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) → CO32- (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:
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