# Chap 20 Thermodynamics(ppt)

Mechanics

Oct 27, 2013 (4 years and 8 months ago)

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Thermodynamics

Chapter 20

Thermodynamics

Prediction of whether change will occur

No indication of timeframe

Spontaneous:

occurs without external intervention

Nonspontaneous:

requires outside “push”

Entropy and Spontaneity

Driving force for a spontaneous change is an

increase in entropy of the universe

Entropy, S
:

measure of disorder

Spontaneous change implies:

more order

less order

fewer ways of arranging particles

more

Second Law of Thermodynamics

In any spontaneous change, there is always an
increase in entropy of the universe.

Units:

J

K

Entropy

1877 Ludwig Boltzmann:

k = Boltzmann constant, R/N
A

W =

no. of possible arrangements

Third Law of Thermodynamics:

The entropy of a perfect crystal at 0 K is zero.

Positional Entropy

Why does a gas expand into a vacuum?

Expanded state has highest positional probability
of states available.

Other factors in entropy

Size:

increase in S with increasing size (mass)

Molecular complexity:

increase in S with increasing complexity

Generally effect of physical state >> complexity

Reactions

For a spontaneous reaction:

NaOH
(s)

+ CO
2(g)

Na
2
CO
3(s)

+ H
2
O
(l)

S
0

64.45 213.7 139 69.94 J/K

Is the reaction spontaneous as written?

Spontaneity and S

Spontaneous:

S
univ

> 0

Nonspontaneous:

S
univ

< 0

At equilibrium:

S
univ

= 0

S
sys

can be positive if

S
surr

increases enough

Surroundings and S
univ

Surroundings add or remove heat

Exothermic:

q
sys

< 0

q
surr

> 0

so

S
surr

> 0

Endothermic:

q
sys

> 0

q
surr

< 0

so

S
surr

< 0

S
surr

and

S
sys

S
surr
:

S
surr

-

q
sys

S
surr

1/T

At constant pressure:

The Math

Given:

@constant P:

Multiply by

T:

Result:

Reactions and

G

G
0
:

Standard Free Energy

Reactants in standard states are

converted to products in standard states

Gibb’s Free Energy

Overall criterion for spontaneity

from the standpoint of the system

A process at constant temp. and pressure is
spontaneous in the direction

G decreases

G =

-

T

S

H

S

G

Spontaneous?

“Good”:

H < 0

“Good”:

S > 0

“Good”:

G < 0

“Good”:

G < 0

-

+

-

At all
temperatures

-

-

?

At low
temperatures

+

+

?

At high
temperatures

+

-

+

Not at any
temperature

Summary

G < 0

Spontaneous as written

G > 0

Not spontaneous as written

Reverse process spontaneous

G = 0

At equilibrium

A Closer Look…

T

energy not avail. for doing work

G:

E avail. as heat

E not avail. for work

max. work available (constant T and P)

Amount of work actually obtained depends on path

G and Work

G

Spontaneous

max. work obtainable

Nonspontaneous

min. work required

Work and path
-
dependence

w
max

(
w
min
)

process performed reversibly

theoretical

w
actual
<
w
max

performed irreversibly

real world

Reversible vs. Irreversible Processes

Reversible:

The universe is exactly the same as it was before
the cyclic process.

Irreversible:

The universe is different after the cyclic process.

All real processes are irreversible.

Some work is changed to heat.

Free Energy and Pressure

Q:

reaction quotient from mass action law

Free Energy and Equilibrium

K:

equilibrium constant

At

equilibrium:

G = 0

K = Q

A B

G and Extent of Reaction

A B

G
0
B

<

G
0
A

Spontaneous

C D

G
0
D
>

G
0
C

Nonspontaneous

Temperature Dependence of K

Plot lnK vs. 1/T

slope =
-

H
0
/R

intercept =

S
0
/R

*assumes

H
0
,

S
0

relatively T independent