Chemistry 30 Lesson 3-01 Page 1© 2012 T. de Bruin

Chemistry 30 lab – An Introduction to Equilibrium Concepts

Equilibrium is a balance between two opposing reactions that takes place in a closed system at the same rate. The tworeactions, known as the forward and reverse reactions, may be represented by a general equation such as:

forward

reverseEntity A Entity B

When the forward and reverse reactions are taking place at the same rate, the system is said to be in a state ofdynamic equilibrium. During such a state, no changes are occurring in the macroscopic (large scale observable)properties of the system in spite of the fact that changes are occurring at the molecular level. Note that equilibriumdoes not mean that the concentrations of Entity A will be the same as Entity B

Experimental DesignIn this chemical analogy, the water in two separate graduated cyclinders represents the reactants and products of anequilibrium system. The volume of water exchanged will represent changes in the concentrations as the total systemapproaches equilibrium.

MaterialsTwo 25 mL graduated cylinders, 2 drinking straws with different diameters

Procedure1. Label one of the graduated cylinders “reactant” and fill it the

25.0 mL mark with water. Label a second cylinder “product”and leave empty (if using a 10 mL cylinder fill “reactant” tothe 10 mL mark)

2. With a lab partner, transfer water simultaneously from onecylinder to the other using straws of different diameters (usethe large bore straw on the “reactant” cylinder). Do this bylowering the straws into your respective cylinders and wheneach straw touches the bottom, place a finger over the openend of the straw. Transfer the water collected to the othercylinder and allow the straws to drain (of course for the firsttransfer, the product cylinder will be empty).

3. Remove each straw and record the volume of water in eachcylinder being careful to read the meniscus to the nearest0.1 mL.

4. Return the straws to their original cylinders and repeat theprocess.

5. Repeat steps 2 through 4 until you observe no change inwater volume in either graduated cylinder. To be certain thatno change has occurred, you will need to complete severaltransfers that result in approximately the same volumereadings.

6. Add 5 mL of water to the “reactant” cylinder (If using a 10mL cylinder, then add 3 mL). Record the new volumes ineach cylinder, then resume the water transfer until you againobserve no further change in volume.

Analysis1. Plot a graph of the cylinder volume (plotting both cylinders A and B on the same graph) versus number of water

transfers and draw smooth “best fit” curves through both data sets. Answer the analysis questions and thenwait for class discussion.

Chemistry 30 Lesson 3-01 Page 2© 2012 T. de Bruin

ManipulatedVariable Responding variable

Number ofTransfers

Volume in(mL)

cylinders

Cylinder A Cylinder B0 25.0 0.012345678910111213141516171819202122232425

Water Transfer Reaction Equilibrium

Cylin

der

Volu

mes

(mL)

Number of Transfers

0 5 10 15 20 25 30

05

1015

2025

30

Chemistry 30 Lesson 3-01 Page 3© 2012 T. de Bruin

Chemistry 30 Water Equilibrium Lab

Analysis questions.

1. In this activity, the volume of water represents the concentration of a reactant or product. Each transfer of waterrepresents an amount of reactant that was converted into product, or product that was converted back intoreactant. (When answering, consider the shapes/slopes of the curves you have drawn).

a) How can you use your graph to compare the rate of the forward reaction (reactant into product) with the rateof the reverse reaction (product into reactant)?

b) What happens to these rates as the reaction proceeds?

2. At the point where the two curves cross, is the rate of the forward reaction equal to the rate of the reversereaction? Explain

3. How did recognize when the system was at equilibrium?

4. What is an empirical (observable) characteristic of a system at equilibrium?

5. Why is the idea of a chemical reaction at equilibrium called “dynamic”?

6. Why is this particular system called a closed system?

Chemistry 30 Lesson 3-01 Page 4© 2012 T. de Bruin

Water Equilibrium Lab Resultant Graph

forward

reverseREACTANTS PRODUCTS

REACTANT

PRODUCTREVERSEREACTION RATEINCREASES

FORWARDREACTION RATEDECREASES [ ]

[ ] =

At equilibrium position 1 [ ][ ] =

At equilibrium position 2

A stress is something done to asystem that unbalances anequilibrium. E.g. in this lab 5 mL of“reactant” water was added.

At equilibrium:Forward reaction rate = Reverse reaction rate

Chemistry 30 lab – An Introduction to Equilibrium ConceptsExperimental DesignMaterialsProcedureAnalysis