Chemistry 102

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EXPERIMENT 3

EQUILIBRIUM GAMES

Fall 2015 www./proffenyes.com 1

PURPOSE:

1. To use model physical systems to illustrate the approach to chemical equilibrium.

2. To understand how the concentration of reactants and products change with time as

equilibrium is achieved.

PRINCIPLES:

In the previous experiment, it has been demonstrated that reaction rates increase with increasing

concentration of reactants and decrease with decreasing concentration of reactants.

Consider the reaction in which reactant A changes into product B:

A B Forward Reaction

In this reaction, as the concentration of B gradually increases and the concentration of A

gradually decreases, B can change back into A.

A B Reverse Reaction

A reaction that can proceed in both direction is referred to as a reversible reaction.

As the concentration of A gradually decreases, the rate of the forward reaction decreases as well.

As the concentration of B gradually increases, the rate of the reverse reaction increases.

At some point, the rate of the reverse reaction becomes equal to the rate of the forward reaction

and a dynamic equilibrium is reached.

Dynamic: Implies that both reactions (forward and reverse) are still proceeding.

Equilibrium: Implies that:

The rate of the forward reaction equals the rate of the reverse reaction

The concentrations of A and B remain constant. This, however does NOT mean that the concentrations of A and B are

necessarily equal to each other.

To sum up:

Dynamic equilibrium for a chemical reaction is the condition in which the rate of the

forward reaction equals the rate of the reverse reaction and the concentration of the

reactants and products no longer change.

Chemistry 102

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EXPERIMENT 3

EQUILIBRIUM GAMES

Fall 2015 www./proffenyes.com 2

A stepwise view showing how a reversible reaction reaches equilibrium is illustrated below:

Forward Reaction

A B Reverse Reaction

STEP 1

A B

Fast

A B

No Reaction

STEP 2

A B

Forward Reaction slows down

A B

Reverse Reaction starts slowly

STEP 3

A B

Forward Reaction slows down further

A B

Reverse Reaction speeds up

STEP 4

A B

Forward Reaction slows down further

A B

Reverse Reaction speeds up further

A B

Rate of Forward Reaction = Rate of Reverse Reaction

The concentrations of A and B no longer change

Dynamic Equilibrium has been reached

Chemistry 102

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EXPERIMENT 3

EQUILIBRIUM GAMES

Fall 2015 www./proffenyes.com 3

PROCEDURE:

1. Working in pairs:

Label a 25 mL graduated cylinder “A” and,

Label a second 25 mL cylinder “B” 2. Fill the graduated cylinder “A” with 25 mL of water

3. Leave the graduated cylinder “B” empty

4. Obtain two glass tubes with different diameters.

Three different physical models of Chemical Equilibrium will be illustrated:

MODEL 1

A. Insert the wider diameter tube in graduated cylinder “A”

B. Insert the narrower diameter tube in graduated cylinder “B”

C. Simultaneously with your partner:

Lower the wider diameter tube in cylinder “A” (contains 25 mL of water)

without blocking the open end of the tube.

Lower the narrower diameter tube in cylinder “B” (empty)

D. After the tubes reach the bottom of the cylinders, place your index finger over the top end of

the glass tubes and hold down tightly.

E. Carefully transfer the contents of the two tubes to the other cylinder and allow the water (if

any) to drain.

F. Read the volumes in the cylinder (nearest 0.1 mL) only after both transfer tubes have drained

completely.

G. Record the total volume in each cylinder (A and B) after every transfer in your Laboratory

Notebook. Expect about 20-30 rows of entries.

H. Repeat steps A through G five more times after equilibrium has been reached.

If you are not sure when equilibrium has been reached, consult your instructor.

MODEL 2

A. Insert the narrower diameter tube in graduated cylinder A”

B. Insert the wider diameter tube in graduated cylinder “B”

Follow steps C through H, as it was done for MODEL 1

MODEL 3

A. Insert a wider diameter tube in graduated cylinder “A”

B. Insert an identical wide diameter tube in cylinder “B”

Follow steps C through H, as it was done for MODEL 1 & 2

Chemistry 102

_____________________________________________________________________________________________

EXPERIMENT 3

EQUILIBRIUM GAMES

Fall 2015 www./proffenyes.com 4

ANALYSIS: The analysis of these three physical models of chemical equilibrium uses the following

analogies:

Physical Model Analogous to

Volume in Cylinder “A”

Concentration of Reactants

Volume in Cylinder “B”

Concentration of Products

Change in volume (ΔVA) in cylinder “A”

Change in concentration (ΔCR) of Reactant

Change in volume (ΔVB) in cylinder “B”

Change in concentration (ΔCP) of Product

Number of Transfers

Time

ΔVA or ΔVB )

Δ# transfers

(The slope of the plot of any given transfer)

ΔCREACTANT or ΔCPRODUCT

Δ time

Volume of water in cylinder B

Volume of water in cylinder A

at the point the system reached equilibrium

The Equilibrium Constant., K

1. Construct three graphs (for the three different models) by recording the data for both

cylinders on each graph (sample graph included on page 8)

Plot the “Volume of Water” on the Y axis versus the “Number of Transfers” on the X axis.

Label each axis

Join each set of points with a smooth curve.

Label all six smooth curves (two for each model) “A” and “B” respectively Attach the graph to your Report Form (page 8 – page before the last)

2. Complete Data Tables 1, 2 and 3 in your Report Form with your recorded data.

3. Answer the questions in Table 4.

Chemistry 102

_____________________________________________________________________________________________

EXPERIMENT 3

EQUILIBRIUM GAMES

Fall 2015 www./proffenyes.com 5

REPORT FORM

Name: ______________________ Date: _____________ Partner: ______________________

DATA TABLE 1

MODEL 1

Number

of

Transfers

Volume

in A

(mL)

ΔV(A)

(mL)

Volume

in B

(mL)

ΔV(B)

(mL)

0 25.0 0 0.0 0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

Chemistry 102

_____________________________________________________________________________________________

EXPERIMENT 3

EQUILIBRIUM GAMES

Fall 2015 www./proffenyes.com 6

DATA TABLE 2

MODEL 2

Number

of

Transfers

Volume

in A

(mL)

ΔV(A)

(mL)

Volume

in B

(mL)

ΔV(B)

(mL)

0 25.0 0.0 0.0 0.0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

Chemistry 102

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EXPERIMENT 3

EQUILIBRIUM GAMES

Fall 2015 www./proffenyes.com 7

DATA TABLE 3

MODEL 3

Number

of

Transfers

Volume

in A

(mL)

ΔV(A)

(mL)

Volume

in B

(mL)

ΔV(B)

(mL)

0 25.0 0.0 0.0 0.0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

Chemistry 102

_____________________________________________________________________________________________

EXPERIMENT 3

EQUILIBRIUM GAMES

Fall 2015 www./proffenyes.com 9

TABLE 4

Answer the questions in the table below, after consulting your data tables and your graphs.

MODEL 1 MODEL 2 MODEL 3

1 Do the volumes of water in the two cylinders

ever become equal? (YES) or (NO)

2 If your answered YES to Question 1 above,

indicate how many transfers are required before

the volumes become equal.

If your answered NO to Question 1 above,

complete the appropriate box by writing N/A

3 From your graphs, determine the approximate

number of transfers needed to reach equilibrium.

Indicate this on your graphs by drawing the

respective vertical lines.

4 Before reaching equilibrium, does the volume of

water in cylinder A increase, decrease or

remains the same?

5 Before reaching equilibrium, does the volume of

water in cylinder B increase, decrease or

remains the same?

6 How do the slopes of curves A and B change as

number of transfer increases, before equilibrium

is reached?

7 At the point equilibrium has been reached and

thereafter, does the volume of water in cylinder

A increase, decrease or remains the same?

8 At the point equilibrium has been reached and

thereafter, does the volume of water in cylinder

B increase, decrease or remains the same?

9 What are the slopes of the curves A and B at

equilibrium and thereafter?

10 What is the volume of water in Cylinder A at

equilibrium?

11 What is the volume of water in Cylinder B at

equilibrium?

12 What is the analogous numerical value of the

Equilibrium Constant, “K” for each model?