Carbon dioxide

Carbon dioxide:

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.

Production

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

Properties

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.

Reducing behaviour:

At elevated temperatures, it acts as a strong reducing agent. For example,

CO₂ + Mg → 2MgO + C

Water gas equilibrium:

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

Acidic behaviour:

The aqueous solution of carbon dioxide is slightly acidic as it forms carbonic acid.

CO₂ + H₂O ↔ H₂CO₃ ↔ H⁺ + HCO₃^-

Structure of carbon dioxide

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.

Uses of carbon dioxide

·           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.