1. Entropy & Chemical Reactions

2. 2nd Law of Thermodynamics
A process will be spontaneous is the entropy of the universe increases
Now we will look at entropy regarding to chemical reactions

3. What is the sign for S?
N2(g) + 3H2(g)  2NH3(g)
First look at the ____________
If all state are the same…then look at # ____________

4. What is the sign for S?
4NH3 + 5O2  4NO + 6H2O

5. Calculating S Calculating S is just like calculating H
S° of any element or diatomic molecule is NOT zero.
You must look these up!

6. Example Calculate S for the following reaction:
2NiS(s) + 3O2(g)  2SO2(g) + 2NiO (s)

7. Example Calculate S for the following reaction:
Al2O3(s) + 3H2(g)  2Al(s) + 3H2O(g)

8. Gibbs Free Energy & Chemical Reactions
You can calculate G in 3 ways…
Like Hess’s Law
Like H°
With the equation G = H - T S

9. Example Calculate H, S, & G at 25°C using the following data…
2SO2 + O2  2SO3
Substance
H (KJ/mol)
S (J/K mol)
SO2
-297
248
SO3
-396
257 O2
205

10. Calculate G Using the following data at 25°C
Cdiamond + O2(g)  CO2 (g) G = -397KJ
Cgraphite + O2(g)  CO2 (g) G = -394KJ
Calculate G for the reaction:
C diamond  C graphite

11. Calculating G
Methanol is a high octane fuel used in high performance racing engines. Calculate G for the following reaction
2 CH3OH(g) + 3O2(g)  2CO2(g) + 4H2O(g)
Given the following free energies of formation:
Substance
G°(KJ/mol)
CH3OH (g)
-163
O2 (g)
CO2 (g)
-394
H2O (g)
-229