- Hydrocarbon - compound of carbon and hydrogen only.
- Crude oil = source of hydrocarbons.
- Complex mixture of hydrocarbons - is seperated into fractions.
- Each fraction contains a mixture of hydrocarbons with the similar boiling points..
- Tre fractions are obtained by fractional distillations.
- Short chains - low Boiling point ... condense near the top.
- long chains - high Bp ... condense near the bottom.
- Gases do not condense and pass through an outlet at the top as petroleum jelly.
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Alkanes And cycloalkanes
- Saturated molecules.
- will not decolourise bromine water.
- Alkanes - single chained hydrocarbons.
- single C-C bond.
- Alkanes - each carbon is bonded to 4 other atoms.
- Each carbon has a tetrahedral shape.
- Each bond angle = 109.5.
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Boiling points Of Alkanes
- Crude oil has many fractions.
- There are weak intermolecular forces (Wan der waals forces) in order to boil these forces have to be broken.
- As the chains length increases the bp increases...more points of contact.
- therefore, more points of contact and so more wan der waals forces of attraction.
- It takes more energy and therefore a higher temperature to seperate the bonds.
- This is why alkanes in crude oil can be seperated into fractions.
- Effects of Branching
- Isomers of alkanes have the same Mr value.
- A branched isomer has a lower bp than a unbranched isomer.
- Because in a branched isomer there are fewer points of contact between molecules and therefore fewer wan der waals forces of attraction.
- Branched molecules can not get as close to each other unbranched molecules ... and therefore fewer WDW's...
- So less heat is required to seperate the molecules.
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The combustion of Alkenes
- Short chain alkanes = valuable ... as they = clean fuel.
- They burn in plentiful supply of oxygen and they for CO2 and H2O.
- Methane - main constituent for natural gas.
- CH4 + 2 O2 -----> CO2 + 2 H2O.
- This shows that methan burns cleanly with oxygen.
- Propane and butane are easily liquified and are used as liquified petroluem gas or LPG.
- IMCOMPLETE COMBUSTION
- Insuffiecient supply of oxygen.
- E.e internal combustion engine in most cars - limited supply of O2.
- Octane (fuel) undergo's incomplete combustion ... to for CO.
- CO= colourless, poisonous gas.
- Produced when a fossil fuel is combusted in insufficient oxygen.
- CO = poisonous.
- prevent RBC's carrying O2 and so the body is starved of oxygen.
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- After fraction distillation there is a surpluss of long chained hydrocarbons.
- BUT... short chained hydrocarbons are in demand.
- short chained alkanes = great fuels.
- short chained alkenes = for polymer production.
- Cracking - breakdown of long chained hydrocarbons to form a mixture of short chain alkanes and alkenes.
- Catalytic cracking ... first used mid 1930's.
- Initially Al2O3 and Silicondioxide were used as the catalysts.
- But now there are more effiecient catalytic systems.
- NOW THEY USE ZEOLITS CATALYST @ 450 degress .
- IN CRACKING - long chained alkanes are broken down randomly!!!
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Producing Cyclic hydrocarbons
- Aliphatic hydrocarbons = carbon atoms attached in straight unbranched chains.
- Can be converted to cyclic and aromatic hydrocarbons.
- BY REFORMING
- H2 gas is always produced as a result of reforming.
- Research octon number rates how well fuels burn.
- Fuels with high octane rating burn close to 100% (efficiently).
- Straight chain - heptane = poor fuel and it has an actane rating of 0.
- Branched and cyclic alkanes = important petrol additives.
- They promote more efficient combustion.
- consequently branched chain alkanes = used in fuel.
- H2 gas which is produced in reforming is then recycled in the production of ammonia and margarine.
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Fossil fuels - fuels of the future
- Crude oil --- and the material produced from it play a big part in our life.
- sf impact on standard of living.
- We rely on crude oil for power for electrical generations.
- Use of crude oil = apparent ... produce alkanes.
- Branched and cyclic alkanes = best of fuel ...
- GOOD FUEL = readily available, inexpensive, and easily transportable.
- Oil price -- increasing.
- Now need an alternative!
- Burning hydrocarbons - increase atmospheric pollution.
- CO - incomplete combustion in the internal of a combustion engine.
- CO2 - contributes to global warming and the greenhouse effect...
- Nitrogen oxides - contribute to aciod rain and the destruction of rain forests
- Hence, increase temp on earth.
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- Biofuel - derived from recently living materials - like plants and animal waste.
- Rapeseed - Agricultural crops... specially grown for energy use ... another eg... sugar cane (biodiesel).
- Ethanol - fermenting sugar and other carbs.
- burns efficiently in plentiful supply of oxygen.
- Ehtanol and petroleum can be blended together to make the fuel burn more efficiently.
- Ethanol can be used (when blended with petroluem ) in engines without any need for majour modifications ...
- sf reduced harmful gases that are produced.
- Bioethanol - getting a big boost.
- First plant located in somerset.
- Capable of producing 105000 tonnes of bioethanol.
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- Fuel oil.
- Derived from rapeseeds.
- Compatible with engines - can be used 100 % pure.
- However, usually plended with normal diesel.
- Motor fuel being produced from sugar can = long term interest ...
- Because crude oil = finite.
- crude oil - millions of year to produce.
- Ethanol - alternative source for fuel (produced from sugar cane).
- sugar cane - annual crop.
- BRAZIL - produces ethanol from sugar cane... climate favour this means of production.
- And it has few oil reserves of its own.
- Fossil fuel = non renewable.
- Ethanol - produced from hydration of ethene and fermentation
- Increased CO2 lvls from combustion of fossil fuels - leads to global warming and climate change.
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Substitution reactions of Alkanes
- Using alkanes and UV radiation.
- by Cl2 and Br2 to form halogenoalkanes.
- Halogen react with alkanes in the presence of UV radiation or at temp above 300degs .
- Methane reacts with chlore = a readical substitution.
- Radical substitution - reaction where radical replaces ... different atom.
- COVALENT BONDS = BROKEN BY HOMOLYTIC FISSION TO FORM RADICALS WITH UNPAIRED ELECTRONS.
- A hydrogen atom in the alkane is substitued with a halogen.
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- The Cl-Cl bonds are broken by homolytic fission ... forming 2 free radicals Cl* and Cl*.
- UV RADIATION - provides the enrgy for the bond fission.
- Cl-Cl -------------> Cl* + Cl* (The * represents the free radical... normall it is a fullstop).
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- Alkenes and cycloalkenes = unsaturated.
- They have atleast one double bond.
- THEY DECOLOURISE BROMINE WATER.
- A sigma bond is formed between the 2 carbon atoms (from the s-subshell).
- Each carbon contributes 1 electron.
- Pi - bonds form above and below the plane of the carbon atoms.
- There is an overalp of the P-orbitals.
- Each carbon atom contributes 1 electron from a p-orbital to the pi bond.
- Pi-bond fixes the carbon atoms in a fixed position - at either end of the double bond.
- This prevent rotation.
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Shape Of Alkene molecules
- 3 regions of electron density surround each carbon in the double bond.
- The Pair of electrons repel each other far apart to minimise repulsion.
- Gives a trigonal planar shape.
- 120 degrees..
- Ethene = flat trigonal planar.
- Cyclic Alkenes.
- Closed rings of carbon atoms - containing one or more double bonds.
- Most common cyclic alkene - cyclohexene.
- Don't follow the same formula as alphatic alkenes.
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