Chemistry I: Equilibrium Notes
Sample Problem
Consider the following reaction and data:
H2O(s) → H2O (l)
Substance
H2O(s)
H2O(l)
∆H°f (kj/mol) S°(J/mol∙k)
-291.84
47.92
-285.83
69.91
1. What is ∆H for this reaction?
2. What is ∆S for this reaction?
3. In which direction is this reaction spontaneous at 20. °C?
4. In which direction is this reaction spontaneous at –20. °C?
5. From personal experience, do your answers to #3 and #4 make sense? Explain.
6. We have learned what sign ∆G is when the reaction is spontaneous to the right. We
have learned what sign ∆G is when the reaction is spontaneous to the left. If the
reaction is not spontaneous in either direction, what do you suppose ∆G equals?
7. Calculate the temperature, in Celsius, at which the condition in #6 is met for the above
reaction.
8. From personal experience, does your answer to #7 make sense? Explain.
What have we learned about the reaction?
Let’s generalize this for many reactions:
Equilibrium:
Consider the equilibrium:
aA + bB ↔ cC + dD
(Label what each letter represents)
When the above reaction reaches equilibrium, is the total amount of A and B equal to the total of
C and D?
When the above reaction reaches equilibrium, what is constant?
Therefore, at equilibrium:
Keq =
Examples
1)
Consider the equilibrium:
H2(g) + F2(g) ↔ 2HF(g)
H2 gas and F2 gas are mixed, and once equilibrium is established, the following
concentrations are observed:
[H2] = 3.2 x 10-2 M
[F2] = 1.032 M
[HF] = 1.936 M
a) What is the Keq expression?
b) What is the value of Keq?
2)
Consider the equilibrium:
CaF2(s) ↔ Ca2+(aq) + 2F-(aq)
Ca2+ and F- are mixed, and once equilibrium is established, the following
concentrations are observed:
[Ca2+] = 6.4 x10-8 M
[F-] = 0.025 M
a)
What is the Keq expression?
b)
What is the value of Keq?
3)
Consider the equilibrium:
2NOCl(g) ↔ 2NO(g) + Cl2(g) Keq = 1.6 x 10-5
NOCl, NO, and Cl2 are mixed at the following concentrations:
[NOCl] = 1.0 M
[NO] = 1.0 M
[Cl2] = 1.0 M
a) At the above concentrations, has the system reached equilibrium? Explain.
b) If not, which way will the system shift in order to reach equilibrium?
LeChatelier’s Principle: famous and fundamental concept in chemistry!
Description:
Sample Problem
1)
Consider the equilibrium:
2SO3(g) ↔ 2SO2(g) + O2(g) DH° = 197 kJ
2)
In which direction will the position of the equilibrium shift for each of the following
stresses? Explain.
a)
Oxygen gas is added.
b) Sulfur trioxide gas is removed.
c) Sulfur dioxide gas is removed.
d) Volume is decreased.
e) Temperature is increased
Keq is constant, but does change with respect to one and only one factor. What is this factor?
______________
If ___________________________ is increased, will Keq increase or decrease? Explain.
(‘fore mentioned factor)
Sample Problem
3)
Consider the equilbrium:
HF(aq) ↔ H+(aq) + F-(aq)
In which direction will the position of the equilibrium shift for each of the following stresses?
Explain.
a) F- is removed.
b) HCl is added.
c) NaOH is added.
d) Hydrofluoric acid is removed.