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