Doc Brown's GCSE/IGCSE/O Level KS4 science–CHEMISTRY Revision Notes

Oil, useful products, environmental problems, introduction to organic chemistry

6. Cracking – a problem of supply and demand, other products

– Cracking is a thermal decomposition process by which large alkane hydrocarbon molecules are broken down by passing them over a heated catalyst at high pressure. The products are smaller alkanes used for fuels (e.g. petrol or diesel) and alkenes which are used to make polymers–plastics and other important compounds. There are two good economic reasons for cracking oil fractions – (i) there isn't enough of fuels like petrol or diesel in the original crude oil and (ii) alkenes are NOT found in oil, so must be manufactured from oil. Either way, it means the vast majority of crude oil can be turned into useful products. These notes on cracking alkanes to make alkenes for plastics and other products from cracking like petrol and diesel fuels are designed to meet the highest standards of knowledge and understanding required for students/pupils doing GCSE chemistry, IGCSE chemistry, O Level chemistry and KS4 science courses. These revision notes on cracking hydrocarbons like alkanes in the petrochemical industry and why the cracking reaction is so important should prove useful for the NEW AQA GCSE chemistry, Edexcel GCSE chemistry & OCR GCSE chemistry (Gateway & 21st Century) GCSE (9–1), (9-5) & (5-1) science courses.

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doc b oil notesdoc b oil notes6a. CRACKING a problem of Supply and Demand in the Oil Industry!

There isn't enough petrol in the original crude oil and crude oil doesn't have alkenes in it for plastics but cracking reactions can help supply them! AND no left over waste oil!

Alkanes can be 'cracked' in a thermal decomposition reaction to make more smaller more useful molecules because, quite simply, we need a lot more petrol and diesel than is in crude oil AND in the cracking process, to make ethene molecules (NOT in oil) from which we make many polymer - plastic products.

Cracking converts longer alkane hydrocarbon molecules into smaller alkane and alkene molecules

The SUPPLY and DEMAND for various fractions of crude oil

A comparison of typical percentages we get, and what we want, from the fractions of crude oil

Crude oil fraction Carbon atoms in hydrocarbon Approx. % in crude oil Approx % required
Gas, LPG etc. 1 - 4 2-4 4
Petrol 5-10 6-13 20-26
Naphtha 8-12 10 5
Paraffin, kerosene 9-16 13-15 8
Diesel, gas oil 15-25 13-20 20-25
Residue - heavy oils, waxes, bitumen etc. 20-30+ 50 38
  • When crude oil has been distilled into useful fractions it is found that the quantities produced do not match the ratio required for commercial needs e.g. we have an insatiable appetite for petrol and diesel in our cars and there are two many left–overs of the larger molecules which do not make good fuels or have other uses

    • e.g. fuel oil, lubricating oil, naphtha, wax and bitumen in crude oil exceed demand.

    • The data table above gives you an idea of the imbalance between what we get from crude oil and what industry and domestic consumers actually want.

      • The data in the table was gleaned from several internet sources and cab a bit variable, so I've quoted typical ranges, so expect the figures to add up to a 100%.

      • The table shows that the hydrocarbon demands don't match the composition of crude oil particularly the demands for petrol and diesel.

      • You can see there is quite a deficiency of gas (half required), petrol (< third required), but a surplus of naphtha, heavy oils, waxes etc., so these heavier fractions are cracked to make more of the smaller molecules we need.

      • Kerosene can be cracked to make more petrol and heavy oil, waxes, bitumen etc. can be cracked to make more petrol and diesel, AND all cracking makes the incredibly useful hydrocarbon molecules called alkenes.

    • So, the excess of these bigger hydrocarbon molecules can be converted to smaller ones by cracking them apart!

  • Also, alkenes are not found in crude oil and they are one of the most valuable types of organic molecule in the chemical industry e.g. to make polymers (plastics) or ethanol (an alcohol).

  • The two deficiencies are remedied by the process of cracking which converts useless big long 'sticky' molecules into useful smaller ones!

    • eg cracking naphtha or diesel oil fractions to convert these 'less demanded' longer molecules to give more shorter useful molecules like petrol fuel molecules AND the extremely useful alkenes.

  • Therefore cracking is an important economic process in the petrochemical industry to make the best and most varied use of the resource we call crude oil.

  • Advanced Chemistry Page Index and LinksCRACKING is done by heating some of the less used fractions to a high temperature vapour and passing over a suitable hot catalyst at high pressure.

    • Sometimes the fraction is heated with steam to a very high temperature.

    • Using different conditions ie by varying with/without steam, temperature, pressure or catalyst you can control the composition of the mixture and make a variety of different hydrocarbon products.

    • The cracking reaction is an example of a thermal decomposition, that is, breaking a molecules down with heat, by heating them to high temperature.

    • The high temperature and catalyst are needed to facilitate the cracking reaction because you have to break the very strong carbon-carbon covalent bonds in the alkane molecule.

    • methods of gas preparation - apparatus, chemicals and equation (c) doc b

    • You can safely demonstrate cracking in the laboratory by heating paraffin grease over an aluminium oxide (or porcelain chips) catalyst at 400–700oC, and collecting the smaller gaseous hydrocarbon molecules over water – readily shown to be flammable.

      • This experiment needs to be done as a teacher demonstration – most carefully!

      • The mineral wool (or glass wool) is soaked in paraffin oil or any other heavy oil, even paraffin grease will do.

      • This and the mineral catalyst are set up as shown in a pyrex boiling tube.

      • The catalyst is strongly heated and the heat will also vapourise some of the oil/grease whose vapour will then pass over the catalyst to 'cracked' i.e. broken down by the combined effect of the heat and catalyst.

      • Some of the hydrocarbon liquid molecules collected in the bottle AND some of the gases in the inverted test tube from the cracking process should decolourise bromine water – the simple test for alkenes.

        • See diagram below, you just add orange bromine water to a sample of the collected hydrocarbon gases or liquids).

        • doc b oil notes

      • The chemistry of ALKENES – unsaturated hydrocarbons – reaction with bromine & hydrogen

      • The experimental setup above allows the safe collection of the very short molecules e.g. methane, ethane, ethene, propene and propane gases and liquid alkanes and alkenes of at least five carbon atoms in the chain.

      • The bottle is a safety measure, if the gases inside the pyrex tube cool down, they contract, but any water sucked back from the gas collection system is trapped in this bottle. This means no cold water gets into the hot pyrex tube to crack it and a cause a nasty accident! We just want to crack the hydrocarbon molecules!

    • The cracking reaction is an example of thermal decomposition – a reaction that breaks down molecules into smaller ones using heat and it takes place on the very hot surface of the catalyst.

      • The main products from cracking alkanes from oil are ...

        • smaller alkanes e.g. for petrol or diesel,

        • alkenes e.g. for polymers–plastics, alcohols and many other important organic compounds.

  • The equations below illustrate the process, small molecules are used to show the overall molecular change clearly BUT in practice the 'starter' molecules are likely to be more like the larger alkane hydrocarbon molecules shown in equations (3) and (4).

    • The cracking involves breaking single carbon–carbon bonds to form the alkanes (saturated hydrocarbons) and alkenes (unsaturated hydrocarbons) products. Examples of word equations and balanced symbol equations for cracking reactions are given below.

      • There are lots of possibilities! eg four examples below show in each case the formation of a shorter alkane and an alkene from a cracking reaction ...

Cracking reaction (1) butane doc b oil notes ethane + ethene

C4H10 ==> C2H6 + C2H4

doc b oil notes doc b oil notes doc b oil notes doc b oil notes doc b oil notes

this is probably not used commercially, but illustrates the principle of cracking with small simple molecules to give a smaller alkane and an alkene eg ethene to make the plastic poly(ethene)

Cracking reaction (2) butane ==> methane + propene

C4H10 ==> CH4 + C3H6

doc b oil notes doc b oil notes doc b oil notes doc b oil notes doc b oil notes

this is probably not used commercially, but illustrates the principle of cracking with small simple molecules to give a smaller alkane methane and an alkene propene to make the plastic poly(propene)

Cracking reaction (3) octane ==> hexane + ethene

C8H18 doc b oil notes C6H14 + C2H4

doc b oil notes doc b oil notes doc b oil notes

this cracking reaction is used commercially to make a volatile petrol fuel molecule hexane plus ethene for polymerisation to make poly(ethene)

Cracking reaction (4) dodecane ==> hexane + propene

C12H26 doc b oil notes C6H14 + 2C3H6

doc b oil notes

doc b oil notes 2 doc b oil notes

this reaction is used commercially to crack a naphtha/kerosene molecule into a petrol molecule plus two alkene propene molecules from which you make the plastic poly(propene)

Cracking reactions (5) Other catalytic cracking reactions at high temperature produce hydrogen as well e.g.

ethane ==> ethene + hydrogen

C2H6 ==> C2H4 + H2

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propane ==> propene + hydrogen

C3H8 ==> C3H6 + H2

doc b oil notes doc b oil notes doc b oil notes doc b oil notes

6b. Other products derived from the compounds in crude oil

  • The petrochemical industry produces lots of basic organic molecules from which molecules with specific uses can be made into valued commercial products.

  • The structural materials, pharmaceutical and food industries etc. have all developed a wide range of products in attempt to enhance our lifestyle and quality of life and most commercial products depend very much on chemical 'feedstock' from the petrochemical and oil industry

  • These include many drugs–medicines, polymers–plastics, dyes for fabrics, soaps–detergents etc. etc.!

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

Advanced Chemistry Page Index and Links

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 (c) doc b 3 linked easy Oil Products gap–fill quiz worksheets

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

... AQA GCSE Science (c) doc b Useful products from crude oil AND (c) doc b Oil, Hydrocarbons & Cracking etc.

... OCR 21st C GCSE Science (c) doc b 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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