thermodynamics is the only science about which i am...

OFP Chapter 11 111/1/2004

Thermodynamics is the only science about which I am firmly convinced that, within the framework of the applicability of its basic principles, it will never be overthrown

- Albert Einstein


Chapter 11Spontaneous Change and

Equilibrium• 11-1 Enthalpy and Spontaneous Change • 11-2 Entropy• 11-3 Absolute Entropies and Chemical

Reactions• 11-4 The Second Law of Thermodynamics• 11-5 The Gibbs Function• 11-6 The Gibbs Function and Chemical

Reactions• 11-7 The Gibbs Function and the

Equilibrium Constant• 11-8 The Temperature Dependence of

Equilibrium Constants


• This entire Chapter deals with a concept called Entropy and the Gibbs function

• Both are new “state properties”• Entropy = S which has the units

JK-1mol-1


• Consider Energy, it is perhaps obvious that processes occur spontaneously to produce a state of lower energy

• But, a chunk of ice at Room Temperature, spontaneously melts, forming a state of higher Energy

• Apparently more than energy is involved in determining the direction of spontaneous change

Solid __ Liquid __ Gas__

E


• This additional factor is the tendency of a system to assume the most random molecular arrangement possible

• Systems become disordered, more random

• Natural processes are favored which result in– Decreased Energy (favored)– Increased Entropy (favored)

• These two factors can oppose each other. Which one wins out?

)(2)(2 sl

StateOrdered

StateDisordered

OHSOHS

SSEntropy

>

>=


Recall from Chapter 10 lecture notes• Third Law of Thermodynamics

∆ S = 0 at 0°K• At absolute zero the Entropy term

contributes nothing to the direction of spontaneous change

• The most stable state has the lowest energy

• A temperature increases, molecular motion increases and the tendency to disorder becomes more important

• At sufficiently high temperatures the Entropy factor becomes large enough to overcome even an unfavorable energy change


For H2O (s) → H2O (l)• Above Tm, the Entropy is

dominant so spontaneous melting takes place

• Below Tm, the energy decrease is dominant so spontaneous freezing takes place

• Temperature is a critical factor

>Tm SH2O is dominant<Tm EH2O is dominant


Disorder and Entropy• Entropy is a quantitative measure of

the number of microstates available to the molecules in a system. It is a measure of the number of ways in which Energy can be arranged.

• Entropy is the degree of randomness or disorder in a system

• The Entropy of all substances is positive

Ssolid < S liquid < Sgas

• ∆S is the Entropy Change in a system

• Entropy = S which has the units JK-1mol-1


For phase transitions, at temperature = T, underequilibrium conditions– Melting (solid to liquid)– Fusion (liquid to solid)– Vaporization (liquid to gas)– Condensation (gas to liquid)

• ∆S =∆H/T units are JK-1mol-1

• E.g., ∆Svap =∆H/Tb at boiling point• Define S° as the absolute molar

Entropy, which is the absolute entropy of 1 mol of a substance in standard state

• Appendix D gives standard molar entropy values, S° in units JK-1mol-1

OFP Chapter 11 1011/1/2004

OFP Chapter 11 1111/1/2004

Entropies of Reaction∆Srxn

° = ΣS°products – ΣS°reactants

∆Srxn° is the sum of products minus the sum of the reactants

For a general reactiona A + b B → c C + d D

(B)S(A)S

(D)S(C)S∆Soo

ooo

∆∆

∆∆

ba

dc

−−

+=

•Appendix D gives standard molar entropy values, S° in units JK-1mol-1

OFP Chapter 11 1211/1/2004

Exercise 11-3(a) Use Data from Appendix D to

calculate ∆Sr° at 298.15 K for the reaction

2H2S(g) + 3O2(g) → 2SO2(g) + 2H2O(g)

(b) Calculate ∆S° of the system when 26.71 g of H2S(g) reacts with excess O2(g) to give SO2(g) and H2O(g) and no other products at 298.15K

OFP Chapter 11 1311/1/2004

Exercise 11-3(a) Use Data from Appendix D to

calculate ∆Sr° at 298.15 K for the reaction

2H2S(g) + 3O2(g) → 2SO2(g) + 2H2O(g)Solution(a)∆Sr°= 2S°(SO2(g) ) + 2S°(H2O(g))

-2S°(H2S(g) ) - 3S°(O2(g))∆Sr°= 2(248.11)+2(188.72)

-2(205.68)-3(205.03)= – 152.8 JK-1mol-1

Notice that this is minus, which is consistent with 5 to 4 decrease in the amount of gas

OFP Chapter 11 1411/1/2004

Exercise 11-32H2S(g) + 3O2(g) → 2SO2(g) +2H2O(g)

(a) ∆Sr= -152.8 JK-1mol-1

(b) Calculate ∆S° of the system when 26.71 g of H2S(g) reacts with excess O2(g) to give SO2(g) and H2O(g) and no other products at 298.15K

Solution:

1

1

22

22

60.0JK∆S

reaction 1mol)152.8JK(

SH mol 2reaction 1mol

SH g 34SH mol 1 SH 26.71g S

−

−

−=°

−•••=°∆

OFP Chapter 11 1511/1/2004

PRS Quiz

Entropy is the thermodynamic property which constitutes a quantitative measure of the

1. heat content of a system. 2. degree of disorder of a system.3. internal energy of system. 4. chaos on Halloween night

OFP Chapter 11 1611/1/2004

PRS Quiz

For a given substance, the most disordered physical state is

1. gas 2. liquid 3. solid4. californium

OFP Chapter 11 1711/1/2004

PRS Quiz

Which of the following processes should show the greatest increase in entropy?

1. C6H6 (g) + 15/2 O2 (g) → 6CO2 (g) + 3H2O (g)

2. 2NO2 (g) → N2O4 (g)

3. C2H4 (g) + H2 (g) → C2H6 (g)

4. BaS (s) + 2 NaNO3 (s) → Ba(NO3)2 (s) + Na2S (s)

OFP Chapter 11 1811/1/2004

Chapter 11Spontaneous Change and

Equilibrium• Second Law of Thermodynamics

In a real spontaneous process the Entropy of the universe (meaning the system plus its surroundings) must increase.∆Suniverse > 0 (spontaneous process)∆Suniverse = ∆Ssystem + ∆Ssurroundings > 0

• if ∆Suniverse = 0, then everything is in equilibrium

• The 2nd Law of Thermodynamics profoundly affects how we look at nature and processes

thermodynamics is the only science about which i am...

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