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Entropy is the most essential element of the statistical thermodynamics since it is a measure of disorder


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Statistical Thermodynamics

Statistical thermodynamics deals with the changes that is hapening to the thermodynamic systems at molecular levels.

Principles of statistics and probability

To understand the equilibria, we should use a language that includes energy, entropy, enthalpy and free energy. Entropy is the most essential element of the statistical thermodynamics since it is a measure of disorder.


If N is the total number of possible outcomes, and nA the outcomes which fall into category A, then PA, the probability of the outcome A is


Below equation is the starting point of statistical mechanics. It defines the relation between entropy and the multiplicity of the microscopic degrees of freedom of a system by using the Boltzman constant, k. k= 1.380662 x 10-23 J/K.

The physical significance of k is the measure of the amount of energy that corresponds to the random thermal motions of the particles within the material.


S=k log W so states that the state that maximizes W also maximizes S.


For a sequence of N distinguishable objects, the number of different permutations W can be expressed in factorial notation

W= N(N-1)(N-2)….3.2.1 = N!

In general, for a collection of N objects with t categories, of which ni objects with t categoris, of which ni objects in each category are indistinguishable from one another, but distinguishable from the objects in the other t-1 categories, the number of permutations W is

W= N !

n1!n2!… . nt!

Suppose that we have a thermodynamic system having two subsystems, A and B, with multiplicities WA and WB, respectively. The multiplicity of the total system will be equal to WAWB and Stotal=k lnWAWB = k ln WA + k ln WB = SA + SB.

Besides, S

k=−piln pi is called Boltzmann distribution law which describes the energy distributions of atoms and molecules.


Low temperature

High temperature



Based on the Boltzmann distribution, states of lower energy are more populated than states of higher energy.

Boltzmann disribution law states that the probability of finding the molecule in a particular energy state varies exponentially as the energy divided by kBT.

N = N0 exp (-E/kBT) Reference:

D. Winterbone and A. Turan , "Advanced Thermodynamics for Engineers", 1996, Butterworth-Heinemann.


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