1. Chapter 19

2. Overview Spontaneous Processes Entropy Second Law of Thermo. Standard Molar Entropy Gibbs Free Energy Free Energy & Temp. & Equil. Const.

3. Thermodynamics and Entropy Gibbs Free Energy  G  H  T  S  Thermo. and the Equilibrium Constant  G  = -RT ln K eq Thermodynamics and Time

4. Spontaneous Processes Energy is conserved in all processes Some processes are spontaneous –occur with no outside intervention Some reactions are reversible In an irreversible rxn the forward reaction is spontaneous, its reverse reaction will be non- spontaneous Dependent on temp, energy & rxn direction

5. Thermo. and Entropy: Thermodynamics: Energy Transfer Difference btw product/reactant energy Spontaneous  product favored Nonspontaneous  reactant favored Kinetics: Speed of Reactions Can be spontaneous (prod. favored) but slow Entropy: Measure of Disorder Disorder is more probable than order Increase in entropy  increase in disorder Is a state function  independent of pathway

6. The more molecules, the smaller the probability of an ordered system AB A A AB B B

7. In General, Entropy (  S = q rev / T)  of gases > liquids > solids more complex molecules > simpler ones ionic solids > as ionic attractions weaken dissolved substances > pure substances evolved gas > dissolved gas

8. small entropy large entropy

9.  S = q rev / T where q is heat transferred in a reversible reaction to the system at constant T  E = q + w q = heat absorbed by system w = work done on the system (review Ch. 5, 1 st Law & Enthalpy)

10. Problem: Calculate the  S for the vaporization of benzene.  H vap = 30.9 kJ/mol at 80.1  C  S = q / T = 30.9 x 10 3 J/mol 353.1  = 87.5 J/mol K liquid to vapor+ 87.5 J/molK vapor to liquid- 87.5 J/molK

11. Problem: Predict if entropy will increase or decrease with each process CO 2(g)  CO 2(s) KCl (s)  KCl (aq) MgCO 3(s)  MgO (s) + CO 2(g) decrease increase

12. Second Law of Thermodynamics: The total entropy of the universe continually increases for an irreversible process For any product-favored reaction,  S universe is +   S universe =  S  system +  S  surroundings   S  surroundings = q surr. / T = -  H system / T For an individual system   S  system =  S  products -  S  reactants

13. Problem: Calculate  S  system,  S  surroundings and  S  universe to determine if H 2(g) + Cl 2(g)  2HCl (g) is product-favored.  S  system = +20.05 J/mol K =  S  products -  S  reactants  S  surroundings  619.5  J/mol K = -  H system / T  S  universe = +639.6 J/mol K =  S  system +  S  surroundings Since  S  universe is positive, product-favored

14. Standard Molar Entropy Denoted as S  Defined at 1 atm and 298 K SME’s of elements not zero  S  products -  S  reactants Calculate the SME of N 2(g) + 3H 2(g)  3(g)

15. Gibbs Free Energy:  S universe =  S surroundings +  S system  S universe = -  H system / T +  S system -T  S universe =  H system  - T  S system  G = -T  S universe = Gibbs Free Energy  G  system =  H  system  - T  S  system

16. Prediction of Spontaneous Rxn:  H system   S system   Spontaneous - +yes, always - - at low temperature + + at high temperature + -no, never  G  system =  H  system  - T  S  system

17. Standard Free Energy of Formation  G  rxn =   G f  prod -   G f  react  H  rxn =   H f  prod -   H f  react Standard Enthalpy of Formation  S  rxn =  S  prod -  S  react Standard Molar Entropy of Rxn

18. Free Energy & Eq. Constant Free energy is related to equil. constant –  G =  G  + RT ln Q for rxn not at equil. –  0 =  G  + RT ln K for a rxn at equil.  G free energy, non-standard conditions  G  free energy, standard conditions  G  negativeK > 1  G  zeroK = 1  G  positiveK < 1