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

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.

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

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)

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

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

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 = kJ ½N 2 (g) + ½O 2 (g) → NO(g)  H = kJ  H changes sign when a process is reversed NO(g) → ½N 2 (g) + ½O 2 (g)  H = kJ

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 = kJ ½N 2 (g) + ½O 2 (g) → NO(g)  H = kJ NO(g) + ½O 2 (g) → NO 2 (g)  H = kJ

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

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 K and 1.00 atm is in a specified state. ◦d = g/mL ◦This density is a unique function of the state. ◦It does not matter how the state was established.

Prentice-Hall © 2007 General Chemistry: Chapter 20 Slide 11 of 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

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

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.

Prentice-Hall © 2007 General Chemistry: Chapter 20 Slide 14 of 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.

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

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

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) _

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.