QUESTION PAPER CODE: 57012
B.E/ B.TECH. DEGREE EXAMINATION, MAY/ JUNE 2014
CY6151 ENGINEERING CHEMISTRY
(Regulation 2013)
Answer all questions.
PART A- (10x 2=20)
1. What is functionality of polymers?
The number of bonding sites or reactive sites or functional groups present in a
monomer is known as its functionality.
xample
Functionality
CH2 = CH2
-2 (Two bonding sites are due to the presence of one
double bond in the monomer. Therefore ethylene is a
bifunctional monomer).
2. Mention any two uses of Nylon 6:6.
 It is used as fibre, the majority of the woven fibre are used in the
manufacture of tyre cards.
 It is used as an engineering plastic.
3. State second law of thermodynamics.
Clausius Statement: It is impossible to construct a machine, which will transfer heat
from a lower temperature to a higher temperature. It is mathematically stated as
ΔS = qrev
T
4. Define entropy.
Entropy is a measure of randomness or disorderness in a molecular system. It is
also considered as a measure of unavailable form of energy.
5. What is phosphorescence?
The emission of radiation due to the transition from triplet excited state T1 to the
ground state S0 is called phosphorescence (T1
S0). This transition is slow and
forbidden transition.
6. Define Beer-lambert’s law
The absorbance (A) is directly proportional to molar concentration (C) and thickness
(or) path length (x). A = εCx
7. Define eutectic point.
It is the point at which two solid and one liquid phase are in equilibrium.
Solid A + Solid B
Liquid
All the eutectic points are melting points. All the melting points need not be eutectic points.
8.Write down the compostion of nichrome.
Nichrome is an alloy of nickel and chromium.
compositions :
Metal
Percentage
Nickel
60%
Chromium
12%
Iron
26%
Manganese
2%
Uses:
 For making resistance coils, heating elements in stoves.
 Electric irons and other household electrical appliances.
 Making parts of boilers, steam-lines stills, gas-turbines, aero-engine valves,
retorts,
annealing boxes.
9.What are nanowires?
Nano wire is a material having an aspect ratio ie., length to width ratio greater than
20. Nano-wires are also referred to as “quantum wires”.
10.Distinguish between bulk materials and nanomaterials.
Nano-materials
Bulk materials
Size is less than 100nm
Size is larger in micron size
Collection of few molecules
Collection of thousands of molecules
Surface area is more
Surface area is less
Strength, hardness are more
Strength, hardness are less
PART B- (5X 16=80 marks)
11.(a) (i) Discuss Bulkpolymerisation technique. Mention any two polymers
synthesized by this technique.
Bulk polymerisation
Polymerisation of the pure liquid or gaseous monomer is called bulk
polymerisation. It can be used for the production of free radical polymers and some
condensation polymers.
In the reaction only monomer, polymer and initiator are present and therefore
a very pure product is obtained.
The polymerisation is very rapid and strongly exothermic. It can lead to
hazardous temperature build up and run away reactions. Overheating can cause
branching and crosslinking and lead to the formation of gels.
The process produces highly transparent polymers. Example: PS
(ii) write any five differences between thermoplastics and thermosetting plastics.
S.No.
Thermoplastic resins
Thermosetting resins
1
They are formed by addition
polymerization.
They are formed by condensation
polymerization
2
They are linear in structure.
They are cross linked polymers.
3
These can be reclaimed from
wastes.
They cannot be reclaimed from
wastes.
4
Weak Vander Waals force is
present.
Strong covalent bonds are
present.
5
They are usually soft, weak
and less brittle.
They are usually hard, strong
and more brittle.
6
They soften on heating and
harden on cooling.
They do not soften on heating.
7
They have low molecular weight.
They have high molecular weight.
8
They are soluble in organic
solvents.
They are insoluble in organic
solvents.
9
By reheating to a suitable
temperature, they can be reused,
reshaped and thus reused.
They retain their shape and structure,
even on heating. Hence, they cannot
be reshaped and reused.
(b) (i) Explain the mechanism of free radical polymerization.
FREE RADICAL MECHANISM
This mechanism occur in three major steps namely,
(i)
Initiation
(ii)
(iii)
Propagation
Termination
1. Initiation
It is considered to involve two reactions.(a) First reaction; It involves production of free
radicals by homolytic dissociation of an initiator (or catalyst) to yield a pair of free radicals
(R.
Examples of some commonly used thermal initiators
Thermal initiator is a substance used to produce free radicals by
homolytic dissociation at high temperature.
(i) Initiation
It is considered to involve two reactions.
(a) First reaction involves production of free radicals by homolytic dissociation
•
of an initiator or catalyst to yield a pair of free radicals (R )
I


Initiator
2R
•
free radicals
Examples:
Some commonly used thermal initiators

090o C  2CH COO  (or) 2R 
(i) CH3COO –– OOCCH3 7
3

free radicals
(ii) C6H5COO –– OOCC6H5
8095o C

 2C6H5COO (or) 2R


(b) Second reaction involves addition of this free radical to the first monomer
to produce chain initiating species.
H
H
R
CH2
C
R
C
CH2
Y
Y
First monomer
Chain initiating polymer
(ii) Propagation
It consists of the growth of chain initiating species by successive additions of large
number of monomer molecules.
R
CH2
C
Y
H
H
H
nCH2
C
R
CH2
Y
C
H
n
CH 2
Y
C
Y
Growing Chain
(living polymer)
The growing chain of the polymer is known as living polymer.
(iii) Termination
The growing chain of polymer is terminated by either
(a) Coupling
(b) Disproportionation
(a) Coupling
It involves coupling of free radical of one chain end to another free radical
forming a macromolecule.
R
CH2
H
H
C
C
Y
Y
CH2
R
R
CH2
H
H
C
C
CH2 R
Y
Y
Macromolecule
(Dead polymer)
(b) Disproportionation
It involves transfer of a hydrogen atom from one reactive chain end to another
t o fo r m mac r o mo lecu les. I n whic h, o ne is sa t ur a t ed and ano t her is
unsaturated.
R
H
H
H
H
C
C
+ C
C
H
Y
Y
H
R
R
H
H
C
C
Y
Unsaturated macromolecule
+
H H
H
C
C R
Y H
Saturated macromolecule
The product of the polymer is called dead p(oDlyemaedr. polymers)
(ii) Write the preparation and uses of epoxide
 These are cross-linked thermosetting resins.

They are polyethers because the monomeric units
ether type of structure i.e., R - O - R.
in the polymer have an
Preparation
 Epoxy polymer (or) Epoxy resins are prepared by
with bisphenol.
 The reactive epoxide and hydroxyl groups give a
structure.
 The value of n ranges from 1 to 20.
Properties
 They have tough.
condensing epichlorohydrin
three dimensional cross-linked
 They have good heat resisting property.
 They are flexible.
 They have good adhesion property due to the presence of polar nature.
 They have good chemical resistance to alkalis, acids, solvents and water
etc., due to the presence of stable other linkage.
Uses

They are employed for the production of components for aircrafts and
automobiles.

They are applied over cotton, rayon and bleached fabrics to impart crease resistance
and shrinkage control.

They are used as laminating materials, used in electrical equipments.

Epoxy resin adhesives are sold in the market as in the name of “Araldite”.

They are used for skid - resistant surfaces for highways.
(ii)Derive any two Maxwell relations.
The different expressions connecting internal energy (E), enthalpy (H), Helmholtz
free energy (A), and Gibbs free energy (G), with relevant parameters such as pressure,
temperature, volume and entropy may be given as
(i)
dE = TdS – PdV
(ii)
dH = TdS + VdP
(iii)
dA = –SdT – PdV
(iv)
dG = –SdT + VdP
From these expressions, the Maxwell relations are obtained.
(a) The combined form of first and second law of thermodynamic is
dE = TdS – PdV
.....(1)
If V is constant, so that dV = 0, the equation (1) yields
  E 
  S  = T

V
.....(2)
If S is constant, so that dS = 0, then equation (1) yields
  E 
  V  = –P

S
.....(3)
Differentiating equation (2) with respect to V at constant S yields

  T 
 2E
=  V 
 S V
S
.....(4)
Differentiating equation (3) with respect to S at constant V yields
 P 
 2E
=   S 
 S V

V
.....(5)
It follows from equation (4) and (5) that,
 T 
 P 
  V  = –   S 

V

S
.....(6)
Equation (6) is a Maxwell relation.
(b)
Enthalpy is defined by
H = E + PV
dH = dE + PdV + VdP
[ dE + PdV = TdS]
dH = TdS + VdP
.....(7)
If P is constant, so that dP = 0, then equation (7) yields
 H 
  S  = T

P
.....(8)
If S is constant, so that dS = 0, then equation (7) yields
 H 
  P  = V

S
.....(9)
Differentiating the equation (8) with respect to ‘P’ at constant ‘S’ yields.
 T 
2H
=   P 
S  P

S
.....(10)
Differentiating equation (9) with respect to ‘S’ at constant ‘P’ yields,
 V 
2H
=   S 
S  P

P
.....(11)
From the equation (10) and (11), we conclude that,
  T 
  V 
  P  =   S 

P

S
.....(12)
13 (a)(i) Explain Fluorescence and Photo sensitization.
Fluorescence
 When a molecule or atom absorbs radiation of higher
frequency (shorter
wavelength), it gets excited.
 Then the excited atom (or) molecule re-emits the radiation of
the same frequency
(or) lower frequency within short time
(about 10-8 sec).
 Fluorescence stops as soon as the incident radiation is cut off.
 This process is called fluorescence. The substance which shows fluorescence is called
fluorescent substance.
Eg: Fluorite (naturally occurring CaF2) , petroleum, organic dyes like eosin, fluorescein,
ultramarine and vapours of Na, Hg and I2.
ii.Sensitized Fluorescence
If the molecule is excited, due to the transfer of part of excitation energy from the foreign
substance, it emits the radiation of lower frequency, the process is known as sensitized
fluorescence.
Eg: If the mercury vapor is mixed with the vapors of silver, thallium, lead or zinc, which do not
absorb radiation at 253.7 nm and then exposed to the radiation, a part of the excitation energy
from mercury is transferred and gets excited to higher energy state. When it returns to its ground
state, it emits radiation of lower frequency.
Photo sensitization
The foreign substance, which absorbs the radiation and transfers the absorbed energy to the
reactants, is called a photosensitizer. This process is called photosensitized reaction (or)
photosensitization.
Eg: (a) Atomic photosensitizers: Mercury, cadmium, zinc
(b) Molecular photosensitizers: Benzophenone, sulphur dioxide
2. Quenching
When the foreign substance in its excited state collides with another substance it gets
converted into some other product due to the transfer of its energy to the colliding substance.
This process is known as quenching.
Mechanism of Photosensitization and Quenching
The mechanism of photosensitization and quenching can be explained by considering a
general Donor (D) – Acceptor (A) system.
 In a donor-acceptor system, only the donor D (ie., the
incident photon.
sensitizer) absorbs the
 When the donor absorbs the photon, it gets excited from ground state (S0) to singlet
state (S1); Then the donor, via inter system crossing (ISC), gives the triplet
excited
state (T1 or 3D). The triplet state of the donor
is
higher than the triplet
state of the acceptor (A).
 This triplet excited state of the donor then collides with the acceptor produces the
triplet excited state of the
acceptor (3A) and returns to the ground state (S0).
 If the triplet excited state of the acceptor (3A) gives
the desired products, the
mechanism is called photosensitization
 However, if the products are resulted directly from the
excited state of the donor
((3D)), then A is called the quencher and the process is called quenching.
Mechanism of photosensitization
 It is necessary that the energy of the triplet excited state
of the donor (sensitizer) must
be higher than the triplet
excited state of the acceptor (reactant).
 So, that the energy available is enough to excite the
reactant molecule to its
excited state.
 The dotted line indicates the transfer of energy from the sensitizer to reactant.
Examples for Photosensitized Reactions
UV light of 253.7 nm does not dissociate H2 molecule, because the molecule is unable to absorb
this radiation. But, if a small amount of mercury vapour is added, dissociation of hydrogen takes
place.
Here Hg acts as photosensitizer.
2. Photosynthesis in Plants
During photosynthesis of carbohydrates in plants from CO2 and H2O, chlorophyll (green
colouring matter ) of plants acts as a photosensitizer . Actually no CO2 (or) H2O absorbs any
radiation in the visible region, but chlorophyll absorbs the radiation. The energy of the light
absorbed by the chlorophyll is transferred to CO2 and H2O molecules, which then react to form
glucose.
In the presence of light and chlorophyll GO becomes (–) ve, there by the reaction proceeds and
produces glucose. Chlorophyll contains an extensively alternate single and doublebonds
(conjugated system), which helps the molecule to absorb visible radiation.
In the presence of light and chlorophyll GO becomes (–) ve, there by the reaction proceeds and
produces glucose. Chlorophyll contains an extensively alternate single and doublebonds
(conjugated system), which helps the molecule to absorb visible radiation.
But in the absence of chlorophyll, the Go for this reaction is + 2875 kJ.
(ii) State and explain Stark- Einstein Law.
 Stark-Einstein law of photochemical equivalence states that, in a primary photochemical
process (first step) each molecule is activated by the absorption of one quantum of
radiation (one photon).
 When a molecule absorbs a photon, it is not necessary that only one molecule should
react.
 The absorption of one photon by a molecule is only the primary step resulting in the
formation of an activated molecule.
 This further may or may not react or may cause the reaction of many molecules through
chain mechanism.
Illustration of law of photochemical Equivalence
The Energy of Photons and Einstein
(ii) Explain the principle and instrumentation of UV- Visible spectroscopy with a neat
block diagram.
Important terms used in UV-visible spectroscopy
1. Chromophores
The presence of one or more unsaturated linkages (-electrons) in a compound is responsible for
the colour of the compound, these linkages are referred to as chromophores.
2. Auxochrome
It refers to an atom or a group of atoms which does not give rise to absorption band on its
own, but when conjugate to chromophore will cause a red shift.
Eg:
INSTRUMENTATION
I Components
The various components of a visible UV spectrometer are as follows.
1.Radiation Source
In visible - UV spectrometers, the most commonly used radiation sources are hydrogen (or)
deuterium lamps.
Requirements of a radiation source
a. It must be stable and supply continuous radiation.
b. It must be of sufficient intensity.
2.Monochromators
The monochromator is used to disperse the radiation according to the wavelength. The essential
elements of a monochromator are an entrance slit, a dispersing element and an exit slit. The
dispersing element may be a prism or grating (or) a filter.
3.Cells (Sample Cell and Reference Cell)
The cells, containing samples or reference for analysis, should fulfil the following conditions.
i. They must be uniform in construction.
ii. The material of construction should be inert to solvents.
iii. They must transmit the light of the wavelength used.
4.Detectors
 There are three common types of detectors used in visible UV spectrophotometers.
 They are Barrier layer cell, Photomultiplier tube and Photocell.
 The detector converts the radiation, falling on which, into current.
 The current is directly proportional to the concentration of the solution.
5.Recording System
 The signal from the detector is finally received by the recording system.
 The recording is done by recorder pen.
II Working of visible and UV Spectrophotometer
 The radiation from the source is allowed to pass through the monochromator unit.
 The monochromator allows a narrow range of wavelength to pass through an exit slit.
 The beam of radiation coming out of the monochromator is split into two equal beams.
 One-half of the beam (the sample beam) isdirected to pass through a transparent cell
containing a solution of the compound to be analysed.
 The another half (the reference beam) is directed to pass through an identical cell that
contains only the solvent.
 The instrument is designed in such a way that it can compare the intensities of the two
beams.
Block diagram of UV-visible spectrophotometer
Applications
1.Predicting relationship between different groups
2.Qualitative Analysis
3.Detection of Impurities
4. Quantitative Analysis
14.(a)(i) Draw a neat two component Lead- silver system and explain.
Lead-Silver System
Since the system is studied at constant pressure, the vapour phase is ignored and the
condensed phase rule is used.
F′ = C − P + 1
The phase diagram of lead-silver system is shown in Fig. 6.5. It contains lines, areas and
the eutectic point.
(i) Curve AO
The curve AO is known as freezing point curve of silver. Point A is the melting point of
pure Ag (961°C). The curve AO shows the melting point depression of Ag by the successive
addition of Pb. Along this curve AO, solid Ag and the melt are in equilibrium.
Solid Ag
Melt
According to reduced phase rule equation.
F′ = C − P + 1; F′ = 2 − 2 + 1; F′ = 1
The system is univariant.
(ii) Curve BO
The curve BO is known as freezing point curve of lead. Point B is the melting point of
pure lead (327°C). The curve BO shows the melting point depression of ‘Pb’ by the successive
addition of ‘Ag’. Along this curve ‘BO’, solid ‘Pb’ and the melt are in equilibrium.
Solid Pb
Melt
According to reduced phase rule equation.
F′ = C − P + 1; F′ = 2 − 2 + 1; F′ = 1
The system is univariant.
(iii) Point ‘O’ (Eutectic point)
The curves AO and BO meet at point ‘O’ at a temperature of 303°C, where three phases
(solid Ag, solid Pb and their liquid melt) are in equilibrium.
Solid Pb + Solid Ag
Melt
According to reduced phase rule equation.
F′ = C − P + 1; F′ = 2 − 3 + 1; F′ = 0
The system is non-variant.
The point ‘O’ is called eutectic point or eutectic temperature and its corresponding
composition, 97.4%Pb + 2.6%Ag, is called eutectic composition.
Below this point the eutectic compound and the metal solidify.
(iv) Areas
The area above the line AOB has a single phase (molten Pb + Ag). According to reduced
phase rule equation.
F′ = C − P + 1; F′ = 2 − 1 + 1; F′ = 2
The system is bivariant.
Both the temperature and composition have to be specified to define the system
completely.
The area below the line AO (solid Ag + liquid melt), below the line BO (solid Pb + liquid
melt) and below the point ‘O’ (Eutectic compound + solid Ag or solid Pb) have twophases and
hence the system is univariant
F′ = C − P + 1; F′ = 2 − 2 + 1; F′ = 1.
Application of Pattinson’s process for the desilverisation of Argentiferous lead
1) The argentiferous lead, consisting of a very small amount of silver (say 0.1%), is heated
to a temperature above its melting point, so that the system consisting of only the liquid
phase represented by the point ‘p’ in the Figure 6.5.
2) It is then allowed to cool. The temperature falls down along the line ‘pq’. As soon as the
point ‘q’ is reached.
3) Pb is crystallised out and the solution will contain relatively increasing amount of ‘Ag’.
On further cooling, more and more ‘Pb’ is separatedalong the line ‘BO’ the melt
continues to be richer and richer in silver until the point O is reached, where the
percentage of
4) Ag rises to 2.6%.
5) Thus, the process of raising the relative proportion of Ag in the alloy is known as
Pattinson’s process.
Uses of Eutectic system
1) Suitable alloy composition can be predicted with the help of eutectic systems.
2) Eutectic systems are used in preparing solders, used for joining two metal pieces
together.
(II) State Phase rule and explain the terms involved in it with examples.
PHASE RULE


If the equilibrium between any number of phases is not influenced by gravity, or
electrical.
Magnetic forces but is influenced only by pressure, temperature and concentration,
then the number of degree of freedom .
F=C−P+2
(F)
System is related to number of components
(C)
Number of phases
(P)
By the following phase rule equation.
Explanation (or) meanings of terms
1. Phase P
It is the temperature at which two solids and a liquid phase are in equilibrium.
a) Gaseous phase
b) Liquid phase
c) Solid phase
(a) Gaseous phase
All gases are completely miscible and there is no boundary between one gas and the
other.
Examples: Air, which is a mixture of O2, H2, N2, CO2 and water vapour, etc., constitutes a
single phase.
(b) Liquid Phase
The number of liquid phases depends on the number of liquids present and their
miscibilities.
i) If two liquids are immiscible, they will form three separate phases two liquid phase and one
vapour phase.
Examples: Benzene - Water.
ii) If two liquids are completely miscible, they will form one liquid phase and one vapour
phase.
Examples: Alcohol - Water.
(c) Solid Phase
Every solid constitutes a separate phase.
Decomposition of CaCO3
CaCO3(s) −−−−−> CaO(s) + CO2 (g)
It involves three phases, solid CaCO3, solid CaO and gaseous CO2.
(d) Consider a water system consisting of three phases.
Ice(s)
Water (l)
Vapour (g)
Each phase is physically distinct and homogeneous and there are definite boundaries
between phases.
So this forms three phases.
(e) A solution of a substance in a solvent consists of one phase only.
Examples: Sugar solution in water.
(f) An emulsion of oil in water forms two phases
(g) MgCO3 (s) −−−−−> MgO(s) + CO2 (g)
It involves three phases, solid MgCO3, solid MgO and gaseous CO2.
(h) Rhombic sulphur (s) −−−−> Monoclinic sulphur(s)
It forms two phases.
(i) Consider the following heterogeneous system.
CuSO4(s) + 5H2O(l) CuSO4 - 5H2O(s)
Number of phase = 3; Number of component = 2
2. Component (C)
Component is defined as, “the smallest number of independently variable constituents, by
means of which the composition of each
phase can be expressed in the form of a chemical
equation”.
(a) Consider a water system consisting of three phases.
Ice(s)
Water (l)
Vapour (g)
The chemical composition of all the three phases is H2O, but is in different physical form.
Hence the number of component is one.
(b) Sulphur exists in 4 phases namely rhombic, monoclinic, liquid and
vapour, but
the chemical composition is only sulphur. Hence it is a one component system.
(c) Thermal decomposition of CaCO3
CaCO3(s)
CaO(s) + CO2(g)
The system consists of three phases namely, solid CaCO3, solid CaO and gaseous CO2.
But it is a two component system, because the composition of each of the above phases can be
expressed in terms of any two of the three components present.
When CaCO3 and CaO are considered as components.
Phase
Components
CaCO3
CaCO3 + 0CaO
CaO
0CaCO3 + CaO
CO2
CaCO3 − CaO
(d) PCl5(s) −−−−−> PCl3 (l) + Cl2 (g)
This system has three phases, but the number of component is only two.
(e) An aqueous solution of NaCl is a two component system. The constituents are
NaCl and H2O.
(f) CuSO4. 5H2O(s) CuSO4 .3H2O(s) + 2H2O(g)
It is also a two component system.
(g) In the dissociation of NH4Cl, the following equilibrium occurs.
NH4Cl(s) −−−−−>NH3 (g) + HCl (g)
The system consists of two phases namely solid NH4Cl and the gaseous mixture
containing NH3 + HCl. When NH3 and HCl are present in equivalent quantities the composition
of both the phases can be represented by the same chemical compound NH4Cl and hence the
system will be a one component system.
3. Degree of freedom
Degree of freedom is defined as, “the minimum number of independent variable factors
such as temperature, pressure and concentration, which must be fixed in order to define the
system completely”.
A system having 1, 2, 3 or 0 degrees of freedom is called univariant, bivariant, trivariant
and nonvariant respectively.
Examples:
(a) Consider the following equilibrium
Ice(s)
Water (l)
Vapour (g)
These three phases will be in equilibrium only at a particular temperature and pressure.
Hence, this system does not have any degree of freedom, so it is non variant or zero variant.
(b) Consider the Following Equilibrium
Water (l)
Water vapour (g)
Here liquid water is in equilibrium with water vapour. Hence any one of the degrees of
freedom such as temperature or pressure has to be fixed to define the system. Therefore the
degree of freedom is one.
(c) For a gaseous mixture of N2 and H2, we must state both the pressure and temperature.
Hence, the system is bivariant.
(b) (i) Discuss any four heat treatment of steel in detail.
HEAT TREATMENT OF ALLOYS (Steel)
Heat treatment is defined as, “the process of heating and cooling of solid steel article
under carefully controlled conditions,”.
During heat treatment certain physical properties
are altered without altering its chemical composition.
Objectives (or) Purpose of Heat treatment
Heat treatment causes
Improvement in magnetic and electrical properties.
Refinement of grain structure.
Removal of the imprisoned trapped gases.
Removal of internal stresses.
Improves fatique and corrosion resistance.
Types of Heat Treatment of Alloys (Steel)
The main characteristics and the relevant heat-treatment processes are
1. Annealing
4.
Normalizing
2. Hardening
5.
Carburizing
3. Tempering
6.
Nitriding
1. Annealing
Annealing means softening. This is done by heating the metal to high temperature,
followed by slow cooling in a furnace.
Purpose of annealing
It increases the machinability.
It also removes the imprisoned gases.
Types of Annealing
Annealing can be done in two ways
Low temperature annealing (or) process annealing.
High temperature annealing (or) full annealing.
(i) Low temperature annealing (or) process annealing
It involves in heating steel to a temperature below the lower critical temperature followed
by slow cooling.
Purpose
It improves machinability by relieving the internal stresses or internal strains.
It increases ductility and shock-resistance.
It reduces hardness.
(ii) High temperature annealing (or) full-annealing
It involves in heating steel to a temperature about 30 to 50° C above the higher critical
temperature and holding it at that temperature
for sufficient time to allow the internal
changes to take place and then cooled to room temperature.
The approximate annealing temperatures of various grades of carbon steel are
mild steel = 840 − 870° C
medium-carbon steel = 780 − 840° C
high-carbon steel = 760 − 780° C
Purpose
It increases the ductility and machinability.
It makes the steel softer, together with an appreciable increase in its toughness.
2. Hardening (or) Quenching
It is the process of heating steel beyond the critical temperature and then suddenly
cooling it either in oil or brine-water or some
other fluid. Hardening increases the
hardness of steel. The faster the rate of cooling, harder will be the steel produced. Medium and
high-carbon steels can be hardened, but low-carbon steels cannot be hardened.
Purpose
It increases its resistance to wear, ability to cut other metals and strength, but steel
becomes extra brittle. It increases abrasion-resistance, so that it can be used for making cutting
tools.
3. Tempering
It is the process of heating the already hardened steel to a temperature lower than its own
hardening temperature and then cooling it slowly.
In tempering, the temperature to which hardened steel is re-heated is of great significance
and controls the development of the final
properties. Thus
(i)
for retaining strength and hardness, reheating temperature should not exceed
400°C.
be
(ii)
for developing better ductility and
within 400 − 600° C.
tough ness, reheating temperature should
Purpose
It removes any stress and strains that might have developed during quenching.
It reduces the brittleness and also some hardness but toughness and ductility are
simultaneously increased.
Cutting-tools like blades, cutters, tool-bites always require tempering.
4. Normalising
It is the process of heating steel to a definite temperature (above its higher critical
temperature) and allowing it to cool gradually
in air.
Purpose
It recovers the homogeneity of the steel structure.
It refines grains.
It removes the internal stresses.
It increases the toughness.
Normalised steel is suitable for the use in engineering works.
A normalised steel will not be as soft as an annealed job of the same material. Also
normalising takes much lesser time than
annealing
process.
5. Carburizing
The mild steel article is taken in a cast iron box containing small pieces of charcoal
(carbon material).
It is then heated to about 900 to 950°C and allowed to keep it as such for sufficient time,
so that the carbon is absorbed to required
depth.
The article is then allowed to cool slowly within the iron box itself. The outer skin of the
article is converted into high-carbon steel containing about 0.8 to 1.2% carbon.
Purpose:
To produce hard-wearing surface on steel article.
6. Nitriding
Nitriding is the process of heating the metal alloy in presence of ammonia at a
temperature to about 550°C. The nitrogen (obtained
by the dissociation of ammonia)
combines with the surface of the alloy to form
(ii) Discuss the compostion, properites and uses of any two ferrous alloys.
Based on the type of base metals,alloys hard nitride.
Purpose:
To get super-hard surface.
are classified into two types
Ferrous alloys.
Non-ferrous alloys.
FERROUS ALLOYS OR ALLOY STEELS
Ferrous alloys are the type of steels in which the elements like Al, B, Cr, Co, Cu, Mn are present
in sufficient quantitites, in addition to carbon and iron, to improve the properties of steels.
Properties of Ferrous alloys
It possesses high yield point and high strength.
It possesses sufficient formability, ductility and weld ability.
They are sufficiently corrosion and abrasion resistant.
Distortion and cracking are less.
High temperature strength is greater.
IMPORTANT FERROUS ALLOYS
Nichrome: Nichrome is an alloy of nickel and chromium. Its composition is
Properties
It shows good resistance to oxidation and heat.
Steels containing 16 to 20% chromium with low carbon content (0.06 to 0.15%) possess
oxidation resistance upto 900°C.
Steel containing 18% nickel, with small amounts of chromium can withstand temperature above
900°C.
It possesses high melting point.
It can withstand heat upto 1000 to 1100°C.
It possesses high electrical resistance.
Uses.
It is widely used for making resistance coils, heating elements in stoves.
It is also used in electric irons and other household electrical appliances.
It is used in making parts of boilers, steam-lines stills, gas-turbines, aero-engine valves,
retorts,annealing boxes.
It is also used in making other machineries or equipments exposed to very high temperatures.
Stainless Steels (or)
Corrosion Resistant Steels
These are alloy steels containing chromium together with other elements such as nickel,
molybdenum, etc., Chromium is effective if its
content is 16% or more. The carbon content
in stainless steel ranges from 0.3 to 1.5%.
Stainless steel resists corrosion by atmos pheric gases and also by other chemicals. Pro
tection against corrosion is mainly due to the
formation of dense, non - porous, tough film
of chromium oxide at the surface of metal. If this film cracks, it gets automatically healed-up
by atmospheric oxygen.
Types of Stainless Steels
There are two main types of stainless steels.
Heat treatable stainless steels.
Non heat treatable stainless steels.
(a)Heat Treatable Stainless Steels Composition
These steels mainly contain upto 1.2% of carbon and less than 12-16% of chromium.
Properties
Heat - treatable stainless steels are magnetic,tough and can be worked in cold condition.
Uses
They can be used upto 800°C.
They are very good resistant towards weather and water.
They are used in making surgical instruments, scissors, blades, etc.,
(b) Non - Heat Treatable Stainless Steels
These steels possess less strength at high temperature.They are more resistant to corrosion.
Types of Non-Heat Treatable Stainless Steel
According to their composition, they are of two types
(i) Magnetic type Composition
It contains 12 - 22% of chromium and 0.35% of carbon.
Properties
It can be forged, rolled and machined by the use of specially designed tools.
It resists corrosion better than heat- treatable stainless steel.
Uses
It is used in making chemical equipments and automobile parts.
(ii) Non - Magnetic Type
Composition :It contains 18 - 26% of chromium, 8 - 21% of nickel and 0.15% of carbon. Total
percentage of Cr and Ni in such steel is more than 23%. 18/8 Stainless Steel If it contains 18%
Cr and 8% Ni, it is referred to as 18/8 stainless steel. It is the most widely used stainless steel.
Properties
It exhibits maximum resistance to corrosion.
Corrosion resistance of which can be further increased by adding a little quantity of
molybdenum.
Uses
It is used in making household utensils, sinks, dental and surgical instruments.
15.(a) (i) Discuss any four salient properties of nanamaterials.
1. Melting Points
Nano-materials have a significantly lower melting point and appreciable reduced lattice
constants. This is due to huge fraction of surface atoms in the total amount of atoms.
2. Optical Properties
Reduction of material dimensions has pronounced effects on the optical properties.
Optical properties of nano-materials are different from bulk forms. The change in optical
properties is caused by two factors
The quantum confinement of electrons within the nano-particles increases the energy level
spacing.
Eg: The optical absorption peak of a semiconductor nano-particles shifts to a short wavelength,
due to an increased band gap.
(ii) Surface plasma resonance, which is due to smaller size of nano-particles than the wavelength
of incident radiation.
Eg: The colour of metallic nano-particles may change with their sizes due to surface plasma
resonance.
3. Magnetic Properties
Magnetic properties of nano materials are different from that of bulk materials. Ferromagnetic behaviour of bulk materials disappear, when the particle size is reduced and transfers to
super-paramagnetics. This is due to the huge surface area.
4. Mechanical Properties
The nano-materials have less defects compared to bulk materials, which increases the
mechanical strength.
(i)
Mechanical properties of polymeric materials
the addition of nano-fillers.
(ii)
As
nano-materials
are
more
wear
resistant
they are used in spark plugs.
can be increased
stronger,
harder
and
corrosion
by
and
resistant,
Eg: Nano-crystalline carbides are much stronger, harder and wear resistant and are used in micro
drills.
5. Electrical Properties
(i) Electrical conductivity decreases with a reduced dimension due to increased surface
scattering. However, it can be increased, due to better ordering in micro-structure.
Eg: Polymeric fiber
(ii) Nanocrystalline materials are used as very good
they can hold more energy than the bulk materials.
separator plates in batteries, because
Eg: Nickel-metal hydride batteries made of nanocrystalline nickel and metal hydride, require far
less frequent recharging and last much longer.
(ii) Describe any two methods of synthesizing nanomaterials.
Nano-materials are synthesised in two methods.
Top-down (or) Physical (or) Hard methods
It involves conversion of larger particles into smaller particles of nano-scale structure.
This methods is carried out by the following process.
1. Laser ablation
2. Chemical Vapour Deposition (CVD)
3. Electro-deposition
1. Laser Ablation
In laser ablation, high-power laser pulse is used to evaporate the matter from the target.
The stoichiometry of the material is preserved in the interaction. The total mass ablated from the
target per laser pulse is referred to as the ablation rate.
Reaction Setup
A typical laser ablation setup in shown in the following figure.
Fig 7.2 Laser ablation chamber equipped with a rotating target holder
 When a beam of laser is allowed to irradiate the
target, a supersonic jet of particles is
evaporated
from the target surface. Simultaneously, an inert gas
such as argon,
helium is allowed into the reactor to sweep the evaporated particles from the furnace
zone to the colder collector.


The ablated species condense on the substrate
The ablation process takes place in vacuum
in the presence of some
background gas.
placed opposite to the target.
chamber, either in vacuum or
2. Chemical Vapour Deposition (CVD)
It is a process of chemically reacting a volatile compound of a material with other gases, to
produce a non-volatile solid that deposits automatically on a suitably placed substrate.
Various steps involved in synthesis of CVD
The various steps involved in synthesis of CVD are summarized as follows.
1. Transport of gaseous reactants to the surface.
2. Adsorption of gaseous reactant on the surface.
3. Catalysed reaction occurs on the surface.
4. Product diffuses to the growth sites.
5. Nucleation and growth occurs on the growth site.
6. Desorption of reaction products away from the surface.
CVD Reactor
The CVD reactors are of generally two types
1. Hot-wall CVD
2. Cold-wall CVD
1. Hot-wall CVD reactors are usually tubular in form, and heating is accomplished by
surrounding the reactor with resistance elements.
2. But in cold-wall CVD reactors, substrates are directly heated inductively by
graphitesusceptors, while chamber walls are air (or)
water-cooled
Advantages of CVD
1. Nanomaterials, proced by this method are highly pure.
2. It is economical.