Oxides of Carbon

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Carbon combines with oxygen at higher temperature to form oxides, viz., carbon monoxide (CO) and carbon dioxide (CO₂).

Carbon monoxide (CO)

Carbon monoxide is formed, when incomplete combustion of carbon or carbon containing fuels takes place

CO is present in automobile exhausts (when there is incomplete combustion), volcanic gases, chimney gases etc.

Preparation

Carbon monoxide can be prepared by any of the following methods:

• By the reduction of carbon dioxide with carbon:

Carbon dioxide is passed through a silica tube packed with charcoal and heated to red heat. The result is a mixture of CO and CO₂. This gas mixture is passed through caustic soda and CO₂ is removed.

• By the reduction of carbon dioxide with zinc or iron.

CO₂ is passed over red-hot zinc dust or iron filings and the resulting gases are passed through caustic soda to remove unreacted CO₂.

• By the reduction of oxides of heavy metals with carbon. For example,

• By heating oxalic acid with sulphuric acid, where sulphuric acid acts as a dehydrating agent.

Passing through caustic soda or caustic potash absorbs carbon dioxide.

• By heating formic acid with sulphuric acid. Here, sulphuric acid acts dehydrating agent.

This method gives pure CO. So, this method is generally used in laboratory to prepare CO.

Properties

• Some physical characteristics of carbon monoxide are given below

• Carbon monoxide (CO) is a colorless, odorless and a poisonous gas.

• Carbon monoxide reacts with the haemoglobin in the red blood cells to form carboxy-haemoglobin. Carboxy-haemoglobin is not able to carry oxygen. This results in a deficiency of oxygen in the body, when CO is inhaled, causing suffocation and even death.

Combustibility

Carbon monoxide is combustible but a non-supporter of combustion. Carbon monoxide burns in air with a pale-blue flame giving carbon dioxide.

Reducing character

Carbon monoxide is a reducing agent. It reduces many oxides to the respective metal. For example,

CO also reduces I₂O₅ to iodine.

With hydrogen

When heated with hydrogen, to 420 - 670 K under 300 atm pressure and in the presence of a catalyst (ZnO + Cu) carbon monoxide reacts to form methyl alcohol

With chlorine

Carbon monoxide reacts with chlorine to give phosgene.

With sodium hydroxide

Carbon monoxide reacts with sodium hydroxide under pressure to form sodium formate.

Formation of metal carbonyls

Many transition metals combine with carbon monoxide under suitable conditions to form carbonyls. For example,

On heating to higher temperatures, these carbonyls decompose to give pure metal.

This reaction forms a basis for purification of nickel by Mond's process. CO forms carbonyls with chromium, molybdenum, iron, cobalt etc. also.

Uses

Carbon monoxide can be used for the following purposes.

• CO is used as a fuel in the form of producer gas or water gas.

(a) Producer gas is obtained by passing air over red hot coke at 1300 K

(b) Water gas is obtained by passing steam over hot coke:

• In the metallurgy of nickel, for its purification by Mond's process.

• For manufacturing methanol and synthetic petrol.

• For manufacture of phosgene used in dye industry and warfare.

• In metallurgy as a reducing agent.

Structure of CO

Electronic structure of carbon monoxide may be represented as follows:

Carbon dioxide (CO₂)

 

Carbon dioxide is present in the Earth's atmosphere to an extent of 0.03% by volume. Carbon dioxide is produced during combustion of fossil fuels, respiration of humans, animals and plants, fermentation and volcanic eruptions.

Preparation

• Carbon dioxide may be prepared by the complete combustion of carbon, hydrocarbons, carbon monoxide etc.

• By heating carbonates of alkaline earth metals and heavy metals or bicarbonates of alkali metals.

• By the action of acids on carbonates: In laboratory, carbon dioxide gas is prepared by the action of dilute HCl on marble chips CaCO3

• As a by-product: Large quantities of carbon dioxide are obtained as by-product, while burning limestone in lime kilns during the manufacture of lime. It is also obtained during the manufacture of alcohol by the fermentation of sugar.

Properties

Some physical characteristics of carbon dioxide are given below

  M.P        B.P.    Density (273K) CO bond length     DHf       
216.4 K    194.5 K   1.977 g/L        116.3pm       -393.5kJ/mol
(5.2 atm) (Sublimes)

• CO₂ is a colorless gas with a faint pungent smell. It is about 1.5 times heavier than air. It can be poured downward like water.

• CO₂ is fairly soluble in water. Its solubility increases with an increase in pressure. Aerated water is a solution of carbon dioxide in water under pressure.

• While CO₂ is not a poisonous gas, it does not support life; animals die in it for want of oxygen.

• CO₂ liquefies and solidifies easily under pressure of 50-60 atmospheres. Solidified carbon dioxide is called dry ice, as it looks like ice. Dry ice sublimes at - 78°C under ordinary conditions and can be used as a refrigerant (coolant) for perishables in food industry.

Stability

Carbon dioxide is a stable compound. Only about 0.32 per cent of the gas is decomposed when heated to 1775 K.

Combustibility

In ordinary conditions, carbon dioxide is neutral towards combustibility, i.e. it neither supports combustion nor is combustible. Certain very active metals, e.g., magnesium, sodium and potassium can continue burning in a jar of the gas and reduce the gas to elementary carbon

(a) With Mg

(b) With Na

Acidic nature

CO₂ dissolves in water to give carbonic acid. Therefore, carbon dioxide is called carbonic anhydride.

Carbonic acid is a very weak dibasic acid. It ionizes in aqueous solutions as,

Carbonic acid being a dibasic acid forms two sets of salts, the hydrogen carbonates HCO₃^- and carbonates, CO₃^2-.

With alkalies

Strong alkalies react with carbon dioxide (which is acidic in nature) to form carbonates and bicarbonates. For example, with NaOH

With lime water

Limewater turns milky when CO₂ is bubbled through it due to the formation of CaCO₃. When more CO₂ is bubbled, the solution becomes clear due to the formation of CaHCO₃. When the clear solution is boiled again, milkiness reappears due to the formation of CaCO₃ once again.

The reaction forms the basis of lime water test for the detection of CO₂^3- and HCO₃^- ions in any salt/salt mixture.

Photosynthesis

Carbon dioxide is absorbed by plants in the presence of chlorophyll and sunlight to produce glucose, starch and cellulose. The process is known as photosynthesis.

Uses

Carbon dioxide is used

• In the manufacture of aerated water (soft drinks).

• As a fire extinguisher.

• For the manufacture of sodium carbonate and sodium bicarbonate by Solvay process.

• As a refrigeration/coolant (in the form of dry ice) for perishables in food industry e.g., for fish, ice creams etc.

• In artificial respiration for the victims of CO poisoning (35% O₂ + 5% CO₂).