Chapter 19
Chemical Thermodynamics
Spontaneous Processes
• Spontaneous processes proceed
without any outside assistance
(thermodynamically favored) and
are irreversible .
• Temperature and pressure can
affect spontaneity.
Entropy
• Entropy is a measure of the randomness in a system.
• Entropy of the universe increases in any spontaneous processes.
+S = final state is more disordered than original
-S = initial state is more disordered than final
S° = S°(products) – S°(reactants)
Effect of Volume and Temperature on Entropy
• If we increase volume, there are more positions possible for the
molecules so increased entropy.
• If we increase temperature, the average kinetic energy increases
which increases entropy.
Entropy and Physical States
• Entropy increases with the freedom of motion of molecules
S(g) > S(l) > S(s)
• Entropy increases with the number of molecules or molecular
complexity
Is S positive or negative?
1) HCl(g) + NH3(g)  NH4Cl(s)
2) 2SO2(g) + O2(g)  2SO3(g)
3) Cooling N2(g) from 20C to -50C
Is S positive or negative?
1) HCl(g) + NH3(g)  NH4Cl(s)
2) 2SO2(g) + O2(g)  2SO3(g)
3) Cooling N2(g) from 20C to -50C
Entropy is negative in all cases (more ordered)
Use Appendix C
What is S for the following reaction?
4HCl(g) + O2(g)  2Cl2(g) + 2H2O(l)
What is S for the following reaction?
4HCl(g) + O2(g)  2Cl2(g) + 2H2O(l)
[2(222.96) + 2(69.91)] –[4(186.69) + 205] = -366.02 J/K
https://youtu.be/MALZTPsHSoo
Video on Entropy
Gibbs Free Energy (amount of energy in a reaction that
can be used for work):
-G = thermodynamically favored, no outside
source of energy is necessary (spontaneous),
product formation favored
G = 0 the reaction is at equilibrium.
+G = not thermodynamically favored
(nonspontaneous), reactant formation favored
Standard Free Energy Changes
G° = Gf° products – Gf° reactants
Use Appendix C
Calculate G for:
4HCl(g) + O2(g)  2Cl2(g) + 2H2O(g)
Calculate G for:
4HCl(g) + O2(g)  2Cl2(g) + 2H2O(g)
[2(0) + 2(-228.57)] – [4(-95.27) + 0] = -76.06 kJ
Free Energy Changes
G° = H°- TS°
• Since reactions are spontaneous if ΔG < 0, the sign of enthalpy and entropy
and the magnitude of the temperature matters to spontaneity.
Free Energy and Equilibrium
Under standard conditions:
G° = -RT ln K
G°< 0, K>1 (favors products)
G°> 0, K<1 (favors reactants)
Determine Keq at 25C for the reaction:
2NO2(g) ⇌N2O4(g)
Determine Keq at 25C for the reaction:
2NO2(g) ⇌N2O4(g)
98.28 kJ – 2(51.84) = -5.4 kJ
-5400 J = (-8.314 J/molK)(298 K) lnKeq
Keq = 8.84
https://youtu.be/huKBuShAa1w
Video on ΔG