Chapter 5: Free Energy and Chemical Thermodynamics

Outline

Free Energy as Available Work

Free Energy as Force toward Equilibrium

Chapter 5: Free Energy and Chemical Thermodynamics

To apply the laws of thermodynamics to chemical reactions and other transformations of matter

X Bai

SDSMT, Physics

Fall 2013

X Bai

Chapter 5: Free Energy and Chemical Thermodynamics

Outline

Free Energy as Available Work

Free Energy as Force toward Equilibrium

1 Free Energy as Available Work Basic tools Electrolysis, Fuel Cells, Batteries Thermodynamic Identities

2 Free Energy as Force toward Equilibrium Non-isolated system

X Bai

Chapter 5: Free Energy and Chemical Thermodynamics

Outline

Basic tools

Free Energy as Available Work

Free Energy as Force toward Equilibrium

What are the typical thermal conditions for chemical reactions and other transformations of matter?

Energy: may vary (reactions may release or absorb heat), chemical reactions happen in non-isolated system Volume: often not, new substances may be created Number of particles: often change in reactions Pressure: often fixed, equal to the environment Temperature: equal to the environment, approximately fixed

We are looking for Tools for fixed T and P !!!

X Bai

Chapter 5: Free Energy and Chemical Thermodynamics

Outline

Basic tools

Free Energy as Available Work

Free Energy as Force toward Equilibrium

To describe the energy budget:

1. Enthalpy :

H U + PV

(1)

H: The total energy to create the system AND let it stay

in the environment with pressure P - Need the work.

2. Helmholtz Free Energy :

F U - TS

(2)

U: The total energy to create the system;

TS = Q: The heat from the environment, with T fixed.

F : energy needed in the format of work IF the

system is created from nothing.

3. Gibbs Free Energy :

G U - TS + PV

(3)

PV is the work done under the constant pressure P.

X Bai

Chapter 5: Free Energy and Chemical Thermodynamics

Outline

Basic tools

Free Energy as Available Work

Free Energy as Force toward Equilibrium

U, H, F , and G are called the Thermodynamic potentials. The correlations among the changes of them under conditions for chemical reactions we are interested:

1, At fix temperature:

F = U - TS = Q + W - TS

(4)

2. At constant pressure:

G = U - TS + PV = Q + W - TS + PV

(5)

Because H = U + PV , in terms of U, we have:

G = H - TS

(6)

In-Class Exercise Ch5-01: Consider the production of ammonia from the reaction N2 + 3H2 2NH3. Assume the reaction was at 298 K and 1 bar . (1) Find out the values of H and S the table on page 404-405 in the textbook; (2) Calculate G for this reaction, (3) Crosscheck the value of G with that in the table.

X Bai

Chapter 5: Free Energy and Chemical Thermodynamics

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