1. Prentice-Hall © 2007 General Chemistry: Chapter 20 Slide 1 of 54 Juana Mendenhall, Ph.D. Assistant Professor Lecture 2 March 31 Chapter 7: Thermochemistry

2. Prentice-Hall © 2007 General Chemistry: Chapter 20 Slide 2 of 54 Objectives 1.Define work and it’s units. 2.Define, explain, and apply the first law of thermodynamics. 3.Apply the direct and indirect method of the standard enthalpy of formation and reaction.

3. Prentice-Hall © 2007 General Chemistry: Chapter 20 Slide 3 of 54  The enthalpy change that occurs in the formation of one mole of a substance in the standard state from the reference forms of the elements in their standard states.  The standard enthalpy of formation of a pure element in its reference state is 0. Hf°Hf° Standard Enthalpies of Formation

4. Prentice-Hall © 2007 General Chemistry: Chapter 20 Slide 4 of 54 Standard Enthalpy of Formation and Reaction  H is an extensive property.  Enthalpy change is directly proportional to the amount of substance in a system. aA + bB → cC + dD  H rxn = [c  H f (C )+ d  H f (D)] - [a  H f (A) + b  H f (B)]  H rxn = [  n  H f (products)] - [  m  H f (reactants)] Direct Method Example with C(graphite)

5. Prentice-Hall © 2007 General Chemistry: Chapter 20 Slide 5 of 54 Standard States

6. Prentice-Hall © 2007 General Chemistry: Chapter 20 Slide 6 of 54 Standard Enthalpies of Formation

7. Prentice-Hall © 2007 General Chemistry: Chapter 20 Slide 7 of 54 Indirect Determination of  H: Hess’s Law  Hess’s Law states: when reactants are converted to products, the change in enthalpy is the same whether the reaction takes place in one step or in a series of steps. N 2 (g) + O 2 (g) → 2 NO(g)  H = +180.50 kJ ½N 2 (g) + ½O 2 (g) → NO(g)  H = +90.25 kJ  H changes sign when a process is reversed NO(g) → ½N 2 (g) + ½O 2 (g)  H = -90.25 kJ

8. Prentice-Hall © 2007 General Chemistry: Chapter 20 Slide 8 of 54 Hess’s Law  Hess’s law of constant heat summation  If a process occurs in stages or steps (even hypothetically), the enthalpy change for the overall process is the sum of the enthalpy changes for the individual steps. ½N 2 (g) + O 2 (g) → NO 2 (g)  H = +33.18 kJ ½N 2 (g) + ½O 2 (g) → NO(g)  H = +90.25 kJ NO(g) + ½O 2 (g) → NO 2 (g)  H = -57.07 kJ

9. Prentice-Hall © 2007 General Chemistry: Chapter 20 Slide 9 of 54 Hess’s Law Schematically

10. Prentice-Hall © 2007 General Chemistry: Chapter 20 Slide 10 of 54 Functions of State  Any property that has a unique value for a specified state of a system is said to be a State Function. ◦Water at 293.15 K and 1.00 atm is in a specified state. ◦d = 0.99820 g/mL ◦This density is a unique function of the state. ◦It does not matter how the state was established.

11. Prentice-Hall © 2007 General Chemistry: Chapter 20 Slide 11 of 54 7-4 Work  In addition to heat effects chemical reactions may also do work.  Gas formed pushes against the atmosphere.  Volume changes.  Pressure-volume work. w = Fd [w ] = kg m s2s2 = J m  Work

12. Prentice-Hall © 2007 General Chemistry: Chapter 20 Slide 12 of 54 Pressure Volume Work w = F  d = (m  g) (m  g) = P  V w = -P ext  V A  h  h =  A  h  h

13. Prentice-Hall © 2007 General Chemistry: Chapter 20 Slide 13 of 54 Definition of Terms  State of a system: the values of all relevant macroscopic properties-example: energy, temp., pressure, volume.  State function: ppts that are determined by the state of the systems.  U is a function of state.  Not easily measured.  U has a unique value between two states.  Is easily measured.

14. Prentice-Hall © 2007 General Chemistry: Chapter 20 Slide 14 of 54 7-5 The First Law of Thermodynamics  Internal Energy, U.  Total energy (potential and kinetic) in a system. Translational kinetic energy. Molecular rotation. Bond vibration. Intermolecular attractions. Chemical bonds. Electrons.

15. Prentice-Hall © 2007 General Chemistry: Chapter 20 Slide 15 of 54 First Law of Thermodynamics  A system contains only internal energy.  A system does not contain heat or work.  These only occur during a change in the system.  Law of Conservation of Energy  The energy of an isolated system is constant  U = q + w

16. Prentice-Hall © 2007 General Chemistry: Chapter 20 Slide 16 of 54 First Law of Thermodynamics

17. Prentice-Hall © 2007 General Chemistry: Chapter 20 Slide 17 of 54 Sign conventions for work & heat ProcessSign Work done by the system on the surroundings _ Work done on the system by the surroundings + Heat absorbed by the system from the surroundings (endothermic process) + Heat absorbed by the surroundings from the system (exothermic procees) _

18. Prentice-Hall © 2007 General Chemistry: Chapter 20 Slide 18 of 54 Example The work done when a gas is compressed in a cylinder like that show in the figure is 462 J. during this process, there is a heat transfer of 128 J from the gas surroundings. Calculate the energy change for this process.