What is the first thing that comes to mind when you hear the word equilibrium? In this experiment, we will study the equilibrium that exists in the reaction shown in Equation 1:

The equilibrium constant for this reaction is represented in the manner shown in Equation 2:
𝐾 𝑒𝑞
[𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠]
=
[𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡𝑠]
=
[𝐶]
[𝐴]
In this experiment, the volume in the graduated cylinder labeled “A” will represent the concentration of the reactants, and the volume in the graduated cylinder labeled “C” will represent the concentration of the products.
In this demonstration, you will explore equilibrium using simple lab materials, and the collected data will be graphed and analyzed to establish a conceptual understanding of equilibrium.
Each group needs two 100mL graduated cylinders (labeled A and C), a 10mL stereological pipette (labeled A), a 5mL stereological pipette, and water.
1.
Have the students place the 10.0 mL pipette labeled “A” next to cylinder A and the 5.0 mL pipette labeled “C” next to Cylinder C. Instruct the groups to fill their graduated cylinders with water according to the following:
Groups 1 and 4: Fill graduated Cylinder A with 50.0 mL of water, none in Cylinder B
Groups 2 and 5: Fill graduated Cylinder B with 50.0 mL of water, none in Cylinder A
Groups 3 and 6: Fill each graduated Cylinder with 25.0 mL of water
Record the starting volume of each cylinder in your data table for “Transfer 0”.

11
12
13
14
15
7
8
9
10
Transfer
0
1
2
5
6
3
4
2.
Hold the 10.0 mL pipette in your left hand and the 5.0 mL pipette in your right hand.
Lower them into their respective cylinders, allow the water to reach its maximum
height in each pipette, and place your thumbs over the ends of the pipettes to create a seal. Keeping the seal with your thumb, lift the pipettes out of the water and cross your arms over each other. With the tip of the 5.0 mL pipette over Cylinder A and the tip of the 10.0 mL pipette over Cylinder B, lift your thumbs to allow the water from the pipettes to dispense into the cylinders. Uncross your arms and put the pipettes down next to their respective cylinders. Record the volume of each cylinder in your data table.
3.
Repeat step 2 indefinitely, transferring amounts of water between the two cylinders using the pipettes until the volume readings normalize, or stay constant. Be sure to always start each transfer with the correct pipette in the correct cylinder and record each volume to the correct digit (for a 100mL cylinder this is 0.1 decimal place).
4.
Once the amount in each cylinder normalizes and you get the same amount after multiple transfers, create an amount over time graph using the data from your table.
Cylinder A (mL) Cylinder B (mL)

1.
Compare and contrast your groups graph to the graphs of the other groups. Sketch a rough version of all 3 graph types below and discuss differences and similarities.

2.
Describe how the graphs provide visual information when equilibrium is established by the system. Do all three graph types show this?
3.
Use your data to calculate the equilibrium constant (K eq
) for the reaction A  C by using the volumes as “concentrations” for A and C. Show all of your work below.
4.
Compare your K eq
value to the other groups. What is the relationship between the calculated values for all groups?
5.
What would you expect to happen to the volume in A and C if the transfers continued after equilibrium was established? Explain.
6.
According to the equilibrium constants calculated, would you classify this process as
“reactant-favored” or “product-favored?” State your reasoning.
7.
How would the equilibrium constant be different if the process was exactly the same but the reaction was written as the reverse reaction: C

A
Support your claim with evidence from the lab and state the relationship to the original equilibrium constant as well as the calculated value of the reverse reaction constant.