Oxidation and Combustion Alkanes as Fuels

The most important use of alkanes is as fuel. With excess oxygen, alkanes burn to form carbon dioxide and water. Most important, the reactions evolve considerable heat (that is, the reactions are exothermic). 

These combustion reactions are the basis for the use of hydrocarbons for heat (natural gas and heating oil) and for power (gasoline). An initiation step is required—usually ignition by a spark or flame. Once initiated, the reaction proceeds spontaneously and exothermically. 

In methane, all four bonds to the carbon atom are C—H bonds. In carbon dioxide, its combustion product, all four bonds to the carbon are C—O bonds. Combustion is an oxidation reaction, the replacement of C—H bonds by C—O bonds. In methane, carbon is in its most reduced form, and in carbon dioxide, it is in its most oxidized form. Intermediate oxidation states of carbon are also known, in which only one, two, or three of the C—H bonds are converted to C—O bonds. It is not surprising, then, that if insufficient oxygen is available for complete combustion of a hydrocarbon, partial oxidation may occur, as illustrated in eqs. 2.5 through 2.8. 

Combustion of hydrocarbons is an oxidation reaction in which C—H bonds are replaced with C—0 bonds. 

Toxic carbon monoxide in exhaust fumes (eq. 2.5), soot emitted copiously from trucks with diesel engines (eq. 2.6), smog resulting in part from aldehydes (eq. 2.7), and acid buildup in lubricating oils (eq. 2.8) are all prices we pay for being a motorized society However, incomplete hydrocarbon combustion is occasionally useful, as in the manufacture of carbon blacks (eq. 2.6) used for automobile tires, and lampblack, a pigment used in ink. 

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