Combustion and Pyrolysis of Alkanes

Combustion. Alkanes burn readily in an excess (plentiful supply) of oxygen to give carbon

dioxide, water and a large amount of heat -- the reaction that makes them valuable as fuels

(LPG, petrol, diesel, kerosene are all alkane mixtures). For propane, the balanced equation for

complete combustion is

and, in general, $\text{C}_n\text{H}_{2n+2} + \dfrac{3n+1}{2}\text{O}_2 \rightarrow n\,\text{CO}_2 + (n+1)\text{H}_2\text{O}$. This is a strongly exothermic reaction (roughly

$45$–$48\ \text{kJ}$ per gram of alkane) and is the basis of every alkane-based fuel's use. When

the oxygen supply is limited, incomplete combustion instead produces carbon monoxide (a toxic,

colourless gas) or even soot (unburnt carbon), which is why appliances burning LPG or natural gas

must be kept well ventilated -- a yellow, smoky flame rather than a clean blue one is the visible

sign of incomplete combustion.

Pyrolysis (cracking). When higher alkanes are heated strongly (typically $400$–$700\,° \text{C}$) in the absence of air, the carbon-carbon and carbon-hydrogen bonds break homolytically

and the molecule fragments into a mixture of smaller alkanes and alkenes -- a process called

pyrolysis, or, in the petroleum industry, cracking. For example, decane cracks to give a

mixture such as

(the exact product mix varies with conditions, but the total carbon count and hydrogen count on

each side must balance). Cracking is carried out on an enormous industrial scale in petroleum

refining because it converts high-boiling, low-demand heavy fractions of crude oil (like the

kerosene and diesel range) into the shorter, more valuable and more useful alkenes and alkanes

needed for petrol and as feedstocks for the petrochemical industry (ethene and propene, in

particular, are the starting materials for polymers such as polyethylene and polypropylene). …