Third Law Of Thermodynamics

Thermodynamics’ Third Law

What is the Thermodynamics Third Law?

The entropy of a flawless crystal at zero Kelvin (absolute zero) is equal to zero, according to the third law of thermodynamics.

The letter ‘S’ stands for entropy, which is a measure of disorder/randomness in a closed system.
It is proportional to the number of microstates (a fixed microscopic state that a system may occupy) that the system has access to, i.e. the more microstates a closed system can occupy, the higher its entropy.
The ground state of the system is the microstate in which the system’s energy is at its lowest.

Contents Table of Contents

Third Law Mathematical Explanation and Applications

FAQs (Frequently Asked Questions)

In a closed system, the following events may be seen at a temperature of zero Kelvin:

There is no heat in this system.

As a result, at absolute zero, a system only has one accessible microstate: the ground state.
The entropy of such a system is precisely zero, according to the third rule of thermodynamics.

Thermodynamics’ Third Law

Between 1906 and 1912, German scientist Walther Nernst formulated this rule.

Alternative Statements of Thermodynamics’ Third Law

According to the Nernst formulation of the third law of thermodynamics, a process cannot reduce the entropy of a system to zero in a limited number of processes.

When the entropy of each and every element (in their completely crystalline forms) is considered as 0 at absolute zero temperature, the entropy of every material must have a positive, finite value, according to American physical chemists Merle Randall and Gilbert Lewis.
The entropy at absolute zero, on the other hand, may be zero, as in the case of a flawless crystal.

The 3rd law of thermodynamics’ Nernst-Simon statement is as follows: given a condensed system conducting an isothermal process that is reversible in nature, the associated entropy change approaches zero as the associated temperature approaches zero.

The exchange of energy between two thermodynamic systems (whose composite represents an isolated system) is limited, according to the third law of thermodynamics.

Why Is It Impossible to Reach Zero Kelvin Temperature?

In order to cool a material to zero Kelvin, an isentropic process that decreases the temperature of the substance by changing some parameter X from ‘X2’ to ‘X1’ must be repeated an endless number of times.

This is because the third rule of thermodynamics stipulates that at absolute zero temperatures, the entropy change is zero.
The graph below depicts the entropy vs. temperature graph for any isentropic process aiming to cool a material to absolute zero.

The Third Law of Thermodynamics and Absolute Zero

It can be seen from the graph that the lower the temperature connected with the chemical, the more steps are necessary to cool the substance further.
The number of steps necessary to cool the material approaches infinity as the temperature approaches 0 kelvin.

The Third Law’s Mathematical Explanation

According to statistical mechanics, a system’s entropy may be stated using the following equation:

S − S0 = 𝑘B ln𝛀


The entropy of the system is S.

The starting entropy is S0.

The Boltzmann constant is denoted by kB.

The total number of microstates that are compatible with the system’s macroscopic configuration is referred to as.

For a flawless crystal with precisely one distinct ground state, the value is 1.
As a result, the equation may be rewritten as:

ln(1) = 0 [since ln(1) = 0] S – S0 = kB

When the system’s initial entropy is set to zero, the following value of ‘S’ is obtained:

S − 0 = 0 ⇒ S = 0

Applications of Thermodynamics’ Third Law

The third law of thermodynamics is useful in calculating the absolute entropy of a material at any temperature ‘T’.
These calculations are based on the substance’s heat capacity data.
Let S0 be the entropy at 0 K and S be the entropy at T K for any solid.

ΔS = S – S0

ΔS =

S0=0 at 0 K, according to the third rule of thermodynamics.

S = dT

Plotting the graph of Cp/ T vs T and then determining the area of this curve from 0 to T yields the value of this integral.
The following is a simplified equation for a solid’s absolute entropy at temperature T:

S = dT

S = d lnT

2.303 Cp log T = Cp ln T

The heat capacity of the material at constant pressure is given by Cp, which is considered to be constant between 0 and T K.


FAQs (Frequently Asked Questions)

What does thermodynamics entail?

Thermodynamics is the field of physical chemistry concerned with the system’s heat, work, temperature, and energy.

What exactly is entropy?

The letter ‘S’ stands for entropy, which is a measure of disorder or unpredictability in a closed system.
It is proportional to the amount of microstates that the system can access, i.e. the more microstates a closed system can access, the higher its entropy.

The first law of thermodynamics is what?

Energy cannot be generated or destroyed, according to the basic rule of thermodynamics, but it may be moved from one form to another.

What is thermodynamics’ second law?

The total entropy of an isolated system increases continually, according to the second rule of thermodynamics.

The third law of thermodynamics is what?

The third law of thermodynamics asserts that the entropy of a system at absolute zero is constant, or that reducing the entropy of a system to zero in a limited number of processes is impossible.

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