1. THERMODYNAMICS Internal Energy Enthalpy Entropy Free Energy Chapter 17 (McM) Chapter 20 Silberberg

2. Goals & Objectives  See the following Learning Objectives on page 914.  Understand these Concepts:  20.1-22.  Master the Skills:  20.1-10.

3. Thermodynamics  the study of the changes in energy and the transfers of energy that accompany chemical and physical processes.  Addresses three fundamental questions Will 2 or more substances react when they are mixed under specified conditions? If they do react, what energy changes and transfers are associated with their reaction? If a reaction occurs, to what extent does it occur?

4. Thermodynamics  Used to determine if a reaction can occur under specified conditions. spontaneous reaction--can occur under the specified conditions nonspontaneous reaction--do not occur to a significant extent under the specified conditions

5. First Law of Thermodynamics  The internal energy of an isolated system is constant.  The total amount of energy in the universe is constant.

6. Some Thermodynamic Terms  System--the substances involved in the chemical and physical changes under investigation  Surroundings--the rest of the universe  Universe--the system and its surroundings

7. Types of Thermodynamic Systems  Open system--can exchange both matter and energy with its surroundings  Closed system--has a fixed amount of matter but can exchange energy with its surroundings  Isolated system--has no contact with its surroundings

8. Thermodynamic State of a System  defined by a set of conditions that completely specifies all the properties of the system  State functions--the properties of a system( pressure, temperature, energy, e.g.) whose values depend only on the state of the system

9. Changes in Internal Energy,  E  Internal energy represents the total energy of a system.  E = q(heat flow) + w(work)  Work is usually defined as PV  If the work term is 0 (no work done) then at constant volume E = q

10. Limitations of the First Law of Thermodynamics  E = q + w E universe = E system + E surroundings  E system = -  E surroundings The total energy-mass of the universe is constant. However, this does not tell us anything about the direction of change in the universe.  E system +  E surroundings = 0 =  E universe

11. Enthalpy  The change in enthalpy () is measured at constant P.  At constant P: H = q

12. Figure 20.1 A spontaneous endothermic chemical reaction. water Ba(OH) 2 8H 2 O( s ) + 2NH 4 NO 3 ( s ) Ba 2+ ( aq ) + 2NO 3 - ( aq ) + 2NH 3 ( aq ) + 10H 2 O( l ).  H 0 rxn = +62.3 kJ

13. Enthalpy Change  H f o (products) - H f o (reactants)  where H f o is the standard molar enthalpy of formation and H is the enthalpy change for the reaction.

14. Enthalpy Change  Calculate the enthalpy change for the following reaction at 298K.  C 3 H 8 (g) + 5O 2 (g) ----> 3CO 2 (g) + 4H 2 O(l)

16. The Second Law of Thermodynamics  In spontaneous changes the universe tends toward a state of greater disorder.  In thermodynamics, entropy is a measure of the degree of disorder.  Entropy tends to increase.

17. The Second Law of Thermodynamics  Likely

18. The Second Law of Thermodynamics  Unlikely

19. The Concept of Entropy (S) Entropy refers to the state of order. A change in order is a change in the number of ways of arranging the particles, and it is a key factor in determining the direction of a spontaneous process. solid liquid gas more orderless order crystal + liquid ions in solution more orderless order more orderless order crystal + crystal gases + ions in solution

20. Entropy  Entropy can be indirectly measured.  Absolute standard molar entropy values can be found in the textbook.  An increase in entropy corresponds to an increase in disorder.  When S is _______, disorder increases.  When S is _______, disorder decreases.

21. Entropy  The Third Law of Thermodynamics states that the entropy of a pure,perfect,crystalline substance at 0K is zero.  The following relationship applies to entropy changes.  S = S o (products) - S o (reactants)

22. Figure 20.4 Random motion in a crystal The third law of thermodynamics. A perfect crystal has zero entropy at a temperature of absolute zero. S system = 0 at 0 K

23. Changes in Entropy  Calculate the entropy change for the following reaction at 298K. Indicate whether disorder increases or decreases.  2NO 2 (g) -----> N 2 O 4 (g)

25. Free Energy Change,  G  If heat is released in a chemical reaction, some of the heat may be converted to useful work. Some of it may be used to increase the order of the universe. If the system becomes more disordered, then more energy becomes available than indicated by enthalpy alone.

26. The Gibbs Free Energy Change  At constant T and P  G = H - TS  When G is > 0, the reaction is nonspontaneous  When G is = 0, the reaction is at equilibrium  When G is < 0, the reaction is spontaneous

27. Gibbs Free Energy Change  The following relationship exists for standard molar Gibbs free energy Gibbs Free Energy Change changes:  G o = G f o (products) - G f o (reactants)

28. Gibbs Free Energy Change  Calculate the Gibbs free energy change for the following reaction at 298K. Indicate whether the reaction is spontaneous or nonspontaneous under these conditions.  C 3 H 8 (g) + 5O 2 (g) ----> 3CO 2 (g) + 4H 2 O(l)

30. Table 20.1 Reaction Spontaneity and the Signs of  H 0,  S 0, and  G 0 H0H0 S0S0 -T  S 0 G0G0 Description -+-- +-++ ++-+ or - --+ Spontaneous at all T Nonspontaneous at all T Spontaneous at higher T; nonspontaneous at lower T Spontaneous at lower T; nonspontaneous at higher T

31. The Gibbs Helmholtz Equation  Calculate S o for the following reaction at 298K.  C 3 H 8 (g) + 5O 2 (g) ----> 3CO 2 (g) + 4H 2 O(l)  From previous examples we found  H o = -2219kJ and  G o = -2107kJ  Indicate whether disorder increases or decreases

33. The Relationship Between  G o and the Equilibrium Constant  The standard free energy change for a reaction is G o. This is the free energy change that would accompany the complete conversion of all reactants, initially present in their standard states, to all products in their standard states. G is the free energy change for other concentrations and pressures.

34. The Relationship Between  G o and the Equilibrium Constant  The relationship between G and G o is  G = G o + RTlnQ where  R = universal gas constant(8.314J/moleK)  T = temperature in K  Q = reaction quotient

35. The Relationship Between  G o and the Equilibrium Constant  When a system is at equilibrium,  G = 0 and Q = K. Then:  0 = G o + RTlnK  Rearranging gives  G o = -RTlnK

36. FORWARD REACTION REVERSE REACTION Table 20.2 The Relationship Between  G 0 and K at 25 0 C  G 0 (kJ) KSignificance 200 100 50 10 1 0 -10 -50 -100 -200 9x10 -36 3x10 -18 2x10 -9 2x10 -2 7x10 -1 1 1.5 5x10 1 6x10 8 3x10 17 1x10 35 Essentially no forward reaction; reverse reaction goes to completion Forward and reverse reactions proceed to same extent Forward reaction goes to completion; essentially no reverse reaction

37. The Relationship Between  G o and the Equilibrium Constant  Calculate the value for the equilibrium constant, K p, for the following reaction at 298K.  N 2 O 4 (g) = 2NO 2 (g)  At 25 o C and 1.00 atmosphere pressure, K p =4.3x10 -13, for the decomposition of NO 2. Calculate G o at 25 o C.