1. Thermodynamics
Part II

2. Entropy, S
Entropy is a measure of the disorder or randomness of a system.
The entropy of the universe is increasing.
When:
DS is positive disorder increases (favors spontaneity)
DS is negative disorder decreases (disfavors spontaneity)
In general: Sgas> Sliquid > Ssolid
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3. Predicting ∆S
Determine whether the entropy value increases or decreases for the following situations
Ex. 1) Melting of ice at room temperature

4. Predicting ∆S
Determine whether the entropy value increases or decreases for the following situations
Ex. 1) Melting of ice at room temperature
Entropy increases: a phase change from an organized solid to a more disorganized liquid
∆S (+)

5. Predicting ∆S
Determine whether the entropy value increases or decreases for the following situations
Ex. 2) Condensation of water vapor on a window

6. Predicting ∆S
Determine whether the entropy value increases or decreases for the following situations
Ex. 2) Condensation of water vapor on a window
Entropy decreases: a phase change from a disorganized gas to a more organized liquid
∆S (-)

7. Predicting ∆S
Determine whether the entropy value increases or decreases for the following situations
Ex. 3) An iron rusting
4Fe(s) + 3O2(g)  2Fe2O3(s)

8. Predicting ∆S
Determine whether the entropy value increases or decreases for the following situations
Ex. 3) An iron rusting
4Fe(s) + 3O2(g)  2Fe2O3(s)
Entropy decreases: there are 4 moles of a solid reacting with 3 moles of a gas to make only 2 moles of the product
∆S (-)

9. Predicting ∆S
Determine whether the entropy value increases or decreases for the following situations
Ex. 4) Baking soda reacting with vinegar
NaHCO3(s) + CH3COOH(l)  NaCH3COO(aq) + H2O(l) + CO2(g)

10. Predicting ∆S
Determine whether the entropy value increases or decreases for the following situations
Ex. 4) Baking soda reacting with vinegar
NaHCO3(s) + CH3COOH(l)  NaCH3COO(aq) + H2O(l) + CO2(g)
Entropy increases: a solid and liquid yield an aqueous soln, a liquid, and a gas
(Also, there are 2 moles of reacts making 3 moles of product)
∆S (+)

11. Entropy changes for reactions can be determined similarly to DH for reactions. As with DH, entropies have been measured and tabulated in for So298.

12. Ex. 5) Find the ∆S for liquid ethanol, C2H5OH, undergoing combustion:
C2H5OH(l) O2(g) → 2CO2(g) + 3H2O(g)
Does this reaction create order or disorder?

13. Enthalpy and entropy can sometimes reinforce each other – this makes the reaction really go or really not go. Ex. Dynamite has a neg Δ H & a pos Δ S so the rxn really goes once started… If the signs don’t reinforce does a rxn occur? This is where Gibbs Free energy addresses the spontaneity of rxns. Ex. Liquid water to water vapor Δ H is + and Δ S is + Δ G is – (neg),the reaction will go on its own once started to make the products. Ex. gas burning. Δ G is + (pos), the rxn won’t go on it’s own, wants to stay as reactants. Ex. batter  cake

14. The change in the Gibbs Free Energy is a reliable indicator of spontaneity of a physical process or chemical reaction.
When:
DG is > 0 reaction is not thermodynamically favored (nonspontaneous) reactant favored
DG is < 0 reaction is thermodynamically favored (spontaneous) product favored
Changes in free energy obey the same type of relationship we have described for enthalpy and entropy changes.

15. C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(g)
Ex. 6) Find the ∆Grxn for liquid ethanol, C2H5OH, undergoing combustion:
C2H5OH(l) O2(g) → 2CO2(g) + 3H2O(g)
Is this reaction spontaneous or nonspontaneous?

16. Using ∆H and ∆S to solve ∆G
The relationship between ∆H, ∆S and temperature can help describe the spontaneity of a system. Whether the reaction is thermodynamically favored or
not thermodynamically favored

17. Ex. 7) Determine if a reaction is spontaneous at room temperature, 25oC, when ∆H = -176 kJ and ∆S = -285 J/K
Units must match!!!

18. Ex. 8) What happens if the reaction is now at 550oC, both ∆H = -176 kJ and ∆S = -285 J/K are the same.
Units must match!!!

19. Ex. 8) What happens if the reaction is now at 550oC, both ∆H = -176 kJ and ∆S = -285 J/K are the same.
Units must match!!!
* At what temperature would ∆G = 0?

20. This general relationship gives us 4 possibilities among the signs
DH DS DG Therefore
forward rxn spontaneous at all T’s
- - ? forward rxn spontaneous at low T’s
+ + ? forward rxn spontaneous at high T’s
forward rxn nonspontaneous at all T’s

21. Copy this into your notes
State function + -
Δ H
Endothermic
(taking in heat)
ice melting
*Exothermic
(giving off heat)
dynamite
Δ S
*Towards disorder
Towards order
Δ G
not thermodynamically favored
nonspontaneous
*thermodynamically favored
spontaneous
Enthalpy (heat changes) H
Entropy (order and disorder) S
Gibbs Free energy (spontaneity) G
The universe prefers exothermic reactions that cause disorder and are spontaneous