THERMODYNAMICS

Internal EnergyEnthalpyEntropyFree EnergyChapter 17 (McM)Chapter 20 Silberberg

Goals & Objectives

See the following Learning Objectives on page 914.

Understand these Concepts: 20.1-22.

Master the Skills: 20.1-10.

Thermodynamics the study of the changes in energy and

the transfers of energy that accompany chemical and physical processes.

Addresses three fundamental questions Will 2 or more substances react when they are

mixed under specified conditions? If they do react, what energy changes and

transfers are associated with their reaction? If a reaction occurs, to what extent does it

occur?

Thermodynamics

Used to determine if a reaction can occur under specified conditions. spontaneous reaction--can occur under

the specified conditions nonspontaneous reaction--do not occur

to a significant extent under the specified conditions

First Law of Thermodynamics

The internal energy of an isolated system is constant.

The total amount of energy in the universe is constant.

Some Thermodynamic Terms

System--the substances involved in the chemical and physical changes under investigation

Surroundings--the rest of the universe

Universe--the system and its surroundings

Types of Thermodynamic Systems

Open system--can exchange both matter and energy with its surroundings

Closed system--has a fixed amount of matter but can exchange energy with its surroundings

Isolated system--has no contact with its surroundings

Thermodynamic State of a System

defined by a set of conditions that completely specifies all the properties of the system

State functions--the properties of a system( pressure, temperature, energy, e.g.) whose values depend only on the state of the system

Changes in Internal Energy,E

Internal energy represents the total energy of a system.

E = q(heat flow) + w(work) Work is usually defined as PV If the work term is 0 (no work done)

then at constant volume E = q

Limitations of the First Law of Thermodynamics

E = q + w

Euniverse = Esystem + Esurroundings

Esystem = -Esurroundings

The total energy-mass of the universe is constant.

However, this does not tell us anything about the direction of change in the universe.

Esystem + Esurroundings = 0 = Euniverse

Enthalpy

The change in enthalpy () is measured at constant P.

At constant P: H = q

Figure 20.1 A spontaneous endothermic chemical reaction.

water

Ba(OH)2 8H2O(s) + 2NH4NO3(s) Ba2+(aq) + 2NO3-(aq) + 2NH3(aq) + 10H2O(l).

H0rxn = +62.3 kJ

Enthalpy Change

Hfo (products) -

Hfo(reactants)

where Hfo is the standard molar

enthalpy of formation and H is the enthalpy change for the reaction.

Enthalpy Change

Calculate the enthalpy change for the following reaction at 298K.

C3H8(g) + 5O2(g) ----> 3CO2(g) + 4H2O(l)

The Second Law of Thermodynamics

In spontaneous changes the universe tends toward a state of greater disorder.

In thermodynamics, entropy is a measure of the degree of disorder.

Entropy tends to increase.

The Second Law of Thermodynamics

Likely

The Second Law of Thermodynamics

Unlikely

The Concept of Entropy (S)

Entropy refers to the state of order.

A change in order is a change in the number of ways of arranging the particles, and it is a key factor in determining the direction of a spontaneous process.

solid liquid gasmore order less order

crystal + liquid ions in solution

more order less order

more order less order

crystal + crystal gases + ions in solution

Entropy

Entropy can be indirectly measured. Absolute standard molar entropy

values can be found in the textbook. An increase in entropy corresponds to

an increase in disorder. When S is _______, disorder

increases. When S is _______, disorder

decreases.

Entropy

The Third Law of Thermodynamics states that the entropy of a pure,perfect,crystalline substance at 0K is zero.

The following relationship applies to entropy changes.

S = So(products) - So(reactants)

Figure 20.4 Random motion in a crystal

The third law of thermodynamics.

A perfect crystal has zero entropy at a temperature of absolute zero.

Ssystem = 0 at 0 K

Changes in Entropy

Calculate the entropy change for the following reaction at 298K. Indicate whether disorder increases or decreases.

2NO2(g) -----> N2O4(g)

Free Energy Change, G

If heat is released in a chemical reaction, some of the heat may be converted to useful work. Some of it may be used to increase the order of the universe. If the system becomes more disordered, then more energy becomes available than indicated by enthalpy alone.

The Gibbs Free Energy Change

At constant T and P G = H - TS When G is > 0, the reaction is

nonspontaneous When G is = 0, the reaction is at

equilibrium When G is < 0, the reaction is

spontaneous

Gibbs Free Energy Change

The following relationship exists for standard molar Gibbs free energy Gibbs Free Energy Change changes:

Go = Gfo(products) -

Gfo(reactants)

Gibbs Free Energy Change

Calculate the Gibbs free energy change for the following reaction at 298K. Indicate whether the reaction is spontaneous or nonspontaneous under these conditions.

C3H8(g) + 5O2(g) ----> 3CO2(g) + 4H2O(l)

Table 20.1 Reaction Spontaneity and the Signs of H0, S0, and G0

H0 S0 -TS0 G0 Description

- + - -

+ - + +

+ + - + or -

- - + + or -

Spontaneous at all T

Nonspontaneous at all T

Spontaneous at higher T;nonspontaneous at lower T

Spontaneous at lower T;nonspontaneous at higher T

The Gibbs Helmholtz Equation Calculate So for the following reaction at

298K. C3H8(g) + 5O2(g) ----> 3CO2(g) + 4H2O(l) From previous examples we found Ho = -2219kJ and Go = -2107kJ Indicate whether disorder increases or

decreases

The Relationship Between Go and the Equilibrium Constant

The standard free energy change for a reaction is Go. This is the free energy change that would accompany the complete conversion of all reactants, initially present in their standard states, to all products in their standard states. G is the free energy change for other concentrations and pressures.

The Relationship Between Go and the Equilibrium Constant

The relationship between G and Go is

G = Go + RTlnQ where R = universal gas

constant(8.314J/moleK) T = temperature in K Q = reaction quotient

The Relationship Between Go and the Equilibrium Constant

When a system is at equilibrium, G = 0 and Q = K. Then: 0 = Go + RTlnK Rearranging gives Go = -RTlnK

FO

RW

AR

D R

EA

CT

ION

RE

VE

RS

E R

EA

CT

ION

Table 20.2 The Relationship Between G0 and K at 250C

G0(kJ) K Significance

200

100

50

10

1

0

-1

-10

-50

-100

-200

9x10-36

3x10-18

2x10-9

2x10-2

7x10-1

1

1.5

5x101

6x108

3x1017

1x1035

Essentially no forward reaction; reverse reaction goes to completion

Forward and reverse reactions proceed to same extent

Forward reaction goes to completion; essentially no reverse reaction

The Relationship Between Go and the Equilibrium Constant

Calculate the value for the equilibrium constant, Kp, for the following reaction at 298K.

N2O4(g) = 2NO2(g) At 25oC and 1.00 atmosphere

pressure, Kp=4.3x10-13, for the decomposition of NO2. Calculate Go at 25oC.