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Carbon and its oxides Concept

(i) Occurrence of carbon

(ii) Position of carbon in the periodic table

Introduction The name carbon is derived from the Latin word ‘carbo’ meaning charcoal. Antoine Lavoisier named the element carbon. Carbon is the 15th most abundant element in the Earth’s crust and the 4th most abundant element in the universe by mass after hydrogen, helium and oxygen. It is present in all known life forms. Carbon is the second most abundant element by mass ( about 18.5%) after oxygen in the human body. This abundance, together with the unique diversity of organic compounds and their unusual polymer – forming ability at the temperatures commonly encountered on Earth, make this element the chemical basis of all known life. Occurrence of carbon Carbon has been known to humans in its various forms since ancient times. Although carbon makes up only 0.032 % of the Earth’s crust, it is very widely distributed and forms a vast number of compounds. Carbon is present in nature in free state as well as in combined form. In free state, carbon is found in the form of coal, charcoal, graphite, diamond and soot. In combined state, it is found in the form of compounds like carbon monoxide, dioxide, carbonates and hydrocarbons. In combination with other elements, it makes up the living tissues and organisms of all living plants and animals. The branch of chemistry which deals with compounds made by and derived from living organisms is given a name ‘organic chemistry’. There are three isotopes of carbon viz. 12C, 13C and 14C.The 12C and 13C isotopes are stable while 14C is radioactive decaying with half life of about 5730 years. The isotope 12C accounts for almost 99 % of naturally occurring carbon while 13C accounts for most of the rest. The isotope 14C occurs only in traces.

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Interesting facts about carbon 1) About 20 % of the weight of living organisms is carbon. 2) More compounds are known which contain carbon than other elements. 3) Diamond is an excellent abrasive because it is the hardest common material and it also has the highest thermal conductivity. It can grind any substance while the heat generated by friction is swiftly conducted away. 4) Car tyres are black because they are about 30 % carbon black which is added to rubber to strengthen it. The carbon black also helps to protect against UV damage to the tyres. 5) Carbon is made(as a product of fusion process) within stars when they burn helium in nuclear fusion reactions. Carbon is part of the ‘ash’ formed by helium burning. 6) Carbon burns in heavy stars to make neon, magnesium and oxygen. Position of carbon in the periodic table Carbon is a chemical element with symbol C, atomic number 6 and mass number 12. It is a member of group 14 ( or group IV A ) of the periodic table and placed in its second row. It is a non-metal and it is tetravalent i.e. it offers four electrons for bonding. Its tetravalency justifies its position i.e. group 14 or group IV A, in the periodic table. Carbon forms four single bonds with other monovalent atoms. Activity 1 – List five items made of carbon which you use in day to day life. Classify whether carbon is in free state or in combined state in them. Activity 2 – Carbon has four electrons in the outermost shell of its atom. Do you see any other reason why carbon should be placed in Group IV A of the periodic table? Test your understanding (i) How does carbon occur in nature? (ii) What is organic chemistry? (iii) Where will you place carbon in the modern periodic table? Why?

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Concept Allotropes of carbon – (i) Crystalline (ii) Amorphous

Introduction The term allotropes refers to the chemical bond structure between atoms of the same kind. This difference in the chemical bond structure may lead to different physical states or forms of the allotropes. So allotropes of the element can exist in any state i.e. gaseous, liquid or solid. The term allotropes is used only for elements and not for compounds. The chemical properties of allotropes are same but their physical properties are different. The phenomenon of existence of an element in different allotropic forms is called allotropy. The property by virtue of which an element exists in more than one form in the same physical state, having same chemical properties but different physical properties is called allotropy. The different forms are called allotropes.

The reasons for allotropy are as follows. 1) The methods of preparation of the element are different. 2) The arrangement of atoms in a given form changes according to its method of preparation. 3) The element has different energies in different forms. 4) The conditions like pressure, light and temperature are different at the time of preparation of the element which gives rise to different forms. Allotropes of carbon Allotropes of carbon  ------------------------------------------------------  Crystalline Amorphous   --------------------------------------------------------------------      Diamond Graphite Fullerene Coal Charcoal Lamp black   ------------------------------------     Coke Gas Wood Sugar Bone carbon charcoal charcoal charcoal

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Carbon exists in two allotropic forms : Crystalline and amorphous.

The allotrope in which there is regular arrangement of atoms and the material has a definite shape is called a crystalline allotrope.

The allotrope in which there is no regular arrangement of atoms and the material does not have a definite shape is called an amorphous allotrope.

There are three crystalline allotropic forms of carbon. They are (i) Diamond (ii) Graphite (iii) Fullerene. They are described in brief below. Diamond – Diamond is the purest form of carbon. Diamonds are formed from carbon under the earth under conditions of extremely high temperature and pressure. Diamonds are generally found in small sizes. Big diamonds are rare and they are given special names according to their features. The unit of weight of diamond is carat. One carat is equal to 200 mg. Structure of diamond Some diamonds have eight sides. They form double pyramids. Some others have six sides. They form cubes. The structure of diamond is shown below.

Fig.1 – Diamond In diamond, each carbon is joined to four other carbon atoms by single covalent bond. Out of the four valence electrons, each carbon atom shares one valence electron with the neighboring carbon atom to form a single bond. Carbon forms a giant molecule. This forms a three dimensional tetrahedral network of carbon atoms. These carbon – carbon bonds are the strongest covalent bonds to be found in any substance at ordinary temperature. Since there are no free electrons , diamond can not conduct electricity. So carbon is a bad conductor of electricity.

5 Properties of diamond (i) Diamond is transparent and extremely brilliant. When light falls on it, it produces a beautiful display of colours. It has refractive index 2.5. (ii) It is transparent to X-rays. (iii) It is brittle and it has density 3.52 g/cm3 . It is the hardest substance known. (iv) It is not soluble in any solvent. (v) When heated to about 8000C, it swells and forms a mass of charcoal which on further heating burns to form carbon dioxide without leaving any ash. If heated above 15000C, in absence of oxygen, it is converted to graphite. (vi) Though diamond is an electrical insulator, it is a good conductor of heat. Uses of diamond (i) Diamond is used as a highly precious jewel. (ii) Being very hard, it is extensively used for cutting and polishing glass and precious stones. (iii) It is used for making the tips of drills used in machinery for mines and oil-wells and also in grinding and engraving operations. (iv) It is used in making windows for space crafts because it checks harmful radiations. (v) Eye surgeons use fine-edged diamond for removing cataract. (vi) Diamond is used in needles in high quality record players. Activity 3 - Mention some properties which distinguish diamond from other crystalline allotropes of carbon. Graphite – Graphite is also known as plumbago or black lead. It was named ‘graphite’ in 1789 by a German Geologist, Abraham Werner. The word graphite is derived from Greek word meaning ‘to write’. Graphite occurs in nature to a small extent as a smooth, black solid that is greasy to touch. Most of the graphite that we use today is prepared synthetically by Acheson process. The process consists of heating a charge consisting of a mixture of coke, sand and iron (III) oxide ( catalyst ) in an electric furnace. The furnace is fitted with two carbon electrodes connected by a core of carbon rods. The temperature of the furnace is about 30000C. Coke and sand react initially to form silicon carbide. This decomposes to silicon which on vaporization leaves behind graphite. 3 C + SiO2  SiC + 2CO ; SiC  Si + C ( Graphite ) Structure of graphite

Fig.2 - Weak van-der-Waals’ forces between layers

6 In graphite, carbon atoms are arranged in flat planes of hexagonal rings, stacked one over the other. This is shown in the figure. Each carbon atom is attached to three other carbon atoms in the same plane. The final three dimensional structure of graphite is hexagonal. to give trigonal geometry. Bond angle (angle between adjacent equal bonds) in graphite is 120oC. In graphite, only three out of four electrons are engaged in carbon – carbon bonding. The remaining fourth electron moves relatively freely between the planes. This free electron gives rise to conductivity in graphite. Hence graphite is a good conductor of electricity. Within each layer, carbon atoms are joined to each other by strong single covalent bonds. The bonds between the adjacent layers or sheets of carbon atoms are weak . They are called van – der – Walls’ forces or bonds. Thus the layers are held together by weak forces. Due to this weak bonding, the planes can slide over each other easily. This makes graphite soft, slippery and a good lubricant. The relatively large distance between the two adjacent carbon layers makes the density of graphite low. Properties of graphite (i) Graphite is a soft, black crystalline substance having metallic luster. (ii) It is so soft that it is soapy to touch and leaves a black mark on the paper and hence the name black lead. (iii) Its specific gravity is about 2.25 so it is lighter than diamond. It is practically infusible. (iv) It is a very good conductor of heat and electricity. (v) At 7000C, it burns in pure oxygen forming carbon dioxide. When heated to about 25000C, it is transformed into diamond. (vi) It is the most stable allotrope of carbon. Uses of graphite (i) Graphite leaves a black mark on paper. So graphite, mixed with clay, is used as the ‘lead’ of the pencils. (ii) Graphite rods are used as electrodes during electrolysis. (iii) Graphite is non-volatile and slippery . So it is used as a lubricant for machines. (iv) Graphite is a good conductor of heat. So it is used in the manufacture of refractory crucibles. (v) Graphite slows down and absorbs the fast moving neutrons. So it is used as a moderator in nuclear reactors.

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Comparison of diamond and graphite Property 1) Nature 2) Physical state 3) Structure

4) Hardness 5) Specific gravity 6) Electrical conductivity 7) Thermal conductivity 8) Reaction with oxygen

Diamond Brittle solid Transparent solid with extraordinary brilliance Compact, three dimensional structure in which the atoms are held firmly by strong covalent bonds Hardest known substance 3.52 Bad conductor of electricity

Graphite Flaky solid Black opaque solid with metallic luster Layers of arranged sheets held by weak physical forces

Good conductor of heat

Good conductor of heat

Catches fire at 8000C

Catches fire at 7000C

Soft and slippery 2.25 Good conductor of electricity

Activity 4 – Mention some properties which distinguish graphite from other crystalline allotropes of carbon. Fullerene – A fullerene is any molecule made entirely of carbon, in the form of a hollow sphere, ellipsoid or tube. Spherical fullerenes are also called bucky balls. Cylindrical ones are called carbon nanotubes or bucky tubes. This form of fullerene, C60, was named Buckminister’s fullerene ( affectionately known as buckyball ) after the American architect Buckminister Fuller whose geodesic domes resemble the fullerene structure. Fullerene was discovered in 1985. Fullerenes occur only in small amounts naturally, but several techniques for producing them in greater volumes have been suggested. Minute quantities of the fullerenes, in the form of C60, C70, C76, and C84 molecules, are produced in nature, hidden in soot and formed by lightning discharges in the atmosphere. Structure of fullerene Fullerene or C60 is soccer – ball – shaped molecule consisting of 20 hexagonal and 12 pentagonal rings as the basis of an icosahedral symmetry closed cage structure. Each carbon atom is bonded to three others . The C60 molecule has two bond lengths - the 6 : 6 ring bonds can be considered "double bonds" and are shorter than the 6 : 5 bonds. Its average bond distance is 1.4 A0. C60 is not "superaromatic" as it tends to avoid double bonds in the pentagonal rings, resulting in poor electron delocalization. The geodesic and electronic bonding factors in the structure account for the stability of the molecule. In theory, an infinite number of fullerenes can exist, their structure based on pentagonal and hexagonal rings, constructed according to rules for making icosahedra. The structure of fullerene – C60 , is shown below.

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Fig.3 - Fullerene – C60 Properties of fullerene (i) Fullerene is a soft and slippery substance. (ii) It is brittle in nature. (iii) Fullerene is bad conductor of electricity. (iv) Its specific gravity 1.65. (v) It is insoluble in water but soluble in common solvents such as benzene, toluene or chloroform. (vi) Fullerene C60 sublimes at 800K. Uses of fullerene Fullerenes are being explored for its uses. The possible uses of fullerene can be listed as follows. (i) Super conductors and semi conductors (ii) Lubricants and catalysts (iii) Electric wires and to reinforce plastic (iv) In medicines and in fibre optics Activity 5 – Find out the number of hexagonal and pentagonal rings in C70 fullerene allotrope of carbon. There are three amorphous allotropic forms of carbon. They are (i) Coal (ii) Charcoal (iii) Lamp black . They are described in brief below. Coal – Coal is a combustible black rock material which consists mainly of carbon. It is the natural form of amorphous carbon formed from the remains of plants that died 1 million to 440 million years ago. The remains of the dead plants formed a thick layer on the swamp floor and gradually hardened into a substance called peat. The buried peat was subjected to tremendous pressure which over a period of millions of years, converted the peat into coal. There are two allotropic forms of coal viz. coke and gas carbon.

9 Coke – Coke is the solid carbonaceous material obtained by destructive distillation of coal (like bituminous coal) which has low ash and low sulphur content. The process by which a substance is decomposed by heat in absence of air is called destructive distillation. When coal is heated between 7000C and 13000C, in oven in absence of air, a mixture of liquids and gases escape from it. These are separated into coal gas, ammoniacal liquor ( the upper layer ), coal tar ( the lower layer ) and a solid residue called coke. Coke contains more than 80 % carbon. Properties of coke (i) Coke is a black porous substance which burns without smoke. (ii) Coke is a bad conductor of heat and electricity. (iii) It acts as a reducing agent. (iv) Coke contains practically no water. Uses of coke (i) Coke is used as a fuel and as a reducing agent in smelting iron ore in blast furnace. (ii) Since it does not produce smoke on burning, it is preferred as a domestic as well as industrial fuel . (iii) It is used for the manufacture of water gas, producer gas and artificial graphite. (iv) It is used to prepare compounds like calcium carbide CaC2 and carborundum SiC. Gas Carbon – When a coal like bituminous coal is subjected to destructive distillation in iron retorts, a hard grey mass is deposited on the inner walls of the retort. This hard mass is called gas carbon. Properties of gas carbon (i) Gas carbon is one of the purest form of carbon. (ii) It is a good conductor of electricity. Uses of gas carbon (i) It is used as rods in arc lamps.

(ii) It is used for making electrodes in batteries.

Charcoal – Charcoal is an impure form of carbon which is obtained from animal and vegetable substances by heating them in absence of air. On heating, the animal and vegetatable substances lose their water and other volatile substances. The resulting, soft, brittle, lightweight, black, porous material called charcoal resembles coal. There are three varieties of charcoal – (a) Wood charcoal (b) Sugar charcoal and (c) Bone charcoal. Their preparation, properties and uses are given in short here. (a) Wood charcoal – Wood charcoal can be prepared by destructive distillation of dry wood . Logs of wood are piled up in rows in the form of kiln. On heating, a major portion of the wood gets charred and thus gets converted in to wood charcoal. ***

10 Properties of wood charcoal (i) It is a black, soft, porous solid and contains about 70 to 75 % carbon. (ii) It has specific gravity 1.5 to 1.9 depending upon the method of preparation but it possesses a large surface area per unit weight and hence it floats on water. (iii) It burns in air forming carbon monoxide and carbon dioxide gases. (iv) It adsorbs gases, decolorizes coloured solutions and deodorizes obnoxious smells due to its highly porous nature. (v) It is a bad conductor of electricity. (vi) It has low ignition point. Uses of wood charcoal (i) It is used as a domestic fuel. (ii) It is used to prepare gas masks which are used in industries, military and by miners because wood charcoal adsorbs gases. (iii) It is used in medicines ( against indigestion ) as an intestinal disinfectant. (iv) Since it has low ignition point, it is used in gun powder. (v) It adsorbs coloring matter. Hence it is used in decolorizing sugar solutions and petroleum products. (b) Sugar charcoal - Sugar charcoal can be prepared by dehydrating cane sugar, either by treating it with concentrated sulphuric acid or by heating it in absence of air. Conc. H2SO4 C12H22O11 --------------- 12 C Cane sugar Sugar charcoal

+

11 H2O

The residue thus obtained is heated in chlorine. This removes combined hydrogen, if any. The residue is then washed and dried. Properties of sugar charcoal (i) It is the purest form of the amorphous variety of carbon. Uses of sugar charcoal (i) Sugar charcoal is used in the manufacture of artificial diamonds. (c) Bone charcoal – This is also called animal charcoal or bone char or ivory black. It is produced by heating bones of animals in the temperature range 4000C to 5000C in a limited supply of oxygen. Its quality depends upon the supply of oxygen during its production. The black variety shows under heating ( less supply of oxygen ) of bones while the white variety shows over heating ( excess supply of oxygen) of bones. The grey variety is the best one ( adequate supply of air ). Bone charcoal contains only 10 to 12 percent elemental carbon and the remainder is mainly calcium phosphate and a little calcium carbonate.

11 Properties of bone charcoal (i) Bone charcoal is a grayish-black porous material. (ii) It has large adsorption capacity. Uses of bone charcoal (i) Bone charcoal is used in the purification of organic liquids. (ii) It is used in decolourising brown sugar. (iii) It is used in paints ( ivory black ) used by artists. Lampblack - It is a material produced by the incomplete combustion of carbon rich material such as tar oil, kerosene , naphthalene or vegetable oil. Lamp black was traditionally produced by collecting soot from oil lamps. Newer methods of producing lamp black ( also called carbon black ) have superseded the traditional methods, although some materials are still produced using traditional methods. For artisanal purposes ( artist’s work ), it is very useful. Properties of lamp black (i) Lamp black is a soft brownish – or bluish – black material that is very stable and unaffected by light, acids and alkalies. (ii) It is one of the purest form of carbon. (iii) It has a high surface-area-to-volume ratio. Uses of lamp black (i) Carbon black helps conduct heat away from the tread and belt area of the tyre, reducing thermal damage and increasing life of tyre (ii) Carbon black particles are also employed in some radar absorbent materials and in photocopier and laser printer toner. (iii) Lamp black is used in the manufacture of printer’s ink, shoe polish, carbon paper, type writer ribbon and black paint and varnishes. (iv) Lamp black is used for colouring products made of rubber and leather. (v) Carbon black is used as a pigment and reinforcement in rubber and plastic products. Activity 6 – List out and compare the properties and uses of different amorphous allotropes of carbon. Activity 7 - Burn vegetable oil in an earthern vessel with the help of a cotton wick. Collect the soot formed and allow it to deposit on the outer surface of a metal bowl. Check whether this soot shows the properties of lamp black.

12 Test your understanding (i) Name the isotopes and allotropes of carbon. (ii) Why does an element show allotropy ? (iii) Give five differences between diamond and graphite. (iv) What is the difference between crystalline and amorphous forms of carbon ? (v) How does structure explain the different properties of diamond, graphite and fullerene ? (vi) How will you classify the amorphous allotropes of carbon ? (vii)Which is the (i) most pure and (ii) most impure form of amorphous form of carbon ?

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Concept Bonding in carbon Carbon is placed in the IV A group of the periodic table. It has four electrons in the outermost shell of its atom. So carbon shows valency 4. We know that the reactivity and tendency of an element to form ionic or covalent bonds depends upon the number of valency electrons. If carbon wants to form ionic bond, it needs to gain or lose four electrons to attain noble gas configuration. These two possibilities would mean : (i) It could gain four electrons forming C4- anion. But it would be difficult for the nucleus with six protons to hold on to ten electrons i.e. four extra electrons. Energetically this will be difficult. So normally C4- anion is not formed unless it is in combination with an extremely electropositive element like Na or Ca and thermodynamic factors are in favour. (ii) It could lose four electrons forming C4+ cation. But it would require a large amount of energy to remove four electrons leaving behind a carbon cation with six protons in its nucleus holding on to just two electrons. This large energy is usually not available and hence C4+ cation is not formed. This means carbon normally does not form ionic bond with other elements. Carbon overcomes the difficulty of attaining the octet by sharing its valence electrons with other atoms of carbon or with atoms of other elements. The shared electrons belong to the outer shells of both the atoms and lead to both atoms attaining the noble gas configuration. The shared electrons result in the formation of covalent bonds. Thus carbon normally forms covalent bonds with carbon atoms or other atoms. Carbon can form four single covalent bonds with four other carbon atoms or with four hydrogen atoms as in alkanes. Carbon can form double bonds with carbon atoms as in alkenes or with other atoms like O or N ( as in C = O, C = N ). Carbon can also form triple bonds with carbon atoms as in alkynes or with other atoms like N (as in C  N ). The bonding of carbon atoms in different allotropes is different. Especially the bonding of carbon atoms in crystalline allotropes of carbon is well defined. In diamond, each carbon atom is bound tetrahedrally to four other carbon atoms with single covalent bonds. This gives rise to a three dimensional carbon network. In graphite, each carbon atom is bound to three other carbon atoms in the same plane by single covalent bonds and the fourth electron is loosely bound to the adjacent layer of carbon atoms by van – der Waals’ bond. This gives rise to a sheet like structure to graphite. In fullerene, each carbon atom is joined to three other carbon atoms partly by single bonds and partly by double bonds. There are 20 hexagonal rings and 12 pentagonal rings in a C 60 fullerene molecule. This gives rise to a foot ball like structure. The bonding in carbon atoms in amorphous allotropes of carbon is not uniform. Activity 7 - Carbon has four electrons in the outermost shell of its atom. Relate this fact with its nature of bonding with other elements.

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Concept

Versatile nature of carbon – (i) Catenation

(ii) Formation of multiple bonds

(iii)Formation of chains, branches and rings

(iv) Polymerisation

Versatile nature of carbon – We have seen that carbon atom contains four electrons in its outermost shell and hence shows covalency four. Due to its tetravalent nature, carbon always forms covalent bonds by sharing electrons with one, two, three or four carbon atoms or atoms of other elements or groups of other atoms. This gives carbon an ability to form a large number of compounds. The estimated number of carbon compounds today has exceeded three millions. This outnumbers the number of compounds formed by all other elements put together by a large margin. The nature of covalent bond enables carbon to form a large number of compounds. (i) Catenation - Carbon has the unique ability to form single bonds with other carbon atoms, giving rise to large chains of molecules. This property is called catenation. The property of direct bonding between atoms of the same element to form chains is called catenation. For carbon, this is possible because the carbon to carbon single covalent bond is very strong. The reason for this is that carbon has small atomic size . The carbon nucleus holds the shared electrons tightly and makes the bond strong. The compounds of carbon may have long chains of carbon atoms , branched chains of carbon atoms or even carbon atoms arranged in rings. For example, pentane, hexane etc. are long chain compounds, iso- pentane, neo- pentane are branched chain compounds and benzene, naphthalene are ring compounds. They are all stable. No other element exhibits the property of catenation to this extent . Silicon forms compounds with hydrogen which have chains of silicon atoms up to seven or eight atoms but these compounds are very unstable and reactive. (ii) Formation of C – C multiple bonds : Due to its small size, carbon atom can also form multiple bonds i.e. double and triple bonds not only with carbon atoms but also with atoms of other elements like oxygen and nitrogen. The formation of these multiple bonds gives rise to a variety in the carbon compounds. Compounds of carbon which are linked by only single bonds between the carbon atoms are called saturated compounds. Compounds of carbon having double or triple bonds between carbon atoms are called unsaturated compounds. (iii) Formation of chains, branches and rings – Since carbon can hold many other carbon atoms, it can form compounds with long chains or branched chains or rings of

15 carbon atoms. The structures of n- pentane and n-hexane which are long chain compounds are shown below.

H H H H H      H–C–C–C–C–C–H      H H H H H

H H H H H H       H–C–C–C–C–C–C–H       H H H H H H

n – pentane ( C5H12 )

n – hexane ( C6H14 )

The structures of iso - pentane and neo-pentane which are branched chain compounds are shown below H H H H     H–C– C–C–C–H     H H3C H H iso – pentane ( C5H12 )

CH3

 H3C – C – CH3  CH3 neo – pentane ( C5H12 )

The structure of benzene and cyclobutadiene which are ring compounds are shown below.

Benzene ( C6H6 )

Cyclobutadiene ( C4H4 )

(iv) Polymerization – Carbon compounds can undergo polymerization i.e. they form polymers. Any process in which relatively small molecules, called monomers, combine chemically to produce a very large chainlike or network molecule, called a polymer, is called polymerization. Since carbon has got unique ability to hold long chains of carbon atoms, its compounds undergo polymerization. The unsaturated carbon compounds can undergo addition polymerization. For example, many ethylene molecules combine together and form poly ethylene polymer.

16 R R R R R R Catalyst       n R2C = CR2 ------------- - RC – CR – CR– CR – CR– CR – Ethylene Polyethylene

( R = H)

Similarly, three ethylene molecules combine to form a benzene molecule. 3 H2C = CH2  C6H6 When two monomers containing different functional groups (e.g. – OH and – H )combine to form long chain molecule ( polymer ), a small molecule like water is eliminated and the two carbon compounds get attached to each other. This process or reaction is called condensation polymerization. Polyesters, polyamides, proteins, polysaccharides are formed in this manner. Some carbon compounds have identical molecular formula but different structural formula. Such compounds are called structural isomers and the phenomenon is called structural isomerism. For example, n-pentane, iso-pentane and neo-pentane all have the same molecular formula C5H12 but they have different structural formulae. So they are structural isomers and they show the phenomenon of structural isomerism. This adds to the variety of carbon compounds. Activity 8 – State the observations which show the versatile nature of carbon. Test your understanding (i) What is the reason that carbon shows great catenation power ? (ii) What do you understand by ‘saturated’ and ‘unsaturated’ compounds of carbon ? (iii) Give one example each of a long chain, branched chain and ring compound of carbon. (iv) Explain the term polymerization with reference to carbon compounds. (v) Explain ‘isomerism’ of carbon compounds with suitable examples.

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Concept Oxides of carbon – (i) Carbon monoxide - Preparation, Properties, Tests, Uses (ii) Carbon dioxide - Preparation, Properties, Tests, Uses

Carbon monoxide – This is a simple diatomic molecule made of one carbon atom and one oxygen atom. It is also called carbonous oxide. The bonding between carbon and oxygen atoms is covalent. Carbon shares four electrons and oxygen shares six electrons for covalent bonding. Thus the molecule has ten electrons in it. It does not obey the octet rule. In terms of electrons, it is an unsaturated molecule. Carbon monoxide contains a triple bond ( C  O ) which consists of two covalent bonds as well as one dative covalent bond. Natural occurrence of carbon monoxide – It is generated by following sources. (i) Decay of swamp gas and other organic materials, in absence of oxygen, gives carbon monoxide. (ii) When substances containing carbon like coal, wood, oil or petrol are burned in limited supply of oxygen, carbon monoxide is formed. (iii) The atmosphere near volcanic regions contains traces of carbon monoxide. (iv) Automobile engines give out carbon monoxide during combustion of fuel. (v) Cigarette smoke contains traces of carbon monoxide. (vi) When carbon dioxide is reduced by coke or coal, carbon monoxide is formed. This carbon monoxide burns with a blue flame. Hence a blue flame is always seen over a coal fire. Preparation of carbon monoxide gas – Carbon monoxide gas can be prepared by heating formic acid or oxalic acid or their salt ( formate or oxalate ) with concentrated sulphuric acid which acts as a dehydrating agent. HCOOH + H2SO4  CO + ( H2O + H2SO4 ) Formic acid ( HCOO )2 + H2SO4  CO + CO2 + ( H2O + H2SO4 ) Oxalic acid In a laboratory method of preparation, crystals of hydrated oxalic acid are taken in a flask provided with a thistle funnel and a delivery tube. Concentrated sulphuric acid is added in to the flask through the thistle funnel. On heating, a mixture of carbon dioxide and carbon monoxide is formed. The mixture of gases coming out from the delivery tube is passed through a wash bottle containing sodium hydroxide solution. It absorbs carbon dioxide and the remaining carbon monoxide gas is collected over water by downward displacement of water. CO2 + 2 NaOH  Na2CO3 + H2O

18 Properties of carbon monoxide gas (i) Carbon monoxide is a colourless, odourless, tasteless and non-corrosive gas. (ii) It is only slightly soluble in water. (iii) It is a poisonous gas. The gas can combine with haemoglobin of blood and can form a stable compound carboxy haemoglobine. Formation of this compound prevents circulation of oxygen in the body tissues. This paralyses the respiratory organs and results into death of a human being. (iv) Carbon monoxide is neutral to litmus. (v) It can act as a reducing agent in metallurgy. Tests for carbon monoxide gas (i) Carbon monoxide burns in air with a pale blue flame. The carbon dioxide thus formed turns lime water milky. Carbon monoxide does not turn lime water milky. (ii) When carbon monoxide gas is passed over a filter paper soaked in a solution of PtCl2 or PdCl2 , the filter paper turns pink or black. The carbon monoxide gas reduces the metal chloride to metal and changes the colour of the filter paper. Uses of carbon monoxide gas (i) Carbon monoxide gas is used in the manufacture of methyl alcohol, synthetic petrol, sodium formate and phosgene gas. (ii) It is used as a reducing agent in metallurgy. (iii) It is used as a fuel especially in the form of water gas ( CO + H2 ) and producer gas ( CO + N2 ). (iv) It is used in the extraction of nickel metal in Mond’s process. (v) It is used in the Fischer – Tropsch process for the manufacture of hydrocarbons and their oxygen derivatives from a combination of hydrogen and carbon monoxide. Activity 9 – Prepare carbon monoxide gas in the laboratory under the guidance of your teacher and test any two properties to identify that the gas is carbon monoxide. Carbon dioxide – It is a chemical compound made up of two oxygen atoms covalently bonded to a single carbon atom. It has a formula O = C = O. It is a gas at room temperature. It occurs in free state as well as in combined state in nature. Natural occurrence of carbon dioxide – It is generated by following sources. (i) As a component of air, it occurs to the extent of about 0.033 % by volume. (ii) In nature, carbon dioxide is released in a number of ways. It is released through carbon cycle and through human activities like burning of fossil fuels or carbonaceous materials like wood, coke, coal and by respiration. It is also obtained in a fermentation process. (iii) Carbon dioxide is evolved during volcanic eruptions.

19 (iv) Carbon dioxide occurs in combined form as metal carbonates like CaCO3, MgCO3, Na2CO3, PbCO3, ZnCO3 etc. Preparation of carbon dioxide gas – Carbon dioxide gas is prepared when carbon and other materials containing carbon burn in air. C + O2  CO2 It can also be prepared by the reaction between any metallic carbonate and an acid like hydrochloric acid. The laboratory method of preparation of CO2 is as follows.

Pieces of marble are placed in a bottle or a flask fitted with a thistle funnel and a delivery tube. These pieces are covered by water. Concentrated hydrochloric acid is poured in the thistle funnel. It falls on the marble chips, reacts with them and produces carbon dioxide gas. Carbon dioxide is heavier than air. Hence it is collected by upward displacement of air. CaCO3 + 2 HCl  CaCl2 + H2O + CO2  Marble chips Carbon dioxide For the preparation of carbon dioxide, marble i.e. calcium carbonate is preferred to other metal carbonates because it is cheap and easily available. Properties of carbon dioxide gas (i) Carbon dioxide is called a green house gas. A green house gas traps infra red rays and does not allow them to escape. (ii) Carbon dioxide is a colourless gas which is slightly acidic in nature. It is 1.5 times heavier than air. (iii) It is fairly soluble in water. It reacts with water and forms a weak acid, carbonic acid, in aqueous medium. H2O + CO2 

H2CO3

20 (iv) Below −78°C it forms a solid, commonly known as dry ice and is found in this state in the Martian polar ice caps and in comets. (v) It is easily liquefied into a colourless liquid at ordinary temperature when compressed to about 70 atmosphere. (vi) Carbon dioxide is not poisonous but living beings suffocate in an atmosphere of carbon dioxide due to lack of oxygen. Tests for carbon dioxide gas (i) Carbon dioxide extinguishes a burning splinter. (ii) It turns moist blue litmus paper red. (iii) In a jar of carbon dioxide, a magnesium ribbon continues to burn giving out black particles of carbon. (iv) It turns moist universal indicator paper orange. (v) Carbon dioxide turns lime water milky due to formation of calcium carbonate. When excess of carbon dioxide is passed through the milky solution, the solution becomes colourless due to the formation of soluble calcium bicarbonate. Ca(OH)2 + CO2  CaCO3  + H2O Lime water Milky solution ( white ppt.) CaCO3 + H2O + CO2  Ca(HCO3 )2 Milky solution Excess Colourless solution Uses of carbon dioxide gas (i) (ii) (iii) (iv)

Carbon dioxide gas is used in the manufacture of aerated waters. It is used in fire extinguishers. It is used in the manufacture of sodium carbonate by Solvay process. Solid carbon dioxide, known as dry ice, is used as a coolant or a refrigerant in ships and in freight cars for preservation of food articles. (v) Carbon dioxide is used in the manufacture of fertilizers like urea. (vi) In hospitals, carbogen, which is a mixture of 5 % carbon dioxide and 95 % oxygen, is used for artificial respiration. It is given to patients suffering from carbon monoxide poisoning, drowning and pneumonia. Activity 10 – Prepare carbon dioxide gas in the laboratory under the guidance of your teacher and test any two properties to identify that the gas is carbon dioxide. Carbon dioxide cycle The proportion of carbon dioxide in the Earth’s atmosphere is maintained at about 0.033 % by volume. This becomes possible due to carbon dioxide cycle. Carbon dioxide is formed due to combustion, respiration, decay of vegetable and animal matter and from volcanic eruptions. Simultaneously, a reverse process of taking up carbon dioxide and liberating oxygen is also in operation by plants. In presence of sunlight, plants absorb carbon dioxide from air, keep carbon with them and release oxygen back to air. This

21 carbon is finally converted to carbohydrates which is the plant food. The reactions taking place are as follows. CO2 + H2O  O2 + HCHO ( Formaldehyde ) 6 HCHO  C6H12O6 ( Glucose ) n (C6H12O6 )  n H2O + (C6H10O5 )n ( Starch ) The process of converting carbon dioxide to carbohydrates in presence of sunlight is called photosynthesis. Living beings take up oxygen in respiration and give off carbon dioxide while plants in sunlight take up carbon dioxide and give back oxygen. This exchange of carbon dioxide and oxygen to maintain the percentage of carbon dioxide in atmosphere is called carbon dioxide cycle. Activity 11- Prepare a line diagram to show the carbon dioxide cycle in nature. Test your understanding (i) Give the sources of carbon monoxide and carbon dioxide in nature. (ii) How will you prepare (a) CO and (b) CO2 in the laboratory? (iii) How will you identify whether the given gas is CO or CO2 ? (iv) Why should we not stand in the vicinity of carbon monoxide for a long time ? (v) Give two uses each of CO and CO2.

22

Concept

Chemical properties of carbon compounds : (i) Combustion (ii) Oxidation

Combustion - Combustion means burning of a substance in oxygen or in air. It is a process that is highly exothermic i.e. it produces a lot of heat. The release of heat can result in the production of light in the form of either glowing or a flame. Such compounds may act as potential fuels. So many organic compounds ( especially hydrocarbons ) in the gas, liquid or solid phase act as fuels. The products of combustion of carbon compounds are carbon dioxide, heat energy and sometimes water vapour. CH4

+ 2 O2 

CO2 + 2 H2O + Heat and light

C2H5OH + 3 O2  2 CO2 + 3 H2O + Heat and light Since carbon combines with oxygen to form carbon dioxide, the combustion reactions are, in a way, oxidation reactions. Oxidation – Many carbon compounds are capable of adding oxygen to them. These reactions are called oxidation reactions. The substances which supply oxygen for these reactions are called oxidisng agents. For example, alkaline KMnO4, acidified K2Cr2O7, bromine water are some such oxidizing agents. The organic compounds like alcohols, aldehydes and ketones show oxidation reactions. Alkaline KMnO4 C3H7OH + 2 [O] ----------------------- C2H5COOH + H2O n - Propyl alcohol Propanoic acid

CH3CHO Acetalehyde

+ [O]

H3C – CO – CH3 + 4 [O] Acetone

Alkaline KMnO4 ------------------------ Acidified K2Cr2O7 ------------------------

CH3COOH Acetic acid CH3COOH + CO2 + H2O Acetic acid

Activity 12 - See whether following compounds will show combustion reaction and or oxidation reaction or both. (i) H2C = CH2 (ii) CH3OH Test your understanding (i) Why do carbon compounds show combustion and oxidation reactions ? (ii) State the difference between combustion and oxidation reactions of carbon compounds.

23 References / Diagrams / Figures 1) Fig. 1 – http://www.chemguide.co.uk/atoms/structures/giantcov.html 2) Fig.2 – http://en.wikipedia.org/wiki/Graphite 3) Fig.3 - http://en.wikipedia.org/wiki/file:C60a.png