5.14 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 600–700C.

Long chain hydrocarbons are passed over a hot catalyst (silica or alumina at 600-700 degrees) this causes them to break down into smaller molecules.

As some atoms are lost from molecules, they become unsaturated and can therefore form a double bond. This is how you get alkenes from the process as well as shorter chain alkanes.

The animation on this page is helpful:
http://www.bbc.co.uk/schools/gcsebitesize/science/aqa_pre_2011/oils/polymersrev1.shtml

5.13 understand that fractional distillation of crude oil produces more long-chain hydrocarbons than can be used directly and fewer short-chain hydrocarbons than required and explain why this makes cracking necessary

Long chain hydrocarbons are less flammable and more viscous.
Short chain hydrocarbons burn well and flow well.

Long chain hydrocarbons can be cracked which breaks them up into short chain ones.

5.12 understand that nitrogen oxides and sulfur dioxide are pollutant gases which contribute to acid rain, and describe the problems caused by acid rain

NO and SO2 are given off into the atmosphere by some industrial processes.
When they are in the atmosphere they react with rain water to create H+ ions.
When the rain falls the acid can corrode rocks and buildings. Acid can also alter the PH in soil or rivers which can effect an ecosystem.
Also acid rain corrodes limestone, which damages buildings and stuff.

5.11 understand that, in car engines, the temperature reached is high enough to allow nitrogen and oxygen from air to react, forming nitrogen oxides

In car engines there is a high enough temperature to cause a reaction between oxygen and nitrogen in the air.
This makes NO.

5.10 understand that incomplete combustion of fuels may produce carbon monoxide and explain that carbon monoxide is poisonous because it reduces the capacity of the blood to carry oxygen

Hydrocarbons (from crude oil) + oxygen > carbon dioxide + water

Unless there is not enough oxygen around. Then fuels will combust incompletely:

Hydrocarbons + oxygen > carbon monoxide + carbon + water

Carbon monoxide combines with heamaglobin in red blood cells, meaning they can't carry oxygen around the body.

5.9 describe the trend in boiling point and viscosity of the main fractions

Fractions with low boiling points are less viscous.

Fractions with high boiling points are more viscous.

5.8 recall the names and uses of the main fractions obtained from crude oil: refinery gases, gasoline, kerosene, diesel, fuel oil and bitumen

bbc

5.7 describe and explain how the industrial process of fractional distillation separates crude oil into fractions

Crude oil is heated until it boils. As a gas it floats upwards.
As the gas goes higher up and further from the heat source the temperature decreases.
When a compound reaches it condensing point it will condense into a liquid and be collected.
This is known as fractional distillation, and groups with similar condensing temperatures are known as fractions; each fraction is a different substance.

5.6 understand that crude oil is a mixture of hydrocarbons

Crude oil contains different molecules made only of hydrogen and carbon.