Thermodynamics 3

2nd Law of Thermodynamics2nd Law of ThermodynamicsThe driving force for a spontaneous process is an increase in the entropy of the universe.

Entropy, S, can be viewed as a measure of randomness, or disorder.

For a given change to be spontaneous, ΔSuniverse must be positive

ΔSuniverse = ΔSsystem + ΔSsurroundings

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 2

http://www.youtube.com/watch?v=5KIhDVLbMeY&feature=related

Entropy on the Molecular Scale

• The number of microstates and, therefore, the entropy tends to increase with increases in– Temperature.– Volume.– The number of independently moving

molecules.

© 2009, Prentice-Hall, Inc.

Entropy on the Molecular Scale

• Molecules exhibit several types of motion:– Translational: Movement of the entire molecule from one

place to another.

– Vibrational: Periodic motion of atoms within a molecule.

– Rotational: Rotation of the molecule on about an axis or rotation about bonds.

© 2009, Prentice-Hall, Inc.

Entropy on the Molecular Scale

• Each thermodynamic state has a specific number of microstates, W, associated with it.

• Entropy is the measure of how many microstates are associated with a particular macroscopic state.

S = k lnW

where k is the Boltzmann constant, 1.38 x 10-23 J/K.

© 2009, Prentice-Hall, Inc.

Entropy on the Molecular Scale

• Ludwig Boltzmann described the concept of entropy on the molecular level.

• The number of microstates available increases

with an increase in VOLUME

TEMPERATURE NUMBER OF MOLECULES

• These factors increase the possible

positions and energies of the molecules. As the number of microstates increase, entropy increases. © 2009, Prentice-Hall,

Inc.

More Disorder???• Entropy of the system increases when

– Gases are formed from either solid or liquids– Liquids or solutions are formed from solids– The number of gas particles increases during

a chemical reaction

SSsolidsolid < S < Sliquidliquid < < < < S < < < < Sgasgas

Third Law of Thermodynamics

The entropy of a pure crystalline substance at absolute zero is 0.

© 2009, Prentice-Hall, Inc.

Standard Entropies

© 2009, Prentice-Hall, Inc.

Standard Molar Entropy is an extensive property (a function of the number of moles) Increases with molar mass Increases with more complex molecules

ΔS = Σ nS (products) –Σ nS(reactants)

Predicting the Sign of Entropy in a System

• a. H2O(l) H2O(g)

• b. Ag+(aq) + Cl-(aq) AgCl(s)

• c. 4 Fe(s) + 3 O2(g) 2Fe2O3 (s)

Thermodynamics 4

Spontaneous Processes

• Spontaneous processes are those that can proceed without any outside intervention.

• The gas in vessel B will spontaneously effuse into vessel A, but once the gas is in both vessels, it will not spontaneously return to vessel B.

© 2009, Prentice-Hall, Inc.

Spontaneous Processes

–Processes that are spontaneous in one direction are non-spontaneous in the reverse direction.

–Spontaneous processes may be fast or slow

– Many forms of combustion are fast

– Conversion of diamond to graphite is slow© 2009, Prentice-Hall,

Inc.

Spontaneous Processes

• Processes that are spontaneous at one temperature may be nonspontaneous at other temperatures.

• Above °C it is spontaneous for ice to melt.

• Below °C the reverse process is spontaneous.

© 2009, Prentice-Hall, Inc.

Spontaneity

• What do these laws (and what we observe) say about entropy and spontaneity?

• The Combination of enthalpy and entropy determines spontaneity of a reaction. Processes in nature are driven in two directions: toward least enthalpy and toward largest entropy.

Process Sign of ΔS system Sign of ΔH system

Product Favored positive negative

Reactants Favored negative positive

Gibbs Free Energy

• -TSuniverse is defined as the Gibbs free energy, ΔG.

• When ΔSuniverse is positive, ΔG is negative.

• Therefore, when ΔG is negative, a process is spontaneous.

© 2009, Prentice-Hall, Inc.

GIBBS FREE ENERGY• ΔG° = ΔH° - TΔS°

• ΔG° Gibbs free energy change measured in Kj mol-1

• ΔH° Enthalpy change measured in Kj mol-1

• T Temperature measured in Kelvin

• ΔS° Gibbs free energy change measured in J K-1 mol-

1

Effect of ΔH and ΔS on spontaneity

ΔG = ΔH - TΔS

Gibbs Free Energy

If G is negative, the forward reaction is spontaneous.

If G is 0, the system is at equilibrium.

If G is positive, the reaction is spontaneous in the reverse direction.

© 2009, Prentice-Hall, Inc.

Predicting with Gibbs Energy

• ΔG rxn = Σ[nΔG (products)] - Σ[nΔG(reactants)]

• ΔG =ΔH – TΔS

• Is the reduction of magnesium oxide, MgO, with carbon a product favored process at 25oC? If not, what temperature does it become so?

• MgO(s) + C(graphite) → Mg(s) + CO(g)

2006 AP Examination, Free-Response Question No. 2.

CO(g) + 1/2 O2(g) → CO2(g)

The combustion of carbon monoxide is represented by the equation above.

(a)Determine the value of the standard enthalpy change, HR xn, for the combustion of CO(g) at 298 K using Hess’ Law and the following information:

C(s) + 1/2O2(g) → CO(g) Δ H = 110.5 kJ /mol

C(s) + O2(g) → CO2(g) ΔH = 393.5 kJ/mol

Answer: ΔH = 283.0 kJ/mol

CO(g) + 1/2 O2(g) → CO2(g)

(b) Determine the value of the standard entropy change, Srxn, for the combustion of CO(g) at 298 K using the information in following table

SRxn = Σ nSProducts - ΣnSReactants

Substance S f[Subst], J mol1 K1

CO(g) 197.7

O2(g) 205.1

CO2(g) 213.7

ANSWER: -86.6 J/K mol

. (d) Is the reaction spontaneous under standard conditions at 298 K? Justify your answer.

(c)Determine the value of the standard free energy change, ΔG, for the reaction at 298 K. Include units with your answerAnswer: Δ G= -257.2 kJ/mol

Answer: Yes. Since ΔG is less than zero the reaction is spontaneous under standard conditions.

This PowerPoint presentation presents materials from Prentice Hall, Houghton Mifflin, Science Geek, YouTube, Google Images.