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Basic concepts of Thermodynamics
Thermodynamics is essentially the study of energy, energy transformations
and its relation to matter
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(ii) Surroundings The part of universe other than the system is known as
surroundings
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(iv) Thermodynamic equilibrium A system in which the macroscopic properties do
not undergo any change with time is called thermodynamic equilibrium
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(vi) Mechanical equilibrium If no mechanical work is done by one part of the
system on another part of the system
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Such
a condition exists when pressure remains constant
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(ii) Closed system The system in which only energy can be exchanged with the
surroundings
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Thermodynamics Properties
1
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g
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Extensive Properties

Properties of the system which are dependent on the quantity of matter are called
extensive properties, e
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, internal energy, volume, enthalpy, etc
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State functions When values of a system is independent of path followed and
depend only on initial and final state, it is known as state function,e
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, Δ U, Δ H, Δ G
etc
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g
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Thermodynamic Process
It is the operation which brings change in the state of the system
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e
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(ii) Isochoric process In which volume remains constant, i
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, (Δ V = 0)
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e
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(iv) Adiabatic process In which heat is not exchanged by system with the
surroundings, i
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, (Δq = 0)
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e
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e
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(vii) Irreversible process The process which cannot be reversed and amount of
energy increases
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Internal Energy (E or U)

It is the total energy within the substance
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etc
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Its absolute value cannot be determined but experimentally change in internal
energy (Δ) can be determined by
ΔU = U2 – U1 or ΣUp – ΣUR
For exothermic process, ΔU = -ve, whereas for endothermic process ΔU = +ve
U depends on temperature, pressure, volume and quantity of matter
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Temperature is used here
to know, the system is in thermal equilibrium or not
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Mathematically, ΔU = q + W
where, ΔU = internal energy change
q = heat added to system
W = work added to system
Sign convention
(i) q is + ve = heat is supplied to the system
(ii) q is – ve = heat is lost by the system
(iii) Wis + ve = work done on the system
(iv) Wis – ve =work done by the system
Modes of Transference of Energy
Heat (q)

It occurs when there is a difference of temperature between system and
surroundings
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Its units are joule or
calorie
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work is referred as pressure – volume work
(WpV)
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]
Heat Capacity of a System
Heat Capacity (c) of a system is defined as the amount of heat required to raise the
temperature of a system by 1° C
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Molar Heat Capacity
It is the heat capacity 1 mole of substance of the system
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Specific Heat Capacity
It is the heat capacity of 1 g of substance of the system
q = mc Δ T
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at constant volume
CV = CV * M
(cp and CV are specific heats at constant pressure and constant volume respectively
and M is molecular weight of gas)
cp – CV = R (R = Molar gas constant)
Cp – CV = R / M

The molar heat capacity at constant volume,
CV = (3 / 2) R
The molar heat capacity at constant pressure,
Cp = (3 / 2) R + R = (5 / 2)R
Poisson’s ratio, γ = Cp / CV = (5 / 3) = 1
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66 for monoatomic gas
γ = 1
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33 for triatomic gas
Enthalpy (H)
It is the sum of internal energy and pV-energy of the system
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Mathematically,
H = U + pV
Like U
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ΔH = H2 – H1
or ΣHp = ΣHR
For exothermic reaction (the reaction in which heat is evolved), ΔH = -ve whereas for
endothermic reaction (the reaction in which heat is absorbed), ΔH = +ve
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Enthalpy or Heat of Reaction (ΔrH)
It is the change in enthalpy that accompanies a chemical reaction represented by a
balanced chemical equation
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Factors affecting enthalpy of reaction
(i) Physical state of reactants and products
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(iii) Chemical composition of reactants and products
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(v) Temperature
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Enthalpy of Formation (ΔHf)
It is heat change when one mole of compound is obtained from Its constituent
elements
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It also gives the idea of stability
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Enthalpy of Combustion
It is the Enthalpy change taking place when one mole of a compound undergoes
complete combustion In the presence of oxygen (ΔHc
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3
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4
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It is an exothermic process
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Enthalpy of Fusion
It is the enthalpy change that accompanies melting of one mole of solid substance
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Enthalpy of Vaporisation
It is the enthalpy change that accompanies conversion of one mole of liquid
substance completely into vapours
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Enthalpy of Neutralisation
It is the enthalpy change that takes place when 1 g-equivalent of an acid (or base) is
neutralised by 1 g-equivalent of a base (or acid) in dilute solution
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e
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1 kJ
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1 kJ due to the fact that here the
heat is used up in ionisation of weak acid or weak base
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]
8
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9
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10
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11
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12
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Laws of Thermochemistry
1
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2
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which takes place in several steps, is the sum
of the standard enthalpIes of the intermediate reactions into which the overall
reactions may be divided at the same temperature
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(b) In determination of heat of transition
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(d) To calculate bond energies
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Trouton’s Rule
According to this law, “The ratio of enthalpy of vaporization and normal boiling point
of a liquid IS approximately equal to 88 J per mol per kelvin
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e
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Dulong and Petit Law
This law states “The product of specific heat and molar mass of any metallic element
is equal to 6
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i
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,
5
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Clausius – Clapeyron Equation
– 2
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Bond Enthalpy
It is the average amount of energy required to break one mole of bonds in gaseous
molecules
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It is definite in quantity and expressed in kJ mol-1
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It is a state
function and extensive property
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Entropy of even elementary substances are not zero
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The entropy changes at the time of phase transition:

Spontaneous Process
The physical or chemical process which proceeds by its own in a particular direction
under given set of conditions without outside heir is called spontaneous process
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All natural processes are spontaneous process
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(ii) Evaporation of water
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Spontaneous process where some initiation is required
(i) Coal keeps on burning once initiated
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Enthalpy Criterion of Spontaneous Process
All the processes which are accompanied by decrease of energy (exothermic
reactions, having negative value of ΔH) occur spontaneously
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Entropy Criterion of Spontaneous Process
A process is a spontaneous if and only if the entropy of the universe increases
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So we need another parameter for
spontaneity viz Gibbs’ energy of system (G)
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Or

All spontaneous processes or natural change are thermodynamically irreversible
without the help of an extemal work
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e
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Joule-Thomson Effect
The phenomenon of cooling of a gas when it is made to expand adiabatically from a
region of high pressure to a region of extremely
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This effect is zero when an ideal gas expands in vacuum
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e
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]
Joule-Thomson Coefficient
The number of degrees of temperature change produced per atmospheric drop in
pressure at constant enthalpy when a gas is allowed to expand through a porous
plug is called Joule-Thomson coefficient
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Inversion Temperature
The temperature below which a gas becomes cooler on expansion is known as the
inversion temperature
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e
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Carnot Cycle
It is an imaginary cycle which demonstrates the maximum conversion of heat into
work
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The efficiency of a heat engine in a Carnot cycle,
η = T2 – T1 / T2
= q2 – q1 / q2 = w / q2
Gibbs Energy or Gibbs Free Energy
It is the energy available for a system at some conditions and by which useful work
can be done
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Mathematically,
G = H – TS
Change in Gibbs energy during the process 1S given by Gibbs Helmholtz equation
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Similarly, endothermic reactions which are
non-spontaneous at low temperature may become spontaneous at high temperature
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e
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Gibbs Energy Change and Equilibrium
Criterion for equilibrium,

⇒ We also know that

Relation between ΔG° and EMF of the Cell

Third Law of Thermodynamics
This law was formulated by Nernst in 1906
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We
can find absolute entropies of pure substances at different temperature
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If there is
imperfection at 0 K, the entropy will be larger than zero

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