热力学第二、第三定律 英文版.ppt

Lecure 8: The Second and Third Laws of Thermodynamics Reading: Zumdahl 10.5, 10.6 Outline Definition of the Second Law Determining DS Definition of the Third Law Weight and Entropy The connection between weight (W) and entropy (S) is given by Boltzmann’s Formula: S = k lnW Calculating Entropy The Second Law The Second Law: there is always an increase in the entropy of the universe. From our definitions of system and surroundings: DSuniverse = DSsystem + DSsurroundings The Second Law (cont.) Three possibilities: If DSuniv 0…..process is spontaneous If DSuniv 0…..process is spontaneous in opposite direction. If DSuniv = 0….equilibrium The Second Law (cont.) Consider a reaction driven by heat flow from the surroundings at constant P. Exothermic Process: DSsurr = heat/T Endothermic Process: DSsurr = -heat/T Example For the following reaction at 298 K: Sb4O6(s) + 6C(s) 4Sb(s) + 6CO2(g) DH = 778 kJ What is DSsurr? The Third Law Recall, in determining enthalpies we had standard state values to use. Does the same thing exist for entropy? What is a Perfect Crystal? Standard Entropies With reference to this state, standard entropies have been tabulated (Appendix 4). Recall, entropy is a state function; therefore, the entropy change for a chemical reaction can be calculated as follows: Example Balance the following reaction and determine DS°rxn. Fe(s) + H2O(g) Fe2O3(s) + H2(g) Big Example Is the following reaction spontaneous at 298 K? Big Example (cont.) Entropy and Phase Changes Phase Change: Reaction in which a substance goes from one phase of state to another. Example: H2O(l) H2O(g) @ 373 K S and Phase Changes (cont.) H2O(l) H2O(g) @ 373 K Example Determine the temperature at which liquid bromine boils: Br2(l) Br2(g) Example (cont.) * k = Boltzmann’s constant = R/Na = 1.38 x 10-23 J/K ? The dominant configuration will have the