Chap 20 Thermodynamics(ppt)

bronzekerplunkMechanics

Oct 27, 2013 (3 years and 11 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