Chemical Engineering - Chemical Engineering Basics - Discussion

CCl4 used as fire extinguisher.

Consider the combustion of a simple molecule methane CH4:

CH4 + 2O2 = CO2 + 2H2O.

However, if we consider a similar molecule CCl4, carbon tetrachloride, which is just a replacement of H by Cl in methane, is incombustible.

This can be explained by observing the reaction above. CH4 burns to give CO2 and Water. Hydrogen being an electropositive element joins oxygen quickly.

However, chlorine being an electronegative element would like to connect to an electropositive element carbon, as it can pull the electron pair closer to itself!

Getting Cl in +1 state is difficult.

So the reaction
CCl4 + 2O2 = CO2 + 2Cl2O is least probable.

Because it doesn't combust, it was used as a fire extinguisher under the name pyrene.

CCl4 is incombustible because the combustion products will have higher energy content than that of the reactants. For combustion to take place, the products should contain stable compounds (lower energy content) eg. H2O, CO2 etc. Than that of the reactants. CCl4 is very stable due to its structure.

Chlorine being an electronegative element, would like to connect to an electropositive element carbon, as it can pull the electron pair closer to itself.

CCL4 is a tetrahydral structure so that it is quit stable compare to this.

Since both, chlorine and oxygen, are electronegative elements therefore the combustion reaction CCl4 + 2 O2 ---> CO2 + 2 Cl2O doesn't take place.

Incombustible means?

Because CCl4 does not have free (delocalized) electrons as does a metal or something like graphite. Nor does CCl4 ionize in solution to form ions. It is a nonelectrolyte.

H2 is combustable too.

The molecular formula of carbon tetrachloride is CCl4. It is a tetrachloro derivative of methane. Its IUPAC name is tetrachloromethane and it doesn't burn.