CHAPTER NO 10

                                    ALLOTROPIC FORMS OF CARBON                                 

ALLOTROPIC FORMS OF CARBON
Carbon exists in two allotropic forms:    (1) CRYSTALLINE FORM   (2) AMORPHOUS FORM
CRYSTALLINE FORMS
There are three crystalline forms of carbon:    (1) Diamond   (2) Graphite   (3) Bucky ball
AMORPHOUS FORMS
There are various amorphous forms of carbon such as    coal   coke   charcoal   lampblack   gas carbon etc.
DIAMOND
In diamond, each C-atom is covalently bonded to four other C-atom to give a tetrahedral unit. In    diamond each C-atom is sp3-hybridized.Therefore each C-atom forms four sigma bonds with    neighbouring C-atoms.    In diamond C-C-C bond angle is 109.5O.These basic tetrahedral units unite with one another and produce    a cubic unit cell.    C-C bond length in diamond is 1.54AO.    C-C bond energy is 347 kj/mole.    In diamond crystal, basic units joined to forms octahedral shape of diamond crystal.
PROPERTIES OF DIAMOND
In diamond each C-atom utilizes its four unpaired electrons in bond formation. These bonding electrons    are localized. Due to this reason diamond is a bad conductor of electricity.    Diamond is the hardest   substance ever known.    Pure diamond is cloudless.    Its melting point is 3500OC.    Pure diamond  is transparent to x-rays.    It has high refractive index i.e. 2.45.    Due to impurities it may be colored.    Its density is 3.5 gm/cm3.
GRAPHITE
In graphite each C-atom is covalently bonded to three C-atom to give trigonal geometry. Bond angle in    graphite is 120O. Each C-atom in graphite is sp2-hybridized. Three out of four valance electrons of each    C-atom are used in bond formation while the forth electrons free to move in the structure of graphite.    Basic trigonal units unite together to give basic hexagonal ring. In hexagonal ring C-C bond length is    1.42AO. In graphite these rings forms flat layers. These layers are arranged is parallel ,one above the    other. These layers 3.35AO apart and are held together by weak van der waals forces only.These layers    can slide over one another.Thus it is very soft. Forth electron of each c-atom forms delocalized    p-system.
PROPERIES OF GRAPHITE
It is dark gray, have dull metallic luster.    It is soft and greasy.    It is used as lubricant.    Its density is 2.2 gm/cm3.    It is used in the preparation of electrodes as it canduct electricity.    It is used as "pencil lead".    It is used as moderator in nuclear reactors.                                                               GLASS                                                        GLASS    Glass is an inorganic super cooled liquid. It has no definite melting point. It has very high viscosity which    prevents its crystallization. Generally glass are the silicates of sodium and calcium. FORMATION OF GLASS    Glasses are formed by allowing molten silicates to cool. TYPES OF GLASS ORDINARY WINDOW GLASS (SODA GLASS)    Preparation:    It is prepared by heating a mixture of silica (SiO2), Na2CO3 and CaCO3 in a furnace. Na2CO3 + SiO2 è Na2SiO3 + CO2 CaCO3 + SiO2 è CaSiO3 + CO2    Properties:    It is not a very expensive kind of glass.    It is easy to work .    It breaks and get scratched easily.       Uses :    Bottles, windows,flat glass sheet. BOROSILICATE OR PYREX GLASS    Preparation:    It is prepared from silica, Boron oxide (B2O3),Aluminum oxide (Al2O3) and sodium oxide (Na2O).    Properties:    It has high softening temperature. It bears high temperature.    Uses :    Laboratory wares such as beakers, test tube, flask etc. CRYSTAL GLASS    Preparation:    It is prepared by silica, lead oxide (PbO) potassium carbonate (K2CO3) and CaCO3.    Properties:    It is heavy glass. It is very expensive.    Uses :    In decorative glass show piece. COLOURED GLASS    Preparation:       It is prepared by mixing different chemical compound in molten glass. These compound give a particular    colour to glass.    e.g:    red colour :Cu2O and AuCl3    Blue : CuO    Green :Cr2O3 WATER GLASS    Na2SiO3 is commonly known as water glass.    Preparation:    It is prepared by heating sodium carbonate with silica. Na2CO3 + SiO2 è Na2SiO3 + CO2    Uses :    It is used in the manufacture of fire proofing material. SILICA GEL    When an acid is added to a solution of water glass, it turns into a jelly like substance known as Gel    (SiO2.H2O) on heating the Gel dehydrates and forms a hard porous material which is known silica Gel.    It is a good moisture absorbent and is used in medicine packing to absorb moisture.

AMMONIA GAS - CEMENT

Laboratory preparation
In Laboratory Ammonia gas is prepared by heating a mixture of Ammonium chloride and calcium Hydroxide.
2NH4Cl + Ca(OH)2 è 2NH3 + CaCl2 + 2H2O
Industrial Preparation
On Industrial scale NH3 gas is prepared by HABER-BOSCH METHOD.
Details of Haber-Bosch Process
Raw materials
1. Nitrogen gas. 2. Hydrogen gas.
Composition of Raw materials
Nitrogen one part of volume. Hydrogen three part by volume
Suitable Conditions for maximum yeild of NH3
Formation of Ammonia in a reversible and exothermic process therefore following conditions are necessary to produce maximum amount of NH3. Temperature: The reaction is carried out at low temperature. Optimum temperature: 400 OC to 450 OC. Pressure : The reaction is carried out at high pressure. Optimum pressure: 400 atmosphere to 1000 atmosphere. Catalyst: To increase the rate of reaction a catalyst of iron oxide and aluminum sulphate with molybdenum (Mo2) is used.
The Process
One part of N2 and three parts of H2-gas by volume are heated upto 400 OC to 450 OC in the presence of catalyst to produce Ammonia gas.
N2+3H2 è 2NH3
CEMENT
Raw material
1. Lime stone (CaCO3) 2. Clay (Al2Si2O7) 3.Gypsum (CaSO4.2H2O)
Manufacturing of cement
First of all lime and clay are pulverized and then mixed in proper proportion. Now this mixture is fed into a revolving Kiln. The mixture in Kiln is heated to 1500 OC. Chemical changes: Limestone decomposes to CaO.
CaCO3 è CaO + CO2
7CaO + Al2Si2O7 è 2Ca2SiO4 (Cement)+ Ca3Al2O6
The cement mixture finally forms into small size clinkers. After cooling, clinkers are ground into fine powder along with gypsum to produce cement.
Setting of cement
Gypsum (CaSO4.2H2O) is added to cement to increase the setting time of cement. When water is mixed, cement sets. The setting of cement is a complex chemical change. It involves bonding forces between calcium ions (Ca+2)and silicates ions (SiO4)-4and water molecules.  Green house A chamber or house made to trap infra red radiation is called green house. Glass of green house allows the thermal radiation to pass through it and in this way the enclosed space is heated. Moreover glass is comparatively opaque to infra red radiation so, the heat is trapped in the green house, green house are used in the field of agriculture. A green house provides a stable and warm environment for plants for their proper growth. Green house effect Gradual rise in average global temperature due to absorption of infrared radiation is called green house effect. Global rise in temperature is due to increase in the concentration of CO2 in the atmosphere. Some gases including CO2 allows short wavelength of energy to pass towards the earth and heat it. However, the hot earth emits only longer wavelengths (infrared) of energy back into space. These gases absorb infrared radiation before it escape. As a result certain amount of heat is retained in our atmosphere. This process which keep the earth warm is called green house effect. (1) Rising sea level: due to increase in a temperature, ice will melt on North & South poles resulting in a rise in sea level. (2) Climatic changes: droughts, floods, cyclones, hurricanes, fire in forest, temperature rise in summer & winter. (3) Ecological changes: rising oceans & climatic changes will change the natural environment and habitat of animals and plants. The effects of these are hard to predict but very serious. In the coming years the world will suffer a great deal due to increasing environmental pollution.