Oxides of carbon

OXIDES OF CARBON

Compounds of carbon with oxygen are called oxides of carbon. There are two kinds of oxides:

Carbon monoxide (CO)

Preparation

• Carbon monoxide is prepared by incomplete combustion of carbon.
• Pure carbon monoxide in the Laboratory is prepared by heating formic acid (HCOOH) with H2 SO4 at about 373 to 413 K. H2SO4 is  a dehydrating agent.

Physical properties 

• It is colourless, odourless and sparingly soluble in water.
• Toxic Nature. CO combines with hemoglobin in human red blood cells and forms carboxy - hemoglobin by a reversible reaction. This destroys the oxygen carrying capability  of the blood causing suffocation and even death in some cases.

Chemical properties 

Combustion Carbon monoxide burns in air with a pale blue flame forming carbon dioxide.

Reducing properties. It is a powerful reducing agent and reduces many metal oxides to metals.

Formation of Metal carbonyls. It combines with many transition metals such as nickel, iron, cobalt, etc, forming a variety of compounds known as metal carbonyls.

Water gas. Water gas is a mixture of equal volume of CO and hydrogen. It is prepared by passing steam through white hot coke.

C+H2OCO+ H2O

Carbon dioxide (CO2)

Occurrence

About 0.03% by volume of air contains carbon dioxide. It occurs in nature both in free and combined state. Plants use carbon dioxide during photosynthesis. During respiration, organisms give carbon dioxide back to air. It is estimated that about 40 liters of CO2

Preparation

carbon dioxide can be prepared by the following methods

• By burning carbon of carbon compounds in air.

C+ O2→ CO2

CH4+2O2→ CO2 +2H2

• By the action of heat on certain metallic carbonates and bicarbonates.

CaCO3 → CaO + CO2

2NaHCO3 → Na2CO3 + H2O +CO2

• By the action of dilute acids on carbonates and bicarbonates.

CaCO3 → 2HCI → CaCI2 →H2O + CO2

NaHCO3 + HCl → NaCl + H2 O + CO2

Physical properties of carbon dioxide

It is a colorless, odourless and tasteless gas.

At lower temperature, the solubility of carbon dioxide is large. At 0°C and 1 atom pressure, 180 ml of carbon dioxide can be dissolved in 100 ml of water. The solubility of carbon dioxide in water decreases with increase of temperature. Carbon dioxide dissolved in water is used by marine plants for photosynthesis.To score More in your class 8 refer NCERT solutions for class 8 .

Density of carbon dioxide 

Carbon dioxide is 1.5 times as dense as air. It can be poured from one container to another container.

Chemical properties of carbon dioxide

Carbon dioxide is very stable and inert. It is neither combustible nor a supporter of combustion. However, some active metals like magnesium continue to burn in it.

2Mg + CO2 → 2MgO + C

Carbon dioxide is reduced to carbon monoxide when passed over red hot coke.

C + CO2 → 2CO

Acidic nature

Carbon dioxide dissolves in water forming carbonic acid.

CO2 + H2O → H2CO3

CO2 neutralizes alkalis to give two salts – normal salts (carbonates) and acid salts (bicarbonates).

2NaOH + CO2 → Na2CO3 + H2O

Na2CO3 + H2O + CO2 → 2NaHCO3

. Action with basic oxides

Being an acidic oxide, carbon dioxide reacts with basic oxides like sodium oxide and calcium oxide to form salts.

Na2O+CO2→Na2CO3

Uses

•  It is used in extinguishing fire.
• Dry ice (solid carbon dioxide) is used in refrigeration.
• It is used in preserving fruits.
•  It is used in bread making.
• . It is used for manufacturing of urea and other nitrogenous fertilizers.

Dry ice

When carbon dioxide is subjected to a pressure of 60 atmospheres and cooled, it is liquefied to a colorless liquid. The colourless liquid CO2 on cooling further freezes like white snow. This is called dry ice.

Uses

• Dry ice is a very good refrigerant. It is used to preserve meat, fish and butter.
• It is also used in creating exceedingly low temperature required for research work in laboratories.
• It is used as a local anesthetic in surgical operations.
• Dry ice is used to produce artificial rash.

Carbon Dioxide (CO2): Carbon dioxide was first prepared by Van Helmont (1630) by burning wood. Joseph Black (1776) obtained it from chalk (CaCO3). Properties of carbon dioxide were studied in detail by Lavoisier (1783).

(i) Occurrence: Carbon dioxide occurs in air. Air contains about 0.03% carbon dioxide. It is produced by the oxidation of organic matter during burning and decay. It is also produced during respiratory process of plants and animals.

(ii) Preparation of Carbon dioxide:

(A)When materials (like wood, charcoal, coal or clothes) are burnt in the presence of excess of air or oxygen, carbon dioxide gas is obtained.

C(s) + O2(g) CO2(g)

Carbon Oxygen Carbon dioxide

CH4(g) + 2O2(g) CO2(g) + 2H2O(l)

Methane Oxygen Carbon dioxide Water

(B) By heating metallic carbonates and bicarbonates 

CaCO3(s) CaO (s) + CO2(g)

Calcium carbonate Carbon oxide Carbon dioxide

2NaCO3(s) Na2CO3 (s) + CO2(g) + 2H2O(l)

Sodium bicarbonate Sodium carbonate Carbon dioxide   Water

(C) By the action of acids on carbonates and bicarbonates:

CaCO3(s) + 2HCl (dil) CaCl2 (s) +  CO2(g)     + 2H2O(l)

Calcium carbonate   Hydrochloric acid Calcium chloride    Carbon dioxide Water

NaHCO3(s)   + HCl (dil) NaCl (aq) +  CO2(g)     + 2H2O(l)

Sodium bicarbonate   Hydrochloric acid Sodium chloride    Carbon dioxide Water

 Laboratory method of preparation of carbon dioxide:

In the laboratory, it is prepared by the action of dilute HCI on marble chips or limestone.

CaCO3(s) + 2HCl (dil) CaCl2 (s) +  H2O(l)    + 2H2O(l)

Marble Hydrochloric acid Calcium chloride    Water Carbon dioxide 

If sulphuric acid is used to react with marble chips, the reaction will stop soon, because the calcium sulphate formed is insoluble and forms a coating over the marble chips. 

CaCO3(s) + H2SO4(dil) ⎯→ CaSO4(s) + H2O(Q) + CO2 (g) 

Marble Sulphuric acid Calcium sulphate Water Carbon dioxide 

(Insoluble)

Physical properties of carbon dioxide - 

• It is colourless and odourless gas.
• It is acidic in nature (Its aqueous solution turns blue litmus red).
•  It is heavier than air and can be poured from one jar to another.
• It is sparingly soluble in water. But the solubility in water increases with increasing pressure.
• CO2when compressed to high pressure and subsequently cooled, condenses into liquid CO2(−78°C). 

On releasing the pressure, liquid CO2 solidifies into dry ice.

It is called dry ice because on being warmed, it vaporizes directly into a gas.

Chemical properties of carbon dioxide: 

• Carbon dioxide turns lime water milky :

Ca(OH)2(aq) + CO2(dil) ⎯→ CaCO3(s) + H2O(l) 

Carbon hydroxide Carbon dioxide Calcium carbonate Water 

(Lime water) (Milky white ppt.)

If CO2 gas is passed in excess, the insoluble calcium carbonate is changed into soluble calcium bicarbonate and the milkiness disappears.

CaCO3(s) + H2O (l) +   CO2 (g) ⎯→ Ca(HCO3)2 (aq)

Calcium carbonate Water Carbon dioxide Calcium bicarbonate 

(Insoluble (In excess) (Soluble)

• CO2 neutralises alkalies to form carbonates:

CO2(s) + 2NaOH (g) ⎯→ Na2CO3(aq)       + H2O (l)

Calcium dioxide Sodium hydroxide Sodium bicarbonate water

• Carbon dioxide dissolves in water to form carbonic acid –

H2O(l) + CO2(g) ⎯→ H2CO3(aq) 

Water Carbon dioxide Carbonic acid

NOTE: Carbon dioxide is also known as carbonic anhydride.

• Carbon dioxide reacts with calcium and magnesium oxides to form calcium and magnesium carbonate respectively 

CaO(s) + CO2 (g) ⎯→ CaCO3(s)

Calcium oxide Carbon dioxide Calcium carbonate 

MgO(s) + CO2 (g) ⎯→ MgCO3(s)

Magnesium oxide Carbon dioxide Magnesium carbonate 

CaCO3 and MgCO3 are used by many animals living in water. For example, molluscs use these carbonates to form protective shells around their body.

Three Properties above show the acidic nature of CO2.

• CO2reacts with water in the presence of sunlight and chlorophyll to form glucose, water and oxygen. This reaction is known as photosynthesis.

6CO2(g) + 12 H2O(l) C6H12O6 (s) + 6O2(g) + 6H2O(l)

Carbon dioxide   water Glucose Oxygen     water

Uses of carbon dioxide :

•  It is used in fire extinguishers. It does not support combustion.
• It is used for making aerated drinks like Coca-cola.
•  Dry ice (solid CO2) is used for refrigeration.
• It is used in making washing soda (Na2CO3.10 H2O).
• It is used by plants to make food during photosynthesis.
• It is used in making bread.
• It is used for preserving food.
•  A mixture of 95% oxygen and 5% CO2is used for artificial respiration. The mixture is sold under the name carbogen.
•  Carbon dioxide is used in the manufacture of urea and other fertilizers.
•  It is used in atomic reactors as a coolant.

 Carbon Monoxide (CO) :

Carbon monoxide was first prepared by Lassone in 1776 by heating zinc oxide with charcoal.

ZnO(s) + C(s) Zn(s) + CO(g)

Zinc oxide   water Zinc Carbon monoxide 

(i) Preparation of carbon monoxide –

2C(s) + O2(s) ⎯→ 2CO(g)

Carbon   Oxygen Carbon monoxide 

(Limited supply)

Physical properties of carbon monoxide –

• (A) It is an odourless and colourless gas.
• (B) It is insoluble in water.
• (C) It is neutral towards litmus paper,
• (D) It is slightly lighter than air. 

(iii) Chemical properties of carbon monoxide 

(A) It is a good reducing agent. It reduces iron oxide to iron.

Fe2O3(s) + 3CO(g) ⎯→ 2Fe(s) + 3CO2(g)

Iron (III) oxide Carbon monoxide Iron Carbon dioxide

(B) It burns with blue flame forming CO Z gas which turns lime water milky. 

2CO(g) + O2(g) → 2CO2(g)

Carbon monoxide   Oxygen Carbon dioxide

(iv) Uses of carbon monoxide

• Mixture of carbon monoxide and other gases are used as fuel,
• It is used as a reducing agent in the extraction of metals (e.g.during metallurgy of iron)
• It is used in the production of synthetic petrol, phosgene gas and methanol.

(v) Harmful effects of carbon monoxide –

• Carbon monoxide is a highly poisonous gas. When inhaled in small quantity, it causes headache, giddiness and unconsciousness.
• Excess breathing of the gas causes death because it readily combines with haemoglobin in the blood, thereby preventing it from carrying oxygen to all parts of the body