Oxides Of Carbon

Oxides Of Carbon

Two oxides of carbon, carbon monoxide and carbon dioxide are important and described below.

(i) Carbon Monoxide, CO

It is found in small amounts in volcanic gases, chimney gases, exhaust gases of internal combustion engines and coal gas.

Preparation

The following methods can be applied for the preparation of carbon monoxide.

(a) By heating oxalic acid with concentrated sulphuric acid: A mixture of CO and CO₂ is obtained. Sulphuric acid acts as a dehydrating agent. CO₂ is removed by passing the gaseous mixture through caustic soda or caustic potash solution.

(COOH)₂ CO + CO₂

Similarly by heating formic acid with concentrated H₂SO₄, only CO is obtained.

HCOOH CO

(b) By the reduction of oxides of heavy metals with carbon: On heating oxides of heavy metals with carbon, CO is formed.

Fe₂O₃ + 3C 2Fe + 3CO

ZnO + C Zn + CO

(c) By reduction of carbon dioxide: CO₂ can be reduced with carbon or zinc at high temperatures. CO₂ when passed over red hot zinc, a mixture of CO and CO₂ is obtained.

Zn + CO₂ ZnO + CO

(d) By heating carbonates of calcium, barium or magnesium with zinc: CO is obtained.

MgCO₃ + Zn MgO + ZnO + CO

(e) By heating potassium ferrocyanide with conc. H₂SO₄: When potassium ferrocyanide in powdered state is heated with concentrated H₂SO₄, CO is evolved. Dilute H₂SO₄ should never be used because it shall evolve highly poisonous gas, HCN.

K₄Fe(CN)₆ + 6H₂SO₄ + 6H₂O 2K₂SO₄ + FeSO₄ + 6CO + 3(NH₄)2SO₄

Properties

(i) It is a colourless and odourless gas.

(ii) It is slightly soluble in water.

(iii) It is a combustible gas but does not support combustion.

(iv) Its density is nearly equal to the density of air.

(v) It is highly poisonous in nature. One part in 100 parts of air causes death in few minutes. The poisonous nature of CO is due to the fact that it combines with haemoglobin (a red colouring matter of blood which is absorber of oxygen) to form carboxy−haemoglobin which is not capable to absorb oxygen and as a result of this, suffocation takes place.

(vi) It is not decomposed by heat.

(vii) It is neutral to litmus.

(viii) It burns with blue flame to form CO₂. This is an exothermic process.

2CO + O₂ 2CO₂ + heat

(ix) It is a good reducing agent as it takes up oxygen and converted into CO₂.

(x) The bonding in carbon monoxide is represented as

: C : : : O : or : C O

Thus, all the four valencies of carbon are not satisfied. It behaves as unsaturated compound and forms addition products with a number of substances.

CO + 2H₂ CH₃OH

Methyl alcohol

CO + Cl₂ COCl₂

Phosgene

(xi) It combines with metals like Cr, Ni, Fe, etc. The compounds thus formed are called carbonyls.

Ni + 4CO Ni(CO)₄

Fe + 5CO Fe(CO)₅

These reactions are used in the extraction and purification of metals.

Uses

(i) It is used as a fuel in the form of water gas (CO + H₂) and producer gas (CO + N₂).

(ii) CO is used in the manufacture of methanol, synthetic petrol, formic acid and phosgene gas (highly poisonous gas).

(iii) It is used as a reducing agent in the extraction of iron.

(iv) It is used in the extraction of nickel (Mond’s process).

(ii) Carbon dioxide, CO₂

Occurrence: It is present in atmosphere to the extent of 0.03−0.05 percent. It comes to the atmosphere from animal breathing, decay of vegetable matter, burning of carbon and carbonous matter etc. It is also utilized by plants in photosynthesis. Thus, a carbon dioxide cycle is operating in nature and the proportion of CO2 in the atmosphere remains about the same. It is found in combined state in the form of carbonates.

Preparation

The following method are used for its preparation.

(a) By the complete combustion of carbon: Carbon is burnt in free supply of air.

C + O₂ CO₂

If CO is formed, it also burns with pale blue flame forming carbon dioxide.

2C + O₂   2CO

2CO + O₂   2CO₂

(b) By the action of dil. mineral acids on carbonates and bicarbonates: Mineral acids react with carbonates and bicarbonates and evolve carbon dioxide.

CaCO₃ + 2HCl CaCl₂ + H₂O + CO₂

Na2CO₃ + 2HCl   2NaCl + H₂O + CO₂

NaHCO₃ + HCl   NaCl + H₂O + CO₂

(c) By heating carbonates and bicarbonates: the carbonates of less electropositive metals on heating decompose evolving carbon dioxide.

ZnCO₃   ZnO + CO₂

CuCO₃ CuO + CO₂

Bicarbonates of all the metals decompose on heating with evolution of CO₂.

2NaHCO₃ Na₂CO₃ + H₂O + CO₂

Ca(HCO₃)₂   CaCO₃ + H₂O + CO₂

Properties

(i) It is a colourless, odourless and tasteless gas.

(ii) It is slightly soluble in water under ordinary pressure but at high pressures, the solubility is high.

(iii) It is heavier than air.

(iv) It is easily liquefied under pressure into a colourless mobile liquid. If CO₂ under pressure is allowed to escape through a nozzle, a white solid, i.e. dry ice is obtained. Solid CO₂ is a soft, white, snow like substance. It sublimes and leaves no residue. Solid CO2 is used as a refrigerant under the commercial name drikold. It is used in the transport of perishable food materials. It provides cold as well as the inert atmosphere which helps in killing the undesirable bacteria.

(v) It is neither combustible nor a supporter of combustion. However, burning magnesium, sodium or potassium continues burning in the gas.

CO₂ + 2Mg 2MgO + C

(vi) CO₂ acts as an oxidizing agent. When heated with Zn, iron or carbon, these are oxidized.

Zn + CO₂ ZnO + CO

CO₂ + C 2CO

(vii) CO2 is an acidic oxide. It dissolve in water forming unstable carbonic acid.

H₂O + CO₂ H₂CO₃

It combines with alkalies to form two series of salts, carbonates and bicarbonates.

2NaOH + CO2 Na₂CO₃ + H₂O

With excess of CO₂, carbonate is converted into bicarbonate.

Na₂CO₃ + H₂O + CO₂ 2NaHCO3

(viii) CO2 reacts with basic oxides like Na₂O, K₂O etc. to form corresponding carbonates.

K₂O + CO₂ K₂CO₃

Na₂O + CO₂ Na₂CO₃

(ix) CO2 is converted by plants in the presence of sunlight and chlorophyll into glucose and higher carbohydrates. This process is known as photosynthesis.

6CO₂ + 6H₂O C₆H₁₂O₆ + 6O₂

(x) Carbon forms some less stable oxides, e.g., C₃O₂, C₅O₂ and Cl₁₂O₉. Graphite oxides are C₂O and C₂O₃ which are still less stable. Carbon suboxide is formed by dehydration of malonic acid.

Uses

(i) It is used in the manufacture of aerated water.

(ii) Solid CO₂ (dry ice) is used in refrigeration. It is superior refrigerant than common ice because it can produce very low temperatures and does not form liquid on melting.

(iii) Carbogen is a mixture of O₂ and CO₂ (5−10%). It is used for artificial respiration in the case of pneumonia patients and victims of CO poisoning.

(iv) It is used in the manufacture of white lead and sodium carbonate (Solvay process).

(v) CO₂ is used as fire extinguisher. The use of common fire extinguisher is based on the production of CO₂.

(vi) CO₂ is used by plants in the form of food.