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A series of free IGCSE Chemistry Activities and Experiments (Cambridge IGCSE Chemistry).

Crude Oil
What is crude oil?
Crude oil is a mixture of hydrocarbons.
Hydrocarbon is a molecule made from hydrogen and carbon atoms.

Explain the process of fractional distillation
Separating molecules according to their boiling points.
1. Crude oil heated.
2. Enters column as a gas.
3. Larger molecules condense at the bottom where it is hottest.
4. Smaller molecules condense at the top where it is cooler.

Describe the uses of different crude oil fractions
Gases - cooking and heating.
Petrol - car fuel.
Naphtha - lighter fuel.
Kerosene - aircraft fuel.
Diesel - lorry/train fuel.
Fuel oil - ship/power station fuel.
Bitumen - road/roof surfacing.



Questions
1. The viscosity of the fractions increases with boiling point. What molecular property would you suggest causes this difference?
2. What differences were there in flammability between the fractions?

Answers
  • Show Solutions
    1. It is related to the increased chain length of the molecules in the higher boiling fractions - the longer the molecules, the more they tangle with each other.
    2. The flammability of the fractions reduces as the molecular size and boiling point increases.
Alkanes and Alkenes
Compare the structure and properties of alkanes and alkenes
Alkane
• Saturated.
• All carbon atoms share single bonds with other atoms.
Alkene
• Unsaturated.
• Contain at least one carbon-carbon double bond.
How to draw alkenes?
Why alkenes decolourise bromine water?

Explain the process and purpose of cracking
Cracking is a thermal decomposition reaction that breaks down long alkenes into smaller useful alkanes and alkenes.
Explain the problems linked to making synthetic polymers.
Requires heat and catalyst.
Long alkanes have limited use eg. road tar.
An experiment to show cracking of alkane.

Explain the problems linked to making synthetic polymers
Polymers are long chain molecules made from repeated links called monomers - requires pressure and catalyst.
Natural polymers eg. starch, proteins and DNA.
Synthetic Polymers (Plastics)
• Poly(ethene) - elastic, light
Eg. Plastic bags, hose pipes
• Poly(propene) - tough, flexible
Eg. Plastic containers, carpets
• Poly(chloroethene) PVC - flexible, water-resistant
Eg. Clothing, electric cables • Poly(tetrafluroethene) Teflon - flame-resistant, unreactive, durable
Eg. non-stick coating for frying pans
Problems of disposing plastics
• Non-biodegradable - remain in landfill sites for years.
• Burning releases toxic gases.
• Only some plastics can be recycled.
Solution:
• Some plastic bags now contain starch granules which are biodegradable.
• some plastics can be broken down by sunlight.

Questions
1. Why is cracking so important in the petrochemical industry?
2. What evidence is there that larger molecules had been converted to smaller molecules in this experiment? Consider the physical nature of the starting material and the product collected.
3. What alternative test should be carried out to prove that the gas collected is unsaturated? Give the test reagent and the expected outcome.

Answers
  • Show Solutions
    1. The exact composition of petroleum varies depending on where it comes rom, but most oils contain more of the larger molecules than the smaller ones. However, the smaller ones are more useful and more economically important. To increase the profit that van be made from a barrel of oil, the larger hydrocarbons are broken down into smaller ones.
    2. The fact that the product is a gas while the reactant is a liquid suggests that the product molecules are smaller (have shorter chains) than the reactants. The boiling point of the product is lower than that of the reactant.
    3. With an alkene, bromine water is immediately decolourised from orange/brown to colourless. With an alkene, there is no change.

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