4.28 describe how bromine water can be used to distinguish between an alkane and an alkene

Testing for alkenes:
- test for alkene: bromine water (orange)
- if you shake alkene with bromine water, then bromine water turns colorless.

The reaction that takes place in this is an addition reaction.

4.17 describe how long-chain alkanes are converted to alkenes and shorter-chain alkanes by catalytic cracking (using silica or alumina as the catalyst and a temperature in the range of 600–700ºC)

- In cracking, a large alkane is cracked to produce smaller, more useful molecules.

• longer chained alkane --> shorter chained alkane + alkene
• OR: longer chained alkane --> alkene + hydrogen
• OR: longer chained alkane --> alkane + alkane + alkene

Why do we crack alkanes?
- shorter chained alkanes are used for fuels
- shorter chained alkenes are used for plastic production

Explanation:
- cracking allows large hydrocarbon molecules to be broken down into smaller, more useful hydrocarbon molecules
- fractions containing large hydrocarbons are heated at 600-700℃ to vaporize them
- vapours will pass over the hot catalyst of silica/alumina
- this process breaks covalent bonds in molecules, causing thermal decomposition reactions
- as a result, cracking produces smaller alkanes and alkenes.
e.g. decane --> octane + ethene

Conditions of Cracking;
2 ways of cracking:
- catalytic cracking: using heat and a catalyst. The catalyst speeds up the reaction
- steam cracking: using heat and steam.

4.25 explain why alkenes are classified as unsaturated hydrocarbons

Saturated: a molecule containing only single bonds between carbon atoms.
e.g. alkanes

Unsaturated: a molecule of C=C double bond.
e.g. alkenes

So in summary, alkanes are saturated and alkenes and unsaturated

4.24 know the general formula for alkenes

General formula for alkenes:

C_nH_2n

4.5 understand how to write the possible structural and displayed formulae of an organic molecule given its molecular formula

Displayed formula: type of structural formula and shows all the bonds in the molecule as individual lines, where each line represents a pair of shared electrons.
 


 Structural Formula:
 

4.8 describe how the industrial process of fractional distillation separates crude oil into fractions

 Process:
- Crude oil is separated by fractional distillation.
- Crude oil is heated and oil evaporates.
- It then goes into the tower. As the vapours rise up the tower, the temperature falls.
- Different sized fractions condense at different heights because they have different boiling points.
- Smaller molecules condense high up the tower. Larger molecules condense low down the tower.
- Fractions are collected in liquid form.