Straw Lab: Introduction to Equilibrium
“What’s Equal about Equilibrium?”
Materials
2 graduated cylinders, 2 drinking straws with different diameters, water, graph paper
Procedure
1. Label a graduated cylinder “A” and fill it to the mark with water. Label a
second cylinder “B”. Record the initial volumes of the two graduated cylinders.
2. Label one straw “A” and place it in the graduated cylinder “A”. Label the other
straw “B” and place it in the other graduated cylinder “B”.
3. This step requires two people – one person for each straw. Lower the straws into your respective cylinders,
and when each straw touches the bottom of the cylinder, place your index finger over the open end of the
straw. Transfer the water collected to the other cylinder and allow the straw to drain.
4. Remove each straw and record the volume of water in each cylinder, being careful to read the meniscus to
the nearest 0.1 mL.
5. Return the straws to their original cylinders and repeat the process, recording the volumes after each transfer.
6. After three successive transfers result in no change in volume, add 1 mL of water to cylinder “A”. Record the
volumes in each cylinder, then resume the water transfer until three successive readings are again identical.
Data Table
Number
of
transfers
Volume of
water in
cylinder “A”
(mL)
Volume of
water in
cylinder “B”
(mL)
Number of
transfers
Volume of
water in
cylinder “A”
(mL)
Volume of
water in
cylinder “B”
(mL)
1
13
2
14
3
15
4
16
5
17
6
18
7
19
8
20
9
21
10
22
11
23
12
24
Calculations
Plot the volume of water for both “A” and “B” on the y-axis of the same piece of graph paper against number of
transfers on the x-axis. Join each set of points with a smooth curve.
Two Possible Graphs could be obtained as shown below:
Questions
1. Assuming that this lab simulates the reaction A (reactant)
B (product) what do these represent: i) volume
of water in cylinder A ii) volume of water in B, iii) transferring water from A to B, iv) transferring water from
B to A, v) number of transfers
Volume of water in cylinder A: concentration of reactant
Volume of water in cylinder B: concentration of product
Transferring water from A to B: forward reaction (rate of disappearance of reactant A or rate of appearance of
product B)
Transferring water from B to A: reverse reaction (rate of appearance of A/ reactant or rate of disappearance of
B/product)
Number of transfers: time/length/duration of reaction
2. By observing your graph, describe the changes in volume (analogous to concentration) and corresponding
rates which occur in each curve up to the point where the extra 2 mL of water was added.
As reactants combine to form products, 1) the concentration of reactants decreases, slowing the rate of the
forward reaction (as determined from the slope of graph), 2) the concentration of products increases, speeding
the rate of the reverse reaction. Eventually both concentrations will be constant (or rates will be equal).
3. Describe the change which occurs in the curve for “A” at the point where the 2 mL is added.
At the point where the 2 mL of water is added, concentrations are no more constant.
4. What is the significance of the following:
a) the point where the two curves meet
b) the first flat portions of the two curves, and
c) the second flat portions of the curves?
a) The two volumes are equal (no significance)
b) Equilibrium is achieved
c) equilibrium is achieved again
5. The additional 2 mL of water constitutes a “stress” on the system.
a) What analogous stress would be involved if the system really represented a chemical reaction in equilibrium?
b) Name two other “stresses” which could be imposed on a chemical system.
a) increasing the concentration of reactant
b) change in temperature, change in pressure for a gaseous system
6. What factor controls the relative volumes of water in each cylinder at equilibrium in this lab? What factor would
control the relative concentrations of reactants and products present at equilibrium in a real chemical system?
The relative volumes of water were determined by the diameter of the tubes, which model
the relative concentrations of reactants and products. At this early point in the chapter,
students likely will suggest temperature as a factor that affects the relative concentrations
of products and reactants at equilibrium. Some students may also suggest pressure. In fact,
any of the factors related to Le Châtelier’s principle are factors; you might want to have
students look again at this activity, and this question in particular, at the end of the chapter.