Alkanes

• Alkanes are saturated hydrocarbons where all carbon-carbon bonds are single bonds. They are part of a homologous series with formula CnH2n+2. Cycloalkanes are an exception to this formula.
• Crude oil is a mixture of different hydrocarbons, and can be separated into separate molecules via fractional distillation.
• Fractional distillation can occur because the different fractions have different chain lengths, hence have different boiling points.
• The first stage of fractional distillation is vapourising the crude oil via heating, then feeding it into a fractionating column. This column has a temperature gradient, with cooler temperature at the top and warmer at the bottom.
• The second stage of fractional distillation is to allow the vapours in the column to rise and condense once they have reached their boiling point.
• During fractional distillation, the larger chain fractions condense towards the bottom of the column since they have a high boiling point.
• Some hydrocarbons in crude oil are dissolved gases, which rise without condensing.
• Cracking is the process of breaking down longer chain hydrocarbons into more useful, shorter chain hydrocarbons.
• In order for cracking to occur, carbon-carbon bonds are broken. The two main types of cracking are thermal cracking and catalytic cracking.
• Thermal cracking produces alkanes and alkenes, and uses a 1200K temperature and 7000kPa pressure.
• Catalytic cracking produces aromatic compounds with carbon rings. It uses a 720K temperature, normal pressure and a zeolite catalyst.
• Reforming is the conversion of straight chain hydrocarbons into branched chain alkanes and cyclic hydrocarbons for more efficient combustion.
• Reforming is achieved through heating with a catalyst, such as platinum.
• Alkanes undergo complete combustion to produce carbon dioxide and water, and release lots of energy as they do so.
• Alkanes can also undergo incomplete combustion if there is a deficit of oxygen, producing carbon monoxide, carbon particulates (soot) and water.
• Complete combustion occurs when all atoms in the fuel have been fully oxidised, producing CO2 and H2O.
• Incomplete combustion occurs when some of the atoms in the fuel aren't fully oxidised.
• Carbon particulates (soot) produced during incomplete combustion are represented as carbon atoms (C) in an equation.
• Carbon monoxide is an odourless, colourless gas which is lethal to humans because it replaces oxygen in the blood, depriving the organs of oxygen and causing suffocation.
• Combustion of alkanes can also produce oxides of sulfur and nitrogen.
• Some sulfur-containing molecules in crude oil may not be removed during the fractional distillation process. During combustion, these sulfur atoms form sulfur dioxide, which then react in the atmosphere to form sulfur trioxide.
• Sulfur dioxide is formed via this equation : S + O2 -> SO2
• Sulfur trioxide is formed via this equation : 2SO2 + O2 -> 2SO3
• Sulfur dioxide and sulfur trioxide are acidic, so they dissolve in water in the atmosphere to produce sulfurous acid and sulfuric acid.
• Sulfurous acid is formed via this equation : SO2 + H2O -> H2SO3
• Sulfuric acid is formed via this equation : SO3 + H2O -> H2SO4
• Sulfurous acid and sulfuric acid can create acid rain, which damages the environment and aquatic life.
• Few nitrogenous molecules exist in alkane fuels, however they can combust at high temperatures, causing them to react with oxygen molecules in the air.
• The main nitrogenous oxides produced via combustion of alkane fuels are nitrogen monoxide and nitrogen dioxide.
• Nitrogen monoxide is formed via this equation : N2 + O2 -> 2NO
• Nitrogen dioxide is formed via this equation : 2NO + O2 -> 2NO2
• Nitrogen dioxide is acidic and can dissolve in water to produce nitrous acid and nitric acid.