Chemistry: Cracking and Alkenes

What type of molecules do crude oil contain?

Many long chain molecules, not many short chain.

Why might the length of the molecules in crude oil be a problem?

the short chains are more useful because we use these as fuel (more flammable and volatile)

What is catalytic cracking?

crack the longer chains we don’t need into shorter chains we do need.

How does catalytic cracking work?

Heating the hydrocarbon molecules to around 600 – 700°C to vaporise them

The vapours then pass over a hot powdered catalyst of alumina or silica

What type of reaction is cracking?

The use of heat means it is a thermal
decomposition reaction.

What are alkenes?

unsaturated hydrocarbons as they have one carbon-carbon double bonds (C=C)

What are alkenes uses?

Making ethanol and polymers.

What is the general formula for alkene?

C_nH_2n

Why are alkenes more reactive than alkanes?

The presence of the double bond, C=C, means they can make more bonds with other atoms by opening up the C=C bond and allowing incoming atoms to form another single bond with each carbon atom.

What reaction can alkene undergo?

Addition reactions

How do you distinguish alkanes and alkenes?

When bromine water is shaken with an alkane the solution remains orange

When bromine water is shaken with an alkene, the solution will decolourise

Why does adding bromine to the solution distinguish them?

the bromine can add across the double bond in an alkene meaning it is no longer in solution.

In presence of unsaturated compounds.

Describe the reaction to test for alkane or alkene.

1. Collect 2 test tubes with bungs.
2. In one test tube, add a few drops of cyclohexane (alkane)
3. In another test test tube, add a few drops of cyclohexene (alkene)
4. Add a few drops of bromine water to each.
5. Add the bung and shake. Note your results.
6. Place these in the fume hood and collect another 2 test tubes with bungs.
7. In one test tube, add a few drops of tea tree oil
8. In the other test tube, add a few drops of peppermint oil.
9. Add a few drops of bromine water to each.
10. Add the bung and shake. Note your results