Alkanes

Petroleum is a mixture consisting mainly of alkane hydrocarbons Petroleum fraction: mixture of hydrocarbons with a similar chain length and boiling point range

• Oil is pre-heated • then passed into column. • The fractions condense at different heights • The temperature of column decreases upwards • The separation depends on boiling point. • Boiling point depends on size of molecules. • The larger the molecule the larger the van der waals forces • Similar molecules (size, bp, mass) condense together • Small molecules condense at the top at lower temperatures • and big molecules condense at the bottom at higher temperatures.

This is a physical process because it involves the splitting of weak van der waals forces between molecules.

Vacuum distillation allows heavier fractions to be further separated without high temperatures which could break them down. • Heavy residues from the fractionating column are distilled again under a vacuum. • Lowering the pressure over a liquid will lower its boiling point.

Heat the flask, with a Bunsen burner or electric mantle • This causes vapours of all the components in the mixture to be produced. • Vapours pass up the fractionating column. • The vapour of the substance with the lower boiling point reaches the top of the fractionating column first. • The thermometer should be at or below the boiling point of the most volatile substance. • The vapours with higher boiling points condense back into the flask. • Only the most volatile vapour passes into the condenser. • The condenser cools the vapours and condenses to a liquid and is collected.

Cracking is the conversion of large hydrocarbons to smaller hydrocarbon molecules by breakage of C-C bonds This is is a chemical process involving the splitting of strong covalent bonds so requires high temperatures. High Mr alkanes —> smaller Mr alkanes+ alkenes + (hydrogen)

• The petroleum fractions with shorter C chains (e.g. petrol and naphtha) are in more demand than larger fractions. • To make use of excess larger hydrocarbons and to supply demand for shorter ones, longer hydrocarbons are cracked. • The products of cracking are more valuable than the starting materials (e.g. ethene used to make poly(ethene), branched alkanes for motor fuels, etc.)

High pressure (7000 kPa) High temperature (400°C to 900°C)

It produces mostly alkenes e.g. ethene used for making polymers and ethanol sometimes produces hydrogen used in the Haber Process and in margarine manufacture.

C8H18 —> C6H14 + C2H4 C12H26 —> C10H22 + C2H4 Bonds can be broken anywhere in the molecule by C-C bond fission and C-H bond fission.

Slight or moderate pressure High temperature (450°C) Zeolite catalyst Therefore cheaper than thermal cracking because it saves energy as lower temperatures and pressures are used

Branched and cyclic hydrocarbons burn more cleanly and are used to give fuels a higher octane number Produces branched and cyclic alkanes and aromatic hydrocarbons

Alkanes readily burn in the presence of oxygen. This combustion of alkanes is highly exothermic, explaining their use as fuels. Fuel : releases heat energy when burnt

In excess oxygen alkanes will burn with complete combustion The products of complete combustion are CO2 and H2O. C8H18(g) + 12.5 O2(g)  8CO2(g) + 9 H2O(l)

Incomplete combustion produces less energy per mole than complete combustion If there is a limited amount of oxygen then incomplete combustion occurs, producing CO (which is very toxic) and/or C (producing a sooty flame) CH4(g) + 3/2 O2(g) —> CO(g) + 2 H2O(l) CH4(g) + O2(g) —> C(s) + 2 H2O(l)

Sufur containing impurities are found in petroleum fractions which produce SO2 when they are burned. S+ O2 —> SO2 CH3SH+ 3O2 —> SO2 + CO2 + 2H2O Coal is high in sulfur content, and large amounts of sulfur dioxide are emitted from power stations. SO2 will dissolve in atmospheric water and can produce acid rain

By flue gas desulfurisation. The gases pass through a scrubber containing basic calcium oxide which reacts with the acidic sulfur dioxide in a neutralisation reaction The calcium sulfite which is formed can be used to make calcium sulfate for plasterboard. SO2 + CaO  CaSO3 calcium sulfite

Nitrogen oxides form from the reaction between N2 and O2 inside the car engine. The high temperature and spark in the engine provides sufficient energy to break strong N2 bond N2 + O2 —> 2NO N2 + 2O2 —> 2NO2

NO is toxic and can form acidic gas NO2 NO2 is toxic and acidic and forms acid rain

Toxic

Contributes to global warming

Contributes to formation of smog

Global dimming and respiratory problems

These remove CO, NOx and unburned hydrocarbons (e.g. octane, C8H18) from the exhaust gases, turning them into ‘harmless’ CO2, N2 and H2O. 2 CO + 2 NO —> 2 CO2 + N2 C8H18 + 25 NO —> 8 CO2 + 12½ N2 + 9 H2O Converters have a ceramic honeycomb coated with a thin layer of catalyst metals platinum, palladium, rhodium – to give a large surface are