Prentice-Hall © 2007 General Chemistry: Chapter 19 Slide 1 of 44 Dr. Mendenhall Lecture 1 April 5, 2010 CHEMISTRY Ninth Edition GENERAL Principles and.

Prentice-Hall © 2007General Chemistry: Chapter 19Slide 1 of 44

Dr. Mendenhall

Lecture 1

April 5, 2010

CHEMISTRYNinth

Edition GENERAL

Principles and Modern Applications

Petrucci • Harwood • Herring • Madura

Chapter 19: Spontaneous Change: Entropy and Free Energy

Prentice-Hall © 2007General Chemistry: Chapter 19Slide 2 of 44

Objectives

1. Qualitatively and quantitatively predict whether reactions are spontaneous or nonspontaneous based on their H or S values.

2. Determine G from tabulated data

3. Use G = H =TS, to determine G at various temperatures.

Prentice-Hall © 2007General Chemistry: Chapter 19Slide 3 of 44

Criteria for Spontaneous Change:The Second Law of Thermodynamics.

ΔStotal = ΔSuniverse = ΔSsystem + ΔSsurroundings

The Second Law of Thermodynamics:

ΔSuniverse = ΔSsystem + ΔSsurroundings > 0

All spontaneous processes produce an increase in the entropy of the universe.

Prentice-Hall © 2007General Chemistry: Chapter 19Slide 4 of 44

Ex. Entropy Changes in the System

Predict whether the entropy change of the system in each of the

following reactions is positive or negative:

A) Ag+(aq) + Cl-(aq) AgCl(s)

B) NH4Cl(s) NH3(g) + HCl(g)

The Ag+ and Cl- ions are free to move in solution, whereas AgCl

is a solid. Furthermore, the number of particles decreases from left To right. Therefore, S is negative.

Since the solid is converted to a gas, S is positive.

Prentice-Hall © 2007General Chemistry: Chapter 19Slide 5 of 44

Free Energy and Free Energy Change

Hypothetical process: only pressure-volume work, at constant T and P.

qsurroundings = -qp = -ΔHsys

Make the enthalpy change reversible. large surroundings, infinitesimal change in temperature.

Under these conditions we can calculate entropy.

Prentice-Hall © 2007General Chemistry: Chapter 19Slide 6 of 44

Free Energy and Free Energy Change

TΔSuniv. = TΔSsys – ΔHsys = -(ΔHsys – TΔSsys)

-TΔSuniv. = ΔHsys – TΔSsys

G = H - TS

ΔG = ΔH - TΔS

For the universe:

For the system:

ΔGsys = - TΔSuniverse

Prentice-Hall © 2007General Chemistry: Chapter 19Slide 7 of 44

Criteria for Spontaneous Change

ΔGsys < 0 (negative), the process is spontaneous.

ΔGsys = 0 (zero), the process is at equilibrium.

ΔGsys > 0 (positive), the process is non-spontaneous.

J. Willard Gibbs

1839-1903

Prentice-Hall © 2007General Chemistry: Chapter 19Slide 8 of 44

Table 19.1 Criteria for Spontaneous Change

Prentice-Hall © 2007General Chemistry: Chapter 19Slide 9 of 44

Standard Free Energy Change, ΔG°

The standard free energy of formation, ΔGf°. The change in free energy for a reaction in which a

substance in its standard state is formed from its elements in reference forms in their standard states.

The standard free energy of reaction, ΔG°.

ΔG° = [ p ΔGf°(products) - r ΔGf°(reactants)]

Prentice-Hall © 2007General Chemistry: Chapter 19Slide 10 of 44

ExampleCalculate the standard free-energy (G) changes for the following

reactions at 25ºC.

A) CH4(g) + O2(g) CO2(g) + 2H20(l)

B) 2MgO(s) 2Mg(s) + O2(g)

C) H2(g) + Br2(g) 2HBr(g)

Prentice-Hall © 2007General Chemistry: Chapter 19Slide 11 of 44

Example

a) Predict the direction in which the G for the equilibrium reaction of

N2(g) + 3H2(g) 2NH3(g)

will change with the increase of temperature. B) Calculate G at 500ºC

Assuming that H and S do not change with temperature.