Chem 102 Recitation
ICE Table in equilibrium calculation
& Le Chatelier’s Principles
& Weak acids
---Dr. Q. Wang
Mar 19, 2015
Initial-Change-Equilibrium Table in K calculations
(1) K value;
(2) Initial concentration(s);
(3) Concentration(s) at equilibrium.
Knowing first two,
Need to calculate the third.
Initial-Change-Equilibrium Table in K calculations
Question 1
For the reaction, Kc = 4.0  103 at 1500 K,
NiO(s) + CO(g) ⇌ Ni(s) + CO2(g)
If a reaction vessel initially contains a mixture of a solid NiO
and 0.20 M CO at 1500 K,
what is the equilibrium concentration of CO2 at this
temperature?
Initial-Change-Equilibrium Table in K calculations
Question 2 – For the reaction, Kc = 4.0  103 at 1500 K,
NiO(s) + CO(g) ⇌ Ni(s) + CO2(g)
If a reaction vessel initially contains a mixture of a solid NiO and 0.20 M CO at 1500 K,
what is the equilibrium concentration of CO2 at this temperature?
NiO(s) + CO(g) ⇌ Ni(s) + CO2(g)
Solution:
Kc 
[CO2 ]eq
[CO ]eq
Assuming at x M of CO will be reacted to reach equilibrium,
NiO(s) + CO(g) ⇌ Ni(s) + CO2(g)
Kc 
Initially
Change
0.20 M
-xM
0M
+xM
At equilibrium
(0.20 - x) M
xM
x
0.20  x
x  0.20M
[CO2 ]eq
[CO ]eq
4.0  103 
Initial-Change-Equilibrium Table in K calculations
Question 2
Consider the reaction, Kc = 102 at 500 K,
CO(g) + H2O(g) ⇌CO2(g) + H2(g)
If a reaction mixture initially contains 0.110 M CO and
0.110 M H2O,
what will the equilibrium concentration of each of the
reactants and products be?
Initial-Change-Equilibrium Table in K calculations
Question 3 – Consider the reaction, Kc = 102 at 500 K,
CO(g) + H2O(g) ⇌CO2(g) + H2(g)
If a reaction mixture initially contains 0.110 M CO and 0.110 M H2O, what will the
equilibrium concentration of each of the reactants and products be?
Solution:
CO(g) + H2O(g) ⇌CO2(g) + H2(g)
Kc 
[CO2 ]eq [ H 2 ]eq
[CO ]eq [ H 2O ]eq
Assuming at x M of CO will be reacted to reach equilibrium,
CO(g) + H2O(g)
Kc 
Initially
Change
0.110 M
-xM
At equilibrium
(0.110 - x) M
[CO2 ]eq [ H 2 ]eq
[CO ]eq [ H 2O ]eq
⇌
0.110 M
-xM
(0.110 - x) M
x2
102 
(0.110  x )2
CO2(g)
+
H2(g)
0M
+xM
0M
+xM
xM
xM
102 
At equilibrium, [CO]=[H2O]=0.010 M, [CO2]=[H2]=0.100M
x
x  0.100M
0.110  x
Le Châtelier’s principle
• Henri LeChâtelier (1850-1936)
• If a system at equilibrium is disturbed by a
change in concentration, pressure, or
temperature, the system will, if possible, shift to
partially counteract the change and restore
equilibrium.
Le Châtelier’s principle
Question 3
This reaction is exothermic.
2 CO(g) ⇌ C(s) + CO2(g)
Predict the effect (shift right, shift left, or no effect) of
increasing and decreasing the reaction temperature.
How does the value of the equilibrium constant depend on
temperature?
Le Châtelier’s principle
Question 3
This reaction is exothermic.
2 CO(g) ⇌ C(s) + CO2(g)
(1) Predict the effect (shift right, shift left, or no effect) of increasing and
decreasing the reaction temperature.
(2) How does the value of the equilibrium constant depend on temperature?
Exothermic:
2 CO(g) ⇌ C(s) + CO2(g) + heat
If increasing the
reaction
temperature
2 CO(g) ⇌ C(s) + CO2(g) + heat
If decreasing the
reaction
temperature
Le Châtelier’s principle
Question 3
This reaction is exothermic.
2 CO(g) ⇌ C(s) + CO2(g)
(1) Predict the effect (shift right, shift left, or no effect) of increasing and
decreasing the reaction temperature.
(2) How does the value of the equilibrium constant depend on temperature?
For any equilibrium:
If temperature does not change, K will not change.
(Changes of volume, pressure, and concentrations will not
be able to change K.)
Le Châtelier’s principle
Question 3
This reaction is exothermic.
2 CO(g) ⇌ C(s) + CO2(g)
(1) Predict the effect (shift right, shift left, or no effect) of increasing and
decreasing the reaction temperature.
(2) How does the value of the equilibrium constant depend on temperature?
Exothermic: 2 CO(g) ⇌ C(s) + CO2(g) + heat
For an exothermic equilibrium,
If T increases, equilibrium shift from products to reactants.
K will decrease.
If T decreases, equilibrium shift from reactants to products.
K will increase.
For an endothermic equilibrium,
If T increases, equilibrium shift from reactants to products.
K will increase.
If T decreases, equilibrium shift from products to reactants.
K will decrease.
Weak Acid Problems
Question 4
A 0.200 M solution of a weak acid (HA) has a pH of 2.95.
Calculate the acid ionization constant (Ka) for the acid.
Weak Acid Problems
Question 4
A 0.200 M solution of a weak acid (HA) has a pH of 2.95.
Calculate the acid ionization constant (Ka) for the acid.
Solution:
HA (aq) ⇌ H+(aq) + A-(aq)
Ka 
[ H  ]eq [ A ]eq
[ HA]eq
Assuming at x M of HA will be ionized to reach equilibrium,
HA (aq)
Initially
Change
⇌
0.200 M
-xM
At equilibrium (0.200 - x) M
Ka 
[ H  ]eq [ A ]eq
[ HA]eq
x x

0.200  x
H+(aq)
+
A-(aq)
0M
+xM
0M
+x M
xM
xM
pH  2.95
 log[ H  ]eq  2.95
[ H  ]eq  102.95  1.1  103 M
x x
(1.110 3 ) 2
1.2 10 6
Ka 


 6.0 10 6
3
0.200  x 0.200  1.110
0.200  0.0011