1. Enthalpy and Entropy
Lesson # 10

2. There are two factors that determine the relative amount of reactants and products at equilibrium.
Reactions tend to proceed toward minimum enthalpy.
Reactions tend to go toward maximum entropy

3. Enthalpy is PE – In an Exothermic Reaction
A B = C energy
Products are Favoured
The low side (side with the energy) is always favoured- minimum

4. Enthalpy is PE – In an Endothermic Reaction
A energy = B C
Reactants are Favoured
The low side (side with the energy) is always favoured- minimum

5. Entropy is randomness or a measure of how spread out, or broken up the system is.
Less entropy (s) (l) (aq) (g) More Entropy
Less Entropy More Entropy
H2O(l)
H2O(g)
Watch Simulation
H2O(s)
H2O(l)
NaCl(aq)
NaCl(s)
N2O4(g)
2NO2(g)
P6O9(g)
3P2O3(g)
glass
broken glass
clean room
messy room

6. You can tell the yeild of a reaction with enthalpy and entropy
√ max entropy √ min enthalpy √ ⇌ √
equlibrium ⇌ √ √
products favoured
If they are both on one side that side is favoured
If they are on different sides it’s an equilibrium

7. ? ↔ + E Are the products or Reactants favoured? 0 gases 1 gas
Are the products or Reactants favoured?
0 gases 1 gas ↔ ?
1. Zn(s) + 2HCl(aq) H2(g) + ZnCl2(aq) ΔH = KJ
+ E
For entropy- only look at phase symbols- mainly gases
√ max entropy
Large Keq
√ min enthalpy
Which Way?
Right or left?
Products are favoured
Goes to Completion
High yield

8. Products
Reactants ⇌

9. ? + E ↔ Are the Products or Reactants favoured? ↔ ?
2. N2O4(g) NO2(g) ΔH = +20 KJ
+ E
1 gas 2 gases
√ max entropy
√ min enthalpy
Which Way?
Right or left?
Keq is about 1
Equilibrium

10. reactants
Products ⇌

11. ? ↔ Are the Products or Reactants favoured? ↔ ?
3. 3C(s) H2(g) KJ C3H6(g)
√ max entropy
√ min enthalpy
Keq is small
Reactants are favoured
Low yield
Reaction does not occur!

12. reactants ⇌
products

13. Describe the change in entropy and enthalpy for an endothermic equilibrium system.
Enthalpy must increase
Entropy must increase
I’m Entropy
Maximum entropy
Must be on the opposite side
Minimum enthalpy

14. 5. Describe the change in entropy and enthalpy for an exothermic equilibrium system.
I’m Entropy
Maximum entropy
Enthalpy is decreasing
Minimum enthalpy
Entropy is decreasing

15. 6. Describe entropy and enthalpy as increasing or decreasing.
Zn(s) + 2HCl(aq) → H2(g) + ZnCl2(aq) ΔH = KJ
√ max entropy
√ min enthalpy
Enthalpy decreases
Entropy increases

16. 7. Describe entropy and enthalpy as increasing or decreasing.
3C(s) + 3H2(g) KJ → C3H6(g)
√ min enthalpy
√ max entropy
Enthalpy increases
Entropy decreases

17. Equilibrium is a compromise between the tendencies toward minimum enthalpy and maximum entropy.