Chemical Equilibrium
There is a balancing act between reactants and products.
Chemical reactions can occur in either direction (reactants to products or products to reactants).
aA + bB  cC + dD
An equilibrium exists when the ratio of the amounts of products to reactants is no longer changing:
products
= CONSTANT
reactants
Using reactant concentrations [Molarity]:
Using gas pressures:
[C ] [ D ]
Kc =
a
b
[ A] [ B ]
PCc PDd
Kp = a b
PA PB
c
d
Example 1:
H 2 + I 2  2 HI
At equilibrium, the following partial pressures are observed:
PH 2 = 0.12 atm
PI 2 = 0.24 atm
PHI = 1.28 atm
What is the equilibrium constant for the reaction?
=
KP
PHI2
(1.28) 2
=
= 57
PH 2 PI 2 (0.12)(0.24)
Example 2: How do we relate equilibrium constants for modified reactions?
H 2 + I 2  2 HI
57
K=
2 HI  H 2 + I 2
1
K=
57
HI 
1
1
H 2 + I2
2
2
2 H 2 + 2 I 2  4 HI
1
K=
57
(57) 2
K=
Solving Equilibrium Problems: ICE Tables
Suppose we introduce 0.100 atm of H2 and 0.100 atm of I2 into an empty flask. After the system is
allowed to equilibrate, what will be the final partial pressures of all gases? Use the reaction and
equilibrium constant found in example 1.
H 2 + I 2  2 HI
Substance
H2
I2
HI
KP
=
Initial
0.100
0.100
0
Change
PHI2
(2 x) 2
=
= 57
PH 2 PI 2 (.10 − x)(.10 − x)
Substance
H2
I2
HI
Equilibrium
-x
-x
+2x
0.100 - x
0.100 - x
2x
solve for x :
x 0.0791
=
Equilibrium Final Pressures
0.100 - x
0.100 - x
2x
0.021 atm
0.021 atm
0.158 atm
Always check your answer:
=
KP
PHI2
(.158) 2
=
= 57
PH 2 PI 2 (.021)(.021)
Equilibrium Problem: Determine which direction reaction will proceed
Sometimes it is not clear which direction a reaction will go in order to reach equilibrium. In the previous
example it was obvious that the reaction goes forward because there were no products present.
Consider a different problem:
Suppose we introduce 0.050 atm of H2 and 0.050 atm of I2 and 0.500 atm of HI into an empty flask. After
the system is allowed to equilibrate, what will be the final partial pressures of all gases?
Evaluate the reaction quotient (Q):
=
Q
PHI2
(0.5) 2
=
= 100
PH 2 PI 2 (0.05)(0.05)
Since Q > K, the reaction will need to go in reverse. Now set up the ICE table accordingly:
H 2 + I 2  2 HI
Substance
H2
I2
HI
Initial
0.050
0.050
0.500
Change
+x
+x
-2x
Equilibrium
0.050 + x
0.050 + x
0.500 - 2x
PHI2
(0.500 − 2 x) 2
=
KP =
= 57
PH 2 PI 2 (.050 + x)(.050 + x)
Substance
H2
I2
HI
Equilibrium Final Pressures
0.050 + x
0.050 + x
0.500 - 2x
Always check your answer:
=
KP
PHI2
(.474) 2
=
= 57
PH 2 PI 2 (.063)(.063)
0.063 atm
0.063 atm
0.474 atm
solve for x :
=
x 0.0128
CHM136: Chemical Equilibrium Practice
1. Consider the following generic gas-phase reaction with stated equilibrium constant:
2A+2BC+3D
K = 2.50 x 10-2
Write the equilibrium expression involving the partial pressures of each gas:
K=
If the equilibrium partial pressures, PA, PB, and PC are all equal to 1.00 atm, what is the equilibrium
partial pressure of D (PD)?
PD =
If we begin with the following partial pressures, PA=5.00atm PB=2.00atm PC=4.00atm PD=1.00atm, which
direction will the reaction proceed in order to establish equilibrium?
2. Consider the following gas-phase reaction with stated equilibrium constant:
A + B  C
K = 1.47 x 10-3 (pressure units)
Write an equilibrium expression for this reaction involving the partial pressures of each gas:
K=
If we begin with the following initial partial pressures: PA=2.00atm PB=1.00atm PC=1.00atm, which
direction will the reaction proceed in order to reach equilibrium?
Using the initial partial pressures given above, solve for the final partial pressures of A, B, and C at
equilibrium:
3. Consider the following chemical equation with stated equilibrium constant:
−
CH 3 I + OH  CH 3OH + I
−
CH 3OH   I − 
=
K
= 10
CH 3 I  OH − 
If 0.50 mol of CH3I is mixed with 0.50 mol OH- (in a one L solution),
a. How much CH3OH and how much I- will form when equilibrium is reached?
b. How much of each reactant will remain at equilibrium?
c. Check to see that the final concentrations when entered into the equilibrium law generate the
equilibrium constant.
4. Again consider the following chemical equation with stated equilibrium constant:
−
CH 3 I + OH  CH 3OH + I
−
CH 3OH   I − 
=
K
= 10
CH 3 I  OH − 
In order to obtain 0.60 mol of CH3OH at equilibrium, how many moles of OH- must be combined with
0.80 mol of CH3I?