Statements of the third law The third law of thermodynamics is concerned with the behavior of systems in equilibrium as temperature approaches absolute zero. As long as we deal in differences in entropy, knowledge of S 0 is unnecessary. However, absolute entropy IS important, e.g. F = U - TS G = H - TS

Statements of the third law The Gibbs-Helmholtz equation: For an isothermal process: These equations imply:

Statements of the third law Nernst postulated the following:

Statements of the third law Nernst postulate implies that:

Statements of the third law The Nernst formulation of the Third Law: ‘All reactions in a liquid or solid in thermal equilibrium take place with no change of entropy in the neighborhood of absolute zero.’

Statements of the third law Planck later postulated that:

Statements of the third law Planck’s statement of the Third Law: ‘The entropy of a true equilibrium state of a system at absolute zero is zero.’

Statements of the third law Another statement of the Third Law is: ‘It is impossible to reduce the temperature of a system to absolute zero using a finite number of processes.’

Step 1: Isothermal increase of H T1T1 T2T2 T3T3 H1H1 H 2 > H 1 S THM

Step 2: Adiabatic demagnetization T1T1 T2T2 T3T3 B1B1 S T M B 2 > B 1