Thermodynamics

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Oct 27, 2013 (3 years and 7 months ago)

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Thermodynamics

Brown,
LeMay

Ch 19

AP Chemistry

Monta Vista High School

Review

2

1
st

Law of Thermodynamics


In any process energy is neither created nor destroyed.


When a system changes from one state to another (
D
E =
q + w), it

1.
Gains heat (+ q) or loses heat (
-

q) and/or

2.
Does work (
-

w) or has work done on it (+ w)


T and internal energy, E, are state functions (depend only
on initial and final states of system and not path taken
between them).


q and w are not state functions.


But
why

does a reaction occur in a particular direction?

19.1: Spontaneous Processes

3


Reversible reaction:
can proceed forward and
backward along same path (equilibrium is possible)


Ex: H
2
O freezing & melting at 0ºC



Irreversible reaction:
cannot proceed forward and
backward along same path


Ex: ice melting at room temperature


Spontaneous reaction:
an irreversible reaction that occurs
without outside intervention


Ex: Gases expand to fill a container, ice melts at room
temperature (even though endothermic), salts dissolve in water

19.2: Molecules & Probability

4


Spontaneity of a reaction is related to the number of
possible
states

a system can have.

Ex: 2 gas molecules are placed in a two
-
chambered container, yielding
4 possible states:


There is a ½ probability that one molecule will be in
each chamber, and a ¼, or (1/2)
2
, probability that both
will be in the right
-
side chamber.

5


With 3 molecules:


There is a ¾ probability that one molecule will be in one
chamber and two in the other, and only a 1/8, or (1/2)
3
,
probability that all 3 molecules will be in the right
-
side
chamber.

All on left Evenly distributed All on right

Frequency

6

All on left Evenly distributed All on right

Frequency


As the number of molecules increases to 100, a bell
-
shaped distribution of probable states, called a
Gaussian

distribution, is observed.

# molecules = 100

Carl Gauss

(1777
-
1855)

7

All on left Evenly distributed All on right

Frequency

# molecules = 10
23


Expanding this to 1 mole of molecules, there is only a (1/2)
10^23
probability that every molecule will be in the right
-
side
chamber.


T
he Gaussian distribution is so narrow that we often forget
that it is a distribution at all,

thinking of the most probable
state as a necessity.

This demonstrates that:


The most probable
arrangements are those in
which the molecules are
evenly distributed.



Processes in which the
disorder

of the system
increases tend to occur
spontaneously.

8

spontaneous

non
-
spontaneous


These probability
distributions apply to the
motion and energy of
molecules, and thus can
predict the
most probable

flow of heat.



We call a process
spontaneous

if it
produces a more probable
outcome, and
non
-
spontaneous
if it
produces a less likely one.

9

spontaneous

non
-
spontaneous


high K.E.


low K.E.

evenly distributed K.E.

Entropy

10

Entropy (S):
a measure of molecular randomness or disorder


S is a state function:
D
S = S
final
-

S
initial

+
D
S

= more randomness

-

D
S

= less randomness



For a reversible process that occurs at constant
T:





Units: J/mol.K

2
nd

Law of Thermodynamics

11


The entropy of the universe increases in a
spontaneous process and remains unchanged in a
reversible (equilibrium) process.


S is not conserved; it is either increasing or constant



Reversible reaction:


D
S
UNIVERSE
= S
SYS
+ S
SURR
= 0



or


S
SYS
=
-

S
SURR



Irreversible reaction:


D
S
UNIV
= S
SYS
+ S
SURR
> 0

Examples of spontaneous reactions:

12


Gases expand to fill a container:





Ice melts at room temperature:





Salts dissolve in water:


Particles are more evenly distributed








Particles are no longer in an ordered crystal lattice

Ions are not locked in crystal lattice

19.3: 3
rd

Law of Thermodynamics

13


The entropy of a crystalline solid at 0 K is 0.


How to predict
D
S
:


S
gas

> S
liquid
> S
solid


S
more gas molecules
> S
fewer gas molecules


S
high T

> S
low T



Ex: Predict the sign of
D
S for the following:

1.
CaCO
3
(s)


CaO (s) + CO
2
(g)

2.
N
2
(g) + 3 H
2
(g)


2 NH
3
(g)

3.
N
2
(g) + O
2
(g)


2 NO (g)

+, solid to gas

-
, fewer moles produced

?

19.4: Standard Molar Entropy, Sº

14


Standard state (º): 298 K and 1 atm


Units = J/mol∙K


D

f

of all elements = 0 J/mol


However, S
°

of all elements ≠ 0 J/mol·K



See Appendix C for list of values.


Where n and m are coefficients in the balanced chemical equation.

19.5: Gibbs free energy, G

15


Represents combination of two

forces that drive a reaction:

D
H (enthalpy) and
D
S (disorder)


Units: kJ/mol


D
G =
D

-

T
D
S


D
G
°

=
D
H
°

-

T
D
S
°


(absolute T)

Josiah Willard Gibbs

(1839
-
1903)

Determining Spontaneity of a Reaction

16

If
D
Gi猠:
ion)


Positive


Forward reaction is non
-
spontaneous; the reverse
reaction is spontaneous



Zero

The system is at equilibrium

19.6: Free Energy & Temperature

17


D
G depends on enthalpy, entropy, and temperature:

D
G =
D
H
-

T
D
S

D
H

D
S

D
Gandreactionoutcome

-


+

Always (
-


2 O
3

(g)


3 O
2

(g)

+


-

Always +; non
-
spontaneous at all T




3 O
2

(g)


2 O
3

(g)

-


-

Spontaneous at low T; non
-
spontaneous at high T




H
2
O (l)


H
2
O (s)

+


+

Spontaneous only at high T ; non
-
spontaneous at



low T




H
2
O (s)


H
2
O (l)

19.7: Free Energy &
Equilibria

18


Nernst Equation
The value of
D
G determines where the
system stands with respect to equilibrium.

D
G =
D
G
°

+ RT ln Q
(Nernst Equation)

where R = 8.314 J/K

mol

-
Used for calculating
D
G
under experimental conditions from
standard conditions
D
G
°
.

-
How do you calculate
D
G
°

?

-
Nernst Equation when the system is at equilibrium:
Note that
D
G
becomes zero at equilibrium and not
D
G
°






19.7: Free Energy & Equilibria

19

D
G

Reaction outcome

Negative

Spontaneous forward rxn, K > 1


Positive

Non
-
spontaneous forward rxn, K < 1


Zero

System is at equilibrium, K = 1