2a. What is in crude oil? Why is it so important?

Crude oil is a finite resource found in rocks - its non-renewable, and will run out eventually.

Crude oil is the remains of an ancient aquatic biomass consisting mainly of plankton that was buried in mud and subjected to heat and pressure and slowly end up as a yellow to brown liquid which may form pools or be absorbed into porous rocks like shale.

Crude oil is an important raw material, and the source of many useful substances such as fuels and a chemical feedstock for the petrochemical industry, from which endless products, including plastics-polymers, dyes, cosmetics and pharmaceuticals like drugs and many more products, are eventually manufactured from crude oil, BUT, it is a finite resource, and won't last forever !

Over 90% of crude oil is used in the manufacture of fuels.

Many useful materials on which modern life depends are produced by the petrochemical industry, such as fuels, solvents, lubricants, polymers, detergents and host of other specialised chemicals - even drugs for medicinal formulations.

When initially pumped out of the ground crude oil is a complex mixture of a very large number of compounds most of which are hydrocarbons, molecules composed of hydrogen and carbon atoms only and many of them are hydrocarbons called alkanes (a particular series of organic compounds, that compounds based on carbon - details in other sections).

The hydrocarbons may consist of molecules based on chains of carbon atoms (mostly 1 to 40), sometimes linear ('straight') and sometimes branched (a side-chain of C atoms) and others are based on rings of carbon atoms.

Examples of hydrocarbons ...

Straight chain or linear alkane hydrocarbons: ,

Branched alkane hydrocarbons: ,

Hydrocarbon ring compounds , , ,

There are lots of other types of organic molecules in crude oil, in fact over 1000 are known, and some of their molecular structures relate to molecules found in marine organisms today.

Full explanation of the structure of the homologous series of alkanes and don't worry about the ring compounds.

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2b. A laboratory demonstration of the fractional distillation of crude oil

The complex mixture of hydrocarbons in crude oil can be separated into fractions by the technique of fractional distillation. The laboratory demonstration of the fractional distillation is illustrated on the right diagram.

A simulated synthetic crude oil is used for health and safety reasons. As the crude oil vapour ascends the fractionating column the highest boiling liquid hydrocarbons condense out first and the lowest boiling hydrocarbon liquid's vapour exits the top of the fractionating column and enters the condenser and runs into the collection tube.

In this way you can distil over progressively higher boiling fractions, which are themselves narrow boiling point ranges of different hydrocarbons of similar carbon chain length e.g. C₆ to C₈ etc.

Crude oil cannot be used directly but must be refined before commercially useful products are produced by the petrochemical industry (collectively called petrochemicals).

The oil refining process principally involves fractional distillation into useful fractions i.e. products with specific uses, but further processing may needed to diversify both the quantity and nature of particular oil based products.

A fraction is a mixture of liquids (in this case hydrocarbons) with a relatively narrow (restricted) boiling point range of molecules, i.e. those with a similar number of carbon atoms in the chain.

Within each fraction obtained from crude oil the hydrocarbon molecules have a similar number of carbon atoms and similar physical properties.

The uses of the fractions very much depends on their physical properties, which in turn are dependant on the length of the molecule i.e. the carbon atom chain in a hydrocarbon molecule.

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2c. The commercial separation of the crude oil mixture into fractions by fractional distillation

• 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.

• All the bonding is covalent C–C or C–H bonds

What goes on in an oil refinery?

• Crude oil is a complex mixture of many compounds, 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.
  • Therefore a mixture can be separated quite easily by physical means eg fractional 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 liquids must also be completely soluble in each other, that is they must all be miscible liquids.
  • When the temperature is high enough, the kinetic energy of a particular hydrocarbon molecule will be sufficient for it to escape the intermolecular forces in the liquid and become a gas.
  • The intermolecular forces are much weaker than the strong carbon - carbon bonds in the hydrocarbon molecule, so it vaporises without decomposes.
• At the bottom of the fractionating column 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 the 'simple' diagram below!
  • A fractionating column acts in the same way as a fractional distillation apparatus in the school/college laboratory but on an industrial scale!
  • In an oil refinery the fractionating columns are very tall with huge surface area to give the best chance of separating the dozens of hydrocarbons in the crude oil (see diagram on the left.
• 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.
• Up the fractioning column the temperature gradually decreases (temperature gradient), so the highest boiling (least volatile) molecules tend to be at the bottom and the lowest boiling (most volatile) hydrocarbons go to the top. The rest of the hydrocarbon molecules then condense out in narrow temperature range i.e. the different fractions condense out in a gradual way from top to bottom depending on their boiling point.
• In other words the most volatile fraction, i.e. the molecules with the lowest boiling points (shortest hydrocarbon molecules), boil or evaporate off first and go higher up the column and condense out at the higher levels in the fractionating column at the lowest temperature.
• The higher the boiling point (the higher the condensation point) the lower down the column the hydrocarbon condenses.
• So all of the hydrocarbon molecules separate out according to their boiling/condensation point so that the highest boiling fraction, i.e. the less volatile molecules with higher boiling points (longest hydrocarbon molecules), tend to condense more easily lower down the column, albeit at the higher temperatures.
• The process is perhaps more correctly called fractional condensation but it is still referred to as fractional distillation.
• The bigger the molecule, the greater the intermolecular attractive forces between the molecules, so the higher the boiling point or condensation point (see physical property trends).
  • This is an important rule to know since the intermolecular forces (intermolecular bonding) affect the physical properties including melting point and viscosity too, and this has a bearing on how each fraction is used, see below.
• Note: The strong covalent chemical bonds like C–C or C–H within the molecules are not broken in the process, only the intermolecular bonding force of attraction between the molecules is weakened to allow the initial evaporation or boiling.
• The fractions are then further processed to produce fuels and chemical feedstock for the petrochemical industry.
• These include fuels such as liquefied petroleum gas, petrol, diesel oil, kerosene, heavy fuel oil which are all non-renewable fossil fuels, as is methane from natural gas.
• From the chemical feedstock and petrochemical industry we produce many useful materials on which our modern life depends e.g. solvents, lubricants, medicines, polymers, detergents etc.
• The fractions are listed below with the approximate boiling point ranges and approximate number of carbon atoms in the molecule
• These tables vary from source to source (internet, textbook etc.), but don't worry about, the oil fractions are all in the same order, and sometimes alternative names are used and a little variation in the carbon numbers range.
• The term chemical feedstock means these hydrocarbons are not used directly but 'feed' into some other processes to this oil fraction into more useful chemicals, and not necessarily hydrocarbons.

You will find small variations of this table in different textbooks

Many are useful fuels - alkane hydrocarbons, but they are non-renewable fossil fuels - specific use depends on physical properties (see later)

Decrease in boiling point, viscosity, number of carbon atoms in molecule.

A simplified diagram of a fractionating column used in the fractional distillation of crude oil

The decrease and increase trends for the hydrocarbon molecules are given on the left of fractionating column

Increase in volatility and ease of ignition

(1–2%)

mainly propane and butane gases which can be compressed or liquefied

(?%)

(20–40%)

(10–15%)

(15–20%)

AND bitumen tar

(40–50%)

bitumen components boil over a 500^oC - 700^oC range

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2d. Relating intermolecular bonding forces in hydrocarbon molecules to their physical properties and uses

For section 2b. a mental picture of the increasing length of hydrocarbon molecules will help you understand more about how, and why, the physical properties of hydrocarbon molecules changes with increasing length AND how their physical properties affect how each fraction is used commercially after the fractional distillation of crude oil.

Note that the longer the hydrocarbon molecule, the more flexible or wiggly it gets!

The important physical properties of hydrocarbons like alkanes all depend on the forces between the molecules - the intermolecular forces (intermolecular bonding). This is NOT covalent bonding, covalent bonds are the much stronger C-C and C-H bonds between the atoms in the molecule itself, these are the weak attractive forces between the individual molecules.

Quite simply, with important consequences (e.g. how they are used), the bigger the molecule the bigger these weak electrical attractive forces are, which I've done my best to illustrate in the diagrams (above and below). As the hydrocarbon molecule gets bigger, the surface to surface contact area increases allowing more points for the intermolecular bonding attractive forces to happen. Hence the increase in viscosity, melting points and boiling points with increase in molecular mass.

The intermolecular bonds are much weaker than the covalent bonds, so when the hydrocarbon molecules have enough kinetic energy, it is the intermolecular forces which are overcome on vaporisation and the molecules stays intact.

CONCEPT: The dotted lines represent the weak intermolecular bonding forces of attraction between the molecules.

The bigger the molecule, the more dotted line 'connections' there are, the bigger the intermolecular forces!

The different fractions are a range of physical properties which vary with molecular size.

Remember for points 1. to 5., 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.

   • The more strongly the hydrocarbon molecules are attracted to each other, the less easily they run over each other, so the liquid itself does not flow as easily - not as fluid.

   • Note for advanced level students: Intermolecular forces or intermolecular bonding, are non–polar weak electrical attractive forces, often described as Van der Waals forces, and correctly described as instantaneous dipole – induced dipole forces.

     • You need to distinguish this intermolecular attractive force from the much stronger force of the covalent bonds between the carbon atoms (C-C) of the chain of the hydrocarbon.

2. ... the molecule has a higher melting point as more vibrational kinetic energy is needed to overcome the increasing intermolecular attractive forces holding the molecules together to form the crystals which 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).

   • The trend in boiling point is the basis for being able to separate the hydrocarbons into fractions by the use of fractional distillation.

   • 

   • The graph shows the boiling point of alkanes from methane CH₄ (boiling point -164^oC/109 K)  to tetradecane C₁₄H₃₀ (boiling point 254^oC/527 K). [Remember K = ^oC + 273]

   • See the ALKANES page for a list of the boiling points of the first 20 linear hydrocarbons found in oil.

4. ... the molecule is less flammable as they become less volatile (less easily vapourised), 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, much longer carbon chain).

   • 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.

   • We can now apply these ideas to explain these hydrocarbons are used, usually as fuels.

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.
   • Note that all fuels are processed at the oil refinery to reduce the concentration of sulfur/sulphur compounds (desulfurisation) to reduce the air pollution effects on burning.
   • 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

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2e. More notes on the USES of these fractions (with reference to 2d. above)

1. CH₄ 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.
2. Ethane C₂H₆, propane C₃H₈, butane C₄H₁₀The very low boiling refinery gas fractions
   • Propane and butane can be stored under pressure as bottled gas (LPG), and because the gas readily flows under the control of a simple valve, they can be conveniently pumped to burner systems.
   • Being gases, they are easily ignited but explosive !
   • The intermolecular forces are very weak in a gas, that's why they are gases at room temperature, and they are very fluid just like a liquid, so easily moved through pipes.
   • In the UK butane gas (C₄H₁₀) is in blue cylinders and propane (C₃H₈) in red cylinders.
3. C₅H₁₂ to C₇H₁₆ The intermolecular forces are now great enough to raise the boiling point of the hydrocarbons to above room temperature and make them liquids at room temperature. Vehicle fuels like petrol must be liquid at room temperature for compact and convenient storage but they must be easily vapourised (low boiling point) to mix with air in the engine prior to ignition. However, the ease of vaporisation due to weak intermolecular forces, does however make them highly flammable !
4. C₁₀H₂₂ to C₁₆H₃₄ Paraffin and kerosene are bigger molecules, bigger intermolecular forces, so less volatile and flammable. This makes safer to use in domestic heating and jet aircraft fuel, but not as easily ignited.
5. C₁₄H₃₀ to C₂₀H₄₂ Diesel oil is not as volatile, flammable or as easily ignited as petrol due to the higher boiling point and intermolecular forces. However as a vehicle fuel it doesn't have to be vapourised first, the diesel fuel is sprayed into the engine cylinder and mixed with air and ignites under compression from the piston action.
6. C₁₄H₃₀ to C₂₀H₄₂ Fuel oil molecules are getting quite big, higher boiling points, higher intermolecular forces and more viscous, but not too viscous to pump to a ships boiler or locomotive diesel engine. Fuel oil is not very volatile and so not as flammable and dangerous to use as petrol or diesel etc.
7. > C₂₁H₄₄ (typically 36 C atoms, C₃₆H₇₄) Lubricating oil must be quite viscous to stick onto surfaces and are virtually non-volatile due to the even higher intermolecular forces. 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.
8. > C₂₁H₄₄ (typically 31 C atoms, C₃₁H₆₄) 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. At this point in the fractions the intermolecular forces are now sufficient to create solids at room temperature, albeit low melting like candle wax.
9. > C₆₀H₁₂₂ (many much bigger) 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 as the top layer in roads. Bitumen is also used to make waterproof roofing felt. Bitumen consists of the largest molecules in crude oil with the greatest intermolecular forces between them, so they are solid at room temperature and very high boiling up to 700^oC.
10. For more products derived from crude oil other than fuels 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
    • so be aware that smaller amounts of crude oil can end up as other synthetic (man-made) products such as plastics, fertilisers, medicines, food additives etc. etc.!
    • As already mentioned, the less useful fractions from crude oilcan be split by Cracking to more useful products like more petrol or alkenes like ethene to make plastics like poly(ethene).

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

(some are not quite as bad as others!)

It may seem a curious question these days, but its only fair to consider what is the best fossil fuel?

but some fossil fuels pollute more than others, and some leave a bigger 'carbon footprint'!

Factors that should be taken into consideration - often factors overlap

• Combustion - burning characteristics (see also toxicity, pollution and climate change):

  • Ease of ignition - how easily does it burn, gas most easy to ignite and control, less so for petrol/diesel and coal the most difficult to ignite and control. Examples described on previous section 2b. on uses.

  • Energy value: e.g. kJ of heat energy released per kg, sometimes referred to as the 'energy density', the bigger the better.

    • See Calorimeter methods of determining energy changes - burning fuels

  • Waste material: Burning hydrocarbons doesn't produce must waste other than the gaseous products, but burning coal produce smoke and large quantities of ash.

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

  • The price of oil can vary with market forces determined by the World's economy AND political instability and wars, particularly in the major oil producing Middle-East Arab Gulf states affect the price too.

  • Therefore, without alternative energy resources, we are at the mercy of forces beyond our control.

  • If stocks or production rates fall, the price of crude oil rises and richer countries can afford more costly oil and can stockpile it, developing countries will struggle to compete.

  • In order to preserve our crude oil and gas supplies we sometimes compromise our ideals when dealing with the politics of countries we may think unsuitable.

• Storage and distribution: Important health and safety issues to consider here e.g.

  • Coal is very dirty but safe to store, difficult to ignite,

  • Natural gas (explosive flammable gas) much more dangerous to store in large tanks under pressure, but the gas is easy and more convenient to distribute via pipes, but gas leaks are potentially VERY dangerous, as you see sometimes on the news!

  • Petrol is quite volatile and the flammable vapour easily ignited.

  • Diesel is not as volatile and central heating oil even less so, but spills of any of these fuels is potentially harmful to the environment or a fire hazard.

  • Again, some specific examples were described in the previous section 2b.

• 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, as already mentioned ...

    • ... 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?, methane is one of the best fossil fuels in this respect.

  • The sulphur content of fuel (most removed before fuel used to minimise sulphur dioxide and acid rain formation), some coals are very high in sulfur, but can't be removed from it, though you can clean the smoke by removing the sulfur dioxide using an alkali like calcium hydroxide (limewater).

  • The efficiency of combustion e.g. minimum carbon monoxide and soot levels, again methane is one of the most clean burning fuels.

  • Coal produces a lot of smoke and larger hydrocarbon fuels like oil, tend to burn with a sooty flame.

• 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.

• Each fossil fuel has a different cost, efficiency and cleanliness on burning.
  • Generally speaking in 'cleanliness' the order is ...
  • methane (natural gas) > alkanes in petrol > heavy oil ...
  • and from left to right there is also an increase in C/H atom ratio in the molecule so more CO₂ is produced too.

2g. Alternative fuels to fossil fuels - more on other energy resources

• OIL IS A VALUABLE CHEMICAL RESOURCE but non-renewable

  • Apart from the obvious value of crude oil as an non-renewable energy source, should we using this very valuable source of organic chemicals by merely burning most of it?

  • AND how long will oil reserves last?

  • AND what happens if the oil runs out?

  • Therefore isn't it in our own interest to manage the finite oil reserves remaining and conserve them?

  • AND seek other sources of energy to power our lives?

  • AND perhaps influence the course of global warming?

  • BUT the trouble is, oil is very convenient, readily available, and new reserves are still being found and the oil and gas trapped in deep layers of shale are being exploited,

  • AND it will take time to develop new technologies.

  • The world's population is steadily increasing and countries like China and India have huge energy demands for home and industry, and are pressured into building many fossil fuel power stations, though China is a leading player in large scale wind power projects.

• 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 progress will be slow.

• HYDROGEN - FUEL of the FUTURE?:

  • Hydrogen gas can be used as fuel and a long-term possible alternative to fossil fuels.

  • It burns with a pale blue flame in air reacting with oxygen to be oxidised to form water.

    • hydrogen + oxygen ==> water

    • 2H_2(g) + O_2(g) ==> 2H₂O_(l) 

  • It is a non-polluting clean fuel since the only combustion product is water and so its use would not lead to all environmental problems associated with burning fossil fuels.

    • It is easily distributed in pipes like natural gas, but there are health and safety issues to do with storage and distribution since it is, like natural gas, highly flammable and explosive.

  • It would be ideal if it could be manufactured by electrolysis of water e.g. using solar voltaic-cells or some kind of but the technology is in its infancy.

  • Hydrogen can be used to power fuel cells see the "Extra Electrochemistry" page

• Other alternative renewable fuels are discussed on other pages ...

  • whatever, be aware at least that nuclear power, wind turbine power, solar energy, biofuels are all under consideration and being developed and you have a few things to say about each of them.

  • See also

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

  • Energy resources: general survey & trends, comparing renewables, non-renewables, generating electricity

  • Renewable energy (1) Wind power and solar power, advantages and disadvantages gcse physics

  • Renewable energy (2) Hydroelectric power and geothermal power, advantages and disadvantages

  • Renewable energy (3) Wave power and tidal barrage power, advantages and disadvantages

GCSE/IGCSE/O Level Oil Products & Organic Chemistry INDEX PAGE

ALL my Advanced A Level Organic Chemistry revision notes

Multiple Choice Quizzes and Worksheets

KS4 Science GCSE/IGCSE m/c QUIZ 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 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

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