The two important oxides of carbon are carbon monoxide (CO) and carbon dioxide (CO2).
Carbon monoxide is formed when carbon or hydrocarbons are burned in a limited supply of oxygen.
2C (s) + O2 (g) → 2CO (g)
It is a colourless and odourless gas. It has a melting point of -199°C and boiling point of -192°C.
It is a major air pollutant and is released in large quantities from automobile engines. Its low level poisoning causes headache and drowsiness whereas its large amounts can cause even death. It is toxic because it reduces the oxygen carrying capacity of blood by binding with hemoglobin, the red pigment of blood.
Carbon monoxide has many uses:
(i) It is used as a reducing agent in metallurgical processes to reduce metal oxides. For example, in the blast furnace, it used to reduce iron oxide to iron.
Fe2O3 (s) + 3CO (g) → 2Fe (s) + 3CO2 (g)
(ii) In the presence of a catalyst, it can combine with hydrogen to give methanol (CH3OH).
(iii) It forms carbonyl compounds. The nickel carbonyl Ni(CO)4 is used in the refinement of nickel.
(iv) It is used as a fuel.
(v) It is used in the synthesis of several organic compounds.
Carbon dioxide is formed when carbon containing substances are burnt in excess of oxygen.
C (s) + O2 (g) → CO2 (g)
CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l)
It is also produced by heating of carbonates.
CaCO3 (s) → CaO (s) + CO2 (g)
It is also released as a by product in the fermentation of sugar to produce alcohol (ethanol).
C6H12O6 (aq) → 2C2H5OH (aq) + 2CO2 (g)
Carbon dioxide is colourless and odourless gas. It is present in very small amount (0.03%) in the atmosphere. It is a major contributor to the green house effect.
The main uses of carbon dioxide are:
(i) Solid carbon dioxide also called dry ice is used as a refrigerant because when it is cooled at atmospheric pressure, it condenses into a solid rather than as a liquid. This solid sublime at -78°C.
(ii) It is used in the production of carbonated drinks.
(iii) It is used in the production of washing soda (Na2CO3.10H2O) and baking soda (NaHCO3).