Entropy & Chemical Reactions
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
What is the sign for S?
• N2(g) + 3H2(g) 2NH3(g)• First look at the states…if you go from a solid
or a liquid to a gas, you will have a + entropy• In this case, all of the states are the same, so
we look at the number of moles• N2(g) + 3H2(g) 2NH3(g)• 1 + 3 vs. 2• The entropy decreases because you go from
4 moles to 2 S is negative
What is the sign for S?
• 4NH3 + 5O2 4NO + 6H2O
• 9 moles vs. 10 molesS increases (+)
Calculating S
• Calculating S is just like calculating H
• Simply use the Appendix…just look at the column for S instead of H
S° of any element or diatomic molecule is NOT zero.
• You must look these up!
Example
• Calculate S for the following reaction:
• 2NiS(s) + 3O2(g) 2SO2(g) + 2NiO (s)
• 2(-53) + 3(-205) + 2(248) + 2(38)
• -149 J/K
Example
• Calculate S for the following reaction:
• Al2O3(s) + 3H2(g) 2Al(s) + 3H2O(g)
• (-51) + 3(-131) + 2(28) + 3(189)
• 179 J/K
Gibbs Free Energy & Chemical Reactions
• You can calculate G in 3 ways…
1. Like Hess’s Law
2. Like H°
3. With the equation G = H - T S
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 0 205
Example
2SO2 + O2 2SO3
H = 2(+297)+(0) + 2(-396) = -198 KJ
S = 2(-248)+ (-205) + 2(257) = -187J/K
G = H – T S
=(-198) – (298 x -0.187)
=-142 KJ
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
G = -3KJ
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) 0
CO2 (g) -394
H2O (g) -229
Calculating G
• 2 CH3OH(g) + 3O2(g) 2CO2(g) + 4H2O(g)
• 2(163) + 3(0) + 2(-394) + 4(-299)
• -1378 KJ