# Notes: Reaction Spontaneity, Thermodynamics

Mechanics

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

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Thermodynamics

Thermodynamics

Thermodynamics

= the study of energy
changes that accompany physical and
chemical changes.

Enthalpy (H):

the total energy “stored”
within a substance

Enthalpy Change (
Δ
H):

a comparison of
the total enthalpies of the product &
reactants.

Δ
H = H
products

-

H
reactants

Exothermic vs. Endothermic

Exothermic reactions/changes: release
energy in the form of heat; have negative
Δ
H signs.

H
2
O
(g)

H
2
O
(l)

Δ
H =
-
2870 kJ

Endothermic reactions/changes: absorb
energy in the form of heat; have positive
Δ
H signs.

H
2
O
(l)

H
2
O
(g)

Δ
H = +2870 kJ

Reaction Pathways

Changes that involve a decrease in
enthalpy are favored!

Endothermic

Exothermic

time

time

E
a

E
a

P

P

R

R

Entropy

Entropy (S): the measure of the degree
of disorder in a system; in nature, things
tend to increase in entropy, or disorder.

Δ
S = S
products

-

S
reactants

All physical & chemical changes involve
a change in entropy, or
Δ
S. (Remember
that a high entropy is favorable)

Entropy

Entropy

Entropy

Entropy

Driving Forces in Reactions

Enthalpy and entropy are DRIVING
FORCES for spontaneous reactions
(rxns that
happen

at normal conditions)

It is the interplay of these 2 driving forces
that determines whether or not a
physical or chemical change will actually
happen.

Free Energy

Free Energy (G): relates enthalpy and
entropy in a way that indicates which
predominates; the quantity of energy that
is available or stored to do work or cause
change.

Free Energy

Δ
G =
Δ
H

T
Δ
S

Where:

Δ
G = change in free energy (kJ)

Δ
H = change in enthalpy (kJ)

T = absolute temp (K)

Δ
S = change in entropy (kJ/K)

Free Energy

Δ
G: positive (+) value means change is
NOT spontaneous

Δ
G: negative (
-
) value means change IS
spontaneous

Relating Enthalpy and Entropy to Spontaneity

Example

Δ
H

Δ
S

Spontaneity

2K + 2H
2
O

2KOH + H
2

-

+

always spon.

H
2
O(g)

H
2
O(l)

-

-

spon. @ lower
temp.

H
2
O(s)

H
2
O(l)

+

+

spon. @
higher temp.

16CO
2
+18H
2
O

2C
8
H
18
+25O
2

+

-

never spon.

Example #1

For the decomposition of O
3
(g) to O
2
(g):

2O
3
(g)

3O
2
(g)

Δ
H =
-
285.4 kJ/mol

Δ
S = 137.55 J/mol∙K @25
°
C

a) Calculate
Δ
G for the reaction.

Δ
G = (
-
285.4
kJ/mol
)

(298
K
)(0.1375
5KJ/mol∙K
)

Δ
G =
-
326.4 kJ

Example #1

For the decomposition of O3(g) to O2(g):

2O
3
(g)

3O
2
(g)

Δ
H =
-
285.4 kJ/mol

Δ
S = 137.55 J/mol∙K @25
°
C

b) Is the reaction spontaneous?

YES

Example #1

For the decomposition of O3(g) to O2(g):

2O
3
(g)

3O
2
(g)

Δ
H =
-
285.4 kJ/mol

Δ
S = 137.55 J/mol∙K @25
°
C

c) Is
Δ
H or
Δ
S (or both) favorable for the
reaction?

Both
Δ
H and
Δ
S are favorable

(both are driving forces)

Example #2

What is the minimum temperature (in
°
C) necessary
for the following reaction to occur spontaneously?

Fe
2
O
3
(s) + 3CO(g)

2Fe(s) + 3CO
2
(g)

Δ
H = +144.5 kJ/mol;
Δ
S = +24.3 J/K∙mol

(Hint: assume
Δ
G =
-
0.100 kJ/mol)

Δ
G =
Δ
H

T
Δ
S

-
0.100
kJ/mol

= (144.5
kJ/mol
)

(T)(0.0243 k
J/K∙mol
)

T = 5951 K
≈ 5950 K

T = 5678
°
C ≈ 5680
°
C