Entropy and Boltzmann. Entropy on the Molecular Scale  Ludwig Boltzmann described the concept of entropy on the molecular level.  Temperature is a measure.

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Presentation transcript:

Entropy and Boltzmann

Entropy on the Molecular Scale  Ludwig Boltzmann described the concept of entropy on the molecular level.  Temperature is a measure of the average kinetic energy of the molecules in a sample. © 2009, Prentice-Hall, Inc.

Entropy on the Molecular Scale  Molecules exhibit several types of motion:  Translational: Movement of the entire molecule from one place to another.  Vibrational: Periodic motion of atoms within a molecule.  Rotational: Rotation of the molecule on about an axis or rotation about  bonds. © 2009, Prentice-Hall, Inc.

Entropy on the Molecular Scale  Boltzmann envisioned the motions of a sample of molecules at a particular instant in time.  This would be akin to taking a snapshot of all the molecules.  He referred to this sampling as a microstate of the thermodynamic system. © 2009, Prentice-Hall, Inc.

Entropy on the Molecular Scale  Each thermodynamic state has a specific number of microstates, W, associated with it.  Entropy is S = k lnW where k is the Boltzmann constant, 1.38  10  23 J/K. © 2009, Prentice-Hall, Inc.

Entropy on the Molecular Scale © 2009, Prentice-Hall, Inc.  The change in entropy for a process, then, is  S = k lnW final  k lnW initial lnW final lnW initial  S = k ln Entropy increases with the number of microstates in the system.

Entropy Changes © 2009, Prentice-Hall, Inc. Entropy changes for a reaction can be estimated in a manner analogous to that by which  H is estimated:  S  =  n  S  (products) —  m  S  (reactants) where n and m are the coefficients in the balanced chemical equation.

Entropy Changes in Surroundings © 2009, Prentice-Hall, Inc.  Heat that flows into or out of the system changes the entropy of the surroundings.  For an isothermal process:  S surr =  q sys T At constant pressure, q sys is simply  H  for the system.

Entropy Change in the Universe © 2009, Prentice-Hall, Inc.  The universe is composed of the system and the surroundings.  Therefore,  S universe =  S system +  S surroundings  For spontaneous processes  S universe > 0

Entropy Change in the Universe © 2009, Prentice-Hall, Inc.  Since  S surroundings = and q system =  H system This becomes:  S universe =  S system + Multiplying both sides by  T, we get  T  S universe =  H system  T  S system  H system T  q system T