2. Chapter 19 Notes Chemical Thermodynamics- Entropy and The Laws of Thermodynamics

3. Spontaneous Reactions CH 4(g) + 2 O 2(g) → CO 2(g) + 2 H 2 O (l) H 2 O (s) → H 2 O (l) at 25 o C Factors Affecting Spontaneity: 1. Energy Factor: at 25 o C, 1 atm, exothermic reactions are ordinarily spontaneous (ΔH<0) 2. Randomness factor: other things being equal, a system tends to move from a more ordered to a more random structure.

4. Entropy Changes ΔS = S products - S reactants = a measure of change in randomness. solid → liquid, ΔS + liquid → gas, ΔS + ΔS is usually positive for a reaction in which the number of moles of a gas or aqueous increases. 2 SO 3(g) → 2 SO 2(g) + O 2(g) Δn = +1, ΔS+ N 2(g) + 3 H 2(g) → 2 NH 3(g) Δn = -2, ΔS- Entropy prediction

5. Ex Calculate ΔS for the following reaction using tabulated entropy values on pgs. 1112 to 1114. Fe 2 O 3(s) + 3 H 2(g) → 2 Fe (s) + 3 H 2 O (g)

6. Reactions for which ΔS is positive tend to be spontaneous, at least at high temperatures. H 2 O (s) → H 2 O (l) ΔS > 0 H 2 O (l) → H 2 O (g) ΔS > 0 Fe 2 O 3(s) + 3 H 2(g) → 2 Fe (s) + 3 H 2 O (g) ΔS > 0 All of these reactions are endothermic (ΔH > 0). They become spontaneous at high temperatures. Ammonium dichromate volcano

7. Second Law of Thermodynamics The entropy of the universe increases in any spontaneous process. Reversible processes have a zero entropy change for the universe but are not real processes, just idealized thought experiments. First Law of Thermodynamics The energy of the universe is conserved

8. Third Law of Thermodynamics The entropy of a pure crystalline substance at 0 K is zero.

9. Notes quiz https://docs.google.com/spreadsheet/embeddedform?formkey=dEx2aUFwaWRZQ nZtdUNhc3lWNE9WN1E6MQ