Combustion Of Hydrocarbons : Alkanes

Table of Contents

• Combustion of alkanes
• Trends in combustion

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 an enormous amount of heat energy is released. For example:

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

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

The above reaction can be generalised as follows:

C_nH_2n+2   +   ((3n+1)/2) O₂ → nCO₂   +   (n+1) H₂O

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

2. 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:

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