2-Day AP/Pre-AP Science Conference
Corpus Christi Omni-Marina Hotel
January 20 & 21, 2006
Introduction to Equilibrium Concepts
John I. Gelder
Former Chief Reader AP Chemistry
Department of Chemistry
Oklahoma State University
Important Web Site
Particulate Level Simulations (Supported by the
National Science Foundation)
http://genchem1.chem.okstate.edu/CCLI/Startup.
html
Login
Free access to simulations and guided inquiry
activities.
Gas Laws
Shifting Reactions (Equilibrium)
Chemical Kinetics
Experimental Basis
Characteristics of a hypothetical chemical reaction?
Investigate the reaction: R + GB 
Investigate the reaction: B + RG 
Reversible reaction
Irreversible reaction
Experimental Basis
How can concentration changes affect a chemical
reaction?
Experiment #1:
Experiment #2: Add reactant R (to 2.0 M)
Experiment #3: Add reactant BG (to 2.0 M)
Experiment #4: Remove reactants R and BG (to 1.0 and
0.8 M)
Experiment #5: Add product RG (to 1.5 M) Predict
Experiment #6: Remove product RG (to 0.5 M)) Predict
Experimental Basis
How can concentration changes affect a chemical
reaction?
Summary of Data
Exp. Stress
#2 Add R
#3
#4
….
Change in Change in
[reactants] [products]
Conc
decrease
Conc.
Increases
Reaction Shifts
Left to right
Experimental Basis
How can concentration changes affect a chemical
reaction?
Review the summary of your experimental observations
and write a statement(s) that generalizes how
stressing a reaction by adding or removing a reactant
of product shifts the chemical reaction.
Experimental Basis
What is the relationship between reactant and product
concentrations at the end of a chemical reaction?
Exp.
Initial Conditions
R
BG
RG
Final Conditions
B
#1
2.0
2.0
0
0
#2
1.0
1.0
0
0
#3
1.5
1.5
0
0
#4
0
0
1.0
1.0
#6
1.0
0.6
1.2
0.8
R
BG
RG
B
Experimental Basis
What is the relationship between reactant and product
concentrations at the end of a chemical reaction?
Exp.
Initial Conditions
R
BG
RG
Final Conditions
B
#1
2.0
2.0
0
0
#2
1.0
1.0
0
0
#3
1.5
1.5
0
0
#4
0
0
1.0
1.0
#6
1.0
0.6
1.2
0.8
R
BG
.64 .64
RG
B
1.36 1.36
Experimental Basis
What is the relationship between reactant and product
concentrations at the end of a chemical reaction?
Exp.
Initial Conditions
R
BG
RG
Final Conditions
B
R
BG
RG
B
#1
2.0
2.0
0
0
.64 .64
1.36 1.36
#2
1.0
1.0
0
0
.32 .32
0.68 0.68
#3
1.5
1.5
0
0
#4
0
0
1.0
1.0
#6
1.0
0.6
1.2
0.8
Experimental Basis
What is the relationship between reactant and product
concentrations at the end of a chemical reaction?
Exp.
Initial Conditions
R
BG
RG
Final Conditions
B
R
BG
RG
B
#1
2.0
2.0
0
0
.64 .64
1.36 1.36
#2
1.0
1.0
0
0
.32 .32
0.68 0.68
#3
1.5
1.5
0
0
.48 .48
1.02 1.02
#4
0
0
1.0
1.0
#6
1.0
0.6
1.2
0.8
Experimental Basis
What is the relationship between reactant and product
concentrations at the end of a chemical reaction?
Exp.
Initial Conditions
R
BG
RG
Final Conditions
B
R
BG
RG
B
#1
2.0
2.0
0
0
.64 .64
1.36 1.36
#2
1.0
1.0
0
0
.32 .32
0.68 0.68
#3
1.5
1.5
0
0
.48 .48
1.02 1.02
#4
0
0
1.0
1.0
.32 .32
0.68 0.68
#6
1.0
0.6
1.2
0.8
Experimental Basis
What is the relationship between reactant and product
concentrations at the end of a chemical reaction?
Exp.
Initial Conditions
R
BG
RG
Final Conditions
B
R
BG
RG
B
#1
2.0
2.0
0
0
.64 .64
1.36 1.36
#2
1.0
1.0
0
0
.32 .32
0.68 0.68
#3
1.5
1.5
0
0
.48 .48
1.02 1.02
#4
0
0
1.0
1.0
.32 .32
0.68 0.68
#6
1.0
0.6
1.2
0.8
.79 .39
1.41 1.01
Experimental Basis
What is the relationship between reactant and product
concentrations at the end of a chemical reaction?
Challenge students to arrive at an algebraic equation
that relates the concentrations of the reactants and
products in the reaction. Hints….
Use the results to predict the final concentrations for a
new initial set of conditions.
Experimental Basis
Additional Investigations:
Try some different reactions:
R + G  RG
BG  B + G
RB  R + B
Change in temperature;
Change the volume of the container;
K, T and ∆H
K and Q
Introduction to Equilibrium
K, the equilibrium constant, measures the extent of a
chemical reaction.
Large K (greater than 1) reaction will proceed towards
products when the initial conditions have all reactants
and products at 1 M or 1 atm.
Small K (less than 1) reaction will proceed towards
reactants when the initial conditions have all reactants
and products at 1 M or 1 atm.
Introduction to Equilibrium
aA(g) + bB(g)
Kp = PcC · PdD/PaA · PbB
KC = [C]c · [D]d/ [A]a · [B]b
Kp = KC·(RT)∆n
cC(g) + dD(g)
1995
H2(g) + CO2(g)
H2O(g) + CO(g)
When H2(g) is mixed with CO2(g) at 2000K, equilibrium is
achieved according to the equation above. In one
experiment, the following equilibrium concentrations were
measured.
[H2] = 0.20 mol/L
[CO2] = 0.30 mol/L
[H2O] = [CO] = 0.55 mol/L
a) What is the mol fraction of CO(g) in the equilibrium mixture?
b) Using the equilibrium concentrations given above, calculate
the value of KC, the equilibrium constant for the reaction.
c) Determine KP in terms KC of for the system.
d) When the system is cooled from 2000 K to a lower
temperature, 30.0% of the CO(g) is converted back to
CO2(g). Calculate the value of KC at this lower temperature.
e) 0.50 mol of H2 is mixed with 0.50 mol of CO2 in a 3.0 liter
container at 2000 K. Calculate the CO(g) at equilibrium.
1995
H2(g) + CO2(g)
H2O(g) + CO(g)
When H2(g) is mixed with CO2(g) at 2000K, equilibrium
is achieved according to the equation above. In one
experiment, the following equilibrium concentrations
were measured.
[H2] = 0.20 mol/L
[CO2] = 0.30 mol/L
[H2O] = [CO] = 0.55 mol/L
a) What is the mol fraction of CO(g) in the equilibrium
mixture?
Mol fraction CO = mol CO/total mol = 0.55/1.60
= 0.34
1995
H2(g) + CO2(g)
H2O(g) + CO(g)
When H2(g) is mixed with CO2(g) at 2000K, equilibrium is
achieved according to the equation above. In one
experiment, the following equilibrium concentrations were
measured.
[H2] = 0.20 mol/L
[CO2] = 0.30 mol/L
[H2O] = [CO] = 0.55 mol/L
b)
Using the equilibrium concentrations given above, calculate
the value of KC, the equilibrium constant for the reaction.
KC = [H2O] · [CO]/ [H2] · [CO2]
= [0.55] · [0.55]/ [0.20] · [0.30]
= 5.0
1995
H2(g) + CO2(g)
H2O(g) + CO(g)
When H2(g) is mixed with CO2(g) at 2000K, equilibrium is
achieved according to the equation above. In one experiment,
the following equilibrium concentrations were measured.
[H2] = 0.20 mol/L
[CO2] = 0.30 mol/L
[H2O] = [CO] = 0.55 mol/L
c)
Determine KP in terms KC of for the system.
Kp = KC·(RT)∆n
∆n = mol product gases - mol of reactant gases
∆n = 1 + 1 - (1 + 1) = 0
Kp = KC
1995
H2(g) + CO2(g)
H2O(g) + CO(g)
When H2(g) is mixed with CO2(g) at 2000K, equilibrium
is achieved according to the equation above. In one
experiment, the following equilibrium concentrations
were measured.
[H2] = 0.20 mol/L
[CO2] = 0.30 mol/L
[H2O] = [CO] = 0.55 mol/L
d) When the system is cooled from 2000 K to a lower
temperature, 30.0% of the CO(g) is converted back
to CO2(g). Calculate the value of KC at this lower
temperature.
1995
H2(g) + CO2(g) H2O(g) + CO(g)
initial 0.20 0.30
0.55
0.55
change
- 0.17
equilibrium
0.30 · 0.55 M = 0.17 M
0.17 M CO reacts
1995
H2(g) + CO2(g) H2O(g) + CO(g)
initial 0.20
0.30
0.55
0.55
change
+ 0.17 + 0.17
- 0.17
- 0.17
equilibrium
0.37
0.47
0.38
0.38
KC = [H2O] · [CO]/ [H2] · [CO2]
= [0.38] · [0.38]/ [0.37] · [0.47]
= 0.83
1995
H2(g) + CO2(g)
H2O(g) + CO(g)
When H2(g) is mixed with CO2(g) at 2000K, equilibrium is
achieved according to the equation above. In one
experiment, the following equilibrium concentrations were
measured.
[H2] = 0.20 mol/L
[CO2] = 0.30 mol/L
[H2O] = [CO] = 0.55 mol/L
e)
0.50 mol of H2 is mixed with 0.50 mol of CO2 in a 3.0 liter
container at 2000 K. Calculate the CO(g) at equilibrium.
1995
H2(g) + CO2(g)
e)
H2O(g) + CO(g)
0.50 mol of H2 is mixed with 0.50 mol of CO2 in a 3.0 liter
container at 2000 K. Calculate the CO(g) at equilibrium.
[H2] = [CO2] = 0.50 mol/3.0 L = 0.17 M
1995
initial
change
equilibrium
H2(g) + CO2(g)
0.17
0.17
H2O(g) + CO(g)
0
0
Q = [H2O] · [CO]/ [H2] · [CO2]
= [0] · [0]/ [0.17] · [0.17]
=0
So Q < Kc (5.0)
So the reaction proceeds from left to right to establish equilibrium
1995
H2(g) + CO2(g)
initial 0.17
0.17
change
-x
-x
equilibrium
0.17 - x 0.17 - x
H2O(g) + CO(g)
0
0
+x
+x
0-x
0-x
Kc = [H2O] · [CO]/ [H2] · [CO2]
5.0 = [x] · [x]/ [0.17 - x] · [0.17 - x]
5.0 = x2/(0.17 - x)2
2.236 = x/(0.17 - x)
x = 0.12 M = [CO]
1998
C(s) + H2O(g)
CO(g) + H2(g) Hº = +131kJ
A rigid container holds a mixture of graphite pellets (C(s)), H2O(g),
CO(g), and H2(g) at equilibrium. State whether the number of
moles of CO(g) in the container will increase, decrease, or remain
the same after each of the following disturbances is applied to the
original mixture. For each case, assume that all other variables
remain constant except for the given disturbance. Explain each
answer with a short statement.
(a) Additional H2(g) is added to the equilibrium mixture at constant
volume.
(b) The temperature of the equilibrium mixture is increased at constant
volume.
(c) The volume of the container is decreased at constant temperature.
(d) The graphite pellets are pulverized.
1998
C(s) + H2O(g)
CO(g) + H2(g) Hº = +131kJ
A rigid container holds a mixture of graphite pellets (C(s)), H2O(g),
CO(g), and H2(g) at equilibrium. State whether the number of
moles of CO(g) in the container will increase, decrease, or remain
the same after each of the following disturbances is applied to the
original mixture. For each case, assume that all other variables
remain constant except for the given disturbance. Explain each
answer with a short statement.
(a) Additional H2(g) is added to the equilibrium mixture at constant
volume.
(b) The temperature of the equilibrium mixture is increased at constant
volume.
(c) The volume of the container is decreased at constant temperature.
(d) The graphite pellets are pulverized.
Kp = PCO · PH2/PH2O
1998
C(s) + H2O(g)
CO(g) + H2(g) Hº = +131kJ
A rigid container holds a mixture of graphite pellets (C(s)), H2O(g),
CO(g), and H2(g) at equilibrium. State whether the number of
moles of CO(g) in the container will increase, decrease, or remain
the same after each of the following disturbances is applied to the
original mixture. For each case, assume that all other variables
remain constant except for the given disturbance. Explain each
answer with a short statement.
(a) Additional H2(g) is added to the equilibrium mixture at constant
volume.
Kp = PCO · PH2/PH2O
Adding H2(g) causes Q to be greater than Kp.
The reaction must shift in the direction to reduce Q, right to left.
The moles of CO(g) will decrease.
1998
C(s) + H2O(g)
CO(g) + H2(g)
Hº = +131kJ
A rigid container holds a mixture of graphite pellets (C(s)), H2O(g), CO(g), and H2(g)
at equilibrium. State whether the number of moles of CO(g) in the container will
increase, decrease, or remain the same after each of the following disturbances is
applied to the original mixture. For each case, assume that all other variables
remain constant except for the given disturbance. Explain each answer with a
short statement.
(b)
The temperature of the equilibrium mixture is increased at constant volume.
The reaction is endothermic. Increasing the temperature
corresponds to adding heat to the reactants side. The
reaction will shift from left to right to relieve the stress. The
moles of CO(g) will increase.
1998
C(s) + H2O(g)
CO(g) + H2(g)
Hº = +131kJ
A rigid container holds a mixture of graphite pellets (C(s)), H2O(g), CO(g), and H2(g)
at equilibrium. State whether the number of moles of CO(g) in the container will
increase, decrease, or remain the same after each of the following disturbances is
applied to the original mixture. For each case, assume that all other variables
remain constant except for the given disturbance. Explain each answer with a
short statement.
(c )
The volume of the container is decreased at constant temperature.
Decreasing the volume of the container, increases the
pressure inside, to relieve the pressure the reaction will
proceed in the direction to decrease the pressure (in the
direcction of the fewest moles of gas) the reaction will
proceed from right to left. The moles of CO(g) will decrease.
1998
C(s) + H2O(g)
CO(g) + H2(g) Hº = +131kJ
A rigid container holds a mixture of graphite pellets (C(s)), H2O(g),
CO(g), and H2(g) at equilibrium. State whether the number of
moles of CO(g) in the container will increase, decrease, or remain
the same after each of the following disturbances is applied to the
original mixture. For each case, assume that all other variables
remain constant except for the given disturbance. Explain each
answer with a short statement.
(d) The graphite pellets are pulverized.
Kp = PCO · PH2/PH2O
Since C(s) does not appear in the equilibrium expression
pulverizing the graphite has no effect of the moles of CO(g).
So the moles of CO(g) remain the same.