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.

       

All about Second Law of Thermodynamics?

Why second law of thermodynamics introduced? 

We know that some processes occur spontaneously but if we try to reverse the direction of process, the process do not occur spontaneously and further some external energy is required to move the given system away from state of equilibrium. 

The question is that "why such reversed processes do not occur spontaneously?" could not be answered by first law because the total energy of system would remain constant in the reversed process as it did in the orginal path and ther is no voilation of first law. Therfore there must be some other natural principle in addition to first law which determines the direction in which a process can take place in an isolated system. This principle is "second law of thermodynamics" .

Second law infers us that "the entropy of universe tends to maximum". 

Second law of thermodynamics in terms of entropy:

  "The entropy of an isolated system is fully conserved in every reversible process i.e. for every reversible process the sum of all changes in entropy taking place in an isolated system is zero. If the process is not a reversible one, then the sum of all changes in entropy taking place in an isolated system is greater than zero. In general we can say that in every process taking place in an isolated system the entropy of system either increases or remains constant."  

Condition for equilibrium of an isolated system

“If an isolated system is in such a state that its entropy is maximum, any change from that state would evidently lead to decrease in entropy and hence will not happen. Thus the necessary condition for equilibrium of an isolated system is that its "Entropy shall be maximum."

 Other forms of second law of thermodynamics

Kelvin-Planck statement : It is impossible to construct a device which, operating in a cycle has the sole effect of extracting heat from a single reservoir and performing equivalent amount of work.

Clausius statement : It is impossible for heat to flow from a cooler body to another hotter body  without the aid of external energy.

 "Study on heat engines is based on the above law"

What is first law of Thermodynamics?

When a definite amount of work is done a certain amount of heat is produced and vice versa.

It can be mathematically expressed as

W = JH

where 'J' is a constant called Mechanical equivalent of heat and 'H' is heat produced. 

But a true version of the law is stated as follows

 “When an amount of heat is supplied to a system a part of it is used in raising internal energy of the system and a part in doing the work.”

                                                      dQ = dU + dW 

 where

dQ - amount of heat supplied ;

  dU - change in internal energy ;

                                                            dW - change in work

 From the first law it could be inferred that it is impossible to derive any work without expenditure of an equivalent amount of energy in some other forms.

 According to first law "The energy of universe remains constant".

 For mathematical calculations it should be kept in mind that heat absorbed by the system should be taken as "positive" and rejected by the system should be taken as "negative".

 Let us now apply first law to some thermodynamic processes.

 i) In an isothermal change dU=0 and dQ = dW; thus in an isothermal change the quantiity of heat absorbed by  a perfect gas is transformed into work by the gas.

 ii) In an adiabatic process, dQ=0 and dU+dW=0 -> dU = - dW

  a) If the system is compressed, work is done on the gas and thus dW is taken as -ve.

                                           Hence  dU = -(-dW) = dW

   b) If the system expands adiabatically 'dW' is positive.

                                           Hence  dU=-(+dW) = -dW

iii) In an adiabatic compression, the decrease in volume is associated with increase in temperature and 

      increase in pressure.