Module 1: Thermodynamics
First Law of Thermodynamics
Second Law of Thermodynamics
Entropy & Gibbs Free Energy
Module 2: Reaction Dynamics
Chemical kinetics
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Chemical kinetics, also known as reaction kinetics, is the study of rates of chemical processes. Chemical kinetics includes investigations of how different experimental conditions can influence the speed of a chemical reaction and yield information about thereaction's mechanism and transition states, as well as the construction of mathematical models that can describe the characteristics of a chemical reaction. In 1864, Peter Waage and Cato Guldberg pioneered the development of chemical kinetics by formulating the law of mass action, which states that the speed of a chemical reaction is proportional to the quantity of the reacting substances.
Chemical kinetics deals with the experimental determination of reaction rates from which rate laws and rate constants are derived. Relatively simple rate laws exist for zero-order reactions (for which reaction rates are independent of concentration), first-order reactions, and second-order reactions, and can be derived for others. In consecutive reactions, the rate-determining step often determines the kinetics. In consecutive first-order reactions, a steady state approximation can simplify the rate law. The activation energy for a reaction is experimentally determined through the Arrhenius equation and the Eyring equation. The main factors that influence the reaction rate include: thephysical state of the reactants, the concentrations of the reactants, the temperature at which the reaction occurs, and whether or not any catalystsare present in the reaction.
Contents |
Factors affecting reaction rate [edit]
Nature of the reactants [edit]
Depending upon what substances are reacting, the reaction rate varies.
Acid/base reactions, the formation of salts, and ion exchange are
fast reactions. When covalent bond formation takes place between the
molecules and when large molecules are formed, the reactions tend to
be very slow. Nature and strength of bonds in reactant molecules greatly
influence the rate of its transformation into products.
Physical state [edit]
The physical state (solid, liquid, or gas) of a reactant is also an important
factor of the rate of change. When reactants are in the same phase, as
in aqueous solution, thermal motion brings them into contact. However,
when they are in different phases, the reaction is limited to the interface
between the reactants. Reaction can occur only at their area of contact;
in the case of a liquid and a gas, at the surface of the liquid. Vigorous
shaking and stirring may be needed to bring the reaction to completion.
This means that the more finely divided a solid or liquid reactant the
greater its surface area per unit volume and the more contact it makes
with the other reactant, thus the faster the reaction. To make an analogy,
for example, when one starts a fire, one uses wood chips and small branches
— one does not start with large logs right away. In organic chemistry, on
water reactions are the exception to the rule that homogeneous reactions
take place faster than heterogeneous reactions.
Concentration [edit]
The reactions are due to collisions of reactant species. The frequency with
which the molecules or ions collide depends upon their concentrations.
The more crowded the molecules are, the more likely they are to collide
and react with one another. Thus, an increase in the concentrations of the
reactants will result in the corresponding increase in the reaction rate, while
a decrease in the concentrations will have a reverse effect. For example,
combustion that occurs in air (21% oxygen) will occur more rapidly in pure
oxygen.
Temperature [edit]
Temperature usually has a major effect on the rate of a chemical reaction.
Molecules at a higher temperature have more thermal energy. Although collision
frequency is greater at higher temperatures, this alone contributes only a very
small proportion to the increase in rate of reaction. Much more important is
the fact that the proportion of reactant molecules with sufficient energy to react
(energy greater than activation energy: E > Ea) is significantly higher and
is explained in detail by the Maxwell–Boltzmann distribution of molecular energies.
The 'rule of thumb' that the rate of chemical reactions doubles for every 10 °C
temperature rise is a common misconception. This may have been generalized
from the special case of biological systems, where the α (temperature coefficient)
is often between 1.5 and 2.5.
A reaction's kinetics can also be studied with a temperature jump approach.
This involves using a sharp rise in temperature and observing the relaxation time
of the return to equilibrium. A particularly useful form of temperature jump apparatus
is a shock tube, which can rapidly jump a gases temperature by more than 1000
degrees.
Generic potential energy diagram
showing the effect of a catalyst in
a hypothetical endothermic chem
-ical reaction. The presence of
the catalyst opens a different reac
-tion pathway (shown in red) with a
lower activation energy. The final
result and the overall thermodynamics
are the same.
Generic potential energy diagram
showing the effect of a catalyst in
a hypothetical endothermic chem
-ical reaction. The presence of
the catalyst opens a different reac
-tion pathway (shown in red) with a
lower activation energy. The final
result and the overall thermodynamics
are the same.
Catalysts [edit]
A catalyst is a substance that accelerates the rate of a chemical reaction
but remains chemically unchanged afterwards. The catalyst increases rate
reaction by providing a different reaction mechanism to occur with a lower
activation energy. In autocatalysis a reaction product is itself a catalyst for
that reaction leading to positive feedback. Proteins that act as catalysts
in biochemical reactions are called enzymes. Michaelis–Menten kinetics
describe the rate of enzyme mediated reactions. A catalyst does not affect
the position of the equilibria, as the catalyst speeds up the backward and
forward reactions equally.
In certain organic molecules, specific substituents can have an influence
on reaction rate in neighbouring group participation.
Agitating or mixing a solution will also accelerate the rate of a chemical
reaction, as this gives the particles greater kinetic energy, increasing the
number of collisions between reactants and, therefore, the possibility of
successful collisions.
Pressure [edit]
Increasing the pressure in a gaseous reaction will increase the number of
collisions between reactants, increasing the rate of reaction. This is because
the activity of a gas is directly proportional to the partial pressure of the gas.
This is similar to the effect of increasing the concentration of a solution.
In addition to this straightforward mass-action effect, the rate coefficients
themselves can change due to pressure. The rate coefficients and products
of many high-temperature gas-phase reactions change if an inert gas is added
to the mixture; variations on this effect are called fall-off and chemical
activation. These phenomena are due to exothermic or endothermic reactions
occurring faster than heat transfer, causing the reacting molecules to have
non-thermal (non-Boltzmann) energy distributions. Increasing the pressure
increases the heat transfer rate between the reacting molecules and the
rest of the system, reducing this effect.
Condensed-phase rate coefficients can also be affected by (very high)
pressure; this is a completely different effect than fall-off or chemical-activation.
It is often studied using diamond anvils.
A reaction's kinetics can also be studied with a pressure jump approach.
This involves making fast changes in pressure and observing the relaxation time
of the return to equilibrium.
Schottky Defects
Module3: Electrochemistry
Module - II
Electrochemistry
Electrochemistry is a branch of chemistry, which deals with the chemical applications of electricity. deals with the chemical reactions produced by passing electric current through an electrolyte or the production of electric current through chemical reactions.
1.1.1 Conductors
1.1.2 Non-Conductors or Insulators
A substance or material that allows electric current to pass through it is called a conductor. The ability of a material to conduct electric current is called conductance.
Example All metals, graphite, fused salts, aqueous solutions of acids, bases, etc.,
Materials which do not conduct electric current are called non-conductors or insulators.
Example Plastics, wood, most of the non metals, etc.,
1.1.2 Non-Conductors or Insulators
The conductors are broadly classified into two types as
follows.
1.1.3 Types of Conductors
1. Metallic conductors or Electronic conductors
Metallic conductors are solid substances, which conduct electric current due to the movement of electrons from one end to another end. The conduction decreases with increase of temperature -
Example All metals, graphite.
2. Electrolytic Conductors
Electrolytic conductors conduct electric current due to the movement of ions in solution or in fused state. The conduction increases with increase of temperature.
Example Acids bases, electrovalent substances
Types of Electrolytic Conductors
The electrolytic conductors are further sub-classified into three types as follows.
(a) Strong electrolytes
Strong electrolytes are substances, which ionise completely almost at all dilutions.
HCI NaOH, NaCt KCI etc
(b) Weak electrolytes
Weak electrolytes are substances, which ionise to a small extent even at high dilutions.
(c) Non electrolytes
Non electrolytes are substances, which do not ionise at any dilutions.
Example Glucose sugar alcohol petrol etc
Table 1.1. Differences between metallic conduction and electrolytic conduction
1.It involves the flow of electrons in a conductor. It involves the movement of ions in a solution.
2. It does not involve any transfer of matter, It involves transfer of electrolyte in the form of ions.
3.Conduction decreases with increase in temperature. Conduction increases with increase in temperature.
4.No change in chemical properties of the c on d u c to it Chemical reactions occur at the two electrodes.
1.1.4 Cell Terminology
(I) Current
Current is the flow of electrons through a wire or any conductor.
(ii) Electrode
Electrode is a material (or) a metallic rod /bar/strip which conducts electrons.
(iii) Anode
Anode is the electrode at which oxidation occurs.
(iv) Cathode
Cathode is the electrode at which reduction occurs.
(v) Electrolyte
Electrolyte is a water soluble substance forming ions in solution, and conduct an electric current.
(vi) Anode Compartment
Anode compartment is the compartment of the cell in which oxidation half-reaction occurs. It contains the anode. (vii) Cathode Compartment
Cathode compartment is the compartment of the cell in which reduction half reaction occurs. It contains the cathode.
(viii) Half-cell
Half cell is a part of a cell, containing electrode dipped in an electrolytic solution. If oxidation occurs at the electrode that is called oxidation half cell. If reduction occurs at the electrode that is called reduction half cell.
(ix) Cell
Cell is a device consisting two half cell. The two half cells are connected through one wire
TYPES OF CELLS
A cell is a device consisting two half cell. Each half cell contains an electrode dipped in an electrolytic solution. The two half cells are connected through one wire. The followings are two types of cells.
1. Electrolytic cells.
2. Electrochemical cells (or) voltaic cells (or) galvanic cells.
1. Electrolytic cells
Electrolytic cells are cells in which electrical energy is used to bring about the chemical reaction.
Electrolysis, electroplating, etc.,
2. Electrochemical cells
Electrochemical cells are cells in which electrical energy is generated by a chemical reaction (redox reaction). Daniel cells, fuel cells, etc.,
1.3 ELECTROCHEMICAL CELLS OR GALVANIC CELLS
Galvanic cells are electrochemical cells in which the electrons, transferred due to red ox reaction, are converted to electrical energy. Daniel cell (Fig 1.3) Cell device (Construction) Daniel cell consists of a zinc electrode dipped in I M ZnS0 solution and a copper electrode dipped in 1 M CuS0 solution. Each electrode is known as a half cell. The two solutions are inter connected by a salt bridge and the two electrodes are connected by a wire through the voltmeter.
At anode: Oxidation takes place in the zinc electrode by the liberation of electrons, so this electrode is called negative electrode or anode.
At cathode: Reduction takes place in the copper electrode by the acceptance of electrons, so this electrode is called the positive electrode or cathode.
Zn —>Zn2 + 2e (at anode)
Cu + 2e —8Cu (at cathode)
Cu + Zn -->+ Cu (net cell reaction
The electrons liberated by the oxidation reaction flow through the external wire and are consumed by the copper ions at the cathode.
Salt bridge
It consists of a U - tube containing saturated solution of KCI or NH in agar-agar gel. It connects the two half cells of the galvanic cells.
Functions of salt bridge
(i) It eliminates liquid junction potential.
(ii) It provides the electrical continuity between the two half cells.
1.3.1 Representation of a galvanic cell (or) Cell diagram
(i) A galvanic cell consists of two electrodes anode and cathode.
(ii) The anode is written on the left hand side while the cathode is written on the right hand side.
(iii) The anode must be written by writing electrode metal first and then electrolyte. These two are separated by a vertical line or a semicolon. The electrolyte may be written by the fommia of the compound (or) by ionic species.
(iv) The cathode must be written by writing electrolyte first and then the electrode metal. These two are separated by a vertical line or a semicolon.
(v) The two half cells are separated by a salt bridge, which is indicated by two vertical lines. Using the above representation, the galvanic cell is
REVERSIBLE AND IRREVERSIBLE CELLS
I.Reversible cells
Daniel cell, secondary batteries (rechargeable batteries).
Daniel cell is a very good example for a reversible cell.
Its emf is 1.1 volt. It is represented as
Zn/ZnSO4(I M)/CuSO4(1 M)/Cu.
Cells which obey the following three conditions of thermodynamic reversibility are called reversible cells.
i) If the aniel cell is connected to an external source of emf equal to 1.1 volt, no current flows and also no chemical reaction takes place in the cell.
(ii) If the external emf is made slightly less than 1.1 volt, small amount of current flows from the cell] and small chemical reaction occurs.
(iii) If the external emf is made slightly greater than 1.1 volt, the current will flow in the opposite direction. Copper will pass into the solution as copper ions and zinc will get deposited on the zinc electrode.
2. Irreversible cells
Example zinc-Silver cell, Dry cell (Primary cells).
Zinc-Silver cell is an example for a irreversible cell. It is represented as
Cells which do not obey the conditions of thermodynamic reversibility are called irreversible cells.
The cell reactions occur at anode and cathodes are,
Zn+H2So4H àZnSO + H (at anode)
2Ag + 2e —>2Ag (at cathode)
When the two electrodes are connected, zinc dissolves with the liberation of hydrogen gas.
When the external emf is slightly greater than the actual emf of the cell is applied to it, the above reactions are not reversed Because one of the product, H gas already escaped. Such cells, which do not obey the conditions of thermodynamic reversibility, are called irreversible cells.
EMF OF A. CELL
Definition
Electromotive force is defined as, “the difference of potential which causes flow of electrons from one electrode of higher potential to the other electrode of lower potential Thus, the emf of a galvanic cell can be calculated by using the following relationship.
Standard reduction EMF = potential of right hand side electrode - Standard reduction potential of left hand side electrode
E cell = E right – E left
Nernst equation for emf of a cell The daniel cell can be represented as
1.5.1 Measurement of EMF of a cell
The potential difference or emf of a cell can be measured on the basis of poggendorff’s compensation principle. Here the emf of the cell is just opposed or balanced by an (emf standard cell) external emf, so that no current flows in the circuit. The potentiometer consists of a uniform wire AB (Fig. 1.4). A storage battery (K) is connected to the ends A and B of the wire through a rheostat (R). The cell of unknown emf (4 is connected in the circuit by connecting its positive pole to A and the negative pole is connected to a sliding contact (D) through a galvanometer (6). The sliding contacts freely moved along the wire AB till no current flows through the galvanometer. Then the distance AD is measured. The emfof unknown cell is directly proportional to the distance AD.
EX * AD
Then the unknown cell (4 is replaced by a standard cell (s) in the circuit. The sliding contact is again moved till there is null deflection in the galvanometer. Then the distance AD’ is measured.
The emf of standard cell E is directly proportional to the distance AD’.
Es * AD1
Then, the emf of the unknown cell can be calculated from the following equation.
Emf of the unknown cell x = Length AD
Emf of the standard cell s = Length AD’
1.5.2 Factors affecting emf of a cell
1. Nature of the electrolytes and electrodes.
2. Concentration and composition of the electrolytes.
3. pH and temperature of the solution.
1.5.3 Applications of emf measurements
1. The valence of an ion can be determined -
2. Solubility of a sparingly soluble salt can be determined
3. Potentiometeric titrations can be carried out.
4 Hydrolysis constant can be determined.
5. Determination of standard free energy change and equilibrium constant
(i) The standard free energy change of a reaction can be calculated as follows where,
n = Number of electrons involved; F = 96,500 coulombs;
E° Standard emf of the cell.
(ii) The equilibrium constant of a reaction can be calculated as follows.
E ° = Standard emf of the cell; K Equilibrium constant
6. Determination of pit by using a standard hydrogen electrode
A hydrogen electrode is introduced into the solution, pH of which is to be determined. It is then coupled with a standard hydrogen electrode through the salt bridge and the emf of the cell is measured If E is the emf of the cell.
1.6 ELECTRODE POTENTIAL
A metal (M) consists of metal ions (MIH) with valence electrons. When the metal (M) is placed in a solution of its own salt, any one of the following reactions will occur.
1. Positive metal ions may pass into the solution.
M à + Mn+Ne- (oxidation)
2. Positive metal ions from the solution may deposit over the metal
Mn+ ne- à M (reduction)
Example1 Zn in ZnSO4
When Zn electrode is dipped in ZnSO solution, Zn goes into the solution as Zn ions. Now, the Zn electrode attains a negative charge, due to the accumulation of valence electrons on the metal. The negative charges developed on the electrode attract the positive ions from solution. Due to this attraction the positive ions remain close to the metal. (Fig. 1 .5a)
Example 2 Cu in CnS04
When Cu electrode is dipped in CuSO solution, Cu ions from the solution deposit over the metal. Now, the Cu electrode attains a positive charge, due to the accumulation of Cu ions on the metal. The positive charges developed on the electrode attract the negative ions from solution.
Thus, a sort of layer (positive (or) negative ions) is formed all around the metal. This layer is called Helmholtz electrical double layer. This layer prevents further passing of the positive ions from or to the metal. A difference of potential is consequently set up between the metal and the solution. At equilibrium, the potential difference becomes a constant value, which is known as the electrode potential of a metal.
Thus, the tendency of an electrode to lose called the oxidation potential, and the tendency of to gain electrons is called the reduction potential.
Factors affecting electrode potential
The rate of the above reactions depend on
(i) The nature of the metal.
(ii) The temperature.
Electrons is an electrode
(iii) The concentration of metal ions in solution.
1.6.1 Single electrode potential (E)
It is the measure of tendency of a metallic electrode to lose or gain e when it is in contact with a solution
of its own salt,
1.6.2 Standard electrode potential (E°)
It is the measure of tendency of a metallic electrode to lose o gain electrons, when it is in contact with a solution of its own salt of I molar concentration at 25°C.
1.6.3 Nernst equation for electrode potential
Consider the following red ox reaction
Mn+Ne- à M
For such a redox reversible reaction, the free energy change (AG) and its equilibrium constant (K) are interrelated as
The above equation (I) is known as Van’t Hoff isotherm.
The decrease in free energy (— AG) in the above reaction will produce electrical energy. In the cell, if the reaction involves the transfer of n’ number of electrons, then n’ faraday of electricity will flow, If £ is the emf of the cell, then the total electrical energy.
1.6.5 Measurement of single electrode potential
It is impossible to determine the absolute value of a single electrode potential. But, we can measure the potential difference between two electrodes potentiometrically, by combining them to form a complete cell. For this purpose, ‘reference electrode’ is used. Standard hydrogen electrode (SHE) is the commonly used reference electrode, who se potential has been arbitrarily fixed as zero. The emf of the. cell is measured and it is equal to the potential of electrode.
In some cases saturated calomel electrode is used as reference electrode.
1.7 Reference Electrodes (or) STANDARD ELECTRODES
The electrode potential is found out by coupling the electrode with another reference electrode, the potential of which is known or arbitrarily fixed as zero. The important primary reference electrode used is a standard hydrogen electrode, standard electrode potential of which is taken as zero,
It is very difficult to set up a hydrogen electrode So other electrodes called secondary reference electrodes like calomel electrodes are used.
1.7.1 Construction
Hydrogen electrode consists of platinum foil, that is connected to a platinum wire and sealed in a glass tube. Hydrogen gas is passed through the side arm of the glass tube. This electrode, when dipped in a IN HCI and hydrogen gas at I atmospheric pressure is passed forms a standard hydrogen electrode. The electrode potential of SHE is zero at all temperatures. (Fig. 1.6)
It is represented as,In a cell, when this electrode acts as anode, the electrode reaction can be written as Standard hydrogen electrode (SHE) When this electrode acts as cathode, the electrode reaction Carl be written as
Limitations
(i ) It requires hydrogen gas and is difficult to set up and transport.
(ii) It requires considerable volume of test solution.
(iii) The solution may poison the surface of the platinum electrode.
(iv) The potential of the electrode is altered by changes in barometric pressure.
1.7. 2 Saturated calomel electrode (Secondary reference electrode)
Construction
Calomel electrode consists of a glass tube containing mercury at the bottom over which mercurous chloride is placed. The remaining portion of the tube is filled with a saturated solution of ICC The bottom of the tube is sealed with a platinum wire . The side tube is used for making electrical contact with a salt bridge. The electrode potential of the calomel electrode is + 0.2422
A glass electrode consists of thin-walled glass bulb (the glass is a special type having low melting point and high electrical conductivity) containing a Pt wire in DiM HCI (Fig. 19). The glass electrode is represented as HO in the bulb famishes a constant IP concentration.
Glass electrode is used as the “internal reference electrode”. The pH of the solutions, especially colored solutions containing oxidizing or reducing agents can be determined. The thin walled glass bulb called glass membrane functions as an ion-exchange resin, and equilibrium is set
up between the N ions of glass and }{t ions in solution. The potential difference varies with the N ion concentration, and is given by the expression.
1.7.3 Ion-Selective electrodes (ISE)
Ion-selective electrodes are the electrodes having the ability to respond only to particular ions, and develop potential, ignoring the other ions in a mixture totally. The potential developed by an ion-selective electrode depends only on the concentration of particular ions. L Glass electrode
The glass membrane of the glass electrode is only selective to only in a mixture.
The glass electrode is placed in the solution under test and is coupled with saturated calome electrode as shown in the figure 1.10. The emf of the cell is measured. From the emf, the pH
of the solution is calculated as follows.
Advantages of Glass Electrode
1 It can be easily constructed and
2 The results are accurate.
3 It is not easily poisoned.
4 Equilibrium is rapidly achieved.
Limitations
(i) Since the resistance is quite electronic potentiometers are measurement.
(ii) The glass electrode can be used in solutions only with pH range of 0 to 10. However above the
pH 12 (high alkalinity), cations of the solution affect the glass and make the electrode useless.
Applications of ISEs
(i) ISEs are used in determining the concentrations of cations like H t Na , K E, Ag , Li
(ii) ISEs are used for the determination of hardness (Ca 2+ and 2* ions).
(iii) Concentrations of anions like NO CN —, 52 —, halides can be determined.
(iv) lSEs are used in the determination of concentration of a gas by using gas-sensing electrodes.
(v) pH of the solution can be measured by using gas-sensing electrode. -
ELECTRODE POTENTIAL OF SOME METALS WITH RESPECT TO SHE
(or) ELECTROCHEMICAL SERIES (or) EMF SERIES
The standard electrode potential (reduction) of a number of electrodes are given in table 1.1. This values are determined potentiometrically by combining the electrode with the another standard electrodes, whose electrode potential is zero.
When the various electrodes (metals) are arranged tn the order of their increasing values of standard red mci/on potential on the hydrogen scale, then the arrangement is called electrochemical series.
1.8.1 Significance of emf series (or) Applications of electrochemical! Series (or)
Applications of Nernst equation
- Calculation of standard emf of the cell
The standard emf of a cell (E°) can be calculated if the standard electrode potential values are known using the following relation.
2. Relative ease of oxidation or reduction
Higher the value of standard reduction potenti& (+ve value) greater is the tendency to get reduced. (i.e. Metals on the top (—ye value) are more easily iodized) (oxidized).
(a) The fluorine has higher positive value of standard reduction potential (-i- 2.87 V), and shows higher tendency towards reduction.
(b) The lithium has highest negative value (—3.01 V) and shows higher tendency towards oxidation.
- Displacement of one element by the other
Metals which lie higher in the series can displace those elements which lie below them in the series.
For example, we may know whether Ca will displace Zn from the solution or vice-versa. We know that standard reduction potenual of Cu & Zn.
Definition
So, cu has a great tendency to acquire Cu form, than Zn has for acquiring Zn form.
- Determination of equilibrium constant for the reaction
Standard electrode potential can also be used to determine the equilibrium constant (K) for the reaction. We know that From the value of E°, the equilibrium constant for the cell reaction can be calculated.
- Hydrogen displacement behaviour
Metals with negative reduction potential (i.e., the metals placed above H in the chi series) will displace the hydrogen from an acid solution. Zinc reacts with gill 11 to give U but Ag does not, why? Zn + H à+ 112 = — 0.76 volt
The metal with positive reduction potential (ie., the metals placed below Li in the emf series) will not displace the hydrogen from an acid solution.
Ag + I-1 àNo reaction E°Ag = + 0.80 volt
Spontaneity of redox reaction can be predicted from the
emf (E°) value of the complete cell reaction.
(i) If the E° of the cell is positive, the reaction is spontaneous.
(ii) If the F° of the cell is negative, the reaction is not feasible.
In genera!, an element having lower reduction potential can displace another metal having higher reduction potential from its salt solution spontaneously.
1.9 POTENTIOMETRIC TITRATIONS
Principle
Emf of a cell depends on the concentration of the electrolytes with which the electrodes are in contact. Therefore, the electrode reaction is,
As the concentration of M ‘ + changes, the emf of the cell also changes correspondingly.
Thus, the potentiometric titrations involve the measurement of emf between reference electrode and an indicator electrode, with the addition of the titrant.
1.9.1 Types of potentiometric titration
Potentiornetric titrations fall into the following three Categories
1. Redox titrations.
2. Precipitation titrations.
3. Acid-base titrations.
- Redox titration
Known amount of FeSO solution is taken in a beaker and the indicator electrode (platinum electrode) is inserted in it. It is then connected to a reference electrode (calomel electrode), to form a galvanic cell. The cell is then connected to the potentiometer and its Ecen is determined.
When it is titrated against the standard K solution, taken in the burette, the crnf is going on decreasing as the concentration of Fe decreases due to the following reaction
At the end point the einf is suddenly decreases. After the end point there is no change in the potential.
When the emf is plotted against the volume of K added, a cure of the type shown in figure 1.12 is obtained. The end point is the point, where the slope of the curve is maximum.
A more sensitive and satisfactory method of detecting
the end point will be the graph of against volume of K The resulting curve rises to a maximum at the equivalent point, which is the end point.
- Precipitation titration (AgNO vs Noci)
Known amount of AgNO solution is taken in a beaker and the indicator electrode (silver-silver ion electrode) is placed in it. It is then connected to a reference electrode (calomel electrode), to form a galvanic cell. The cell is then connected to the potentiometer nd its Eceii is determined.
When the AgNO solution is titrated against the standard MaC solution, taken in the burette, the emf is going on decreasing as the Ag+ ion concentration decreases.
AgNO + NaCI à 4 AgCLLà + NaNO3
At the end point, emf is suddenly decreases. After the end point there is no noticeable change in the potential. When the emf is plotted against the volume of NaCI added a curve of the type shown in figure 1.13 is obtained. The end point is the point, where the slope of the curve is maximum.
The graph vs vol. of NaCl is plotted as above, to get the accurate end point.
1.9.2 Advantages of potentiometric titrations
1 The necessary apparatus required is cheap and easily available.
2 This method can be used for colored solution. Fixing up end point is easier when compared to the titratious in which indicators are used to fix up end points.
3 Very dilute solutions can be titrated with accuracy. Several components may be titrated in the same solution.
1.10 CONDUCTOMETRIC TITRATION
Principle
Conductometric titration is a volumetric method based on the measurement of conductance of the solution during the titration.
The conductance of a solution depends on
(1) the number and charge on the free ions, and
(ii) the mobility of the ions.
1.10.1 Types of conduct metric titrations
(a) Acid-Base titrations.
(b) Precipitation titrations.
(c) Replacement titrations.
(d) Redox (oxidation-reduction) titrations.
(e) Complexometric titrations.
1.10.2 Acid-Base titration
(i) Strong acid Vs Strong base (R Vs NaOH)
Known amount of acid (HCI) is taken in the conductivity cell and the alkali (NaOH) in the burette. Initially the conductivity of the HCI is high, this is due to the presence of fast moving H ions (Point A in the graph). As the NaOH is added gradually, conductance will be going on decreasing until the acid has been completely neutralized (indicated by the line AB). This is due to the replacement of fast moving Ht ions by slow moling Na ions. The point ‘B’ indicates complete neutralization of al }F ions.
Further addition of NaOH will introduce the fast moving
OW ions. Therefore the conductance, after reaching a certain minimum value, will begin to increase (indicated by the line BC).
On plotting the conductance against the volume oi alkali added, the two lines intersect at a point ‘B’ gives the end point. This corresponds to the volume of NaOH required for neutralization.
1.10.2 Advantages of conductometric titration
(i) It gives more accurate end point.
(ii) It is also used for the analysis of dilute solutions and weak acids.
(iii) Since the end point is detected graphically, no keen observation is necessary near the end point.
out.
1.10.4 Disadvantages of conductonietric titration
(i) Only limited number of redox titration can be carried
(ii) It becomes less accurate and less satisfactory, when the total electrolytic concentration is high.
Module 4: Organic Chemistry
Polymerisation
Module 5: Industrial Chemistry
INTRODUCTION
A fuel is a combustible substance, containing carbon as the main constituent, which on burning gives large amount of heat. During the process of combustion of a fuel, the atoms of carbon, hydrogen, etc., combine with oxygen with simultaneous liberation of heat.
C + O2 —>C02 + 94k cals.
2H2 + 02 —> + 68.5 k cals.
The main source of fuel is coal and crude petroleum oil. These are stored fuels available in earth’s and are generally called fossil fuels, because they were formed from fossilized remains of pants and animals.
I. SOLID FUELS
3.1 COAL
Coal is an important primary solid fuel, that has been formed as a result of alteration of vegetable matter under some favorable conditions.
Clarification (or) Metamorphism
The process of conversion (or alteration) of vegetable matter to anthracite (coal) is called calcification or of coal.
(i) it reduces the calorific value of coal,
(ii) moisture in coal consumes more heat in the form of latent heat of evaporation and hence more heat is to be supplied to the coal,
(iii) it increases the transport cost.
(iv) Volatile matter
High percentage of volatile matter is undesirable because
(i) it reduces the calorific value of coal,
(ii) Large ptoportrnn of fuel on heating will distill over as vapor, which escapes out un brut,
(iii) coal with high percentage of volatile matter burns with a long flame with high smoke,
(iv) The coal containing high percentage of volatile matter do not coke well.
(v) Ash content
High percentage of ash content is undesirable because
(i) It reduces the calorific value of coal,
(ii) Ash causes hindrance to heat flow as well as produces clinkers, which blocks the air supply through the fuel,
(iii) It increases the transporting, handling and storage costs,
(iv) It involves additional cost in ash disposal
(iv) Fixed carbon
(i) High percentage of the iced carbon is desirable because higher the percentage of fixed carbon in a coal, greater is its calorific value,
(ii) the percentage of fixed carbon helps in designing the furnace and the shape of the fire-box.
3.2 Ultimate Analysis
It involves the determination of percentage of
(1) carbon and hydrogen contents
(2) Nitrogen content
(3)sulphur content ash content
(4)oxygen content
1. - Carbon and Hydrogen contents
A known amount of the coal sample is burnt in a current of 02 i a combustion apparatus. The carbon and hydrogen, pr in the coal sample, are converted into CO2 and H2O respectively according to the following equations.
H2O+ 1/202 —>H2O
The liberated CO and H vapors are absorbed respective KOH and anhydrous Cad tubes of known weights. The increase in weight of KOH tube is due to the formation of CO while increase in weight of CaCI tube is due to the formation of 1-120. Froth the weights of CO and F1 formed, the % of carbon and hydrogen present in the coal can be m = weight of the coal sample taken. x increase in weight of KOH tube.
y = increase in weight of CaCl2 tube.
2. Sulphur content
A known amount of coal sample is burnt completely in a bomb calorimeter. During this process sulphur is converted into sulphate, which is extracted with water. The extract is then treated with Bacl2 solution so that suiphates are precipitated as BaSO The precipitate is filtered, dried and weighed. From the weight of BaSO4 obtained, the sulphur present in the coal is calculated as follows.
3. Strength
The coke should have very high mechanical strength in order to withstand high pressure of the overlying material in the furnace.
4. Calorific value
The calorific value of coke should be very high.
5. Combustibility
The coke should bum easily.
6. Reactivity
The reactivity of the coke should be low because .reactive cokes produce high temperature on combustion.
7. Cost
It should be cheap and readily available.
3.2.a.MANUFACTURE OF METALLURGICAL COKE
There are so many types of ovens used for the manufacture of metallurgical coke. But the important one is
Otto-Hoffman’s by product oven.
3.2.1 Otto-Hoffman’s by-product oven
In order to
(i) Increase the thermal efficiency of the carbonisatioll process and,
(ii) Recover the valuable by products (like coal gas ammonia, benzol oil, etc.) Otto-Hoffman develop modem by product coke oven.
The oven consists of a number of silica chambers. Each chamber is about 10 — 12 m long, 3 —4 m height and 0.4—0.45 m wide. Each chamber is provided with a charging hole at the top, it is also provided with a gas off take valve and iron door at each end for discharging coke (Fig 4.1).
Coal is introduced into the silica chamber and the chambers are closed. The chambers are heated to 1200°C by burning the preheated air and the producer gas mixture in the interspaces between the chambers.
The air and gas are preheated by sending them through 2nd and 3rd hot regenerators. Hot flue gases produced during carbonisation are allowed to pass through 1st and 4th regenerators until the temperature has been raised to 1000°C. While 1st and 4th regenerators are heated by hot flue gases, the 2nd and 3rd regenerators are used for heating the incoming air and gas mixture.
For economical heating, the direction of inlet gases and flue gases are changed frequently. The above system of recycling the flue gases to produce heat energy is known as the regenerative system of heat economy. When the process is complete, the coke is removed and quenched with water.
Time taken for complete carbonisation is about 12-20 hours. The yield of coke is about 70%.
The valuable by products like coal gas, tar, ammonia, FI and benzol, etc. can be recovered from flue gas.
Recovery of by-products
(i) Tar
The flue gases are first passed through a tower in which liquor ammonia is sprayed. Tar and dust get dissolved and collected in a tank below, which is heated by steam coils to recover back the ammonia sprayed.
(ii) Ammonia
The gases are then passed through another tower in which water is sprayed. Here ammonia gets converted to NH4O4.
(iii) Naphthalene
The gases are again passed through a tower, in which cooled water is sprayed. Here naphthalene gets condensed.
(iv) Benzene
The gases are passed through another tower! where petroleum is sprayed. Here benzene gets condensed to liquid.
(v) Hydrogen Suiphide
The remaining gases are then passed through a purifier packed with moist Fe Here H is retained.
The final gas left out is called coal gas wlj is used as a gaseous fuel.
Advantages of Otto Hoffman’s process
1. Valuable by products like ammonia, coal gas, naphthalene etc., are recovered.
2. The carbonisation time is less.
II LIQUID FUELS
3.3 PETROLEUM.
Petroleum or crude oil is naturally occurring liquid fuel. It is dark brown or black colored viscous oil found deep in earth’s crust. The oil is usually floating over a brine solution and above the oil, natural gas is present. Crude oil is a mixture of paraffinic, olefinic and aromatic hydrocarbons with small amounts of organic compounds like N, 0 and S.
The average composition of crude oil is as follows
Percentage *
C 80-87 H 11 – 15 S -0.1 – 3.5 N + O0.1 – 0.5
3.4 Classification of Petroleum
Petroleum is classified into three types.
1. Paraffinic-Base type crude oil
It contains saturated hydrocarbons from CM to C H with a smaller amount of naphthenes and aromatics.
2. Naphthenic or Asphaltic Base type crude oil
It contains cycloparaffins or naphthenes with a smaller amount of paraffins and aromatics.
3. Mixed Base type crude oil
It contains both paraffinic and asphaltic hydrocarbons.
3.5 REFINING OF PETROLEUM OR CRUDE OIL
The crude oil obtained from the earth is a mixture of oil, water and unwanted impurities. After the removal of water and other impurities, the crude oil is subjected to fractional distillation. During fractional distillation, the crude oil is separated into various fractions.
Thus, the process of removing impurities and separating the crude oil into various fractions having different boiling points I catted Refining of Petroleum. The process of refining involves the following steps.
Step I: Separation of water (Cottrell’s process)
The crude oil from oil well is an extremely stable emulsion of oil and salt water. The crude oil is allowed to flow between two highly charged electrodes, here colloidal water droplets combine to form large drops, which is then separated out from the oil.
Step 2: Removal of harmful sulphur compounds
Sulphur compounds are removed by treating the crude oil with copper oxide. The copper su formed is separated out by filtration.
Step 3: Fractional distillation
The purified crude oil is then heated to about 400°C in an iron retort, where the oil gets vapourised. The hot vapors are then passed into the -bottom of a “fractionating column” (Fig 4.2). The fractionating column is a tall cylindrical tower containing a number of horizontal stainless steel trays at short distances. Each tray is provided with small chimney covered with a loose cap.
When the vapors of the oil go up in the fractionating column, they become cooler and get condensed at different trays. The fractions having higher boiling points condense at lower trays whereas the fractions having lower boiling points condense at higher trays. The gasoline obtained by this fractional distillation is called straight-run gasoline.
3.6 CRACKING
Definition:-
Cracking Lc defined as “the decomposition of high boiling hydrocarbons of high molecular weight into simpler, low boiling hydrocarbons of low molecular weight.”
3.6.1 Necessity for Cracking
The crude oil on fractional distillation yields only about 15 - 20% gasoline. This is known as straight run gasoline.The unity of straight run gasoline is not so good. It contains mainly straight chain paraffins, which ignite readily and more rapidly than any other hydrocarbons and hence it produces knocking (unwanted sound) in IC engines.
In order to overcome these difficulties and also to improve the quality and quantity of gasoline, high boiling fractions are cracked into more valuable low boiling fractions suitable for SJ engines. Thus gasoline obtained by cracking is called Cracked Gasoline.
During cracking the following changes take place
(i) Straight chain alkanes are converted to branched chain hydrocarbons.
(ii) Saturated higher hydrocarbons are converted to mixture of saturated and unsaturated lower hydrocarbons.
(iii) Aliphatic alkanes may also be converted to cyclic compounds.
(iv) All the hydrocarbons obtained by cracking have lower boiling point than the parent hydrocarbons.
3.6.2 Types of Cracking
There are two kinds of cracking.
I. Thermal Cracking.
2. Catalytic Cracking.
1. Thermal Cracking
If the cracking is carried out at higher temperature and pressure without any catalyst, it is called thermal cracking. There are two types of thermal cracking.
(I) Liquid Phase Thermal Cracking
In this method, the heavy oil is cracked at a temperature of 475— 530°C under high pressure of 100 kg/cm to keep the reaction product in liquid state. The cracked products are then separated into various fractions in a fractionating column. The yield of gasoline is about 50 - 60% and the octane number is
65 - 70.
(ii) Vapour phase thermal cracking
In this method, the heavy oil is first vapourised and then cracked at a temperature of 600 — 650°C under a lower pressure of 10—20 kg/cm the yield of gasoline is about 70%. This process is suitable only for those oils which are readily
2. Catalytic Cracking
lf the cracking is carried out at lower temperature and pressure in the presence of suitable catalyst, it is called catalytic cracking. The catalyst used is aluminum silicate or alumina. There are two types of catalytic cracking.
(i) Fixed Bed Catalytic Cracking
The heavy oil vapour is heated to 420— 450°C in a preheated (Fig 4.3). The hot vapours are then passed through a catalytic chamber, maintained at 425 — 450°C and 1.5 kg/cm pressure, where catalysts (artificial clay mixed with zirconium oxide), are kept in fixed beds.
Converted into gasoline and about 2-4 % carbon is formed. The carbon gets adsorbed on the catalyst bed. The cracked vapours are then passed through the fractionating column, where heavy oil gets condensed at the bottom. The vapours of gasoline are then sent through the cooler where gasoline gets condensed along with some gases. The gasoline containing some dissolved gases is then sent to a stabilizer, where the dissolved gases are removed and pure gasoline is recovered.
After 8-10 hours, the catalyst loses its activity due to the deposition of carbon. It is reactivated by burning off the deposited carbon.
(ii) Moving bed (or) Fluid bed catalytic cracking
In this process, the solid catalyst is finely powdered, so that it behaves as a fluid, which can be circulated in oil vapour.
The heavy oil vapor is heated to 420 450°C in a preheater and it is mixed with the catalyst powder. Then this mixture is forced into the reactor, which is maintained at a temperature of 500°C and a pressure of 5 kglcm where
Cracking takes place. Near the top of the reactor, there is a centrifugal separator (called cyclone), which allows only the cracked oil vapours to pass on to the fractionating column leaving behind the catalyst powder in the reactor itself. The catalyst powder gradually becomes heavier, due to coating of carbon and it settles down at the bottom of the reactor. Then it is forced into the regenerator maintained at 600°C, where Carbon is burnt and the regenerated catalyst is again recirculated along with the heavy oil vapour.
From the reactor the cracked oil vapours are passed into the fractionating column, where heavy oil settles down and the vapours are then passed through the cooler where gasoline condenses along with some gases. The dissolved gases are from gasoline by passing it through a stabilizer.
3.6.3 Advantages of Catalytic Cracking over Thermal Cracking
1. The yield of petrol is higher.
2. The quality of petrol produced is better.
3. The production cost is very less, since high temperature and high pressure are not required.
4. No external fuel is necessary for cracking. The heat required for cracking is derived by burning the carbon deposited on the catalyst.
5. The percentage of gum and gum forming compounds is very low.
6. The products contain less sulphur compounds.
7. The octane number of cracked gasoline is higher when compared to straight-mn gasoline. This is due to the presence of branched paraffins and aromatic hydrocarbons in cracked gasoline.
8. The cracking process can be easily controlled, so the desired products can be maintained.
3.7 SYNTHETIC PETROL
The gasoline obtained from the fractional distillation of curde petroleum oil, is called straight run petrol As the use of gas Use is increased, the amount of straight gasoline is not enough to meet the requirement of the present we are in need of finding out a method of synthesizing petrol.
Hydrogenation of coal (or) Manufacture at synthetic petrol
Coal contains about 4.5% hydrogen compared to about 18% petroleum so, coal is a hydrogen deficient compoud.
If coal is heated with hydrogen to high temperature under high pressure it %s converted to gasoline. The preparation of liquid fuels from solid coal called hydrogen of coal (or) synthetic petrol
There are two methods available for the hydrogenation of coal
1. Bergius process (or direct method).
2. Fischer process (or indirect method).
1. Bergius process (or) (direct method)
In this process (fig. 4.5) the finely powdered coal is made into a paste with heavy oil and a catalyst powder (tin or nickel oleate) is mixed with it, The paste is pumped along with hydrogen gas into the converter where the paste is heated to 400 — 450°C under a pressure of 200—250 atm.
During this process hydrogen combines with coal to of saturated higher hydrocarbon which undergo fourth decomposition at higher temperature yield mixer of loW6I hydrocarbons The mixture is led to a condenser, where the crude oil is obtained.
The middle oil is further hydrogenated in vapour phase to yield more gasoline. The heavy oil is recycled for making paste with fresh coal dust. The yield of gasoline is about 60% of the coal used.
The crude oil is then fractionated to yield
(1) Gasoline (ii) Middle oil (iii) Heavy oil
2. Fischer-Tropsch process (or) (indirect method)
In this process (Fig 4.6) coal is first converted into coke. Then water gas (CO + 2) is produced by passing steam over red hot coke.
The water gas is mixed with hydrogen and the mixture ‘Purified by passing through Fe (to remove H and then, a mixture of Fe + Na (to remove organic Sulphur compounds). The purified gas is compressed to 5 to 5 trn and then led through a converter, which is maintained.
Fiscker process at a temperature of 200 — 300°C. The converter is provided with a catalyst bed consisting of a mixture of 100 parts cobalt parts thoria, 8 parts magnesia and 200 parts keiseighur earth. A mbaure of saturated and unsaturated hydrocarbon is produced as a result of polymerization
The corning gaseous mixture is led to a condenser, where the liquid crude of is obtained. The crude oil is fractionated to yield (i) gasoline and (ii) heavy the heavy oil is used for cracking to get more gasoline.
3.8 KNOCKING
Definition:-
Knocking I a kind of explosion due to rapid pressure I rise occurring in an IC engine.
Causes of knocking in S.I (Spark Ignition) Engine
[Petrol engines]
In a petrol engine, a mixture of gasoline vapour and air ratio is used as fuel.
This mixture is compressed and ignited by an electric spark. The product of oxidation reaction (combustion) increases the pressure and pushes the piston down the cylinder. If the combustion proceeds in a regular way, there is no problem in knocking. But in some cases, the rate of combustion (oxidation) will not be uniform due to unwanted chemical constituents of gasoline. The rate of ignition of the fuel increases and the final of the fuel-air mixture gets ignited instantaneously producing an explosive sound known as “Knocking”. Knocking property of the fuel reduces the efficiency of engine. So a good gasoline should resist knocking.
Chemical structure and knocking:-
The knocking tendency of fuel hydrocarbons mainly depends on their chemical stmctures. The knocking tendency decreases in the following order.
Straight chain paraffins > Branched chain paraffins > Cycloparaffins > Olefins >Aromatics.
Thus olefins of the sale carbon-chain len possess 4*better anti-knock properties than the corresponding paraffins.
3.8.1 Improvement of antiknock characteristics
The octane number of fuel can be improved by
(1) Blending petrol of high octane number with petrol octane number, so that the octane number of the latter be improved,
( now a days aromatic phosphates are used as antiknock agent because it avoids lead pollution)
3.9 OCTANE NUMBER (or) OCTANE RATING
Octane number is introduced to express the knocking characteristics of petrol. It has been found that n knocks very badly and hence, its anti value has been given zero. On the other hand, iso gives very little Knocking and so, its anti value has been given 100.
Definition:-
Thus octane number is defined as ‘the percentage of iso present in a mixture of is and n-heptanes.’
3.10 LEADED PETROL (ANTI.KNOCK AGENT)
The antiknock properties of a gas oil can be by the addition of s additives. Tetra ethyl lead (TEL is an important ad added to petrol. Petrol contain tetra ethyl. Mechanism of knocking:-
TEL reduces the knocking tendency of hydrocarbon. Knocking follows a free radical mechanism, leading to a chain growth which results in an explosion. If the chains are terminated before their growth, knocking will cease. TEL decomposes thermally to form ethyl free radicals which combine with the growing free radicals of knocking process and thus the chain growth is stopped.
Disadvantages of using TEL:-
When the leaded petrol is used as a fuel, the TEL is converted to lead oxide and metallic lead. This lead deposits on the spark plug and on cylinder walls which is harmful to engine life. To avoid this, small amount of ethylene dibromide is added along with TEL. This ethylene dibromide reacts with Pb and PhO to give volatije lead bromide, which goes out along with exhaust gases.
But this creates atmospheric pollution. So now a days aromatic phosphates are used instead of TEL.
3.11 CETANE NUMBER (or) CETANE RATING
Cetane number is introduced to express the knocking of diesel. Cetane (hexa decane)
Very short ignition lag and hence its cetane number is taken100. On the other hand a-methyl naphthalene has a long lag and hence its cetane number is taken as zero.
Definition:-
Thus the cetane number is defined as ‘the percentage Of present in a mixture of hexa decane and a which has the same ignition lag as the fuel under test”.
The center number decreases in the following straight chain The octane number of a d oil c he increase by adding additives called dopes.
III GASEOUS FUELS
3.12 COMPRESSED NATURAL GAS (CNG)
When the natural gas is compressed, it is ca compressed Natural Gas (CNG). The primary component present in CNG is methane. It is mainly derived from natural gas.
The natural gas can either be stored in a tank of a vehicle as compressed natural gas (CNG) at 3,000 or 3,600 psi or as liquid feed natural gas (LNG) at typically 20-150 psi.
Properties:-
1. CNG is the cheapest, cleanest and least environmentally impacting alternative fuel.
2. Vehicles powered by CNG produce less carbonmonoxide and hydrocarbon (BC) emission.
3. It is less expensive than petrol and diesel.
4.The ignition temperature of CNG is about 550°C.
5..CNG requires more air for ignition.
Uses
CNG is used to run an automobile vehicle just like LPG.
3.12.1 Comparison of emission levels between CNG-driven vehicles and petrol driven vehicles
LIQUEFIED PETROLEUM GAS (LPG) of propane and butane. It can be readily liquefied under pressure, so it can be economically stored and transported in cylinders. The average composition of LPG is as follows. Its calorific value is about 25,000 kcal/m3
Uses:-
1. It is used as a domestic and industrial fuel
2. It is also used as a motor fuel.
3.13 PRODUCER GAS
It is a mixture of CO & N with small amount of H Its average composition is as follows.
Its calorific value is about 1300 kcal / m Manufacture
The reactor used for the manufacture of producer gas IS known as gas producer. It consists of a tall steel vessel of which is lined with refractory bricks. It is provided with cup and cone feeder at the top and a side opening for producer gas exit. At the bottom, it is provided with a inlet pipe for passing air and steam .
When a mixture of air and steam is passed over a red hot coke maintained at about 1100°C in a reactor, the producer gas is produced
Various Reactions
The reactions of producer gas producer can be divided fl four Zones as follows.
(i) Ash Zone
This is the lowest Zone consists mainly of ash. The incoming and steam is preheated in this Zone.
Gas
(ii) Combustion or Oxidation Zone
This is the zone next to ash zone, here the coke is oxidized to CO and CO2 Both the reactions are exothermic.
Hence, the temperature of the bed reaches around 1,100°C.
C+ 1/ 2 àCO exothermic
C + 02 -->C0 exothermic
Reduction Zone
This is the middle zone. Here both CO and steam are reduced.
C + CO2 à2C0 : endothermic
C+H20àCO+H2: endothermic
The above reactions are endothermic. Hence the temperature of the coke bed falls to 1000°C.
(iv) Distillation or Drying Zone
This is the upper most layer of the coke bed. In this zone (400— 800°C) the incoming coke is heated by the outgoing gases.
1. It is used as a reducing agent in metallurgical operations.
2. It is also used for heating muffle furnaces, open-hearth furnaces etc.
3.14 WATER GAS
It is a mixture of CO and H2 with small amount of N2. The average composition of water gas is as follows
Uses
Constituents Percentage
CO 41%
H2 51%
N2 4%
CO2+CH4 Rest
Its calorific value is about 2800 kcallm
Manufacture
The water gas producer consists of a tall steel vessel, lined . inside with refractory bricks. It is provided with cup and cone feeder at the top and a side opening for water gas exit. At the bottom it is provided with two inlet pipes for passing air and steam.
When steam and little air is passed alternatively over a 900— 1000°C in a reactor, red hot coke maintained at about water gas is produced.
3.15 Combustion of Fuels
3.15.1 INTRODUCTION
Combustion is a process of rapid exothermic oxidation, in which a fuel burns in the presence of oxygen with the evolution of heat and light.
Aim of combustion is to get the maximum amount of heat from a combustible substance in the shortest time. Most of the combustible substances are enriched with carbon and hydrogen. During combustion they undergo thermal decomposition to give simpler products, which are oxidized to C0 H etc.,
C + O2 —>C02 Exothermic.
H + ½ O2 ——>F1 Exothermic.
Above reactions are so large quantity given out INSTITIT OF TECHNOLOGY
3.16 CALORIFIC VALUE
The efficiency of a fuel can be understood by its calorific value. The calorific value of a fuel is defined as “the total amount of heat liberated, when a unit mass of fuel is burnt completely.”
Units of calorific values
The quantity of heat can be measured by the following
units:
(i) Calorie.
(ii) Kilocalorie
(iii) British Thermal Unit (B.T,U).
(iv) Centigrade Heat Unit (C.H.U).
Calorie: It is defined as the amount of heat required to raise the temperature of I gram of water through IT (15 to 16°C).
3.17 HIGHER AND LOWER CALORIFIC VALUES
3.17.1 Higher (or) Gross calorific value (GCV)
It is defined as the total amount of heat produced; when a unit quantity of the fuel is completely burnt and the products of combustion are cooled to room temperature.
When a fuel containing hydrogen is burnt, the hydrogen is convened into steam, if the combustion products are cooled to room temperature, the steam gets condensed into water and latent heat is evolved. Thus, the latent heat of condensation of steam is also included in gross calorific value.
3.17.2 Lower (or) Net Calorific Value (NCV)
It is defined as the net heat produced; when a unit quantity of the fuel is co burnt and the products of combustion are allowed to escape.
= GCV — Latent heat of condensation of water vapour produces
= GCV — Mass of hydrogen x 9 x Latent heat of condensation of water vapour.
I part by weight of H2 produces 9 parts by weight of H as follows. The latent heat of steam is 587 cal/gm.
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