Key Terminology
Term Definition
Alkanes Saturated hydrocarbons with the maximum number of hydrogen atoms
Complete Combustion Occurs in plentiful supply of oxygen, and produces only CO2 and H2O
Incomplete Combustion Occurs in limited oxygen supply and produces poisonous CO gas or soot.
Free Radicals Atoms with an unpaired electron produced from homolytic fission of a bond
Crude oil is separated into useful products by heating in a
fractionating tower, where different fractions are collected
by condensing across a range of temperatures. Each
fraction is a group of hydrocarbons of similar chain length
and properties.
1. Crude oil heated in a furnace
2. Mixture of liquid and vapour passes into a tower which
is hottest at the bottom
3. Vapours pass up the tower via a series of trays until they
condense. The bubble caps increase surface area of
contact with cold material to ensure condensation at
the correct fraction, forcing bubbles through the liquid
4. Mixture of condensed liquids is piped off
• Molecule size increases down the column
• Stronger van der Waals between larger molecules so boiling point increases down the column
• Longer chains also mean harder to ignite, and longer hydrocarbons have higher viscosity
• Primary distillation still produces groups of hydrocarbons. Fractions can be further separated into
purer products by secondary distillation. Fractions with bp > 350°C must be distilled under lower
pressure to allow distillation at lower temperatures
• Cracking is the chemical splitting of long-chain alkanes, often from the naptha and kerosene
fractions, into shorter hydrocarbons, mostly to produce useful hydrocarbons
Thermal Cracking Catalytic Cracking
Temperature, Pressure 700°C, 70atm for 0.5s 450°C, lower pressure
Catalysts Superheated steam Zeolite, silica, Al2O3
Produces Alkenes Branched alkanes, aromatic
hydrocarbons
Other Higher temperatures favour
cleavage near ends of molecule
Produces radicals
Zeolite structure has large lattice,
making process efficient
Combustion of Alkanes
Fuels are substances which release heat energy during combustion. Examples of alkane fuels include
methane (natural gas), petrol and paraffin. The longer the carbon chain, the greater the heat output.
Complete combustion occurs in plentiful supply of oxygen, and produces only CO2 and H2O. Incomplete
combustion, often occurring with larger hydrocarbons, occurs in limited oxygen supply and produces
poisonous CO gas or soot.
All hydrogen-based fuels from crude oil may produce pollutants when burnt, including:
• CO – from incomplete combustion; prevents O2 binding to haemoglobin in red blood cells causing
poisoning
• NOX – from reaction of N2 and O2 at high temperatures in the engine. Oxides react with H2O and O2
to form nitric acid, contributing to acid rain and photochemical smog
• SO2 – produced by sulfur-containing impurities. Sulfuric acid contributes to acid rain
• C – soot is particulate and can exacerbate asthma
• Unburnt Hydrocarbons – contribute to smog, causing health problems
• CO2 and H2O vapour, produced by complete combustion, are themselves greenhouse gases
All cars with petrol engines have catalytic converters to reduce CO, NOX and hydrocarbon output. Its
honeycomb structure, providing a huge surface area, is made of a ceramic material coated with
platinum and rhodium metal catalysts.
Greenhouse gases trap rays reflected from the Earth and re-radiate the energy as infrared with a longer
wavelength which cannot escape the atmosphere. While this effect is crucial in maintaining heat to
sustain life, levels of these gases has increased rapidly since the Industrial Revolution. Concentration of
water tends to stay constant due to the equilibrium of the water cycle, but rising temperatures cause
greater evaporation. Carbon-neutral activities produce no overall CO2 and so are vital in sustainability.
Fossil fuels containing sulfur impurities cause SO2 formation, which in turn leads to acid rain. Flue gas
desulfurisation is the process of sulfur dioxide removal from power station gases. Lime (calcium oxide)
or limestone (calcium carbonate) and water are sprayed into flue gas, eventually forming calcium
sulfate, or gypsum, used to make plasterboard.
CaO(s) + 2H2O(l) + SO2(g) + ½O2 → CaSO4∙2H2O(s) is the overall equation using lime
CaCO3(s) + SO2(g) + ½O2 → CaSO4 (s) + CO2(g) is the overall equation using limestone
Formation of Halogenoalkanes
Radicals are the only species reactive enough to overcome the high EA to break C-C and C-H bonds to
form halogenoalkanes.
Free radicals are atoms with a single unpaired electron produced from the homolytic fission of a bond.
Chloromethane is made from the reaction of methane with chlorine following the following steps:
1. Initiation – Frequency of UV light produces enough energy to break a Cl-Cl
bond and produce chlorine radicals; Cl2 UV⃗⃗⃗⃗⃗ 2Cl∙
2. Propagation – a chlorine radicals collides with a methane molecule, removing a hydrogen atom
and producing HCl and a methyl radical, ∙CH3. The methyl radical is also highly reactive and
reacts with chlorine to produce chloromethane. Chlorine radical is regenerated.
CH4 + Cl∙ → ∙CH3 + HCl
∙ CH3 + Cl2 → CH3Cl + Cl∙
3. Termination – the propagation step continues to produce more radicals which then react to
form a relatively unreactive 2Cl∙ → Cl2
This process can continue to knock off H atoms to add chlorine atoms to form CH2Cl2 , CHCl3 or CCl4.
As the reaction proceeds, the original product is used up and the likelihood of the radical colliding with
this reactant decreases steadily over time, with an increased chance of colliding with a substituted
product. Chain reactions occur purely by chance, so a mixture of products is usually produced. Excess
chlorine favours the product with highest halogen proportion (in this example, tetrachloromethane),
while a huge excess of methane favours a less substituted product.
