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© Boardworks Ltd 2003 Cracking. © Boardworks Ltd 2003 Cracking Crude oil contains many large molecules. If these are to be used as fuels or feedstock.

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Presentation on theme: "© Boardworks Ltd 2003 Cracking. © Boardworks Ltd 2003 Cracking Crude oil contains many large molecules. If these are to be used as fuels or feedstock."— Presentation transcript:

1 © Boardworks Ltd 2003 Cracking

2 © Boardworks Ltd 2003 Cracking Crude oil contains many large molecules. If these are to be used as fuels or feedstock for the chemical industry then they have to be cracked into smaller molecules. When hydrocarbons burn they are reacting with oxygen in the air. In general, the smaller the molecule the better it will mix and then react with the air. Big molecules Small molecules Medium molecules

3 © Boardworks Ltd 2003 Involves the breaking of C-C bonds in alkanes Converts heavy fractions into higher value products THERMALproceeds via a free radical mechanism CATALYTICproceeds via a carbocation (carbonium ion) mechanism CRACKING Cracking

4 © Boardworks Ltd 2003 High Pressure kPa High Temperature °C to 900°C Free Radical Mechanism Homolytic fission Produces mostly alkenes... e.g. ethene for making polymers and ethanol Produces Hydrogen... used in the Haber Process and in margarine manufacture Bonds can be broken anywhere in the molecule by C-C bond fission or C-H bond fission Thermal Cracking

5 © Boardworks Ltd 2003 Slight pressure High Temperature °C Zeolite catalyst Carbocation Mechanism Heterolytic fission Produces branched and cyclic alkanes, aromatic hydrocarbons used for motor fuels ZEOLITES are crystalline aluminosilicates; clay like substances Catalytic Cracking

6 © Boardworks Ltd 2003 Catalytic Cracking Large hydrocarbons are broken into smaller molecules using heat and a catalyst. This process is known as catalytic cracking. The small molecules produced are then separated by distillation. Catalytic cracker Heat to vaporise Distillation tower pressure Big Molecules Smaller molecules Molecules break up

7 © Boardworks Ltd 2003 Catalytic Cracking In the catalytic cracker long chain molecules are ‘cracked’. An example of such a reaction is: C 8 H 18  C 6 H 14 + C 2 H 4 + ethene Octane hexane Ethene is used to make plastics Heat pressure catalyst Used as a fuel

8 © Boardworks Ltd 2003 Draw out displayed formulae of a pair of products formed by cracking decane Heat pressure catalyst HH CC H HH HH C C HH HH CC HH HH CC HH HH CC HH H decane + ethene HH CC H HH HH C C HH HH CC HH H C H H C H H octane Activity

9 © Boardworks Ltd 2003 Poly(e)thene One important reaction of alkenes involves the joining together of alkene molecules. HH CC H HH HH C C HH HH CC HH HH CC HH HH CC HH And lots more.. addition polymerisation This is called addition polymerisation and is written as: Pressure high temperature catalyst n n ethene poly(e)thene thousands

10 © Boardworks Ltd 2003 Polypropene polymerisedEthene is only one alkene. Other unsaturated molecules such as propene, vinyl chloride and styrene can also be polymerised to produce a range of plastics. E.g. propene Poly(propene) n n propene

11 © Boardworks Ltd 2003 PTFE Tetrafluoroethane is another alkene that is made into an important plastic used to coat non-stick pans: polytetrafluoroethane or PTFE. n tetrafluoroethene Poly(tetrafluoroethane) or PTFE n

12 © Boardworks Ltd 2003 pvc CC HH H Cl n n Vinyl chloride CC Cl H HH Fill in the products that will be obtained from vinyl chloride Activity

13 © Boardworks Ltd 2003 Some uses of plastics Poly(e)thene Shopping bags Bottles Buckets Washing up bowls Polypropene Milk crates Rope Carpet fibres Polystyrene packing insulation Ball pens

14 © Boardworks Ltd AcrossDown 3) separate substances with different boiling points 1) Contain a double bond 7) saturated hydrocarbon 2) a series of molecules differing by a CH2 8) joining of many small molecules 4) breaking up a large molecules 9) full up: unable to add more atoms 5) the different substances collected from distillation 6) used to test for unsaturated molecules.

15 © Boardworks Ltd 2003 AcrossDown 3) separate substances with different boiling points DISTILLATION 1)Contain a double bond ALKENE 7) saturated hydrocarbon ALKANE 2) a series of molecules differing by a CH2 HOMOLOGOUS 8) joining of many small molecules POLYMERISE 4) breaking up a large molecules CRACKING 9) full up: unable to add more atoms SATURATED 5) the different substances collected from distillation FRACTIONS 6) used to test for unsaturated molecules. BROMINE Answers

16 © Boardworks Ltd 2003 Which of these is an alkane? A.C 6 H 14 B.C 4 H 8 C.C 12 H 24 D.C 102 H 204

17 © Boardworks Ltd 2003 Which of these is a true statement about alkenes? A.They turn bromine water from colourless to red B.They contain a double bond C.The smallest alkene has 1 carbon atom D.They have names that end in “ane.”

18 © Boardworks Ltd 2003 Which of these is a true statement about cracking? A. it is the separation of molecules into fractions of different sizes. B. it is carried out at low temperatures C. it uses a catalyst. D. It produces polymers

19 © Boardworks Ltd 2003 Which of these is a true statement about polymerisation? A.it is the joining together of many small molecules. B. it is the thermal decomposition of plastics C.it is carried out using saturated molecules D.it is a multiplication reaction

20 © Boardworks Ltd 2003 Which of these is an addition polymer? A.styrene B.ethene C.p.v.c. D.propane

21 © Boardworks Ltd 2003 How might you test to see if polystyrene still contained some unsaturated monomer (styrene)? A.Crush it up and burn it. B.Crush it up and add it to bromine water C.Crush it up and dissolve it in petrol D.Crush it up and add hydrochloric acid


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