A fraction is a mixture of a restricted boiling point range of molecules, they have a similar number of carbon atoms and physical properties. The uses of the fractions depend on their physical and chemical properties.

• Hydrocarbon molecules are only  made of a chemical combination of carbon and hydrogen atoms.

• They are compounds because they consist of atoms of at least two different elements.

• What goes on in an oil refinery?
• Crude oil is a complex mixture of many, but mainly hydrocarbon compound molecules.
  • A mixture consists of two or more elements or compounds which are NOT chemically combined.
  • The chemical properties of each substance in the mixture is unchanged as the there are no chemical bonds between the hydrocarbon molecules.
  • A mixture can be separated by physical means eg distillation.
  • See notes on

    • Definitions in Chemistry - Elements, Compounds & Mixtures

    • Examples of methods of Separating Mixtures

• This means crude oil can be separated by physical methods, in this case by fractional distillation, because they have different boiling and condensation points.
• The crude oil is heated to vapourise it (evaporated or boiled) and the vapour passed into the fractionating column - a large construction of many levels and pipes, see 'simple' diagram below!
• This is a continuous process (not a batch process). The fractionating column works continuously with heated-vapourised crude oil piped in at the bottom and the various fractions condensed and constantly tapped off from various levels, each with a different condensation temperature range.
• The most volatile fraction, i.e. the molecules with the lowest boiling points, boil or evaporate off first and go higher up the column and condense at the higher levels in the column at the lowest temperatures.
• The rest separate out according to their boiling/condensation point so that the highest boiling fraction, i.e. the less volatile molecules with higher boiling points, tend to condense more easily lower down the column, albeit at the higher temperatures.
• The process is perhaps more correctly called fractional condensation.
• The bigger the molecule, the greater the intermolecular attractive forces between the molecules, so the higher the boiling or condensation point (see physical property trends).
• Note: Covalent chemical bonds like C-C or C-H are not broken in the process, only the intermolecular force of attraction is weakened to allow the initial evaporation or boiling and this .

(% in crude oil)

mainly depends on its physical properties - see below this table

(1-2%)

(?%)

(20-40%)

(10-15%)

(15-20%)

AND bitumen

(40-50%)

AND over 70

AND

2b. More on relating the physical properties of the fractions to their uses and dangers

The different fractions are a range of physical properties which vary with molecular size. Down the list above (and below) the longer the carbon chain, the bigger the molecule gets ...

1. ... the more viscous the molecule (stickiness! less runny, more sticky) as the intermolecular attractive forces between molecules increases the bigger the molecule in a series of molecules of similar structure.

   • Note that intermolecular forces are non-polar weak electrical attractive forces, often described as Van der Waals forces, and correctly described as instantaneous dipole - induced dipole forces (by advanced level students only!).

2. ... the molecule has a higher melting point as more vibrational kinetic energy is needed to overcome the intermolecular attractive forces holding the molecules together to form the crystals increases with increase in size of molecule.

3. ... the molecule has a higher boiling point as more particle kinetic energy is needed to overcome the increasing intermolecular forces between the liquid molecules. 3.follows from 2. ie the intermolecular forces increase between the hydrocarbon molecules increases as they get bigger (longer carbon chain).

   • Trends 2. and 3. are readily appreciated e.g. methane gas (CH₄), liquid petrol (about C₅H₁₂ to C₇H₁₆) and solid candle wax (over C₂₁H₄₄).

4. ... the molecule is less flammable as they become less volatile, again due to increasing intermolecular forces with increasing size of molecule so for example, petrol (small molecules) is much more flammable than lubricating oil (much bigger molecules).

   • This raises health and safety issues about handling, distributing and storing flammable hydrocarbons. The smallest molecules (natural gas to petrol) are the most volatile and therefore the most easily ignited. Any naked flame or spark could set off a fire and explosion and even

5. Further comments on the use of the fractions related to the use of the hydrocarbons from crude oil
   • The examples are discussed in order of increasing molecular size - increase in carbon chain length.
   • All fuels are processed at the oil refinery to reduce the concentration of sulfur/sulphur compounds (desulfurisation/desulphurisation).
   • It should be noted that liquid fuels like petrol, diesel, central heating oil etc. are east to store and distribute to wherever they are need in homes or factories and they are so readily available, that change may be necessary, but it will be slow.
     • See notes on pollution, global warming
6. Methane natural gas, either from gas fields (eg under North Sea) or from an oil refinery can be piped to power electricity generation or domestic heating in the home.
7. The refinery gas fractions, can be stored under pressure as bottled gas, and because the gas readily flows under the control of a simple valve, so can be conveniently pumped to burner systems, but it is easily ignited and explosive.
8. Vehicle fuels like petrol must be liquid at room temperature for compact and convenient storage but they must be easily vapourised to mix with air in the engine prior to ignition. The ease of vaporisation does however make them flammable!
9. Paraffin and kerosine are bigger molecules, less flammable and safer, but not as easily ignited.
10. Diesel is not as volatile or flammable as petrol and doesn't have to be vapourised first, the diesel fuel is sprayed into the engine cylinder and mixed with air and ignites under compression.
11. Fuel oil molecules are getting quite big, but not too viscous to pump to a central heating burner for domestic use. Fuel oil is not very volatile and so not as flammable and dangerous to use as petrol or diesel etc.
12. Lubricating oil must be quite viscous to stick onto surfaces. Smaller molecules might be more runny but they would evaporate away! It is also water repellent and helps reduce corrosion on moving metal parts from factory machines to cars and bicycles.
13. Candle wax is very convenient as a solid for a humble lamp (especially in power cuts!), but via a wick, the heat from the flame is sufficient to vaporise the hydrocarbons to burn them and give a big enough luminous yellow flame to act as a source of light.
14. Bitumen is a water repellent solid at room temperature but is readily melted (sometimes too easily in hot weather). Used as base for a road chipping top surface or sometimes directly. It is also used to waterproof roofing felt.
15. For more products derived from crude oil see ...
    • Section 6. Cracking - a problem of supply and demand, other products
    • Section 7. Polymerisation - Polymers and plastics
    • Section 8. Alcohols - the production and uses of ethanol

2c. Energy resource evaluation - What makes a good fossil fuel?

Factors that should be taken into consideration - often factors overlap

• Energy value: e.g. kJ of heat energy released per kg;

• Availability: Geographical convenience - is it imported?, fluctuations in oil production levels and the market price

• Storage and distribution: Health and safety issues e.g. coal very safe, natural gas (explosive flammable gas) much more dangerous to store, but the gas is easy and more convenient to distribute via pipes.

• Production Costs:

  • Costs of exploration and extraction can be high for oil

  • Coal mines are dangerous to operate, good health and safety policies don't come cheaply, and its the same for operating oilfields and petrochemical complexes - oil refineries.

  • Costs of transporting the fuel

  • AND even after considering all of these factors ...

    • ... the market price can fluctuate with market forces and may also depend on political situations too!

    • eg the politics of the Middle East or European countries importing Russian natural gas etc.!

• Toxicity, Pollution and climate change:

  • Greenhouse effect - which fuel produces the least or most carbon dioxide for the energy released?

  • The sulphur content of fuel (most removed before fuel used to minimise sulphur dioxide and acid rain formation)

  • The efficiency of combustion e.g. minimum carbon monoxide and soot levels.

• Ease of use: Transferred easily e.g. oil and gas readily piped around and readily ignited for a quick start in power station. Coal is more trouble to transport and does not ignite as easily.

• See also

  • Section 4. Fossil fuels, pollution and climate change

  • Section 9. biofuels eg bioethanol, biodiesel, alternative renewable fuels

  • Some other ideas for alternative fuels (page originally designed for AS level).

• also on OP04

Multiple Choice Quizzes and Worksheets

KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products (easier-foundation-level)

KS4 Science GCSE/IGCSE m/c QUIZ on Oil Products (harder-higher-level)

KS4 Science GCSE/IGCSE m/c QUIZ on other aspects of Organic Chemistry

and 3 linked easy Oil Products gap-fill quiz worksheets

ALSO gap-fill ('word-fill') exercises originally written for ...

... AQA GCSE Science Useful products from crude oil AND Oil, Hydrocarbons & Cracking etc.

... OCR 21st C GCSE Science Worksheet gap-fill C1.1c Air pollutants etc ...

... Edexcel 360 GCSE Science Crude Oil and its Fractional distillation etc ...

... each set are interlinked, so clicking on one of the above leads to a sequence of several quizzes

Advanced Level Organic Chemistry revision notes

Revise KS4 Science GCSE/IGCSE/O level Chemistry Revision-Information Study Notes for revising for AQA GCSE Science, Edexcel GCSE Science/IGCSE Chemistry & OCR 21stC Science, OCR Gateway Science WJEC/CBAC GCSE science-chemistry CCEA/CEA GCSE science-chemistry (and courses equal to US grades 8, 9, 10)