Summary AQA AS Level Chemistry - Organic Chemistry: Alkanes and Halogenoalkanes Full Notes (Unit 3.3.2 and Unit 3.3.3)
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Course
Unit 3.3.1-6 (7404)
Institution
AQA
Detailed and comprehensive notes covering the alkanes and halogenoalkanes sub-topics of AQA AS organic chemistry (units 3.3.2 and 3.3.3). This includes: fractional distillation, pollution, free radical substitution, the ozone layer, haloalkane hydrolysis nucleophilic substitution with OH- / NH3 / C...
Fractional Distillation
1. Crude oil is vaporised to ~350°C
2. The vaporised crude oil goes into a fractionating
column and rises up through the trays.
3. The largest hydrocarbons don’t vaporise at all
because their boiling points are too high, so they are
drawn off at the bottom and form residue.
4. As the crude oil vapour goes up the fractionating
column, it gets cooler.
5. Since the alkane molecules have different chain
lengths, they have different boiling points, so each
fraction condenses at different temperatures.
6. The fractions are drawn off at different levels in the
column.
7. The hydrocarbons with the lowest boiling points
don’t condense; they’re drawn off as gases at the top
of the column.
Hydrocarbon Properties
As the number if carbon atoms increases:
• Boiling point increases • Volatility decreases
• Viscosity increases • Flammability decreases
Cracking
Cracking is the chemical splitting of long chain alkanes into shorter hydrocarbons, mostly to produce alkanes for
petrol.
• The process produces a shorter alkane and shorter akenes(s).
• E.g. C15H32 → 2C2H4 + C3H6 + C8H18
• Two methods: Thermal Cracking and Catalytic Cracking
Thermal Cracking
• Alkanes heated to 450°C - 900°C for half a second (any longer and the hydrocarbon would decompose).
• Under high pressure (up to 70 atm)
• Sometimes in the presence of superheated steam, in order to break strong C-C bonds.
• Gives a mixture of products containing high proportions of hydrocarbons with double bonds (alkenes), which
are useful starting points of the chemical industry, e.g. making polymers.
Catalytic Cracking
• Does not require such high temperatures (450°C and moderately low pressures suitable) as it uses a catalyst
(finely divided mixture of silica and Al2O3.
• Zeolites (crystalline aluminosilicates) are used, which have large lattice structures and make the process
extremely efficient.
• Zeolites chosen to give high percentages of C5-C10 hydrocarbons (useful for petrol) and tend to produce high
proportions of branched alkanes and aromatic hydrocarbons, like benzene due to reforming.
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, Pollution
Carbon Dioxide, CO2
Source: Complete combustion of fossil fuels, decay of plants and animals
Fungi in soil act as ‘soil sinks’, converting CO into CO2: 2CO (g) + O2 (g) → 2CO2 (g)
Problem: Major greenhouse gas contributing to global warming – increased temperatures, sea level rise, etc.
Solution: Reduce combustion of fossil fuels, planting trees
Carbon Monoxide, CO
Source: Incomplete combustion of fossil fuels
Atmospheric oxidation of methane: 2CH4 (g) + 3O2 (g) → 2CO (g) + 4H2O (l)
Problem: Toxic – interferes with O2 transport as it prevents haemoglobin from carrying O2, depriving body cells
of O2, leading to asphyxiation
Solution: Catalytic converters remove CO from engine, lean burn engine, thermal exhaust reactor
Nitrogen Dioxide, NO2
Source: High temperature combustion in automobiles can produce NO2: N2 (g) + 2O2 (g) → 2NO2 (g)
NO gas present in the atmosphere (a primary pollutant) is rapidly converted to NO 2 (a secondary
pollutant): 2NO (g) + O2 (g) → 2NO2 (g)
Problem: Acid rain
NO2 causes breathing problems & respiratory distress
Solution: Catalytic converters remove NO2 from engine, lean burning engines, recirculation of exhaust gases
Sulphur Dioxide, SO2
Source: Combustion of sulphur-containing fossil fuels, such as coal: S (s) + O2 (s) → SO2 (g)
Smelting plants oxidise sulphide ores to metal ores, e.g. Cu2S (g) + 2O2 (g) → 2CuO (s) + SO2 (g)
SO2 is formed as a secondary pollutant in the atmosphere as a result of the atmospheric oxidation of
hydrogen sulphide gas (H2S): 2H2S (g) + 3O2 (g) → 2SO2 (g) + 2H2O (l)
Problem: Acid rain formed from SO2 pollution
Solution: Alkaline scrubbing, limestone-based fluidised beds, sulphur removed from fossil fuels before burning
Soot, C (soot)
Source: Incomplete combustion of fossil fuels
Problem: Global dimming
Solution: Electrostatic precipitators, gravity settling chambers, cyclone separators
Unburnt Hydrocarbons
Source: Petroleum extraction/refining and incomplete combustion of coal/gasoline
Problem: Aromatic compounds cause irritation, some are carcinogenic
Solution: Catalytic converters
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