Enthalpy and Entropy
Lesson # 10
There are two factors that determine the relative amount of reactants and products
at equilibrium.
1. Reactions tend to proceed toward minimum enthalpy.
2. Reactions tend to go toward maximum entropy
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
Enthalpy is PE – In an Endothermic Reaction
A
+
energy
=
B
+
Reactants are Favoured
The low side (side with the energy) is always favoured- minimum
C
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)
H2O(s)
Watch Simulation
H2O(l)
NaCl(s)
NaCl(aq)
N2O4(g)
2NO2(g)
P6O9(g)
3P2O3(g)
glass
broken glass
clean room
messy room
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
Are the products or Reactants favoured?
0 gases
1.
Zn(s)
+ 2HCl(aq)
?
↔
1 gas
H2(g) + ZnCl2(aq)
+E
ΔH = -252 KJ
For entropy- only look at phase symbols- mainly gases
√ max entropy
√ min enthalpy
Large Keq
Products are favoured
Which Way?
Goes to Completion
Right or left?
High yield
Reactants
⇌
Products
Are the Products or Reactants favoured?
2.
+E
N2O4(g)
?
↔
1 gas
ΔH = +20 KJ
2NO2(g)
2 gases
√ max entropy
√ min enthalpy
Which Way?
Keq is about 1
Right or left?
Equilibrium
reactants
⇌
Products
Are the Products or Reactants favoured?
3.
3C(s)
+
3H2(g)
+
45 KJ
?
↔
C3H6(g)
√ max entropy
√ min enthalpy
Keq is small
Reactants are favoured
Low yield
Reaction does not occur!
reactants
⇌
products
4.
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
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
6.
Describe entropy and enthalpy as increasing or decreasing.
Zn(s)
+ 2HCl(aq) → H2(g) + ZnCl2(aq)
ΔH = -252 KJ
√ max entropy
√ min enthalpy
Enthalpy decreases
Entropy increases
7.
Describe entropy and enthalpy as increasing or decreasing.
3C(s)
+
3H2(g) + 45 KJ
→
√ min enthalpy
√ max entropy
Enthalpy increases
Entropy decreases
C3H6(g)
Equilibrium is a compromise between the tendencies toward minimum enthalpy and
maximum entropy.