Alkanes

The reaction of methane with chlorine

Methane and chlorine react in ultraviolet light to form chloromethane and hydrogen chloride. CH4(g) + Cl2(g) -> CH3Cl(g) + HCl(g)

How can sulfur dioxide be removed from flue gases?

A slurry of calcium oxide (lime) and water is sprayed into the flue gas which reacts with the calcium oxide and water to form calcium sulfite, which can be further oxidised to calcium sulfate. This is a neutralisation reaction: calcium oxide is basic and sulfur dioxide is acidic. CaO(s) + 2H2O(l) + SO2(g) + 1/2O2(g) -> CaSO4•2H2O(s) An alternate process uses calcium carbonate (limestone). CaCO3(s) + 1/2O2(g) + SO2(g) -> CaSO4(s) + CO2(g)

What are alkanes?

Alkanes are saturated hydrocarbons

Complete combustion

Alkanes burn completely in a plentiful supply of oxygen to give carbon dioxide and water Example: CH4(g) + 2O2(g) -> CO2(g) + 2H2O(g)

How can gaseous pollutants from internal combustion engines be removed?

By catalytic converters. The catalytic converter is a honeycomb made of a ceramic material coated with platinum and rhodium catalysts. The honeycomb shape provides an enormous surface area. The polluting gases react with each other to form less harmful products when they pass over the catalyst. 2CO(g) + 2NO(g) -> N2(g) + 2CO2(g) C7H16(g) + 22NO(g) -> 11N2(g) + 7CO2(g) + 8H2O(g)

Pollutant: carbon (C)

Carbon particles or particulates produced by incomplete combustion can exacerbate asthma and cause cancer

What is cracking?

Converting longer chain hydrocarbons into shorter chain hydrocarbons by breaking C-C bonds in alkanes

Fractional distillation of crude oil

Crude oil can be separated into fractions by fractional distillation. Each fraction is a mixture of hydrocarbons of similar chain length and therefore similar properties. Crude oil is heated and then passed into a fractionating column. The temperature of the column decreases upwards. The crude oil vaporise and the vapours pass up the column. The vapours then condense at different heights of the column depending on their boiling points. Molecules with similar boiling points condense together. The shorter chain hydrocarbons condense nearer to the top of the column where it is cooler, because the molecules are smaller so the Van der Waals forces between them are weaker and they have lower boiling points. The longer chain hydrocarbons condense nearer to the bottom of the column where it is warmer, because the molecules are larger so the Van der Waals forces between them are stronger and they have higher boiling points.

What can alkanes be used for?

Fuels. They release heat energy when they undergo combustion.

Pollutant: carbon dioxide (CO2)

Greenhouse gas, level of CO2 is rising and this is a cause of the increase in the Earth's temperature

Incomplete combustion

In a limited supply of oxygen, carbon monoxide or carbon (soot) is formed CH4(g) + 3/2O2(g) -> CO(g) + 2H2O(g) CH4(g) + O2(g) -> C(s) + 2H2O(g)

Free-radical substitution mechanism: termination

In termination the free radicals are removed: 1. Two chlorine free radicals react together to give chlorine. Cl• + Cl• -> Cl2 2. Two methyl free radicals react together to give ethane. •CH3 + •CH3 -> CH3CH3 3. A chlorine free radical and a methyl free radical react together to give chloromethane. Cl• + •CH3 -> CH3Cl

Pollutant: nitrogen oxides (NOx)

Include NO, NO2 and N2O4 In a petrol engine temperature is high when the sparks ignite the fuel. There is enough energy for nitrogen and oxygen in the air to combine. N2(g) + O2(g) -> 2NO(g) N2(g) + 2O2(g) -> 2NO2(g) NOx can react with water vapour and and oxygen in the air to form nitric acid. They contribute to acid rain and photochemical smog.

What pollutants does the internal combustion engine produce?

NOx, CO, carbon, unburnt hydrocarbons

Economic reasons for cracking alkanes

Petroleum fractions with shorter chain are in more demand and economically more valuable than those with longer chain. To make use of excess longer chain hydrocarbons and to supply demand for shorter chain hydrocarbons, longer chain hydrocarbons are cracked.

What is petroleum?

Petroleum is a mixture consisting mainly of alkane hydrocarbon

Pollutant: carbon monoxide (CO)

Poisonous gas produced by incomplete combustion

Pollutant: sulfur dioxide (SO2)

Produced from sulfur-containing impurities present in crude oil. SO2 combines with water vapour and oxygen in the air to form sulfuric acid and contribute to acid rain. SO2(g) + H2O(l) + 1/2O2 (g) -> H2SO4(l)

Pollutant: unburnt hydrocarbons

Significant greenhouse gases and contribute to photochemical smog

Other products of the reaction of methane and chlorine

Some ethane is produced in the termination stage. The chlorine free radical can react with chloromethane that has already formed: Cl• + CH3Cl -> HCl + •CH2Cl, •CH2Cl + Cl2 -> CH2Cl2 + Cl• Cl• + CH2Cl2 -> HCl + •CHCl2, •CHCl2 + Cl2 -> CHCl3 + Cl• Cl• + CHCl3 -> HCl + •CCl3, •CCl3 + Cl2 -> CCl4 + Cl• The free radicals produced in these reactions can also combine to form (di/tri/tetra/penta/hexa)chloroethane.

Catalytic cracking

Takes place at a slight pressure (100-200kPa), high temperature (450°C) and in the presence of a zeolite catalyst and is used mainly to produce motor fuels and aromatic hydrocarbons

Thermal cracking

Takes place at high pressure (7000kPa) and high temperature (900°C) and produces a high percentage of alkenes

Free-radical substitution mechanism: initiation

The Cl-Cl bond in the chlorine molecule absorbs the energy of one ultraviolet photon and breaks homolytically (one electron goes to each chlorine atom). This results in two separate chlorine atoms or free radicals. Cl-Cl -> 2Cl• The C-H bond in methane is stronger than the Cl-Cl bond so it does not break.

Free-radical substitution mechanism: propagation

This takes place in two stages: 1. The chlorine free radical takes a hydrogen atom from methane to form hydrogen chloride and leaves a methyl free radical. Cl• + CH4 -> HCl + •CH3 2. The methyl free radical takes a chlorine atom from chlorine molecule to form chloromethane and leaves a chlorine free radical. •CH3 + Cl2 -> CH3Cl + Cl• The Cl• produced in step 2 can react with more methane molecules and repeat the two steps, so this is a chain reaction.