• VARIOUS TYPE OF DISORDER
• EXAMPLES OF DISORDER
• DEFINITION AND EXPRESSION OF ENTROPY
• IMPORTANCE OF ENTROPY IN GEOCHEMICAL
• APPLICATION OF ENTROPY
What is entropy?
The word entropy is sometimes confused with energy.
Although they are related quantities, they are distinct.
or energy measures the capability of an object or system
to do work.
on the other hand, is a measure of the "disorder" of a
system. What "disorder refers to is really the number of
different microscopic states a system can be in, given that the
system has a particular fixed composition, volume, energy,
pressure, and temperature. By "microscopic states", we mean
the exact states of all the molecules making up the system.
Entropy = (Boltzmann's constant k) x logarithm of number of
= k log(N).
Entropy - thermodynamic property-- a
quantitative measure of disorder
Entropy traces out its origin –molecular
movement interpretation-Rudolf Clausias in
The concept of entropy -thermodynamic
laws(i.e. the 2nd law of thermodynamics)
It can be visualised due to the process of
expansion, heating, mixing and reaction.
Entropy is associated with heat and
Entropy-reflects the degree of disorderness.
Diorderness can be pointed out in three different
types. They are:
whether the atoms are free to move or not
Vibrational disorder(thermal disorder)
whether the atoms vibrate about an
this refers to the distribution of different
atoms or sites in lattice.
Various types of disorder
This is one
Definition and expression of entropy
Entropy may be defined as the property of a system
which measure the degree of disorder or randomness
in the system
It is a Greek word which means
It is denoted by the symbol ‘S’
Clausius was convinced of the significance of the ratio
of heat delivered and the temperature at which it is
Entropy is the sum total of entropy due to
positional disorder, vibrational disorder and
configurational disorder. i.e randomness due to
change of state
When a system is undergoing change then the entropy
change is equal to the heat absorbed by the system
divided by the temperature at which change taken
ΔS = S2 –S1
= ∫ dq / T
T ΔS = dq or TdS = dq
this is the II law expression.
Suppose the process is undergoing change at constant
From I Law we know that
ΔE = q – w or dE = dq – dw or dE = dq – PdV
At constant temperature ΔE = 0, therefore dq =
From II law we know that dq = TdS ,
Substituting this in the above we get,
Tds = Pdv
ΔS = PdV / T,
Suppose the process is undergoing change at constant pressure
T ΔS = (q)p -
but we know that (q)p = CpdT
T ΔS = Cp dT,
Or TdS = Cp dT
1∫2dS = 1∫2 Cp dT /T
S2 – S1 = Δ Cp ln (T2 / T1)
This is the entropy change of the system at constant pressure condition
from room temperature to the reaction temperature.
Importance of entropy in
The aim of the thermodynamics in geochemical
term is to generate a set of properties, which helps
us to predict the direction of chemical processes.
The 2nd law starts with simplest term is that there is
an increase in entropy in every natural processes.
The degree of order or disorder in a system may be
described in terms of the probability or
improbability of the observed state,
With the statistical conception of entropy, the possible
application to geochemical systems become recognisable,
For e.g.- the distribution of energy in geomorphic system is
one way of expressing the relative elevation of particle of
water and sediments etc. in the evolution of landscape.
It is noted that all natural processes are spontaneous,
unidirectional –where there is increase in randomness.
Thus this disorderness in the geochemical processes can be
determined by the concept of entropy
Applications of entropy
Experimental work in the mineralogy,
Thermobarometric models are various
thermodynamic formulas or equation by
which pressure temperature are determined,
The model is calibrated through
Thus entropy is applied in the model to
measure the disorderness of the system
through temperature , pressure of the
Thermobarometry is thus an excellent
case study when the application of
the thermodynamic parameters are
Entropy is the thermodynamic property which is
the measure of disorder in a system.
It can be expresses by ‘S’=q/t
The term is coined by Rudolf Clausius.
Entropy is mainly associated with heat and
Disorder can be of 3 types- Positional, Vibrational
Thermobarometric models is an excellent case
study when the application of thermodynamic
parameters are involved.
It can also be concluded that-
(a) when heat is transferred at a high temperature,
entropy change is small whereas when heat is
transferred at low temperature, entropy change
(b) When heat is supplied-entropy increases and
when heat is removed- entropy decreases
Entropy is not perceptible to our sense and there is
no such instrument to measure its effect. Only
changes in entropy can be determined by
It is usually determined from some specified
arbitrary datum of temperature only.
Alok K.Gupta and Sisir K. Sen, a short course on elementary
thermodynamics for earth scientist,96
Guirlo Ottonello, principles of geochemistry, Columbia University press,
New York, 1893, 151
Roger Powell, equilibrium thermodynamics in petrology,Hasper and
row publishers, pp 231-232
W.M White, geochemistry, 2007, pp 44-54