Biology/Chemistry

 

Equilibrium:  What Is It?

 

Patricia Ann Riley

Lincoln Park High School

2001 N. Orchard St. Mall

CHICAGO IL 60614

(773) 534-8130

 

Objective(s):

 

To have the students define the term “equilibrium” through mechanical, chemical, and physical examples of equilibria.  This lesson is designed for 10th grade chemistry students.

 

Materials:

 

Demonstrations:  chairs        2 2-liter beakers      1 250-mL beaker           1 100-mL beaker

                            water        food coloring           overhead marker pen     various other sized beakers

 

Per group of 3 or 4 students:

 2 18 x 150 mm test tubes                              2 jars                               Epsom salts

             concentrated ammonia, NH3                         hot water                         eyedroppers

             0.1M copper(II) sulfate, CuSO4                   stirring rod                       plastic spoon

             1M hydrochloric acid, HCl                          ice    

              

Strategy:

 

.     1) Ask the class what they think the word “equilibrium” means.  Students will commonly

     suggest such synonyms as “balance” or “equal states”.

2)  Place two chairs face-to-face several feet apart in the front of the room.  Ask for two volunteers. Have one student sit on one of the chairs, while the other student stands in front of the other chair.  Explain that the sitting student is to stand up slowly and the standing student is to sit down slowly.  The two students must watch each other so that they complete their action at the same time.  Count aloud to help them start and end at the same time.  As soon as they have completed their action have them repeat the process doing the opposite action, so that the class can see that there are two actions occurring, sitting ® standing and standing ® sitting, occurring simultaneously and continuously.

a.  Explain that an equilibrium consists of two opposite actions, for example standing up and

           sitting down, that occur at the same time, in the same place, at the same speed.

a.      Ask the class how they know the two actions were occurring at the same speed.  Students

      should realize that only one student in the room was standing and all the others were

      sitting at any given moment in time.

c.  Now have the class as a whole participate in the equilibrium.  Ask for a volunteer to be the standing student.  Explain that as this student begins to sit down some other student must stand up without being called on.  This means everyone must pay attention to what   everyone else is doing.  Continue the actions for several minutes.

           d.   Have a student summarize what an equilibrium is and how he/she would know it exists.     

 e.  On the chalkboard show the class how to write an equation for this equilibrium:

                                              Standing student  W Sitting student

                 Point out that the two actions are included in the equation through the double arrows.

                 The action of standing changing to sitting can be seen when the equation is read from

                 left to right (the forward direction or top arrow), while the opposite action of sitting

                 changing to standing can be seen when the equation is read from right to left (the

                 reverse direction or bottom arrow).

 3)  Again ask for two volunteers.  Have each student stand behind a 2-liter beaker.  One of the beakers is completely empty, the other is ¾ full of water (use food coloring for better visibility).  One of the students is given a 100-mL beaker and the other a 250-mL beaker.  Tell the class that each volunteer will submerge his/her small beaker in any water in his/her large beaker and then empty the small beaker into the other student’s large beaker and that the exchange must be done at the same time and the same speed.  Ask the class for predictions as to what will happen to the water levels in the two large beakers once the exchange process has occurred for a while.  Now start the action and ask the class to make observations.  The class should notice that eventually the water levels in the two large beakers stop changing and that the water is not evenly divided between the two large beakers.  Have the class explain what has happened and why:  the levels stop changing when equilibrium is reached since the rate of exchange is constant.  How does the class know when equilibrium has been reached?  Students should answer that there are no more changes observed in the water levels.  Mark the water level on each large beaker.

         a. Ask the class what they think will happen if the process were repeated but this time starting

        with all the water in the other large beaker.  What if the water levels are initially the same?

        Test the predictions by repeating the process.  Again mark the resulting water levels.

        Students should notice that the levels are exactly the same as those in the first trial!  The

        same equilibrium will be reached no matter how the water is apportioned between the two

        large beakers initially.

    b. Ask the class what they think will happen if the process were repeated but this time the

      volunteers switched their small beakers or used different sized small beakers.  Allow them to

      test their predictions.  The class should note that different sized small beakers would result in

      a different set of equilibrium water levels.

   c. Have a volunteer write an equation for this equilibrium on the board. 

   d. Tell the class that each equilibrium has a balance or equilibrium point.  When that point has

      been reached, no further changes will be observed with instruments or our senses.  This

      balance point can be approached from either direction or action.

 4)  At this point divide the class into groups of 3 or 4 students each.  Each group will study two

      actual equilibria.  Be sure that all students are wearing safety goggles and aprons.

   a.   Solid Epsom salt crystals (MgSO4) dissolve in water to make a solution of ions:

                MgSO4 (s)     X     Mg+2(aq)  +  SO4—2(aq)

b.  Direct the students to fill a test tube half full of hot water and then to keep adding

       Epsom salt crystals with a spoon or spatula to the water until no more crystals will

       dissolve.  They should stir the contents of the test tube with a stirring rod.  There must

       be a layer of undissolved crystals on the bottom of the test tube.  Why won’t any more

       crystals dissolve?  Does an equilibrium exist?  If so, what are the two actions?  How

       do you know that two actions are still occurring?

   c.   Now the students should place the test tube into a jar of ice water and record any observations.  Students can even place the test tube in a freezer, if available, for a few minutes.  Students should notice that more crystals have formed.

      d.  Finally the students should place the test tube into a jar of hot water and record any      observations.  Students should notice that the crystals have again dissolved.

e.  Ask students to describe what they observed and to answer the above questions. 

5)  Copper(II) sulfate reacts with ammonia to make a solution of ions:

                   Cu(H2O)4+2(aq)     +     4 NH3(aq)    W    Cu(NH3)4+2(aq)     +    4 H2O(l)

                         Copper(II) sulfate       ammonia

a.   Direct the students to use a calibrated eyedropper to put 1 mL of copper(II) sulfate

      solution into a clean test tube and record the color of the copper(II) sulfate solution

      (pale baby blue).

b.  Now the students should add drops of concentrated NH3 (For safety reasons the NH3 should be given to the students in small dropper bottles and students should be cautioned not to leave the bottles open.) to the test tube until a color change has occurred and the solution is clear. (Remind students that “clear” is not a synonym for “colorless” but for “transparent”.)  What is the new color?  (Deep royal blue)  Why did the change occur?  (The copper(II) sulfate reacted with the ammonia.) 

   c.  Finally have the students add drops of 1M HCl to the test tube until the color again changes and the solution is clear.  (Like the NH3 the HCl should be available in small dropper bottles.)   What is the new color?  (Pale baby blue)  Why did the color change?  (There is an equilibrium.)  How do you know there is an equilibrium?  If time permits, have the students add more NH3 so that they can see that an equilibrium is indeed there.

            d. Have the students discuss their experimental observations and explain what happened and

                why.

6)    Have students summarize what is meant by an equilibrium and how they recognize that one exists.  Assign homework:  students are to finish writing up their lab and should read and study the pertinent pages in their text dealing with equilibrium.

                                                              

Performance Assessment:

 

Students will be assessed on their participation in class discussions and on their written lab report.  Each student must be able to define what an equilibrium is and how he/she would recognize when an equilibrium has been established.

 

Conclusions:

 

Subsequent topics will include the equilibrium constant expression, LeChatelier’s Principle, and sample math problems.  Once students have learned about equilibrium, then discussions involving its applications to acids and bases and oxidation-reduction can follow.