Modelling the Equilibrium

9.1 Practical Skills: Practical Skills

Equipment:

• Two 50mL measuring cylinders
• Two pipettes of differing diameters (e.g. one 10mL, one 5mL)

Method:

1. Fill one 50mL measuring cylinder with water, leaving the other measuring cylinder empty.
2. Place one pipette into first measuring cylinder, letting the pipette lightly touch the bottom. Place your finger over the top of the pipette and move its contents to the second measuring cylinder, letting your finger off the top and releasing the water. Do so carefully to ensure minimum spillage.
3. Place the other pipette into the second measuring cylinder, repeating the second step by emptying the contents of the second pipette into the first measuring cylinder.
4. Note the amount of water in both measuring cylinders and tabulate the results. These results represent the results after 'Cycle 1'. Repeat steps 2 & 3 until Cycle 30 is recorded. Graph the results against each other, with volume on the y axis and cycle count on the x-axis.
5. Repeat steps 1 to 4 but transfer 10mL of water from the second measuring cylinder to the first after the 15th cycle and continue as normal. Graph the results using the same axis as before.

Expected Results:

You will find the equilibrium will be seen as the water levels on both cylinders tend towards, but do not reach the halfway mark - 25mL of water in each measuring cylinder. The change will be relatively quick at first, but will eventually slow down to the point where the water level does not change considerably.
Step number 5 will lead to a temporary spike in the volume of water in the first measuring cylinder, and a decrease in the second measuring cylinder. However, this will also be rapidly corrected at first, and then more gradually after a while.

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