Combustion Of Hydrocarbons : Alkanes


Let us first understand the terms combustion and alkanes individually then move towards the topic. Combustion or burning is a high temperature exothermic reaction. It happens between a fuel and oxygen (oxidant), giving out gaseous products, also termed as smoke. Alkane is referred to as a saturated open chain hydrocarbon consisting of carbon-carbon single bonds.

Combustion of alkanes:

  1. Complete combustion of alkanes: When alkane is heated in the presence of sufficient air or dioxygen it forms carbon dioxide and water and enormous amount of heat energy is released. For example:

CH4 (g)   +   2O2 (g) → CO2 (g) + 2H2O (l);                                ∆cHΘ = – 890 kJ mol-1

C4H10        + 13/2 O2(g) → 4CO2 (g) + 5H2O (l);                         ∆cHΘ = – 2875.84 kJ mol-1

The above reaction can be generalised as follows:

CnH2n+2   +   ((3n+1)/2) O2 nCO2   +   (n+1) H2O

Alkanes can also be used as a fuel, as they produce large amounts of heat.

  1. Incomplete combustion of alkanes: It occurs when there is not enough amount of oxygen for fuel to react completely. This leads to the formation of carbon or carbon monoxide. Carbon monoxide formed as a by product is a colourless poisonous gas. For example:

\(~~~~~~~~~~~~~~~\)Methane + (little) Oxygen –> Carbon + Water

\(~~~~~~~~~~~~~~~\)CH4 + O2 → C + 2H2O

\(~~~~~~~~~~~~~~~\)Methane + (some) Oxygen →  Carbon Monoxide + Water

\(~~~~~~~~~~~~~~~\)CCH4(g)   +   (3/2)O2 (g)  →   CO(g)  + 2H2O(l)

The carbon black produced during the incomplete burning of alkane is used in the manufacturing of inks.

Trends in combustion:

For a hydrocarbon, if complete ignition occurs then it burns with a blue flame. As the molecular mass of hydrocarbon increases it starts burning with a yellow coloured flame showing incomplete burning. The burning of hydrocarbons becomes difficult with increasing molecular weights.

Know more about Pyrolysis of Alkanes


Practise This Question

The major  product of the following reaction sequence is :