Carbon & Its Compounds

Allotropy — It is the existence of an element in

• more than one physical form
• having different physical properties
• but the same graded chemical properties.

It is the result of difference in atomic arrangement in the crystal structure of the element. Hence they differ in physical properties but are forms of the same element carbon, and hence have similar chemical properties.

Crystalline allotropes of carbon —

1. Diamond
2. Graphite

Amorphous allotropes of carbon —

1. Coal
2. Coke
3. Lampblack
4. Wood charcoal

Coal is formed by slow bacterial decomposition of vegetable matter. It is formed under the influence of heat, pressure & limited air, over millions of years. The different stages of transformation of vegetable matter results in a residue rich in varying amounts of carbon, hence giving different types of coal

• Peat [60% C]
• Lignite [65% C]
• Bituminous [85% C]
• Anthracite [90% C ]

(a) Uses of coal —

1. Mainly as a cheap fuel.
2. Manufacture of coke, coal gas and synthetic petrol.

(b) Uses of coke —

1. As a fuel.
2. As a reducing agent in the iron & steel production.

(c) Uses of lampblack or soot —

1. For making printers ink, black shoe polish, typewriter ribbons & black paints.
2. As a filler in rubber tyres.

ZnO + C ⟶ Zn + CO

Carbon dioxide is added to the atmosphere by:

1. Respiration in living organisms.
2. Burning of carbon compounds - e.g. wood, petroleum.
3. Decay & decomposition of organic matter - e.g. animals, plants.

(a) CaCO₃ + 2HCl [dil.] ⟶ CaCl₂ + H₂O + CO₂

(b) (i) The evolved carbon dioxide is passed through the washer bottles containing conc. H₂SO₄, to absorb moisture.

(ii) Carbon dioxide is fairly soluble in water, hence, it is not collected over water,

(iii) Dilute sulphuric acid reacts with calcium carbonate forming a coating of insoluble calcium sulphate on the marble chips thereby the reaction slowly comes to a stop.
CaCO₃ + H₂SO₄ [dil.] ⟶ CaSO₄ [insoluble] + H₂O + CO₂

(a) Insert a burning candle inside a jar containing carbon dioxide gas. The burning candle is extinguised. It is concluded that carbon dioxide does not support combustion.

(b) Insert a moist blue litmus paper in a jar containing carbon dioxide gas. The paper turns pink or scarlet. It is concluded that carbon dioxide is slightly acidic in nature.

(a) CO₂ dissolves in water under pressure to give a weak acid called carbonic acid [H₂CO₃].

CO₂ + H₂O ⟶ H₂CO₃

(b) CO₂ reacts with ammonia gas at elevated temperatures & pressures forming Urea [(NH₂)₂CO], an important nitrogenous fertilizer.

2NH₃ + CO₂ ⟶ (NH₂)₂CO + H₂O

(c) CO₂ when passed over heated coke [carbon] is reduced to Carbon monoxide [CO] which is a highly poisonous gas.

CO₂ + C ⟶ 2CO

(d) Burning magnesium burns in CO₂ producing Magnesium oxide [MgO] & black particles of carbon [C].

2Mg + CO₂ ⟶ 2MgO + C

Basic oxides such as Na₂O & MgO react with CO₂ to give metal carbonates such as Na₂CO₃ & MgCO₃ respectively

Na₂O + CO₂ ⟶ Na₂CO₃

When carbon dioxide is bubbled into lime water, the lime water turns milky due to the formation of insoluble calcium carbonate.

Ca(OH)₂ + CO₂ ⟶ CaCO₃ ↓ + H₂O

When CO₂ is bubbled in excess the milkiness disappears due to the formation of soluble calcium bicarbonate [Ca(HCO₃)₂]

CaCO₃ + H₂O + CO₂ ⟶ Ca(HCO₃)₂

Liquid carbon dioxide on sudden evaporation results in freezing and formation of solid carbon dioxide which is called 'dry ice'.

It is used as a refrigerant for preservation of foods.

Three reasons for using carbon dioxide in fire extinguishers:

1. It is non-combustible.
2. It is non-supporter of combustion.
3. It is heavier than air, hence cuts out the fire.

On burning magnesium ribbon in a jar of carbon dioxide, the black particles of carbon are deposited on the walls of the jar. Hence, the presence of carbon dioxide can be detected.

CO₂ + 2Mg ⟶ 2MgO + C

Conc. sulphuric acid acts as a dehydrating agent & removes a molecule of water from oxalic acid to produce carbon monoxide and carbon dioxide. The reaction is given below:

HOOC-COOH $\xrightarrow{\text{Conc. H}_2\text{SO}_4}$ H₂O + CO + CO₂

Carbon monoxide combines with the haemoglobin in the red cells forming a stable compound carboxyl-haemoglobin which prevents the haemoglobin from taking up oxygen. A person thus dies due to lack of oxygen resulting in asphyxia which is a condition caused by lack of oxygen resulting in suffocation or death.

(i) It is dangerous to sleep in a closed room with a coal fire burning since carbon burns in a limited supply of air forming carbon monoxide, which is fatal to life.

(ii) As fumes of burning tobacco contains traces of carbon monoxide, which is a poisonous gas, hence it it dangerous to be in the vicinity of smokers.

(i) Carbon monoxide burns in oxygen forming carbon dioxide.

2CO + O₂ ⟶ 2CO₂

(ii) Carbon monoxide reduces heated metallic oxides to metals and evolve carbon dioxide gas.

ZnO + CO ⟶ Zn + CO₂

(i) Carbon monoxide is used in the extraction of metals from their ores — Reduces heated metallic oxides e.g. iron [III] oxide to iron.

Fe₂O₃ + 3CO ⟶ 2Fe + 3CO₂

(ii) Two volumes of H₂ reacts with one volume of carbon monoxide to form methyl alcohol.

CO + 2H₂ ⟶ CH₃OH [methyl alcohol]

In diamond, due to the linking of each carbon atom with four neighbouring carbon atoms forming a rigid tetrahedral structure, there are no free or mobile electrons. This makes diamond inert.
Graphite has a more open structure than diamond with one valence electron of each carbon atom remaining free which makes it more reactive than carbon.

(a) Diamond has a sparkling brilliance due to high refractive index hence, it is used as the most precious & expensive item of jewellery.

(b) (i) Ordinary lubricating oils gets charred due to high temperature whereas graphite is non-volatile and non-sticky and can withstand high temperatures.

(ii) Graphite can withstand very high temperature as it has a high melting point and is a good conductor of heat. So it is used as a lining for crucibles used in manufacture of high grade steel.

(iii) Graphite is a good conductor of electricity and is relatively inert. Chemically it is almost inactive with acids hence is used as an electrode in electroplating.

In the free state —

• carbon occurs free in nature in the crystalline form [i.e. crystal] as diamond & graphite & in the

• amorphous form [impure] as coal formed below the earth's surface.

In the combined state —

• Carbon occurs as carbon dioxide in the atmosphere [about 0.03% by vol.]
• It also occurs as hydrocarbons in natural gas & petroleum.
• In the solid state in wood [cellulose].
• In plants & animals as carbohydrates & in minerals as limestone - CaCO₃

1. Graphite

2. Coal

3. Wood charcoal

4. Carbonic acid [H₂CO₃]

5. Carbon dioxide

1. B: Diamond

2. A: Anthracite

3. E: Lampblack

4. D: Urea

5. C: Carbogen

1. $\underset{\text{coke}}{\text{C}} + \underset{\text{steam}}{\text{H}_2\text{O}} \xrightarrow{1000 \degree \text{C}} \underset{\text{water gas}}{[\text{CO} + \text{H}_2 ]} - \Delta$

2. $\underset{\text{ calcium bicarbonate} }{\text{Ca}\text{(HCO}_3)_2} + \underset{\text{Dil acid}}{2\text{HNO}_3} \longrightarrow \text{Ca(NO}_3)_2 + 2\text{CO}_2 + 2\text{H}_2\text{O}$

3. Ca(OH)₂ + CO₂ ⟶ CaCO₃ ↓ + H₂O
   CaCO₃ + H₂O + CO₂ ⟶ Ca(HCO₃)₂

4. CO₂ + 2Mg ⟶ 2MgO + C

5. Na₂O + CO₂ ⟶ Na₂CO₃

1. The crystal of graphite is opaque to light and is a good conductor of heat.

2. A graphite-clay mixture baked at high temperature is used in making refractory crucibles.

3. Adsorption capacity of wood charcoal is increased by passing steam over wood charcoal, at high temperatures.

4. Organic Chemistry is the Chemistry of carbon compounds mainly of 'Carbon', 'Hydrogen' & 'Oxygen'.

5. Sodium oxide combines with carbon dioxide to give sodium carbonate.

1. Diamond & graphite are different in atomic arrangement in the crystal structure of the element. Hence they differ in physical properties but are forms of the same element carbon, and hence have similar chemical properties and are called allotropic forms of carbon.

2. It is dangerous to stand behind a running engine of a vehicle since in exhaust fumes carbon monoxide is present due to incomplete combustion of fuel in the engine.

3. SO₂ turns potassium permanganate paper from purple to colourless whereas CO₂ does not. Hence, the two gases can be distinguished.

4. Carbon monoxide has a very strong affinity for haemoglobin, binding to it about 200-250 times more strongly than oxygen to form a stable compound called carboxyhaemoglobin which prevents the haemoglobin from carrying oxygen efficiently. Carbon dioxide also combines with haemoglobin to form carbaminohaemoglobin but its affinity for haemoglobin is much weaker compared to oxygen and carbon monoxide. This makes carbon monoxide and not carbon dioxide a highly poisonous gas.

5. When carbon dioxide is bubbled into lime water, the lime water turns milky due to the formation of insoluble calcium carbonate.
   Ca(OH)₂ + CO₂ ⟶ CaCO₃ ↓ + H₂O
   When CO₂ is bubbled in excess the milkiness disappears due to the formation of soluble calcium bicarbonate [Ca(HCO₃)₂]
   CaCO₃ + H₂O + CO₂ ⟶ Ca(HCO₃)₂
   On the other hand lime water does not directly react with carbon monoxide.
   CO + lime water ⟶ No milkiness.
   However, on ignition, carbon monoxide burns with a pale blue flame forming carbon dioxide which reacts with lime water.