2. Unit 10:
Thermodynamics

3. Entropy, S
A measure of how energy is dispersed (spread out) among atoms and molecules in a system.
Think DISORDER !!!
Mathematical expression and units -

4. C (s, diamond) S = 2.38 𝑱 𝑲 𝒎𝒐𝒍 CCl4 (l) S = 214.4 𝑱 𝑲 𝒎𝒐𝒍
Entropy, S
Entropy values are assigned to substances based on the third law of thermodynamics …
3rd Law of Thermo: A perfect crystalline solid at 0 Kelvin has zero entropy, S = 0.
C (s, diamond) S = 𝑱 𝑲 𝒎𝒐𝒍
CCl4 (l) S = 𝑱 𝑲 𝒎𝒐𝒍
CCl4 (g) S = 𝑱 𝑲 𝒎𝒐𝒍

5. Entropy, S Predicting entropy changes: Entropy increases when …
1. Solids  liquids or gases
2. Liquids  gases
3. Less moles  more moles (if all states of matter are the same)
4. Solid  aqueous (dissolving)
5. Mixing or heating occurs

6. Entropy, S Mg (s) + O2 (g)  MgO (s) N2 (g) + 3 H2 (g)  2 NH3 (g)
Is entropy increasing or decreasing in each of the following reaction?
I2 (g)  I2 (l)
Mg (s) + O2 (g)  MgO (s)
N2 (g) H2 (g)  2 NH3 (g)
What is the sign of the entropy change for each reaction?

7. Determining Entropy Values:
The best thing about thermodynamics is that whatever works for one thermo property, works for the others!!!
2 NO (g) + O2 (g)  2 NO2 (g)
Substance
∆Hf S NO
KJ/mol
J/K mol O2
0 KJ/mol
J/K mol
NO2
33.1 KJ/mol
J/K mol
a. Determine the ∆H for the reaction shown.
Endo- or exothermic?
b. Determine the ∆S for the reaction shown.
What you would expect?