Third Law of Thermodynamics ( The Law of zero entropy )

"Nernst" in 1906 proposed a general priniciple supported by series of experimental tests on problem of atomic heat at low temperatures. it was proposed as " The new heat theorem " and is called as third law of thermodyanmics.

Nernst statement

                       " The heat capacities of all solids tend to zero as the absolute zero of temperature is approached and   that the internal energies and entropies of all substances become equal there, approaching their common value asymptotically".

This law neither follows from first law or second law nor is totally a new law.

Other statement of Nernst:

                        " No entropy change takes place when pure crystalline solid reacts at absolute zero".

 Plank statement:

                        " The entropy of a solid or a liquid is zero at absolute zero of temperature".

Lewis and Randall statement 

                       "Every system has finite positive entropy, but at the absolute zero of temperature the entropy may become zero and does so become in the case of a pure crystalline substance".      

But this statement is confined to pure crystalline solids because theoretical argument and experimental evidence have shown that the entropy of solutions and super cooled liquids is not zero even at absolute zero.

For instance, ice always has residual entropy at absolute zero. It also doesn't apply to amorphous class of substances like glass etc.

Importance of third law of thermodynamics

• Third law is useful in explaining the nature of bodies in neighborhood of absolute zero.
• It permits the calculations of absolute values of entropy and physical interpretation of thermodynamic properties such as Helmholtz & Gibbs free energies etc.
• It can be conceived that as the temperature of system tends to absolute zero, its entropy tends to a constant value which is of pressure and state of aggregation etc. 

 "Nernst" formulated that "the entropy change in isothermal reversible process of condensed system approaches zero as temperature at which the process occurs approaches zero".

The principle of Barthelot states that "every chemical transformation which takes place with out the intervention of external energy tends towards the production of that substance or systems of substance which will give the greatest development of heat i.e that process is realized which is most exothermic.

       

What is the difference between absolute, Gauge and vacuum pressures?

To answer this question let us know the various types of pressures

1) Atmospheric pressure

2) Absolute pressure

3) Gauge pressure

4) Vacuum pressure

Atmospheric pressure:

The atmospheric pressure exerts a normal pressure upon all surfaces with which it is in contact, and is known as atmospheric pressure.

It varies with altitude and it can be measured by means of a barometer.As such it is also called "Barometric pressure".

Absolute pressure: -

When pressure is measured above absolute zero it is called absolute pressure. All values of absolute pressure are positive. The lowest absolute pressure which can probably exist corresponds to absolute zero or complete vacuum.

Gauge pressure: -

When pressure is measured either above or below atmospheric pressure as a arbitrary datum then it is called as gauge pressure. All pressure gauges read zero when open to atmosphere.
Pressure gauges read only the difference between, pressure of a fluid to which they are connected & atmospheric pressure.

Vacuum pressure:-

If pressure of a fluid is below atmospheric pressure it is designated as vacuum pressure. Its gauge value is the amount by which it is below the atmospheric pressure. A gauge which measures vacuum pressure is vacuum gauge.