2-Day AP/Pre-AP Science ConferenceCorpus 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 SiteParticulate Level Simulations (Supported by the

National Science Foundation)http://genchem1.chem.okstate.edu/CCLI/Startup.htmlLoginFree access to simulations and guided inquiry

activities.

Gas LawsShifting Reactions (Equilibrium)Chemical Kinetics

Experimental BasisCharacteristics of a hypothetical chemical reaction?

Investigate the reaction: R + GB

Investigate the reaction: B + RG

Reversible reaction

Irreversible reaction

Experimental BasisHow 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) PredictExperiment #6: Remove product RG (to 0.5 M)) Predict

Experimental Basis

How can concentration changes affect a chemical reaction?

Summary of Data

Exp. Stress

Change in [reactants]

Change in [products] Reaction Shifts

#2 Add R Conc decrease

Conc. Increases

Left to right

#3

#4….

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 BasisWhat is the relationship between reactant and product

concentrations at the end of a chemical reaction?

Exp. Initial Conditions Final Conditions

R BG RG B R BG RG 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

Experimental BasisWhat is the relationship between reactant and product

concentrations at the end of a chemical reaction?

Exp. Initial Conditions Final Conditions

R BG RG 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

#3 1.5 1.5 0 0

#4 0 0 1.0 1.0

#6 1.0 0.6 1.2 0.8

Experimental BasisWhat is the relationship between reactant and product

concentrations at the end of a chemical reaction?

Exp. Initial Conditions Final Conditions

R BG RG 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 BasisWhat is the relationship between reactant and product

concentrations at the end of a chemical reaction?

Exp. Initial Conditions Final Conditions

R BG RG 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 BasisWhat is the relationship between reactant and product

concentrations at the end of a chemical reaction?

Exp. Initial Conditions Final Conditions

R BG RG 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 BasisWhat is the relationship between reactant and product

concentrations at the end of a chemical reaction?

Exp. Initial Conditions Final Conditions

R BG RG 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 + GRB R + B

Change in temperature;Change the volume of the container;K, T and ∆HK 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) cC(g) + dD(g)

Kp = PcC · Pd

D/PaA · Pb

B

KC = [C]c · [D]d/ [A]a · [B]b

Kp = KC·(RT)∆n

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/La) 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/La) 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 0.55change - 0.17 equilibrium

0.30 · 0.55 M = 0.17 M0.17 M CO reacts

1995 H2(g) + CO2(g) H2O(g) + CO(g)

initial 0.20 0.30 0.55 0.55change + 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) H2O(g) + CO(g)

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.

[H2] = [CO2] = 0.50 mol/3.0 L = 0.17 M

1995 H2(g) + CO2(g) H2O(g) + CO(g)

initial 0.17 0.17 0 0changeequilibrium

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) H2O(g) + CO(g)

initial 0.17 0.17 0 0change -x -x +x +xequilibrium 0.17 - x 0.17 - 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.

• Additional H2(g) is added to the equilibrium mixture at constant volume.

• The temperature of the equilibrium mixture is increased at constant volume.

• 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.

• Additional H2(g) is added to the equilibrium mixture at constant volume.

• The temperature of the equilibrium mixture is increased at constant volume.

• 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.

• 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.