1. Spontaneity & Entropy

2. Spontaneous Changes H2O(s) ---> H2O(l)
Occurs with no outside intervention; the rate of change may be fast or slow
H2O(s) ---> H2O(l)
Reverse rxn is NOT spontaneous
And equilibrium

3. NonSpontaneous Changes
Occur with outside intervention
H2O --> H2 + O2
Reverse rxn IS spontaneous
Reactions are spontaneous in the rv
H2O --> H2 + O2
Demo decomposition of H2O

4. Equilibrium “Product Favored” - more products around at equilibrium
“Reactant Favored” more reactants around at equilibrium
May be spontaneous or not

5. Heat and Spontaneity Exothermic rxn are _____________
Endothermic rxn are ____________

6. Entropy (S) measures the disorder or randomness in a system.
Nature favors disorder
2nd Law of Thermodynamics
- in any process, the entropy of the universe increases

7. Entropy Happens

8. Entropy (S) Universe includes the system & surroundings
∆Suniv = ∆ Ssys + ∆ Ssurr
The entropy of a system may decrease, as long as the surroundings increase

9. Entropy (S) and Spontaneity
For a spontaneous process: ∆ Suniv > 0
For a non-spontaneous process: ∆ Suniv < 0
At equilibrium: ∆ Suniv = 0
Entropy is not conserved, it is continually increasing.

10. Entropy Changes of Surroundings (∆Ssurr°)
∆ Ssurr = - ∆Hrxn T
∆Hsys° = ∆H°f(Products) −  ∆ H°f (Reactants)
Kelvin T of
surroundings

11. Entropy Changes of Surroundings (∆Ssurr°)
Calculate the ∆Ssurr° using Appendix L in this reaction:
N2(g) + 3H2(g) --> 2NH3(g)

12. Entropy Changes of System (∆Ssys°)
∆Ssys° = S°products − S°reactants
Example: N2(g) + 3H2(g) --> 2NH3(g)
Calculate the ∆Ssys° using Appendix L
S = J
H= kJ

13. More About Entropy Larger S, more entropy (disorder) State function
3rd Law of Thermodynamic is a reference point
A crystal at 0 Kelvin has no entropy (S=0), its perfectly ordered
Nature tends to be disorganized or randomness

14. The entropy of any substance can be obtained by measuring the heat added to a substance to raise its temp. from 0 K

15. Standard Entropy , S°
Entropy gained by converting a crystal at 0 K to standard conditions
Units: J/K mol
Generalizations:
For similar substances:
Increasing S
Solutions are
comparable to
liquids
S l g

16. Generalizations
Boiling has much greater change in entropy than melting

17. Generalizations
Larger molecules have more entropy than smaller molecules
Entropy increases as temperature increases

18. Generalizations
Any rxn that increases the # gas molecules has higher entropy at the end
2 H2O (l) --> 2H2(g) + O2(g)