Entropy and Gibbs Free Energy Chemistry Mrs. Coyle

Spontaneous Process  A process that occurs without outside intervention.  Spontaneous processes can be fast or slow!

What Factors Determine the Spontaneity of a Reaction?  Entropy  Enthalpy  What ties the two together is: Gibbs Free Energy.

Entropy  Symbol S  A measure of molecular randomness or disorder.

Disorder  Chemical processes spontaneously go to a direction of increased entropy.  Why?  Probability

Probability of Disorder  Is there a higher probability your room will be messy or neat as time goes on?

System the part of the universe under investigation.

Entropy of the System  Is greater in: Gases than solids. Larger volumes of gases than smaller volumes. Larger number of gas molecules than smaller number of gas molecules.

Example:  Which has more entropy in its system? H 2 O (s) orH 2 O (g)

Enthalpy (Heat of Reaction)  Spontaneity is favored when the process is exothermic ( H<0).

Example of a spontaneous exothermic reaction:  2SO 2 (g)+ O 2 (g)  2SO 3 (g) + heat   H<0

So…… :  When  H 0 (exothermic) (Greater Disorder) the reaction would be spontaneous.

 Are all spontaneous reactions exothermic and with a greater system disorder?  Answer: No.

What about exothermic and less disorder? Use Gibbs Free Energy  Gibbs Free Energy can be used to predict the spontaneity and it ties together the H and the S, the two driving forces of reactions. G= H-TS  (all quantities refer to the system)

Gibbs Free Energy  Energy that can be converted to work.  G<0 for spontaneous processes.  G=0 at equilibrium.