Nernst Heat Theorem

Introduction : In the chemical thermodynamics, it was difficult to find a quantitative relation between ∆G and ∆H in chemical reaction. & To find out ∆G from thermal data i.e.… ∆H
Various attempts to relate ∆G & ∆H are as follow

(1) Joule-thomsan concept : They found that ∆G & ∆H values are same in case of Daniel cell :: they proposed that ∆G & ∆H are identical. (2) Berthelot’s concept :

He suggested that “when heat is given out in a reaction, the free energy of
System decreases. ”

qt describes the qualitative relationship between ∆G & ∆H qt was found to be true in case of condensed system at ordinary imperative but failed in no. of other cases. (3) Gibbs – Helmholtz Concept :

For the first time they deduced quantitative relation between ∆G & ∆H by the Gibbs – Helmholtz equation,

The limitation of the Gibbs- Helmholtz equation that it does not allow to calculate ∆G from thermal date i.e. ∆H.

*1*

(4) Richard’s concept:

In 1902 Richard measured the emf of cells at law temperature and Concluded that…… ∂ (∆G / ∂T) gets decreased gradually with lowering of temperatures.

i.e. ∆G and ∆H approach each other marl closely at extremely low temperature.

i.e. Lit ∆G = ∆H T -> O

* The Nernst Heat Theorem: From the data of Richard in 1906, Nernst postulated that………. “For a process in condensed system the value of [∂(∆G) / ∂T]p approaches zero asymptotically as the absolute zero is approached ”

i.e. ∆G and ∆H curses meet each other slightly above absolute zero.
&
Run coinciding with each other up to absolute zero.

This behavior is shown by full lines.

The dotted line curve reveals that values of ∆G and ∆H become equal at absolute zero only.
&
Their