Carbon dioxide occurs in nature in free state as well as in the combined state. It is a constituent of air (0.03%). It occurs in rock as calcium carbonate and magnesium carbonate.
On industrial scale it is produced by burning coke in excess of air.
2CO + O2 → 2CO2
ΔH = 394 kJ mol-1
Calcination of lime produces carbon dioxide as by product.
CaCO3 → CaO + CO2
Carbon dioxide is prepared in laboratory by the action of dilute hydrochloric acid on metal carbonates.
CaCO3 + 2HCl → CaCl2 + H2O + CO2
It is a colourless, nonflammable gas and is heavier than air. Its critical temperature is 31⁰ C and can be readily liquefied.
Carbon dioxide is a very stable compound. Even at 3100 K only 76 % decomposes to form carbon monoxide and oxygen. At still higher temperature it decomposes into carbon and oxygen.
At elevated temperatures, it acts as a strong reducing agent. For example,
CO2 + Mg → 2MgO + C
The equilibrium involved in the reaction between carbon dioxide and hydrogen, has many industrial applications and is called water gas equilibrium.
CO2 + H2 ↔ CO + H2O Water gas
The aqueous solution of carbon dioxide is slightly acidic as it forms carbonic acid.
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-
Carbon dioxide has a liner structure with equal bond distance for the both C-O bonds. In this molecule there is one C-O sigma bond. In addition there is 3c-4e bond covering all the three atoms.
· Carbon dioxide is used to produce an inert atomosphere for chemical processing.
· Biologically, it is important for photosynthesis.
· It is also used as fire extinguisher and as a propellent gas.
· It is used in the production of carbonated beverages and in the production of foam.