The Second and Third Laws of Thermodynamics

Introduction

• System: the part of the universe that is of specific interest.
• Surroundings: constitute the rest of the universe outside the system.
• Correctly predicting the spontaneity of a process requires the consider entropy changes in both the system and the surroundings.
• The change in entropy of the surroundings (ΔSsurr) is directly proportional to the change in enthalpy of the system.
  • ΔSsurr is also inversely proportional to temperature.

The Second Law of Thermodynamics

• The entropy change of the universe is the sum of the entropy changes for the system and surroundings. 
• Second law of thermodynamics: all spontaneous changes cause an increase in the entropy of the universe.
  • For a spontaneous process, ΔSuniverse must be positive.
  • A process with –ΔSsystem can still be spontaneous if …
  • ΔSsurroundings  is + and larger. 
  • It would have to be an exothermic process, which transfers heat to the surroundings.
• ΔSuniverse > 0 for a spontaneous process 
• ΔSuniverse < 0 for a nonspontaneous process (spontaneous in the reverse direction). 
• ΔSuniverse = 0 for a process at equilibrium 

The Third Law of Thermodynamics

• Third law of thermodynamics: the entropy of a pure perfect crystalline substance at zero Kelvin is zero. 
  • Zero Kelvin is called absolute zero. 
  • There is no lower temperature than zero Kelvin.
  • At zero Kelvin, all molecular movement completely stops. 
  • There is only one possible way to arrange the molecules.

Standard Entropies

• It is possible to determine the absolute entropy of a substance.
• Standard Entropies, S°: these values are for 1 mole of a substance at a pressure of 1 bar and a temperature of 298 K. 
  • Aqueous species at 1 M concentration.
• Standard entropy values can be used to calculate the standard entropy change (ΔS°) for a process.