1. Enthalpy (H) is a measure of the total energy of a system at constant pressure. It can be used to determine if a chemical reaction is exothermic or endothermic. 2. Entropy (S) is a measure of disorder or randomness in a system. Reactions that increase disorder have a positive change in entropy. 3. Gibbs free energy (G) takes into account both enthalpy and entropy to determine if a reaction is spontaneous. A reaction is spontaneous if the change in Gibbs free energy (ΔG) is negative.

1. ENTHALPY, ENTROPY, &
GIBB’S FREE ENERGY

2. Chemical energy is stored in:
– moving electrons
– vibration of chemical bonds
– rotation and translation of
molecules
– stored nuclear energy of protons
& neutrons
– energy stored in chemical bonds
ENTHALPY

3. Enthalpy
enthalpy (H) – total kinetic and potential energy of a
system at a constant pressure
change in enthalpy ( H) – change in heat of a system
H = Hfinal - Hinitial
H = Hproducts – Hreactants
The units for enthalpy are in J or kJ per mol (i.e. kJ/mol)
ENTHALPY

4. Enthalpy
For an exothermic reaction:
Hproducts < Hreactants
H is negative
Describe what has happened.
H
reaction progress
reactants
products
ENTHALPY

5. Enthalpy
For an endothermic reaction:
Hproducts > Hreactants
H is positive
Describe what has happened.
H
reaction progress
reactants
products
ENTHALPY

6. ENTHALPY
N2O4 (g)  2 NO2 (g) H° = +57.93 kJ
2 NO2 (g)  N2O4 (g) H° = -57.93 kJ
Reversing a chemical reaction causes a sign
change in front of the H° value.
Enthalpy

7. ENTROPY

8. entropy ( S)- a measure of disorder or randomness
entropy =  disorder
The units for entropy are J/mol·K or J/K
ENTROPY

9. ENTROPY
SECOND LAW OF
THERMODYNAMICS
the entropy of the
universe is constantly
increasing

10. S solid < S liquid < S gas
ENTROPY AND GIBBS FREE ENERGY

11. ΔS = Sproducts – Sreactants
When Sproducts>Sreactants, ΔS>0
ENTROPY AND GIBBS FREE ENERGY

12. ΔS = Sproducts – Sreactants
When Sproducts<Sreactants, ΔS<0
ENTROPY AND GIBBS FREE ENERGY

13. ENTROPY
Predict the sign of S for the following reactions:
H2(g)  2 H(g) S
2 H2(g) + O2(g)  2 H2O(l) - S
2 NO2(g)  N2O4(g) - S
C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(g) + S
condensation of steam to liquid - S

14. ENTROPY
Predict the sign of S for the following reactions:
sublimation of dry ice + S
2 NH3(g)  N2(g) + 3 H2(g) + S
C6H12O6(s) + 6O2(g)  6CO2(g) + 6H2O(g) + S
2 H2O(l)  2 H2(g) + O2(g) + S
NaCl(s)  NaCl(aq) + S

15. ENTROPY
Example 1: Calculate the standard entropy of the
following reaction:
C2H4(g) + H2(g)  C2H6(g)
Do you think this reaction will spontaneously occur
at room temperature?
Since molecules tend to
become more and more
disordered, and this reaction
results in more organization
(fewer moles of gas), then
this reaction is most likely
not spontaneous at room
temperature

16. GIBBS FREE ENERGY
How are entropy and enthalpy related?
ΔGº = ΔHº - TΔSº
Gibbs free energy
(J or kJ)
enthalpy Temperature (K)
entropy
Gibbs free energy is the energy that is available to do useful work.
A reaction will spontaneously occur if ΔG<0 (exergonic reaction)
A reaction will NOT spontaneously occur if ΔG>0 (endergonic reaction)

17. GIBBS FREE ENERGY
A reaction will spontaneously occur if ΔG<0 (exergonic reaction)
A reaction will NOT spontaneously occur if ΔG>0 (endergonic reaction)

18. GIBBS FREE ENERGY
Reactions with a negative ΔH and positive ΔS all
have a negative ΔG
ΔGº = ΔHº - TΔSº
(-) - T(+)
Exergonic & spontaneous at all temperatures
Reactions with a positive ΔH and negative ΔS all
have a positive ΔG
ΔGº = ΔHº - TΔSº
(+) - T(-)
Endergonic & nonspontaneous at all temperatures and
will only occur with the continuous input of energy

19. GIBBS FREE ENERGY
Reactions with a negative ΔH and negative ΔS all
have a negative ΔG
ΔGº = ΔHº - TΔSº
(-) - T(-)
ΔG will be negative in temperature conditions where the
value of TΔS is lower than the value of ΔH. Thus the
reaction is spontaneous only at lower temperatures.
Example: 2SO2(g) + O2(g)  2SO3(g)
This reaction is spontaneous at temperatures below
786ºC (1059K) and nonspontaneous above this
temperature

20. GIBBS FREE ENERGY
Reactions with a positive ΔH and positive ΔS all
have a negative ΔG
ΔGº = ΔHº - TΔSº
(+) - T(+)
ΔG will be negative in temperature conditions where the
value of TΔS is higher than the value of ΔH. Thus the
reaction is spontaneous only at higher temperatures.
Example: H2O(s)  H2O(l)
This reaction is spontaneous at temperatures above 0ºC
(273K) and nonspontaneous below this temperature

21. GIBBS FREE ENERGY
ΔH>0 ΔH<0
ΔS>0 Spontaneity depends
on T
(spontaneous at higher
temperatures)
Spontaneous at all
temperatures
ΔS<0 Nonspontaneous
(proceeds only with a
continuous input of
energy)
Spontaneity depends
on T
(spontaneous at lower
temperatures)
Summary: Spontaneous and Non-Spontaneous Reactions

22. GIBBS FREE ENERGY
Example 2: Is the following reaction spontaneous at
SATP?
CO(NH2)2(aq) + H2O(l)  CO2(g) + 2NH3(g)
ΔHº =119kJ
ΔSº = 354.8J/K = 0.3548KJ/K
T = 25ºC = 298K
ΔGº = ΔHº - TΔSº
= (119kJ) – (298K)(0.3548kJ/K)
= (119kJ) – (105.7kJ)
= 13kJ
ΔGº>0, so this reaction is not spontaneous
Therefore this reaction is not spontaneous at SATP

23. GIBBS FREE ENERGY