Entropy and Free Energy Thermodynamics: the science of energy transfer – Objective: To learn how chemists predict when reactions will be product-favored vs. when they will be reactant-favored

What is thermodynamics  Thermodynamics tells us NOTHING about the rate of reaction.  The study of rates and why some reactions are fast and others are slow is called kinetics (Ch. 15.)

Entropy Entropy, S: Measure of dispersal or disorder.  Can be measured with a calorimeter. Assumes in a perfect crystal at absolute zero, no disorder and S = 0.  If temperature change is very small, can calculate entropy change,  S = q/T (heat absorbed / T at which change occurs)  Sum of  S can give total entropy at any desired temperature.

Entropy Examples (positive  S) Boiling water Melting ice Preparing solutions CaCO 3 (s)  CaO (s) + CO 2 (g)

Entropy Examples (negative  S) Molecules of gas collecting Liquid converting to solid at room temp  2 CO (g) + O 2 (g)  2 CO 2 (g)  Ag + (aq) + Cl - (aq)  AgCl (s)

Entropy Generalizations S gas > S liquid > S solid Entropies of more complex molecules are larger than those of simpler molecules (S propane > S ethane >S methane ) Entropies of ionic solids are higher when attraction between ions are weaker.  Entropy usually increases when a pure liquid or solid dissolves in a solvent. Entropy increases when a dissolved gas escapes from a solution

Laws of Thermodynamics First law: Total energy of the universe is a constant. Second law: Total entropy of the universe is always increasing. Third law: Entropy of a pure, perfectly formed crystalline substance at absolute zero = 0.

Calculating  S o system  S o system =  S o (products) -  S o (reactants) Can also relate surroundings to the system!  S o surroundings = q surroundings / T = -  H system / T

Calculating  S o universe  S o universe =  S o surroundings +  S o system  S o universe = -  H system / T +  S o system Can use 2 nd law to predict whether a reaction is product-favored or reactant-favored! The higher the temperature, the less important the enthalpy term is!

Gibbs Free Energy  G system = - T  S universe =  H system - T  S system  G o system =  H o system - T  S o system  G o rxn =  H o rxn - T  S o rxn

Gibbs Free Energy  G o system or  G o rxn If negative, then product- favored. If positive, then reactant-favored.  G o reaction =  G f o (products) -  G f o (reactants)

Gibbs Free Energy   G is a measure of the maximum magnitude of the net useful work that can be obtained from a reaction!  Know the meaning of enthalpy-driven vs. entropy-driven reactions.  Gs are additive!

Thermodynamics and K If not at standard conditions,  G =  G o + RT ln Q (Equilibrium is characterized by the inability to do work.) At equilibrium, Q = K and  G = O Therefore, substituting into previous equation gives 0 =  G o + RT ln K and  G o = - RT ln K(can use Kp or Kc)

First law: Total energy of the universe is a constant. Second law: Total entropy of the universe is always increasing. Third law: Entropy of a pure, perfectly formed crystalline substance at absolute zero = 0. Entropy : time’s arrow Absolutely MUST learn table in Chapter highlights! Thermodynamics and Time

Thermodynamics and K  Understand relationship between  G o, K, and product-favored reactions!  G o 1product-favored  G o =0 K=1  G o >0 K<1reactant-favored