CONDUCTOMETRIC TITRATIONS

Conductometric titration of strong acid with strong base

Consider the conductometric titration of a strong acid, HCl with a strong base, NaOH. Since both the acid HCl and the base NaOH are strong, so complete dissociation occurred. Here neutralization mean replacement of H⁺ ions by an equal number of Na⁺ ions.

H⁺ + Cl^- + [Na⁺ + OH^-] -----> Na⁺ + Cl^- + H₂O

H⁺ ion has much higher ionic conductance than that of Na⁺ ion. So, the conductance of the solution will decrease as more and more NaOH is added. When all the H⁺ ions replaced by the Na⁺ ions, conductance will reach the lowest limit. Any subsequent addition of NaOH means increase in the numbers of both Na⁺ ions and fast moving OH^- ions. These are no longer removed, the conductance therefore increases linearly with addition of NaOH. On plotting the conductance against the volume of alkali added, the point of intersection of two lines, that is the lowest point, corresponds to the minimum conductance, which is the exact neutralization point of this conductometric titration between HCl and NaOH.

Strong acid with strong base

Conductometric titration of weak acid with strong base

Consider the conductometric titration of a weak acid, CH₃COOH with a strong base NaOH. Since the acid is weak, the conductance of the acid will be low on account of its poor dissociation. As NaOH is added the OH^- ion removed the H⁺ ion by producing H₂O. Undissociated CH₃COOH molecules produce fresh H⁺ ions. The conductivity of the solution rises due to presence of some Na⁺ ions. At the starting stage, due to common ion effect, there may be a little fall in conductivity, but later on this fall will be insignificant compared to rise in the conductivity of the solution due to the added new Na⁺ ions.

CH₃COOH+[Na⁺ + OH^-] ----> CH₃COO^- + Na⁺ + H₂O

The total volume of CH₃COOH dissociates slowly in this way and is neutralized. When the acid is completely neutralized, further addition of NaOH will increase the conductivity of the solution, because OH^- ions have large ion-conductance. On plotting the conductance against the volume of alkali added, the point of intersection is obtained by extending the lines, which gives the end point.

Weak acid with strong base

Conductometric titration of mixture of strong and weak acids with strong base

Suppose of a mixture of strong acid HCl and weak acid CH₃COOH, is to be titrated with a strong base NaOH. Since HCl is much stronger acid than CH₃COOH, HCl will get titrated first. When HCl is completely neutralized by NaOH, then titration of CH₃COOH will start. On plotting the conductance against the volume of alkali added, the following type of curve is obtained.

Mixture of strong and weak acids with strong base

Conductometric titration of strong acid with weak base

Consider the conductometric titration of a strong acid, HCl by a weak base, NH₄OH. The conductance of the solution will decrease due to the replacement of fast-moving H⁺ ions by slow moving NH₄⁺ ions. When all the H⁺ ions replaced by the NH₄⁺ ions, conductance will reach the lowest limit and neutralization point comes.

H⁺ + Cl^- + NH₄OH -----> NH₄⁺ + Cl^- + H₂O

Due to weakly ionized nature of NH₄OH, further addition of NH₄OH will not cause any appreciable change in the conductance. On plotting the conductance against the volume of alkali added, the following type of curve is obtained.

Strong acid with weak base

Conductometric titration of weak acid with weak base

Consider the conductometric titration of a weak acid, CH₃COOH by a weak base NH₄OH. Since the acid is weak, the conductance of the acid will be low on account of its poor dissociation. As NH₄OH is added, the OH^- ions removed the H⁺ ions. Undissociated CH₃COOH molecules produced fresh H⁺ ions. Besides the solution has some NH₄⁺ ions.  At the starting stage, due to common ion effect there may be little fall in conductivity. But later on, the conductivity of the solution increases slowly due to the presence of slow moving NH₄⁺.

CH₃COOH + NH₄OH ----> CH₃COO^- + NH₄⁺ + H₂O

The total volume of CH₃COOH dissociates slowly in this way and neutralized. Due to weakly ionized nature of NH₄OH, further addition of NH₄OH will not cause any appreciable change in the conductance. On plotting the conductance against the volume of alkali added, the following type of curve is obtained.

Weak acid with weak base

 COLLISION THEORY

The first theory to explain the mechanism of chemical reaction was the collision theory. According to this theory the reactant molecules collide and due to this collision, some structural rearrangement occurs which is responsible for chemical reaction.

However, if each collision leads to reaction then number of molecules reacting per c.c. per second should be equal to frequency. But the number of molecules reacting per c.c. per second much less than the collision frequency. Further more while rate of a reaction increases by 100% to 200% for 10⁰C rise in temperature, collision frequency increases by only 2% to 3% for the same change in temperature. Hence all collision may not lead to reaction. It was at this time; Arrhenius introduced the concept of activation energy. According to which for a molecule to react, it must have energy equal to or greater than activation energy (E). These molecules are called active molecules. Collision between active molecules are fruitful and lead to reaction where as collision between passive molecules are fruitless.

Now, according to Boltzmann’s distribution law, if n₀ is the total number of molecules, out of which n_E molecules have energy greater or equal to E, then—

n_E = n₀ e^{- E/RT}

Hence, if Z is the total number of collisions per c.c. per second, out of which Z_E is the number of collisions between activated molecules, then---

Z_E = Z e^{- E/RT}

Hence number of molecules reacting per c.c. per second is proportional to Z_E. This is collision theory.

Test of this theory was made from kinetic study of the decomposition of HI.

2HI <========> H₂ + I₂

If, Z is the collision frequency, then since each collision involves two molecules, number of molecules reacting per c.c. per second is---

 – dn/dt = 2 Z e^{- E/RT}

Now from kinetic theory----

Z = (1/√2) Π σ² C_a n²

Where,

σ = Collision diameter

C_a = Average velocity

n = Number of molecules per c.c.

So,

 – dn/dt = 2 (1/√2) Π σ² C_a n² e^{- E/RT} ----- (1)

Now, we calculate the rate in another way.

For the above reaction rate----

 – dc/dt = K₂C²

Where,

K₂ = 2 nd order rate constant

C = Molar concentration

N = Avogadro number

But, C = (n x 10³)/N

Or, – dc/dt = - (10³/N) dn/dt

Hence, - (10³/N) dn/dt = K₂ [(n x 10³)/N]²

Or, - dn/dt = K₂ (10³/N) n² ----- (2)

Comparing (1) and (2), we get---

K₂ (10³/N) n² = √2 Π σ² C_a n² e^{- E/RT}

Or, K₂ = √2 Π (N/10³) σ² C_a e^{- E/RT}

Or, K₂ = A e^{- E/RT}

[Where, A = √2 Π (N/10³) σ² C_a and it is known as frequency factor.]

LIMITATIONS OF THE THEORY

Though collision theory explains many reactions very well, it fails to explain many other cases. Thus, in many cases, the observed rate constant is much different from A e^{- E/RT}. The cases where it is greater than A e^{- E/RT} are chain reaction. However, the cases where it is less than A e^{- E/RT} are explained on the basis of orientation of molecules during collisions. If the collisions are not perfect oriented than collisions between active molecules also may not lead to reaction. In these aspects the expression for rate constant K is now written as—

K = P A e^{- E/RT}

Where P is known as steric factor or probability factor, generally its value is less than 1 (excepting chain reaction).

Another defect of collision theory is that it does not consider the entropy change for the reaction. Actually, probability factor is related with entropy change, which can be shown below.

For reversible reaction, the forward and backward rate constant K₁ and K₂ can be written as---

K₁ = P₁ A₁ e^{- E}₁^/RT

K₂ = P₂ A₂ e^{- E}₂^/RT

Or, K₁/K₂ = [(P₁ A₁) / (P₂ A₂)] e^{- (E}₁^{- E}₂^)/RT

But, K₁/K₂ = Equilibrium constant (K)

So, K = [(P₁A₁) / (P₂A₂)] e^{- (E}₁^{- E}₂^)/RT ----- (3)

Again, from Vant Hoff’s isotherm----

ΔG = - RT ln K

Or, K = e^{- ΔG/RT}

Putting, ΔG = ΔH – TΔS

K = e^{- ΔH/RT} e^ΔS/R ----- (4)

Comparing (3) and (4), we get---

[(P₁A₁)/(P₂A₂)] = e^ΔS/R

Thus, probability factor is related with entropy change of the reaction. This factor has not been maintained in collision theory.

 EFFECT OF CATALYST ON SUBSTITUTION REACTION

Catalytic effect on S_N¹ reaction---

The rate of S_N¹ reaction increases by the addition of Lewis acids such as AlX₃, Ag⁺ ion and the Bronsted acids. The more acidic the nucleophilic solvent, the faster is the rate of S_N¹ reaction. Ag⁺ or Al³⁺ has a stronger affinity for X^- than has a solvent molecule. The formation of AgX or AlX₄^- accelerates the dissociation of X^-, thus increases the S_N¹rate. This is an example of electrophilic catalysis. The effectiveness of the H bonding, a factor that accelerates the dissociation of X^-, increases with the acidity of the H of H―A, the Bronsted acid.

Catalytic effect on S_N² reaction---

Crown ethers and phase transfer catalyst accelerate the rate of S_N² reaction.

Crown Ethers

The crown ethers contains at least four oxygen (O) atoms, they are the heterocyclic poly ether. The naming of crown ethers is done by the following way x-crown-y, where ‘x’ represents the total number of atoms in the ring, and ‘y’ represents the number of oxygen atoms (O). The structure of crown ethers are:

Crown Ethers

Crown ethers strongly complex with metallic cation in the cavity of the ring by forming ion-dipole bonds. 18-crown-6 strongly complexes and traps K⁺ as shown in the figure.

18-crown-6 traps potassium cation

There are a host-guest relationship between the crown ether and the ion that is transports. There host is crown ether, and guest is the coordinated cation. Similarly, 12-crown-4 forms complex with Li⁺ and 15-crown-5 forms complex with Na⁺ ion.

Ion pairing diminishes the reactivity of the anion which is intended to act as a nucleophile in substitution reaction. By complexing the cation, the crown ethers leave a ‘bare’ anion with a greatly enhanced reactivity. Since the nucleophile comes after the rate determining step in an S_N¹ reaction, enhancement of nucleophilic character of an anion does not influence the rate of an S_N¹ reaction, but it increases the rate of S_N² reaction. For example, the rate of the reaction---

CH₃―CH₂―Br + KF -------> CH₃―CH₂―F

Increases several times by the addition of 18-crown-6.

Phase Transfer Catalyst

A difficulty that occasionally arises when carrying out nucleophilic substitution reaction is that the reactants do not mix. For a reaction to take place the reacting molecule is usually insoluble in water and other polar solvents, while the nucleophile is often an anion which is soluble in water, but the nucleophile is insoluble in the substrate or other organic solvents. To overcome this problem is to use a solvent that will dissolve both species. A dipolar aprotic solvent may serve this purpose. Another way, which is used may often is phase transfer catalysts.

An example of phase transfer catalyst (Q⁺X^-) is usually a quaternary ammonium halide (R₄N⁺X^-), such as tetra butyl ammonium chloride (Bu₄N⁺Cl^-), benzyl triethyl ammonium chloride [Ph-CH₂-N⁺ (CH₂CH₃)₃Cl^-]. These are soluble in organic phase because of the four hydrocarbons substituents on nitrogen as well as these are soluble in aqueous phase due to the ionic in nature. The transfer of the nucleophile (for example CN^-) as an ion pair (Q⁺CN^-) into the organic phase, is causes by the phase transfer catalyst. The reaction between the nucleophile of the ion pair (CN^-) and the organic substrate RX, occur in the organic phase. The cation (Q⁺) then migrates back into the aqueous phase to complete the cycle. Until all of the nucleophile or the organic substrate has reacted, the process goes continues.

Phase transfer catalyst

An example of a nucleophilic substitution reaction carried out with phase transfer catalysis is the reaction of 1-chlorooctane (in decane) and sodium cyanide (in water). The reaction (at 105⁰C) is complete in less than 2 hours and gives a 95% yield of the substitution product.

Another example of phase transfer catalyst is crown ethers. They render many salt soluble in non-polar solvents. Salts such as KF, KCN and CH₃COOK for example can be transferred into aprotic solvents by using 18-crown-6. A nucleophilic substitution reaction carry out by the relatively unsolvated anions of these salts on an organic substrate in the organic phase.

Phosphazenes

 PHOSPHAZENES OR PHOSPHONITRILIC HALIDES

Nitrogen and phosphorus catenate together, forming an interesting series of polymers containing P-N single bonds and P=N double bonds. In these compounds, the phosphorus atom is in a +5 oxidation state, and the nitrogen atom is in a +3 oxidation state. The compounds are formally unsaturated.

PREPARATION OF PHOSPHAZENES

When PCl₅ is refluxed with NH₄Cl in presence of C₂H₄Cl₂ or C₆H₅Cl, or heated with solid NH₄Cl at 145⁰C–160⁰C, ring compounds of the type [NPCl₂]_n where (n = 3-8) and chain compounds of the type Cl₄P(NPCl₂)_nNPCl₃ are produced. The newly synthesized compounds are called phosphazenes or phosphonitrilic halides.

Preparation of phosphazenes

The bromo derivative is obtained by heating PBr₅ with NH₄Br and the fluoro derivative is obtained from the chloride on treatment with NaF in acetonitrile.

[NPCl₂]_n + 2n NaF -----> [NPF₂] + 2n NaCl

Alkoxy groups can be introduced in the polymer by replacing Cl.

[NPCl₂]_n + 2n NaOR -----> [NP(OR)₂]_n + 2n NaCl

The iodide derivatives are not yet known. The rings compounds where n=3 and n=4 are stable and can be prepared by under controlled conditions.

STRUCTURE OF PHOSPHAZENES

The structure involves sp² hybridised trivalent nitrogen (N) and roughly sp³ hybridised pentavalent phosphorus (P). Alternate single and double bonds are present between N and P. All the P―N bond distances (167 pm) become same through resonance. This distance is shorter than P―N single bond distance (178 pm). The multiple bonding can be explained by pπ―dπ bonding between filled pπ orbital of N and empty dπ orbital of P. Electron delocalisation similar to benzene and graphite occurs here also, this gives a pseudo aromatic character to the ring.

Cyclophosphazene

INORGANIC RUBBER

When excess PCl₅ is refluxed with NH₄Cl in S-tetra chloro ethane linear polymers are obtained. They are rubbery in nature, elastic, thermally stable, and insoluble in petroleum ether. A long chain polymer [NPCl₂]_x of molecular weight of 20,000 or more is obtained by heating [NPCl₂]₃ to 250⁰C-350⁰C. The end group of such high molecular weight polymers are not known. These rubbery substances are called inorganic rubber. These are water proof and fire proof.

Inorganic rubber

PROPERTIES OF PHOSPHAZENES

The phosphazenes range from fluids and definite solids to polymeric elastomers. The lower polymers are insoluble in water but soluble in organic solvents and resemble vulcanized rubber in properties and are often called inorganic rubber.

REACTIONS OF PHOSPHAZENES

The chlorine atoms in chlorophosphazenes are reactive and may be substituted by a variety of reagents.

[NPCl₂]₃ + 6CH₃MgI ------> [NP(CH₃)₂]₃ + 3MgCl₂ + 3MgI₂

[NPCl₂]₃ + 6C₆H₅Li ------> [NP(C₆H₅)₂]₃ + 6LiCl

[NPCl₂]₃ + 6NaOR ------> [NP(OR)₂]₃ + 6NaCl

[NPCl₂]₃ + 6NaSCN ------> [NP(SCN)₂]₃ + 6NaCl

[NPCl₂]₃ + 6H₂O ------> [NP(OH)₂]₃ + 6HCl

[NPCl₂]₃ + 6NH₃ ------> [NP(NH₂)₂]₃ + 6HCl

The lone pair of electrons present on each N atom in [NPCl₂]₃ molecule makes it basic and hence it forms adduct with Lewis acids like HClO₄, SO₃, AlCl₃ etc.

[NPCl₂]₃ + 2AlCl₃ ------> [NPCl₂]₃•2AlCl₃

[NPCl₂]₃ + 3SO₃ -------> [NPCl₂]₃•3SO₃

[NPCl₂]₃ + 2HClO₄ ------> [NPCl₂]₃•2HClO₄

USES OF PHOSPHAZENES

Phosphazenes or phosphonitrilic halides find uses in flame-proofing of fabrics, as plasticisers and as catalysts in the manufacture of silicones.

About Lithium

 LITHIUM

Symbol --- Li

Abundance --- 0.0017% in earth's crust.

Physical state --- solid

Elemental Category --- Alkali metal

Colour --- silver

Discovery --- Johan August Arfwedson (1817)

Atomic no --- 3

Atomic weight --- 6.93

Period --- 2

Group --- 1

Block --- s-block

Known Isotopes--- ₃Li³ ₃Li⁴ ₃Li⁵ ₃Li⁶ ₃Li⁷ ₃Li⁸ ₃Li⁹ ₃Li¹⁰ ₃Li¹¹ ₃Li¹²

Stable Isotopes --- ₃Li⁶ ₃Li⁷ 

Isotopic abundance --- [₃Li⁶ (7.59%), ₃Li⁷ (92.41%)]

Melting Point --- 453.65 K (180.5⁰C)

Boiling Point --- 1603 K (1330⁰C)

Density --- 0.534 g/cc

Electron Configuration --- [He] 2s¹

Oxidation State --- +1

Valence --- 1

Electronegativity --- 0.98

Electron Affinity --- 59.6 KJ/mol

Ionisation Energy --- 520.2 KJ/mol (1st), 7298.1 KJ/mol (2nd), 11815 KJ/mol (3rd)

Covalent Radius --- 128 pm

Van der Waals radius --- 182 pm

Crystal Structure --- B.C.C (Body Centered Cubic) 

Heat of fusion --- 3 KJ/mol

Heat of vaporisation --- 136 KJ/mol

Critical Point --- 3220 K, 67 MPa (661.239 atm)

Molar heat capacity --- 24.86 J/mol-K

Specific heat --- 3570 J/(Kg-K)

Thermal conductivity --- 84.8 W/(m k)

Molar volume --- 0.00001297

Speed of sound --- 6000 m/s

Magnetic type --- paramagnetic

Mass magnetic susceptibility ---   +2.56x10^-8 m³/kg

Molar magnetic susceptibility --- + 1.78 x 10^-10 m³/mol 

Lattice angles --- π/2, π/2, π/2

Lattice constants --- 351 pm,351 pm,351 pm

Quantum numbers --- ²S_1/2

Neutron cross section --- 71

Electrical conductivity --- 1.1 x 10⁷ S/m

Electrical resistivity --- 92.8 nΩ m

Mohs hardness --- 0.6

Brinell hardness --- 5 MPa

Young's modulus --- 4.9 GPa

Bulk modulus --- 11 GPa

Shear modulus --- 4.2 GPa

ABOUT LITHIUM

 Fullerenes

Fullerene is the allotropes of carbon and commonly refers to a molecule with 60 carbon atoms, C₆₀, and with an icosahedral symmetry. Now a days there are several such hollow closed-cage (polyhedral) cluster molecules, C_n (n only even in the range 30-600), and all of them have structures based on polyhedra formed by fusing pentagons and hexagons. Larger molecular weight fullerenes are C₇₀, C₇₆, C₇₈, C₈₀ which possess different geometric structure, such as C₇₀ has a rugby ball-shaped symmetry. The structure of fullerenes very much comparable to geodesic domes used in architecture built by R. Buckminster Fuller. The most widely used fullerene C₆₀, was called buckminster fullerene or buckyball.

Preparation of Fullerenes

Synthesis of fullerenes is done by arc discharge between graphite electrodes in approximately 200 Torr of He gas. The formation of fullerenes occurs due to evaporation of carbon, because of heat generated at the contact point between the electrodes. This discharge produces a carbon soot that can contain upto 15% fullerenes, C₆₀ (13%) and C₇₀ (2%). The fullerenes are separated according to their mass, by use of liquid chromatography and using a solvent such as a toluene.

Properties of Fullerenes

The colour of thin films of C₆₀, are mustard-coloured, but in bulk it appears dark brown. Where as the colour of thin films of C₇₀, are red-brown coloured, but in bulk it appears grey-black. The colour of the solutions of C₆₀, is magenta colour, where as the colour of the solutions of C₇₀, are dull red. Fullerenes are dissolved slowly in organic solvents, indicating close packing in the crystal and have high melting points. The C-13 nmr spectrum of C₆₀, shows a single peak (142.68 pm), suggesting uniform environment for all 60 carbon atoms.

Structure and Bonding of Fullerene

In fullerene molecule, each carbon may be supposed to sp² hybridized, and forming three σ bonds to other three carbon atoms. Therefore, the remaining electron at each carbon is delocalized into a system of molecular orbitals that imparts some aromatic character to the whole molecule. One mechanism suggested that, the addition of carbon atoms occur, first in pentagonal and hexagonal patterns which gradually curl under appropriate conditions to form the cage-like structure. To close into a spheroid, a fullerene must have exactly 12 five membered faces, but the number of six membered faces can vary widely (1/2 n - 10).The number of five membered or pentagonal faces are 12 in C₆₀, whereas number of six membered or hexagonal faces are [1/2 (60) - 10] = 20. In case of C₇₀, the number of five membered or pentagonal faces are 12, and number of six membered or hexagonal faces are [1/2 (70) - 10] = 25. In the case of C₆₀, each carbon atom is at the junction of two six membered rings and one five membered ring. Each six membered rings containing three double bonds. Thus C₆₀ has the structure of a truncated icosahedron. Fullerenes have two different types of C―C bonds:

(a) Those at an edge shared between two fused hexagons, that is ( 6 : 6 rings), the bond length is 139.1 ± 1.8 pm. This is also the distance between pentagons.

(b) Those at an edge between a pentagon and a hexagon or the bonds within a given pentagon are somewhat longer, the bond length is 145.5 ± 1.2 pm.

Structural unit of fullerene

The unit cell of solid C₆₀ consists of a face centered cubic array of C₆₀ molecule in close packing. The molecules can rotate freely at their lattice sites by the thermal energy available at room temperature and can be considered to be spherical.

Structure of fullerene

 Manufacture of Cement

At the present time, Cement is one of the most important building materials.

When a strongly heated mixture of limestone and clay is mixed with a small amount of water, and this mixture allowed to set for a few hours,  this mixture resembles a hard stone-like substance. After setting, the stone-like mass looks like as famous Portland rock of England, and hence it was named Portland cement.

Chemically cement is defined as the varying composition of a finely ground mixture of calcium aluminates and silicates, and this mixture is hydrated by mixing with water, therefore, form a rigid solid structure with good compressive strength.

Cement is a mixture of the following compounds:

Composition of cement

Average chemical composition of cement:

Average chemical composition of cement

Raw Materials

The essentials raw materials for the manufacture of cement are limestone and clay which supply all the four principal ingredients, such as CaO, Al₂O₃, SiO₂, and Fe₂O₃. Calcium oxide and iron oxide, these both substance are obtained from limestone, while silica and alumina are obtained from the clay. Thus raw materials are two types---

1. Calcareous materials: limestone, calcium carbonate sludge, Marl, Chalk, and Alkali waste. These supply mainly CaO.

2. siliceous materials: Clay, Blast furnace slag, siliceous stone, Shale, Slate, etc. These supply silica, iron oxide, and alumina.

Cement making process

There are two methods of manufacturing cement.

1. Wet process.

2. Dry process.

The choice between two processes is usually governed by the following factors---

1. Physical conditions of raw materials.

2. Climate surrounding the place of manufacture.

3. Cost of fuel.

If limestone and clay are soft, the climate is fairly moist and the cost of fuel is cheap, the wet process is preferred.

In the wet process, the limestone is crushed in a suitable mill to prepared particles of suitable size, and clay is washed with water in a wash mill to remove flint and a slurry containing about 60% water is obtained. Crushed limestone and clay slurry are mixed in a special type of ball mill. The resulting slurry called raw slurry which further ground in tube mills and then stored in the collecting tanks, where after analysis addition is made to adjust the proportion. The raw slurry from the wet process is now introduced into the hopper provide on the upper part of a rotary kiln, which consists of an inclined steel cylinder which is about 200-300 ft long and 7-12 ft in diameter and the lower end provided with a fire hood, containing a short rotating cylinder. The charge moves forward slowly, due to the rotary motion (30-60 turns per hour) given to the kiln. The upper portion of the kiln is lined with ordinary brick, the middle portion is lined with thick fire brick and the lower firing zone is lined with fire clay bricks. A blast of burning coal dust and the air is blown from the lower end for a long time. The hot air obtained by cooling the hot clinker is also introduced into the kiln from the lower end. During its passage, the slurry first loses water in the upper portion and the remaining water is eliminated at about 750⁰C region when the charge centre in the middle portion of the kiln, the temperature rises to about 1000⁰C. At this high temperature, limestone is decomposed into CaO and CO₂. When the charge moves into the lowest portion, the hottest zone of the kiln, the temperature rises to about 1400⁰C to 1600⁰C. At this temperature, the mixture is partly fused and the chemical reaction between CaO and aluminium silicate (clay) takes place resulting in the formation of calcium silicates and aluminates.

Reactions

 CaCO₃ ----> CaO + CO₂

2CaO + SiO₂ ---> 2CaO.SiO₂ (Dicalcium silicate)

3CaO + SiO₂ ---> 3CaO.SiO₂ (Tricalcium silicate)

2CaO + Al₂O₃ ---> 2CaO.Al₂O₃ 

(Dicalcium aluminate)

3CaO + Al₂O₃ ---> 3CaO.Al₂O₃ 

(Tricalcium aluminate)

4CaO + Al₂O₃ + Fe₂O₃ ---> 4CaO.Al₂O₃.Fe₂O₃ 

(Tetracalcium alumino ferrite)

The resulting mass is either greenish-black coloured or grey coloured, and is called clinker. These are cooled in a rotary cooler and crushed. Generally, (2-3)% gypsum is added and finely ground to an exceedingly fine powder in grinding machines. The cement is filled in airtight bags to exclude moisture.

 

Flow sheet for manufacture of cement