3.1.6 Alkanes - Combustion of alkanes

Specification

Students should:
  • know that alkanes are used as fuels and understand that their combustion can be complete or incomplete and that the internal combustion engine produces a number of pollutants (e.g. NOx, CO and unburned hydrocarbons)
  • know that these pollutants can be removed using catalytic converters
  • know that combustion of hydrocarbons containing sulfur leads to sulfur dioxide that causes air pollution and
  • understand how sulfur dioxide can be removed from flue gases using calcium oxide
  • know that the combustion of fossil fuels (including alkanes) results in the release of carbon dioxide into the atmosphere
  • know that carbon dioxide, methane and water vapour are referred to as greenhouse gases and that these gases may contribute to global warming

Alkanes

Compounds containing only hydrogen and carbon. There are three types alkanes, alkenes and alkynes.

Alkanes have a -CH3 group at each end (except methane has only one CH4) and fill out the required number with -CH2- groups.


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Combustion

This is effectively a technical word for burning. Most organic compounds burn with the exception of chlorinated (halogenated ) hydrocarbons.

Complete combustion produces CO2 and H2O, incomplete combustion produces CO, C and H2O (usually occurs with unsaturated compounds, where there is a limited supply of oxygen). C produces a 'dirty' flame leaving carbon deposits on everything, CO is toxic and CO2 is a greenhouse gas.

C3H8 + 5O2 3CO2 + 4H2O

Incomplete combustion is where the carbon is not completely oxidised. In this case some carbon monoxide and carbon may be formed. The hydrogen always combines first and makes water.

C3H8 + 4O2 CO2 + 2CO + 4H2O

The combustion of hydrocarbons is an exothermic process (otherwise there wouldn't be much point in burning them to produce energy for fuel and heat). This is because the O-H bond is stronger than the C-H bond, and the C=O bond is stronger than the C-C. This means that, the C-C and C-H bonds breaking requires energy, but this is more than made up for by the energy released by the formation of the C=O and O-H bonds.


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Internal combustion engines

These usually use hydrocarbons as fuel. Petrol engines burn isomers of octane, C8H18, with many additives designed to increase flow efficiency, burning, engine lubrication, etc.

Invariably, the sparks that ignite the petrol air mixtures also cause combination between nitrogen and oxygen in the air producing oxides of nitrogen, NOx, which are a pollutant health hazard as well as unburnt hydrocarbons and carbon monoxide gas, both of which are health hazards.

N2 + O2 2NO

Nowadays, these exhaust gases are passed through a catalytic converter that reacts these gases together to form environmentally harmless nitrogen and other compounds.

2CO + O2 2CO2

2NOx xO2 + N2

unburnt hydrocarbons carbon dioxide + water


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Pollution

Hydrocarbon fields originally came from living matter and as living matter contains amino acids, all of the elements present in amino acids must also be present in the hydrocarbon mixtures found in crude oil. One of these elements is sulfur. This burns when fuels burn producing sulfur dioxide, a poisonous gas responsible for acid rain.

It can be removed by passing burnt gases through a flue contining calcium oxide. This is a basic oxide which reacts with the sulfur dioxde forming calcium sulfite, an ionic solid whiich is easily removed as a white powder.

CaO + SO2 CaSO3


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The greenhouse effect.

Burning fossil fuels produces carbon dioxide, a major contributor to the greenhouse effect.

Greenhouse effect


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quick test