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Carbon Dioxide

What is carbon dioxide?

Carbon dioxide (molecular formula CO2) is a colorless, orderless gas that can be easily liquefied under critical temperature and pressure. It is the most important industrial chemical or greenhouse gas that causes global warming of our environment. Carbon dioxide is fairly soluble in water by forming weak acidic solutions of H3CO3. It is a major industrial chemical obtained from several industrial processes. For example fermentation industries, calcination of limestone and flue gases from coal-based power plants.

Carbon dioxide in atmosphere

Atmospheric carbon dioxide is the main source of carbon in our life. The major consumers of CO2 gas from the atmosphere are green plants and oceans. The green plants absorbed CO2 from the atmosphere to prepare carbohydrates through the process of photosynthesis. While oceans dissolve CO2 to form carbonate rocks.

Carbon dioxide, water and oxygen in atmosphere for photosynthesis

Today, with the increased burning of carbonaceous fuels and decreases in forest areas, the concentration of atmospheric CO2 has been in an alarming stage. It causes to raise the global temperature of our earth environment. The effect of rising global temperature is called the greenhouse effect. Carbon dioxide molecules in the atmosphere trap the infrared radiation coming from the earth’s surface.

Formula and Structure

Carbon dioxide molecular formula, structure and dipole moment

The carbon dioxide molecule is linear and symmetrical with a C=O chemical bond length of 116.3 pm. The carbon atom in linear CO2 molecule is sp hybridized. Two half-filled sp hybridized carbon atoms form two sigma bonds with two oxygen atoms. Two singly occupied p-orbitals of carbon atom formed two pi-bonds each of the p-orbital of two oxygen atoms. Therefore, the linear CO2 molecule has two sigma and two pi bonds in its structure.

Physical Properties

CO2 is a colorless, orderless gas and fairly soluble in water. The solubility increases remarkably under pressure.

Properties of carbon dioxide
Chemical formula CO2
Molar mass 44.009 g/mol
Appearance Colorless gas
Density Gas Liquid Solid
1562 kg/m3 at 1 atm and −78.5 °C 1101 kg/m3 at −37 °C 1.977 kg/m3 at 1 atm and 0 °C
Critical Constants Critical temperature (TC) Critical pressure (PC)
30.978 °C 72.808 atm
Dipole moment 0 Debye
Molecular shape Linear
Heat capacity 37.135 J K-1 mol-1

Facts about Carbon dioxide

Carbon dioxide is mild Lewis acid by the formation of carbonates with oxide or hydroxide ions. It also acts as a Lewis base towards metal atoms in a low oxidation state. Neutralization of the dissolved CO2 occurs by two routes depending upon the pH scale of the solution.

At pH < 8
H2O + CO2 → H2CO3 (slow)
H2CO3 + OH → HCO3 + H2O (fast)

At pH < 8, the overall reaction follows pseudo-first-order chemical kinetics since H2O is always in large excess.

At pH > 10
CO2 + OH- → HCO3 (slow)
HCO3 + OH → CO3-2 + H2O (fast)

At pH > 10, the overall rate is first order. Between pH 8 to 10, all the chemical equilibrium is involved.

Chemical Reaction

CO2 reduces electrochemically to CO in an aqueous medium.
CO2 + 2H+ + 2e → CO + H2O

Electrolysis by platinized titanium oxide (TiO2) or transition metal complexes reduces CO2 to various products like carbon monoxide, methane, methyl alcohol, etc.

Reduction of CO2 to organic compounds under ordinary conditions occurs during photosynthesis.
6CO2 + 6H2O → C6H12O6 + 6O2

Uses of carbon dioxide

Liquid carbon dioxide is used in the refrigeration of ice cream, meat, fish, etc. Over the last two centuries, compressed liquid CO2 is gradually replaced by dry ice due to lower production costs and transport facilities. Liquid CO2 is also used as a substitute for chlorofluorocarbons in aerosol propellants.

Gaseous carbon dioxide is extensively used to prepare soda water and other soft drinks preparation. It provides an inert atmosphere for welding and neutralizes the alkali. In the manufacture of sodium carbonate, bicarbonate, and urea, we used a huge quantity of gaseous carbon dioxide.