it’s mixed with alkenes, all of the bromine will react, and so the solution loses its orange colour, and turns colourless.
• This doesn’t happen when bromine water is mixed with alkanes, because alkanes are not reactive enough to react with bromine water. So when mixed with an alkane, the solution will stay orange.