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Alkanes From Crude Oil (a mixture of Hydrocarbons) Fractional Distillation Uses Non Renewable source Reactivity No Reaction with acid or alkalis No Reaction.

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Presentation on theme: "Alkanes From Crude Oil (a mixture of Hydrocarbons) Fractional Distillation Uses Non Renewable source Reactivity No Reaction with acid or alkalis No Reaction."— Presentation transcript:

1 Alkanes From Crude Oil (a mixture of Hydrocarbons) Fractional Distillation Uses Non Renewable source Reactivity No Reaction with acid or alkalis No Reaction with Br 2 ? Cracking Pass Alkane over HEATED Al 2 O 3 catalyst

2 Fractional Distillation Separation by Boiling Point Each fraction has a different number of Carbons (C 1 -C 4 ) (C 5 -C 7 ) (C 8 -C 10 )…(C 30 -C 40 ) What varies as the molecule size changes? Home

3 Uses Fuels What do we do when we use them as fuels? Feedstock for alkenes

4 Practical One Place a small amount of a.Hexane (C 6 H 14 ) b.Octane (C 8 H 18 ) c.Paraffin (C 20 H 42 ) Onto a heat proof mat in turn and ignite with a Bunsen Burner How easy to they ignite? How sooty is the flame? What products are formed in a good supply of Oxygen? What products are formed in a poor supply of Oxygen? Why do the flames get more sooty?

5 Combustion Ease of combustion decreases as more C in alkane Flame gets more sooty as more C in alkane CO 2 and H 2 O formed ONLY if oxygen supply is good CO and C (soot) formed if poor air supply (Called INCOMPLETE combustion) As alkanes have more C in molecule more moles of O 2 per mole of alkane are required for complete combustion WRITE EQUATIONS

6 Ease of combustion Vapour Burns Less Vapour as molecule increases in size at same temperature Stronger VDW HOME

7 Isomerisation and Reforming Weakens VDW so more vapour Better ignition Straight to Branched Straight to cyclic

8 Practical 2 Reactions of alkanes Observations of Bromine reaction Br 2 decolourised AND an acidic gas produced Reaction only occurs when UV light is present

9 Bond Breakage X-X  X. + X. X-X  X + + X -

10 CH 4 + Cl 2 CH 3 Cl + HCl Overall reaction equation Conditions ultra violet light excess methane i.e. homolytic breaking of covalent bonds to reduce further substitution Free radical substitution chlorination of methane

11 CH 4 + Cl CH 3 + HCl Cl 2 Cl + Cl CH 3 + Cl 2 CH 3 Cl + Cl CH 3 Cl CH 3 + Cl initiation step two propagation steps termination step ultra-violet CH 3 CH 3 + CH 3 minor termination step Already leads to multiple products Free radical substitution mechanism

12 CH 3 Cl + Cl 2 CH 2 Cl 2 + HCl Overall reaction equations Conditionsultra-violet light CH 2 Cl 2 + Cl 2 CHCl 3 + HCl CHCl 3 + Cl 2 CCl 4 + HCl excess chlorine Further free radical substitutions

13 CH 3 Cl + Cl CH 2 Cl + HCl CH 2 Cl + Cl 2 CH 2 Cl 2 + Cl CH 2 Cl 2 CH 2 Cl + Cl two propagation steps termination step CH 3 CH 3 + CH 3 termination step Further propagation steps and termination steps

14 Cracking Is the product a bigger or smaller molecule than the paraffin? Is the product more or less flammable than the paraffin? What happens when Br 2 is added to the product? Smaller as it is a gas (so weaker VDW so lower bp) More flammable (as it is a gas already) Br 2 decolourised without UV light (it is an alkene) HOME


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