3.1.6 Alkanes - Combustion of alkanes
| Students should:
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.
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.
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
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
The greenhouse effect.
Burning fossil fuels produces carbon dioxide, a major contributor to the greenhouse effect.