3.3.2 Alkanes

• The general formula for alkanes is CnH2n+2.
• The general formula for cycloalkanes is CnH2n.
• Bond enthalpies of C—C bond and C—H bond are quite high that makes them difficult to be broken.
• Alkanes are non polar as Carbon and Hydrogen have similar electronegativities, making the bonds difficult to be broken and making alkanes unreactive except towards free radicals.
• The longer hydrocarbons, which are in high supply and low demand, are broken into shorter hydrocarbons in a process called cracking.
• Cracking produces shorter alkanes and alkenes which are in high demand because they are more useful, but are in low supply.
• Alkenes are important as chemical feedstock, which means that they are the starting materials to make other chemicals, such as plastics, polymers, paints, pharmaceuticals.
• There are two types of cracking process: thermal cracking and catalytic cracking.
• Thermal cracking involves breaking a carbon-carbon bond by homolytic fission, so one electron from the bonding pair goes to each half of the molecule, forming fragments of the original molecule called free radicals.
• In order to become more stable, the fragments become an alkane and an alkene, as there are not enough hydrogens to form two alkanes in thermal cracking.
• Zeolite catalysts contain silicon dioxide and aluminium oxide, have a honeycomb structure, and have a large surface area.
• The products of catalytic cracking are aromatic compounds and fuels.
• Crude oil is refined and its components are separated using the process called fractional distillation.
• The lower the molecular mass of a hydrocarbon, the more volatile it is and lower its boiling point as it has weaker van der Waals forces between the molecules.
• Describe the process of fractional distillation

  1. crude oil is heated until evaporated
  2. then fed into a fractionating column. 
  3. the hydrocarbons have different chain lengths so have different boiling points; condense at different fractions 
  4. the fractions at the bottom are hotter so longer hydrocarbons are collected as liquids here due to their high BPs because of their strong Van der Waals forces
  5. shorter hydrocarbons continue to rise to the top where it is cooler as they have lower BPs (due to weak Van der Waals forces), and are collected here as liquids. 
• The boiling points of alkanes increases as the carbon chain length increases becuase there is more electron density, resulting in stronger Van der Waals forces.
• Alkanes are not soluble in water.
• Alkanes are good fuels as the reaction is very exothermic with large negative enthalpies of combustion.
• In excess oxygen, short chain alkanes can undergo complete combustion: alkane + oxygen → carbon dioxide + water.
• In the presence of limited supply of oxygen, incomplete combustion takes place and carbon monoxide is produced.
• In extremely limited supplies of oxygen, carbon is produced in incomplete combustion.
• Incomplete combustion often happens with longer chain hydrocarbons which require more oxygen to completely combust.
• Alkane + oxygen → carbon monoxide + water is the equation for incomplete combustion of alkanes.
• Carbon dioxide and water are both greenhouse gases, contributing to the enhanced greenhouse effect.
• Sulphur dioxide is produced by the combustion of fossil fuels which contain sulphur impurities, dissolving in water in the atmosphere to produce acid rain.
• Incomplete combustion produces carbon particulates, which can trigger asthma attacks and cause cancer.
• Unburnt hydrocarbons can enter the atmosphere and act as greenhouse gases. They also contribute to photochemical smog.
• However since the industrial revolution the massive increase in the combustion of fossil fuels has dramatically increased carbon dioxide levels in the atmosphere, leading to an enhanced greenhouse effect which causes the Earths average temperature to increase.
• N2 + O2 → 2NO

  2NO is toxic and produces smog.
• NO2 is toxic and produces acid rain.
• Soot released during combustion of alkanes is responsible for respiratory problems.
• Petroleum fractions, coal and natural gas contain sulphur impurities that burn in the presence of oxygen to form sulphur dioxide.

  S + O2 → SO2
• In power stations that burn coal or natural gas the sulphur dioxide that is produced is removed from the flue (waste) gases through a chemical reaction called flue gas desulfurisation
• Calcium oxide, which is a base, neutralises the acidic sulphur dioxide. It is then further oxidised to form calcium sulphate. The overall reaction is:

  
  

  CaO(s) + 2H2O(l) + SO2(g) + 0.5O2(g) → CaSO4.2H2O (s)
• Calcium sulphate is then sold and used to make plasterboards and builders plaster.
• Alternatively, calcium carbonate can be used for flue gas desulfurisation, however this process produces carbon dioxide.

  CaCO3(s) + SO2(g) + 0.5O2(g) → CaSO4(s)+ CO2(g)
• Crude oil is a non-renewable source of energy.
• In car engines, nitrogen and oxygen combine to form nitrogen oxides (NOx) because of sparks igniting the fuel and high temperatures. 

  N2+ O2 → 2NO

  N2 + 2O2 → 2NO2
• A fuel is a substance that releases heat energy during combustion. 
• The catalysts in a catalytic converter are platinum and rhodium. These are very expensive metals. In order to reduce the cost and increase the effectiveness, the metals are coated onto a ceramic honeycomb, giving an enormous surface area for the reaction. The reactions take place on the surface of the catalyst.