Monday, January 28, 2008

IIT JEE Revision - Combustion of Alkanes

Complete combustion

Complete combustion (given sufficient oxygen) of any hydrocarbon produces carbon dioxide and water.

C3H8 + 5O2 --> 3CO2 + 4H2O

2C4H10 + 13O2 --> 8CO2 + 10H2O

Incomplete combustion (where there isn't enough oxygen present) can lead to the formation of carbon or carbon monoxide.

A simple explanation is that, hydrogen in the hydrocarbon gets the first chance at the oxygen, and the carbon gets whatever is left over.

The presence of glowing carbon particles in a flame turns it yellow, and black carbon is often visible in the smoke.

Carbon monoxide is produced as a colourless poisonous gas.

Controlled combustion

a) When a mixture of methan and oxygen in the molar ratio of 9:1 is compresed to abut 1100 atmospheres and passed through copper tubes at 575 K, methane is oxidized to methanol - Output is methanol

b) When methane and oxygen are passed through heated molybdenum oxide (Mo2O3), it is oxidized to methanal (formaldehyde).

Alkanes having teriary hydrogen atom can be oxidized to alcohols in the presence of potassium permangante.

Alkanes are oxidized to carboxylic acids by silver oxide (Ag2O)

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