Statements of the third law The third law

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 S H 1 M H 2 > H 1 T 3 T 2 T 1 T H

Step 2: Adiabatic demagnetization S B 1 M B 2 > B 1 T 3 T 2 T 1 T