Notes-Class 9-Science-Chapter-13-Carbon: An Important Element-Maharashtra Board

Topics to be learn :

Carbon – occurrence, properties, allotropes
Hydrocarbons
Carbon dioxide- occurrence, properties, uses
Methane – occurrence, properties, uses

An Element : A substance which cannot be broken down into simple substances by any chemical reaction is called an element.

Types of elements : Metals, non-metals and metalloids are the different types of elements.

Carbon : Carbon is a non-metallic element.

Carbon is the main constituent of all plant and animal products.
In nature, it is found in the free state and also in compounds.

On complete combustion of any organic compound, carbon dioxide and water are formed.

1. Symbol of carbon – C 2. Atomic number – 6

3. Atomic mass – 12 4. Electron configuration – 2,4

5. Valency – 4 6. Non-metallic element

Compound : A substance produced by a chemical reaction of two or more elements is called a compound. Compounds are obtained directly or indirectly from plants or animals and compounds are also obtained from minerals.

Occurrence of carbon : In the free state, carbon is found as diamond and graphite. In the combined state, carbon is found in the following compounds :

Carbon dioxide, calcium carbonate, marble and calamine (ZnCO₃).
Coal, petroleum and natural gas.
Carbohydrates, proteins and fats.
Natural fibres such as cotton, wool, silk.

In the earth’s crust, carbon is present to the extent of approximately 0.27% in the form of carbonate, coal and petroleum. The proportion of carbon in the form of carbon dioxide, in the atmosphere is approximately 0.03 %.

Allotropes of an element : Some elements are found in nature in more than one form. They have same chemical properties but different physical properties. These forms are called allotropes of an element.

Properties of Carbon : Carbon exhibits allotropy. It exists in more than one form. The chemical properties of these different forms are the same but their physical properties are different.

Carbon allotropes have two forms : crystalline form and amorphous form..

Crystalline forms : (i) It has a regular and definite arrangement of atoms. (ii) They have high melting points and boiling points. (iii) A crystalline form has a definite geometrical shape, sharp edges and plane surfaces. (iv) Carbon has three crystalline allotropes : 1. Diamond 2. Graphite 3. Fullerene.
N0n-crystalline/ Amorphous forms : The arrangement of carbon atoms is irregular. Examples : Coal, charcoal, coke.

Diamond :

Diamond : Diamonds are found in India in Govalkonda (Karnataka) and Panna (Madhya Pradesh).

In diamond, each carbon atom is linked to four other carbon atoms by covalent bonds. Therefore, diamond has tetragonal three dimensional structure.

Brilliant, pure diamond is the hardest known substance.
The density of diamond is 3.5 g/ cm³ and melting point of diamond is 3500 °C. When it is heated at 800 °C, it gives out CO₂.

It does not dissolve in any solvent. Diamond

does not react with acids or alkalies.
It is a bad conductor of electricity.
Diamonds are used in glass cutting and rock drilling machines
It is used in ornaments.
Diamond knives are used for eye surgery.
In artificial satellites diamonds are used to make windows giving protection from radiation in space.

[collapse]

Graphite :

Graphite : In the natural state, graphite is found in Russia, New Zealand, America and India.

In graphite, each carbon atom is bonded to three other carbon atoms in such a way that a hexagonal layered structure is formed.
Thus, the regular arrangement of atoms in graphite is in layers.
The graphite that occurs in nature is black, soft and smooth and it is a good conductor of electricity.
Due to the layered structure, graphite can be used for writing on paper.
The density of graphite is 1.9 to 2.3 g/ cm³.
It does not dissolve in most solvents.
Graphite is used for making lubricant sand carbon electrodes. It is used in paints, polish and in pencils for writing.
Graphite is used in arc lamps which give a very bright light.

[collapse]

Fullerene :

Fullerene :

Fullerene, an allotrope of carbon, is rarely found in nature. C₆₀, C₇₀,

C₇₆, C₈₂ and C₈₆ are other examples of fullerene found in small numbers in soot.

Fullerene molecules are found in the form of buckyballs and buckytubes.
These molecules are made of 30 to 900 carbon atoms.
Fullerenes are soluble in carbon disulphide and chlorobenzene.
Fullerenes are used as insulators.
Fullerenes are used as a catalyst in water purification.
Fullerenes exhibit super conductivity at a certain temperature.

[collapse]

Amorphous Forms of Carbon :

Amorphous Forms of Carbon : The arrangement of carbon atoms in amorphous form is not regular. Coal, charcoal, coke are the non-cystalline forms of carbon.

Coal is a fossil fuel.
The types of coal with the percentage of carbon are : peat less than 60% of carbon, lignite 60 to 70 % of carbon, bituminous coal 70 to 90% of carbon, anthracite about 95% of carbon.
Coal is used as fuel in factories and homes.
To obtain coke, coal gas and coal tar, coal is used.
Coal is used in thermal power plants for generation of electricity.
The pure coal that remains when coal gas is removed from coal, is called coke.
Coke is used as domestic fuel.
It is used as a reducing agent.
Coke is used in production of water gas (CO + H₂) and producer gas (CO + H₂ + CO₂ + N₂).

[collapse]

Fossil fuels :The fuels formed over millions of years by the burial of plants and animals are called fossil fuels. They contain energy rich carbon compounds originally made by plants. Coal, petroleum and natural gas are fossil fuels.

Q. How did coal form in nature?

Answer :

Ans. Millions of years ago the remains of plants and animals got buried under the earth. Due to action of microorganisms, the gaseous substances in these remains were released into the atmosphere and the left behind was carbon compounds. After this, under tremendous pressure, the liquid substances in these compounds drained away and the remaining mineral solidified into compact and hard rock and this material is called coal.

Carbon dioxide : Molecular formula : CO₂, Molecular mass 44.

Occurrence : Carbon dioxide occurs in free state in atmosphere to the extent of about 0.03 %. Exhaled air contains about 4% of CO₂.

It is found in chalk and shahabad tiles/marble/ limestone in compound form.
In the combustion of wood and fossil fuel coal, CO₂ is given out.

Properties of Carbon dioxide :

Physical Properties :

Carbon dioxide is a colourless, tasteless and odourless gas.
It is heavier than air and sparingly soluble in water.
Carbon dioxide neither burns nor supports burning.

Chemical properties :

CO₂ + Ca(OH)₂ -> CaCO₃+ H₂O (Calcium Carbonate + Water)
CO₂ + H₂O -> H₂CO₃(Carbonic Acid)
Sodium carbonate is formed when carbon dioxide is passed through an aqueous solution of sodium hydroxide (Sodium carbonate = washing soda) Chemical equation of the reaction : CO₂ + 2NaOH -> Na₂CO₃+ H₂O
Sodium bicarbonate is formed on passing CO2 through an aqueous solution of sodium carbonate. (sodium bicarbonate = baking soda) Chemical equation of the reaction :-CO₂ + Na₂CO₃ + H₂O -> 2NaHCO₃
2NaHCO₃ + H₂SO₄ -> Na₂SO₄ +2 H₂O + 2CO₂

[collapse]

Uses of Carbon dioxide :

Uses :

Carbon dioxide is used in the manufacture of aerated drinks, solid carbon dioxide, baking soda and washing soda.
Liquefied CO₂ is used to remove caffeine from coffee.
It is used in fire extinguishers.
Liquefied CO₂ is used as solvent in modern eco-friendly dry cleaning.
Plants use CO₂ to make their own food.

[collapse]

Hydrocarbons : Carbon and hydrogen are included in most organic compounds. These basic organic compounds are called hydrocarbons. The electronic conﬁguration of carbon is 2,4, therefore, the valency of carbon is 4.

A carbon atom can form four covalent bonds with other carbon atoms or atoms of different elements.

Properties of covalent bond :

Properties of covalent compounds:

Covalent compounds have low melting and bioling points.
They are insoluble in water and soluble in organic solvents.
They are poor conductors of heat and electricity.

[collapse]

Saturated and Unsaturated Hydrocarbons : Carbon atom exhibits a characteristic property. They can form a chain of carbon atoms by forming covalent bonds with other carbon atoms. The hydrocarbons having only single bonds between carbon atoms are called saturated hydrocarbons.

Examples : Ethane (C₂H₆) which is (CH₃—CH₃), propane (CH₂ — CH₂ — CH₃).

Some hydrocarbons have a multiple bond (a double bond or a triple bond) between two carbon atoms. Hydrocarbons having at least one multiple bond are called unsaturated hydrocarbons.

Examples : Ethene (CH₂ = CH₂), ethyne (HC Ξ CH), propene (CH₃—CH=CH₂), propyne (CH₃—C Ξ CH).

Methane : Molecular formula : CH₄, Molecular mass 16.

Occurrence :

Methane occurs in natural gas to the extent of 87%.
The decomposition of vegetable matter in the absence of air produces methane.
It is obtained from the biogas.
It is found in coal mines.
Methane is found at the surface of marshy places, therefore, it is also called marsh gas.
In the laboratory, when a mixture of hydrogen and carbon monoxide gases are heated at 300°C in the presence of nickel as a catalyst, methane gas is formed.
Methane gas in pure form is obtained by fractional distillation of natural gas.

Physical & Chemical properties of methane :

Physical properties of methane :

Melting point of methane is (-182.5 0C).
Boling point of methane is (-161.5 0C).
It is a colourless gas.
The density of liquid methane is less than that of water.
Methane is sparingly soluble in water. It is highly soluble in organic solvents like gasoline, ether and alcohol.
Methane is in gaseous state at room temperature.

Chemical properties of methane : Methane is highly inflammable. It burns by reacting with oxygen to give a bluish flame.

In this reaction, 213 kcal/mol of heat is given out. Methane burns completely.

Chemical reactions :

CH₄+2O₂ -> CO₂+ 2H₂O + heat

CH₄+Cl₂-> CH₃Cl +HCl

[collapse]

Uses of methane :

Uses of methane :

Methane burns in air with a pale blue non-luminous ﬂame to form carbon dioxide and water. The reaction evolves large amount of heat. Hence, methane is used as a fuel.

Methane in the form of natural gas is used in industries such as fabric mills, paper mills, food processing industry, petrol purification.
Being the smallest hydrocarbon, the proportion of CO₂ released in the combustion of methane is small and, therefore, it is used as a domestic fuel.
Methane is used for production of organic compounds such as ethanol, methyl chloride, methylene chloride and acetylene.

[collapse]

Biogas Plant :

Biogas plant : In a biogas plant, animal dung, wet garbage is decomposed by anaerobic microbes. This produces methane gas and carbon dioxide and it is called biogas.

Biogas contains about 55% to 60% methane (CH₄) and the rest is carbon dioxide (CO₂).
It is a cheap fuel and meets the demand for cooking gas. It is also used in the production of electricity. It burns instantly and provide heat quickly.
Biogas is convenient to use and in addition to this the by-product of the process can be used as a very good manure.
It does not produce solid waste. Therefore it is a eco-friendly fuel

The production of biogas is an anaerobic process. It takes place in two stages.

Production of acids : The biodegradable complex organic compound is decomposed by microbes leading to the formation of organic acids-
Methane gas production : The methanogenic bacteria act on the organic acids to produce methane gas.

[collapse]

Fire extinguisher :

Fire extinguisher :

A fire extinguisher contains sodium bicarbonate powder. Dilute sulphuric acid (H₂SO₄) is placed in glass capsule.
The capsule breaks on pressing the knob, the dil. H₂SO₄ comes in contact with NaHCO₃, the chemical reaction [takes place to produce carbon dioxide
(CO₂).
CO₂ is not combustible does not support combustion. When it is spread out on a burning object, it covers the object and separates it from air. Thus, the fire is extinguished.

Chemical reaction : 2NaHCO₃ + H₂SO₄ -> Na₂SO₄ + 2H₂O + 2CO₂

CO2 based fire extinguishers are non-conductors of electricity and it prevents corrosions. Therefore, these are used when electrical equipment catches fire.
In modern fire extinguishers liquid and solid carbon dioxide is filled under pressure. When the pressure is reduced it becomes gaseous and comes out forcefully through the horn-like hose pipe.
Nowadays many types of ﬁre extinguishers are available.

[collapse]

PDF-Notes, Solution, Text Book