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CARBON AND ITS COMPOUNDS

CARBON AND ITS COMPOUNDS

  • The element carbon is a non-metal. Earth’s crust contains only 0.02% carbon in the form of minerals (like coal, petroleum, carbonates, etc.).
  • The atmosphere has only 0.03% of carbon dioxide gas. All living things (plants and animals) made up of compounds of carbon are called organic compounds.
  • The atomic number of carbon is 6 (electronic configuration: 2, 4).
  • Carbon is tetravalent, i.e., its valency is 4. It forms covalent bonds by sharing of electrons.
  • Carbon has the unique property of self combination (known as catenation) to form long chains, which gives rise to an extremely large number of carbon compounds (organic compounds).

Allotropes of Carbon

  • Allotropes are the various physical forms in which an element can exist.
  • In the free state, carbon occurs in nature mainly in two forms: Diamond (a colourless, transparent substance) and Graphite (a grayish-black opaque substance).
  • Another naturally occurring form of carbon, called Buckminsterfullerene, has been recently discovered. These three are called allotropes of carbon.
  • Diamond and graphite have entirely different physical properties. Example: Diamond is extremely hard whereas graphite is soft; diamond is a nonconductor of electricity whereas graphite is a good conductor of electricity.
  • Their chemical properties are the same. Example: Both form carbon dioxide on burning in oxygen. The difference in their physical properties arises because of the different arrangements of carbon atoms in them.
  • The compact and rigid structure of diamond makes it a very hard substance, due to which it is used for making rock borers for drilling oil wells, and for making glass cutters.
  • A sharp, diamond edged knife (called keratome) is used by eye-surgeons to remove cataract from the eye. Diamonds can be made artificially by subjecting pure carbon to very high pressure and temperature. Diamonds are used for making jewellery.
  • There are ‘no free electrons’ in diamond. Therefore, it does not conduct electricity. Graphite consists of sheets (or layers) of carbon atoms, due to which it is a comparatively soft substance. It contains ‘free electrons’ because of which it conducts electricity.
  • Therefore, graphite is used for making electrodes in dry cells. The carbon brushes of electric motors are also made of graphite. Powdered graphite is used as a lubricant for machine parts especially those which operate at very high temperatures.
  • Mixed with clay, graphite is used for making the cores of pencils, called pencil leads. The tiles on the nose cone of space shuttle contain graphite because it does not melt easily.
  • The third allotrope of carbon, Buckminsterfullerene (also called fullerene), contains clusters of 60 carbon atoms joined together to form spherical molecules. Its formula is C60. It is a football shaped molecule with 60 carbon atoms arranged as 20 hexagons and 12 pentagons which are interlocked.
  • It has been named after the American architect, Buckminster Fuller, because its structure resembles the framework of dome-shaped halls designed by Fuller.
  • Buckminsterfullerene is a dark solid, which is neither very hard nor soft. It bums in oxygen to produce only carbon dioxide. It is a much smaller molecule compared to diamond and graphite, which are giant molecules. The figure below shows the structure of all the above allotropes of carbon.

Some amorphous allotropic forms Of carbon are:

  1. Coke is a greyish-black hard solid obtained by destructive distillation of coal during manufacture of oil gas.
  2. Charcoal is of four types:
  3. Wood Charcoal obtained by strong heating of wood in a limited supply of air.
  4. Animal Charcoal obtained by heating of bones in the absence of air.
  5. Sugar charcoal is obtained by the action of sulphuric acid on cane sugar.
  6. Activated charcoal is prepared by heating charcoal at 1273K in a current of super heated steam. It is highly porous and is an excellent adsorbent.
  7. Carbon black (or lamp black) is the soot obtained when natural gas, kerosene, petroleum, etc., are burnt in a limited supply of air. It contains 98-99% carbon.

Apart from the above, two new forms of carbon recently discovered are as follows

Carbon Nanotubes:

  • A Carbon Nanotube is a tube-shaped material, made of carbon, having a diameter measuring on the nanometer scale.
  • The graphite layer appears somewhat like a rolled-up chicken wire with a continuous unbroken hexagonal mesh and carbon molecules at the apexes of the hexagons as shown in the adjoining figure.
  • They have a long, hollow structure with the walls formed by one-atom-thick sheets of carbon, called graphene.
  • These cylindrical carbon molecules have unusual properties, which are valuable for nanotechnology, electronics, optics and other fields of materials science and technology.
  • In particular, owing to their extraordinary thermal conductivity and mechanical and electrical properties, carbon nanotubes find applications as additives to various structural materials.

Graphene:

  • Graphene is a substance made of pure carbon, with atoms arranged in a regular hexagonal pattern similar to graphite, but in a one-atom thick sheet as shown in the adjoining figure.
  • Graphene is the basic structural element of some carbon allotropes including graphite, charcoal, carbon nanotubes and fullerenes. The Nobel Prize in Physics for 2010 was awarded to Andre Geim and Konstantin Novoselov at University of Manchester for “groundbreaking experiments regarding the two-dimensional material graphene”.
  • Since it is practically transparent and a good conductor, graphene is suitable for producing transparent touch screens, light panels, and maybe even solar cells. When mixed into plastics, graphene can turn them into conductors of electricity while making them more heat resistant and mechanically robust.
  • This resilience can be utilised in new super strong materials, which are also thin, elastic and lightweight. In the future, satellites, airplanes, and cars could be manufactured out of the new composite materials.

Organic Compounds

  • Compounds of carbon and hydrogen (hydrocarbons) and their derivatives (containing oxygen or other elements) are known as organic compounds.
  • Examples: Methane (CH4); Ethane (C2H6), Ethene (C2H4), Acetylene (C2H2), Ethyl alcohol (C2H5OH), Acetaldehyde (CH3CHO), Acetic acid (CH3COOH), Chloroform (CHCI3), and Urea [CO(NH2)2]. Organic compounds are covalent compounds having low melting and boiling points. Most of them do not conduct electricity.

Organic compounds occur in all living things like plants and animals. The oxides of carbon, carbonates, hydrogen carbonates, and carbides are inorganic compounds. More than 5 million carbon compounds are known at present. The reasons for the existence of a large number of organic compounds are:

  • Catenation (Self Linking): Carbon atoms can link with one another by means of covalent bonds to form long chains or rings of carbon atoms, so that a large number of organic compounds are formed. This property of self-linking is called
  • Tetravalency: The valency of carbon is 4 (tetravalency). Due to this large valency, a carbon atom can form covalent bonds with a number of carbon atoms as well as other atoms like hydrogen, oxygen, nitrogen, sulphur, chlorine, etc., to form a large number of organic compounds.

TYPES OF ORGANIC COMPOUNDS | CARBON AND ITS COMPOUNDS

Hydrocarbons

  • A compound made up of only carbon and hydrogen is called a hydrocarbon.
  • Example: Methane, ethane, ethane (ethylene), and ethyne (acetylene).
  • Petroleum, which is obtained from underground oil deposits by drilling oil wells, is an important natural source of hydrocarbons.
  • Petroleum in oil fields is covered with natural gas, which also contains hydrocarbons.

Types of hydrocarbons

  • Saturated Hydrocarbons (Alkanes)
  • An alkane is a hydrocarbon in which the carbon atoms are connected only by single bonds.
  • The names of alkanes end with ‘ane’ and their general formula is CnH2n+2 where n is the number of carbon atoms in one molecule of the alkane.
  • Examples: Methane (CH4), ethane (C2H6), propane (C3H8), butane (C4H10), and pentane (C5H12).

 Unsaturated Hydrocarbons (Alkenes and Alkynes)

  • A hydrocarbon in which the two carbon atoms are connected by a double bond or a triple bond is called an unsaturated hydrocarbon.
  • The unsaturated hydrocarbons are obtained mostly from petroleum by a process called cracking.
  • Alkenes contain a double bond between two carbon atoms which is formed by the sharing of two electron pairs (i.e., four electrons).
  • The names of alkenes end with ‘ene’ and their general formula is CnH2n. Examples: Ethene (C2H4), propene (C2H6), and butene (C4H8). Ethene is used for ripening many raw fruits. Polymerisation of ethene gives polythene.
  • Alkynes contain a triple bond between two carbon atoms which is formed by the sharing of three electron pairs (or six electrons).
  • The names of alkynes end with ‘yne’ and their general formula is CnH2n-2 . Examples: Ethyne (C2H2), propyne (C3H4), and butyne (C4H6). Ethyne (Acetylene) forms a polymer called polyacetylene.

Cyclic Hydrocarbons

  • The hydrocarbons in which the carbon atoms are arranged in the form of a ring are called cyclic hydrocarbons.
  • They may be saturated or unsaturated. Saturated cyclic hydrocarbons are called cycloalkanes. The general formula of cycloalkanes is CnH2n, which is the same as that of alkenes.
  • Examples: Cyclopropane (C3H6), cyclobutane (C4H8), cyclopentane (C6H10), and cyclohexane (C6H12). An important example of an unsaturated cyclic hydrocarbon is benzene (C6H6). It contains 3 carbon-carbon double bonds and 3 carbon-carbon single bonds. Compounds containing benzene rings are called aromatic compounds.
  • Isomers The systematic names of hydrocarbons were given by International Union of Pure and Applied Chemistry (IUPAC) in 1958, so they are called IUPAC names.
  • The organic compounds having the same molecular formula but different structures are known as isomers.
  • For example, both n-butane and iso­-butane have the same molecular formula (C4H10) but they have different structures.
  • LPG cylinders (cooking gas cylinders) contain a mixture of n-butane and iso-butane, along with small amounts of propane and ethane. Isomerism is possible only with hydrocarbons having 4 or more carbon atoms. Methane, ethane, and propane do not have isomers. Butane, Pentane, and Hexane have 2, 3, and 5 isomers, respectively.

COAL AND PETROLEUM | CARBON AND ITS COMPOUNDS

  • Most of the fuels are obtained from coal, petroleum, and natural gas. Energy is released mainly in the form of heat (and some light) when a fuel is burnt.
  • This energy can be used to cook food, run generators in thermal power stations, machines in factories, and engines of vehicles.
  • Fuels such as coal, coke, and charcoal contain free carbon whereas fuels such as petrol, LPG, kerosene, and natural gas are all carbon compounds.
  • When carbon burns in oxygen (of air), it forms carbon dioxide and releases a large amount of heat.
  • Coal, petroleum, and natural gas are known as fossil fuels because they were formed by the decomposition of the remains of plants and animals, which got buried under the surface or the earth millions of years ago, under high temperature and pressure.

Coal is a complex mixture of compounds of carbon, hydrogen and oxygen, and some free carbon. Small amounts of nitrogen and sulphur compounds are also present in coal. Petroleum (or rock oil) is a dark coloured, viscous, foul smelling crude oil.

  • It is a complex mixture of hydrocarbons (some nitrogen and sulphur containing compounds are also present). Petrol, diesel, LPG, and kerosene are obtained from petroleum.
  • Due to the presence of nitrogen and sulphur compounds in them, combustion of coal and petroleum fuels leads to the formation of oxides of nitrogen and sulphur, which are major air pollutants.
  • When the supply of oxygen is sufficient, the fuel bums completely, producing a blue flame (non-luminous flame). In a gas stove, cooking gas (LPG) burns with a blue flame because the stove has holes (inlets) for air, which allows complete combustion of cooking gas.
  • When the supply of oxygen is insufficient, then the fuel burns incompletely, producing a yellow flame (luminous flame).
  • The yellow colour of the flame is due to the glow of hot, unburnt carbon particles produced by incomplete combustion of the fuel. Since incomplete combustion of wax takes place in a candle, it bums with a yellow flame.
  • Fuels which do not vaporise on heating, bum without producing a flame. Thus, coal and charcoal bum without producing a flame.
  • They just glow red and give out heat. The largest supply of fossil fuels is in the form of coal. Most of the coal is burned to make electricity. Coal can be converted into a relatively clean-burning fuel by a process known as gasification.

Refining of Petroleum

  • Crude petroleum has to be refined before being put to commercial use. Two important operations are involved in refining of petroleum: Fractional distillation and Cracking.
  • Fractional distillation leads to the separation of crude petroleum into a number of fractions, each passing over .a definite temperature range. Each fraction is a mixture of different hydrocarbons which can be used for a definite purpose as stated in the table below.
  Fraction Uses
1 Gaseous hydrocarbons Industrial and domestic fuel
2 Petroleum ether Solvent in perfumery, for dry cleaning of clothes
3 Gasoline Fuel
4 Kerosene Fuel in lamps, burners, etc.
5 Diesel oil or Gas oil Fuel for diesel engines, for industrial heating
6 Lubricating oils and Greases Lubricants
7 Paraffin wax Candles, polishes, waxed paper, ointments and cosmetics
8 Asphalt and coke Roofing,    road    building,    cables,   battery

‘boxes and electrodes

Cracking: 

  • Cracking is a process by means of which higher hydrocarbons are degraded to give smaller hydrocarbons.
  • High-boiling fractions can be converted into gasoline by cracking. The quality of petrol used in car engines is denoted by their anti-knock properties.
  • The anti-knock property of gasoline (petrol) is expressed in terms of the Octane number and that of diesel in terms of the Cetane Number.
  • The higher the Octane or Cetane Number, the better is the fuel. The Octane Number can be increased by adding tetraethyl lead (TEL) to gasoline. Gasoline treated in this way is called ethyl gasoline or leaded gaso­line.

Compressed Natural Gas (CNG)

  • Compressed natural gas (CNG) is a fossil fuel substitute for gasoline (petrol), Diesel fuel, or propane/LPG.
  • Although its combustion does produce greenhouse gases, it is a more environmentally clean alternative to those fuels, and it is much safer than other fuels in the event of a spill (natural gas is lighter than air, and disperses quickly when released).
  • CNG may also be mixed with biogas, produced from landfills or wastewater, which doesn’t increase the concentration of carbon in the atmosphere.
  • CNG is made by compressing natural gas (which is mainly composed of methane [CH4]), to less than 1% of the volume it occupies at standard atmospheric pressure.

Biofuels

  • A biofuel is a type of fuel whose energy is derived from biological carbon fixation.
  • Biofuels include fuels derived from biomass conversion, as well as solid biomass, liquid fuels and various biogases.
  • Biofuels are gaining increased public and scientific attention, driven by factors such as oil price hikes and the need for increased energy security.

Bioethanol

  • Bioethanol is an alcohol made by fermentation, mostly from carbohydrates produced in sugar or starch crops such as corn or sugarcane.
  • Cellulosic biomass, derived from non-food sources, such as trees and grasses, is also being developed as a feedstock for ethanol production.
  • Ethanol can be used as a fuel for vehicles in its pure form, but it is usually used as a gasoline additive to increase octane and improve vehicle emissions.

Biodiesel

  • Biodiesel is made from vegetable oils and animal fats. Biodiesel can be used as a fuel for vehicles in its pure form, but it is usually used as a diesel additive to reduce levels of particulates, carbon monoxide, and hydrocarbons from diesel-powered vehicles.
  • Biodiesel is produced from oils or fats using transesterification, i.e., by chemically reacting lipids (e.g., vegetable oil, animal fat (tallow)) with an alcohol producing fatty acid esters.

Petrochemicals

  • Petroleum and natural gas are excellent sources for the manufacture of a large number of compounds called petrochemicals.
  • The important petrochemicals, which serve as building blocks for products like plastics, synthetic fibres, rubber, detergents, pesticides, dyes, drugs, etc., are obtained directly or indirectly from petroleum.

SOAPS AND DETERGENTS | CARBON AND ITS COMPOUNDS

Soap

  • A soap is the sodium or potassium salt of a long chain carboxylic acid (fatty acid) which has cleansing properties in water.
  • A soap has a large non-ionic hydrocarbon group and an ionic group, COONa+. Examples: Sodium stearate (C17H35COONa) and sodium palmitate (C15H31COONa).
  • A soap is the salt of a strong base and a weak acid, so a solution of soap in water is basic in nature. Soaps are biodegradable. Sodium soaps are hard in consistency and are called hard soaps.
  • Potassium soaps are soft in consistency and are called soft soaps. Shampoos and shaving creams contain potassium soaps.
  • Soap is manufactured by the hydrolysis of oils and -fats with sodium or potassium hydroxide.
  • Animal fats or vegetable oils like castor oil, cotton seed oil, soyabeen oil, linseed oil, coconut oil, palm oil and olive oil are used for making soaps. 
  • Sometimes, common salt (sodium chloride) is added to precipitate out all the soap from the solution.
  • This is known as ‘salting out’. On adding common salt, the solubility of soap in water decreases due to which it separates out easily.

Cleansing Action of Soap

  • Soaps are molecules in which the two ends have differing properties, one is hydrophilic, that is, it dissolves in water, while the other end is hydrophobic, that is, it dissolves in hydrocarbons.
  • When soap is at the surface of water, the hydrophobic ‘tail’ of soap will not be soluble in water and the soap will align along the surface of water with the ionic end in water and the hydrocarbon ‘tail’ protruding out of water.
  • Inside water, these molecules have a unique orientation that keeps the hydrocarbon portion out of the water. This is achieved by forming clusters of molecules in which the hydrophobic tails are in the interior of the cluster and the ionic ends are on the surface of the cluster.
  • This formation is called a micelle. Soap in the form of a micelle is able to clean, since the oily dirt will be collected in the centre of the micelle, due to the presence of hydrophobic tails.
  • The micelles stay in solution as a colloid and will not come together to precipitate because of ion-ion repulsion.
  • Thus, the dirt suspended in the micelles is also easily rinsed away. The soap micelles are large enough to scatter light. Hence a soap solution appears cloudy.
  • Soap is not suitable for washing clothes with hard water. The calcium and magnesium ions present in hard water form insoluble calcium and magnesium salts of fatty acids with soaps.
  • This insoluble precipitate, known as scum, makes cleaning of clothes difficult and also harms the fabric.

Detergents

  • A detergent (also called synthetic detergent) is the sodium salt of a long chain benzene sulphonic acid or the sodium salt of a long chain alkyl hydrogen sulphate, which has cleansing properties in water. Detergents are also called soap-less soap.
  • They are better cleansing agents than soaps because they do not form insoluble calcium and magnesium salts with hard water.
  • They can, therefore, be used for washing even with hard water.
  • The cleansing action of a detergent is similar to that of soap.
  • Detergents are usually used to make washing powders and shampoos. Some of the detergents (which have branched chains) are not biodegradable. They are called hard detergents.
  • Biode­gradable detergents are called soft detergents.
  • Non-biodegradable detergents cannot be decomposed by micro-organisms like bacteria in sewage discharge. Therefore, they cause water-pollution. Detergents have a stronger cleansing action than soaps. Deter­gents are also more soluble than soaps.

Synthetic detergents are of three types:

  • Sodium alkyl sulphates and sodium alkylbenzene sulphonates are called anionic detergents. Alkyl benzene sulphonates with straight chain alkyl groups are called LAS detergents (Linear Alkyl Su!phonates) while those having branched chains are called ABS detergents (Alkyl Benzene Sulphonates).
  • Quaternary ammonium salts containing one or more long chain alkyl groups are called cationic detergents (or invert soaps). They are extensively used as germicides. Example: cetyltrimethylammonium bromide (used in hair conditioners).
  • Non-ionic detergents are obtained from long chain alcohols by treatment with ethyl­ene oxide. Some dish washing detergents are of non-ionic type.

 

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