Carbon Dioxide, CO2

Carbon Dioxide, CO₂

It is prepared by complete combustion of carbon and carbon containing fuels in excess of air.

C (g)   +   O₂ (g) (Heat)    →    CO₂ (g)

CH₄ (g)   +   2O₂ (g) (Heat)    →    CO₂ (g)   +   2H₂O (g)

In the laboratory it is conveniently prepared by the action of dilute HCl on calcium carbonate.

CaCO₃ (g)   +   2HCl (aq) (Heat)    →    CaCl₂ (aq)   +   CO₂ (g)   +   H₂O (I)

On commercial scale it is obtained by heating limestone.

It is a colorless and odorless gas. Its low solubility in water makes it of immense biochemical and geo-chemical importance. With water, it forms carbonic acid, H₂CO₃ which is a weak dibasic acid and dissociates in two steps.

H₂CO₃ (aq)   +   H₂O (I)    →    HCO₃^- (aq)   +   H₃O⁺ (aq)

HCO₃^- (aq)   +   H₂O (I)    →    CO₃^2- (aq)   +   H₃O⁺ (aq)

H₂CO₃/HCO₃^- buffer system helps to maintain pH of blood between 7.26 to 7.42. Being acidic in nature, it combines with alkalies to form metal carbonates.

Carbon dioxide, which is normally present to the extent of 〜0.03% by volume in the atmosphere, is removed from it by the process known as photosynthesis. It is the process by which green plants convert atmospheric CO₂ into carbohydrates such as glucose. The overall chemical change can be expressed as:

By this process plants make food for themselves as well as for animals and human beings. Unlike CO, it is not poisonous. But the increase in combustion of fossil fuels and decomposition of limestone for cement manufacture in recent years seem to increase the CO₂ content of the atmosphere. This may lead to increase in green house effect and thus, raise the temperature of the atmosphere which might have serious consequences.

Carbon dioxide can be obtained as a solid in the form of dry ice by allowing the liquefied CO₂ to expand rapidly. Dry ice is used as a refrigerant for ice-cream and frozen food. Gaseous CO₂ is extensively used to carbonate soft drinks. Being heavy and non-supporter of combustion it is used as fire extinguisher. A substantial amount of CO₂ is used to manufacture urea.

In CO₂ molecule carbon atom undergoes sp hybridization. Two sp hybridized orbitals of carbon atom overlap with two p orbitals of oxygen atoms to make two sigma bonds while other two electrons of carbon atom are involved in pn-pn bonding with oxygen atom. This results in its liner shape with both C-O bonds of equal length (115 pm) with no dipole moment. The resonance structures are shown below: