Carbon Monoxide

By | December 18, 2017


1) It is formed by incomplete combustion of carbon and carbon containing fuels.

2 C + O2 ——> 2 CO

This type of incomplete combustion occurs during burning of petrol or diesel in automobile and therefore CO is always present in automobile exhausts.It is also present in volcanic gases and gases coming out of furnaces.

2) In the laboratory , pure carbon monoxide is obtained by dehydration of formic acid with Conc H2SO4 at 343 K.

HCOOH ———–> CO + H2O

CO can also be prepared in the laboratory by the action of conc H2SO4 on potassium ferrocyanide.

K4[Fe(CN)6] + 3 H2SO4 ——> 2 K2SO4 + FeSO4 + 6 HCN

HCN + 2 H2O ——-> HCOOH + NH3 ] × 6

HCOOH ———-> H2O + CO ] × 6

2 NH3 + H2SO———> (NH4)2SO4 ] × 3


K4[Fe(CN)6] + 6 H2SO4  + 6 H2O ——->2 K2SO4 + FeSO4  + 3 (NH4)2SO4  + 6 CO

Commercial preparation

1) It is commercially produced by passing steam over red hot cake.

C(s) + H2O (g) ——–> CO (g) + H2 (g)

A mixture of CO and H2 is called water gas or synthetic gas or syngas.

2) When air is used instead of steam , a mixture of CO and N2 are produced.This mixture is called producer gas.

2C + O2 + 4 N2 ———-> 2 CO + 4 N2

CO present in water gas or producer gas can further undergo combustion forming CO2 with evolution of heat.

2 CO + O2 —–> 2 CO2

Water gas and producer gas are two industrially important fuel gases.

Structure of CO

In CO molecule, both C and O atoms are sp – hybridised. One sp- hybrid orbital each of C and O overlap to form a C – O , σ bond while the other sp- orbital on each atom contains the lone pair of electrons.The two unhybridized p-orbital of C and O form pπ-pπ bonds.Thus CO, is a linear molecule.

Due to the presence of a lone pair of electrons on the carbon atom, CO acts as a Lewis base or a ligand and can form a coordinate bond with metal to form metal carbonyls.

C has 6 while O has 8 electrons in the valence shell.To complete the octet around  C , O donates a pair of electrons.

Resonating structure of CO

The carbon-oxygen bond length is just 113 pm which corresponds to a carbon-oxygen triple bond.

The dipole moment of CO is very low due to back donation of a pair of electrons from the more electronegative O to the less electronegative C atom.


1) It is a neutral oxide.

2) It is a colourless and odourless gas which is slightly soluble in water.

3)Poisonous nature

CO is highly poisonous in nature. Its toxic nature is due to its ability to form a complex with haemoglobin which is about 300 times more stable than the oxygen-haemoglobin complex formed in the lungs.

This prevents haemoglobin present in the RBC from carrying oxygen from the lungs to all parts of the body thereby causing suffocation ultimately leading to death.

4) Reducing properties

Since CO can be easily oxidised to CO2 , it acts as a powerful reducing agent. As such, it reduces many metal oxides to their respective metals.

ZnO + CO —-> Zn + CO2

CuO + CO ——> Cu + CO2

Fe2O3 + 3 CO ——–> 2 Fe + 3 CO2

PdCl2 + CO + H2O —> Pd + CO2 + 2 HCl

I2O5 + 5 CO ——> I2 + 5 CO2

5) Formation of metal carbonyls

It combines with many transition metals such as iron, cobalt , nickel forming metal carbonyls.

Ni + 4 CO ——–> Ni(CO)4

Fe + 5 CO ——> Fe (CO)5

Nickel carbonyl is volatile, When heated to 440 – 450 K , it decomposes to form pure nickel.

Ni(CO)———–> Ni + 4 CO

Nickel carbonyl is used for purification of nickel by Mond’s process.

6) Formation of Phosgene

It readily combines with Cl2 in presence of sunlight to give carbonyl chloride or phosgene which is an extremely poisonous gas.

CO (g) + Cl2 (g) ——-> COCl2 (g)

7) Absorption

CO is readily absorbed by a solution of CuCl in conc HCl or NH3 due to the formation of soluble complex.

CuCl + NH3 + CO ——–> [Cu(CO) NH3 ]+ Cl‾

CuCl + HCl + CO ——> H+[Cu(CO)Cl2]‾


1) It is an important constituent of two industrial fuels i.e. water gas and producer gas.

2) It is used in Mond’s process for purification of nickel.

3) It is used in manufacture of methyl alcohol, acetic acid, synthetic petrol, sodium formate.

4) It is used in metallurgy of iron as a reducing agent

5) Iron carbonyl is used in the manufacture of magnetic tapes for videoes and recorders.

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