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THERMODYNAMICS

THERMODYNAMICS. Internal Energy Enthalpy Entropy Free Energy Chapter 17 (McM) Chapter 20 Silberberg. Goals & Objectives. See the following Learning Objectives on page 914. Understand these Concepts: 20.1-22. Master the Skills: 20.1-10. Thermodynamics.

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THERMODYNAMICS

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  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,DE • Internal energy represents the total energy of a system. • DE = q(heat flow) + w(work) • Work is usually defined as PDV • If the work term is 0 (no work done) then at constant volume DE = q

  10. Limitations of the First Law of Thermodynamics DE = q + w Euniverse= Esystem + Esurroundings DEsystem = -DEsurroundings DEsystem + DEsurroundings = 0 = DEuniverse The total energy-mass of the universe is constant. However, this does not tell us anything about the direction of change in the universe.

  11. Enthalpy • The change in enthalpy (DH) is measured at constant P. • At constant P: DH = q

  12. water . Ba(OH)2 8H2O(s) + 2NH4NO3(s) Ba2+(aq) + 2NO3-(aq) + 2NH3(aq) + 10H2O(l) Figure20.1 A spontaneous endothermic chemical reaction. DH0rxn = +62.3 kJ

  13. Enthalpy Change • DH = SDHfo (products) - SDHfo(reactants) • where DHfo is the standard molar enthalpy of formation and DH is the enthalpy change for the reaction.

  14. Enthalpy Change • Calculate the enthalpy change for the following reaction at 298K. • C3H8(g) + 5O2(g) ----> 3CO2(g) + 4H2O(l)

  15. 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.

  16. The Second Law of Thermodynamics • Likely

  17. The Second Law of Thermodynamics • Unlikely

  18. more order less order solid liquid gas more order less order crystal + liquid ions in solution more order less order crystal + crystal gases + ions in solution 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.

  19. 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 DS is _______, disorder increases. • When DS is _______, disorder decreases.

  20. 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. • DS = SSo(products) - SSo(reactants)

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

  22. Changes in Entropy • Calculate the entropy change for the following reaction at 298K. Indicate whether disorder increases or decreases. • 2NO2(g) -----> N2O4(g)

  23. Free Energy Change, DG • 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.

  24. The Gibbs Free Energy Change • At constant T and P • DG = DH - TDS • When DG is > 0, the reaction is nonspontaneous • When DG is = 0, the reaction is at equilibrium • When DG is < 0, the reaction is spontaneous

  25. Gibbs Free Energy Change • The following relationship exists for standard molar Gibbs free energy Gibbs Free Energy Change changes: • DGo = SDGfo(products) - SDGfo(reactants)

  26. 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. • C3H8(g) + 5O2(g) ----> 3CO2(g) + 4H2O(l)

  27. - + - - + - + + + + - + or - - - + + or - Table 20.1 Reaction Spontaneity and the Signs of DH0, DS0, and DG0 DH0 DS0 -TDS0 DG0 Description Spontaneous at all T Nonspontaneous at all T Spontaneous at higher T; nonspontaneous at lower T Spontaneous at lower T; nonspontaneous at higher T

  28. The Gibbs Helmholtz Equation • Calculate DSo for the following reaction at 298K. • C3H8(g) + 5O2(g) ----> 3CO2(g) + 4H2O(l) • From previous examples we found • DHo = -2219kJ and • DGo = -2107kJ • Indicate whether disorder increases or decreases

  29. The Relationship Between DGo and the Equilibrium Constant • The standard free energy change for a reaction is DGo. 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. DG is the free energy change for other concentrations and pressures.

  30. The Relationship Between DGo and the Equilibrium Constant • The relationship between DG and DGo is • DG = DGo + RTlnQ where • R = universal gas constant(8.314J/moleK) • T = temperature in K • Q = reaction quotient

  31. The Relationship Between DGo and the Equilibrium Constant • When a system is at equilibrium, • DG = 0 and Q = K. Then: • 0 = DGo + RTlnK • Rearranging gives • DGo = -RTlnK

  32. Essentially no forward reaction; reverse reaction goes to completion Forward and reverse reactions proceed to same extent FORWARD REACTION REVERSE REACTION Forward reaction goes to completion; essentially no reverse reaction Table 20.2 The Relationship Between DG0 and K at 250C DG0(kJ) K Significance 200 9x10-36 100 3x10-18 50 2x10-9 10 2x10-2 1 7x10-1 0 1 -1 1.5 -10 5x101 -50 6x108 -100 3x1017 -200 1x1035

  33. The Relationship Between DGo and the Equilibrium Constant • Calculate the value for the equilibrium constant, Kp, for the following reaction at 298K. • N2O4(g) = 2NO2(g) • At 25oC and 1.00 atmosphere pressure, Kp=4.3x10-13, for the decomposition of NO2. Calculate DGo at 25oC.

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