Handouts for Inquiry-Based Redox and Electrochemistry Labs

Handouts for Inquiry-Based Redox and Electrochemistry Labs

Steve Sogo

Laguna Beach High School

Instructor’s Notes for #138: Color by Number

Instructional video from ACR92651:

Each lab group needs:

1x Erlenmeyer flask (125 or 250 mL)

1x 150 mL beaker

One or two graduated cylinders (25 mL recommended size)

Set-Up for 32 lab groups

• Need about 1-liter of 3 M H2SO4 (count on each lab group using 30 mL). Distribute into 4 or 5 reagent bottles on buffet table
• Make about 1-liter of 0.015-M KMnO4 solution (2.37 g KMnO4 per 1-liter distilled water). Distribute into 4 or 5 reagent bottles on buffet table (keep remainder in reserve). If doing the Shadow of Doubt and Let There be Light labs, make 2-liters of 0.015 M KMnO4.
• Set out solid oxalic acid next to weighing scales (actual compound is the dehydrate, but you can pretend that it is anhydrous for student calculations)
• 1-liter waste beaker on buffet table

Have available (in reserve--produce these upon student request):

• manganese(II) salts (MnSO4, MnCl2, Mn(NO3)2) solids are fine--students to use 0.10 grams for catalytic trials
• 6 M H2SO4 (2 x 200 mL bottles)
• Sparkling water (club soda)
• 3x concentrated KMnO4 solution (.045 M) (one or two 200 mL bottles)

What to expect:

Typically, the initial reaction and analysis requires 45 minutes of class time. Part Two of the lab (unscripted experiments) will take 30-45 minutes for students to formulate a hypothesis, design an experiment (with some input from the instructor), and carry out the experiment. The instructor will need to provide materials for many of the experiments--the goal is for each lab group to try a different experiment, so limiting the numberof bottles of each “extra” chemical helps-- students should be instructed not to share their special reagents with any other groups.

If students are investigating the use of heat, it is best to start the experimental reaction with pre-heated reactants (the oxalic acid + sulfuric acid mixture can be preheated using a microwave or Bunsen burner). A temperature of 50-70C is a good place to start, but even near boiling is OK (reaction will be instantaneous at such a high temperature).

Assignment #138: Color By Number (Lab)

Permanganate reaction with oxalic acid

Write-Up: Everyone will turn in the question page of the lab (on Monday) for 7.5 points. If you want a better grade than this, you will also need to make a paragraph-based write-up describing the experiments you designed in step #8. Your paragraphs should be thoughtful, explaining your ideas instead of just stating what you did.

Procedure: (Bring a calculator, periodic table, and pink page to your table)

1. You will need 1 clean beaker and 1 clean flask for this lab. The flask will be to create a solution of oxalic acid solution, and the beaker will be used to run redox reactions.

2. Weigh out between 0.62 and 0.74 grams of solid oxalic acid. Then dissolve this solute in enough water to make the concentration equal to 0.20-molar. Hint: your calculated volume should be in the range between 30 and 45 mL.

3. Using a graduated cylinder, measure out 10 mL of your oxalic acid solution into the
reaction beaker (save the remaining 30-45 mL of oxalic acid solution for future experiments). Place a smallmagic bean in the reaction beaker and place it atop the magnetic stir plate.

4. Add 10 ml of 3-molar sulfuric acid (H2SO4) to the reaction beaker. Sulfuric acid is a strong acid that is very effective at burning holes in clothing. Because the H2SO4 molecule can form many friendship bonds (hydrogen bonds), the acid evaporates very slowly, so a drop that gets on your clothes will remain there for hours, slowly eating away the fabric. Note: don’t expect to see any reaction between the sulfuric acid and the oxalic acid.

5. To start the redox reaction, add 10 ml of 0.015-molar potassium permanganate solution (KMnO4) to your reaction beaker and stir with the magic bean for a few seconds. After the solutions are mixed, stop stirring. Watch the beaker as it goes through several color changes and record the time it takes for the reaction to go to completion (start timing as soon as you mix the purple KMnO4 into the beaker). Expect the reaction to take a few minutes to run to completion. The reaction is done when the solution is colorless.

6.Look carefully at the solution once the reaction is doneto see subtle clues as to the products of the reaction (in particular, look for some tiny bubbles that may accumulate on the magic bean).

7. After the reaction has finished, answer the questions on the next page. You should be able to figure out all the answers if you apply the theories you have learned this week!

8. Once you understand the chemistry of the reaction, try to perform experiments of your own design to accomplish the following tasks:

i. eliminate the "lag time" in the beginning of the reaction (when it is purple and just sits there)

ii. arrest the progress of the reaction at one of the intermediate colors so that you can keep the color permanently (or at least for a long time)

Note: When faced with a challenge of this sort, a chemist would consider three general ways of changing a reaction’s rate:

a) Alter the temperature of the reaction

b) Alter the concentrations of reactants (your instructor has solutions of varying molarities available for your use!)

c) Include a catalyst in the reaction (this is the most interesting idea to try if you have a clue as to what the catalyst might be!!!!)

Assignment #138 (continued) Color By Number Lab Questions:

A. What is the oxidation number of the Mnatom in KMnO4? Hint: K is a +1 ion.

B. The manganese atom ends this reaction as a free-swimming Mn2+ ion. Based on your experimental results, what is the color of manganese in a +2 oxidation state?

C. Explain how the reduction of manganese can produce the many colors seen during the reaction. Hint: consider the title of this lab!!!

D. The structure of the oxalic acid molecule is shown below. Assign an oxidation number to each atom in the structure.

E. When manganese is reduced (gaining electrons), something must be oxidized (losing electrons). In this case, the element that is oxidized is in the oxalic acid molecule. Based on your understanding of oxidation numbers, which element in the oxalic acid is being oxidized? Hint: Your choices are limited to H, C, and O. Only one of these is a “variable” that can change its number. . .

F. Determine the oxidation number of carbon in each of the molecules shown below. Then explain why only CO2is a plausible identity for the molecule produced when oxalic acid is oxidized.

H2COCO2HCOOH

G. Using the idea of wolves and goats, fill in the appropriate coefficients to balance the chemical equation for the reaction of permanganate with oxalic acid:

___ MnO4- + ___ H2C2O4 + ___ H+  ___ Mn2+ + ____ CO2 + ____ H2O

Mn (wolf) goes from +7 to +2, gobbling up ___ e-.

C (goat) goes from ___ to ___, providing ___ e-.

Need___ carbons to “feed” each Mn.

H. You may have noticed that the reaction in this lab starts very slowly, but appears to speed up after a minute or two. The reason for this acceleration in rate is that the reaction is “autocatalytic”. Use the suggestions below to come up with a hypothesis for how the catalyst works its “magic”. Note: you are encouraged to test your hypothesis by performing appropriate experiments in procedure #8.

a) The catalyst is one of the reactant molecules

b) The catalyst is one of the product molecules

c) The catalyst is a substance that does not appear in the chemical equation

Explain your choice in the space below.
Note: after completing thequestion page, return to the previous page to complete step #8.

Notes for #142: Oxidizing anions (now called A Shadow of Doubt)

Instructional Video from ACR92651:

This is a beautiful lab--many vibrant colors and miraculous transformations. The write-up for this lab requires students to develop cogent arguments that take into account various types of data in an internally consistent manner. This is quite a challenge for high school sophomores, who often double back in their logic to contradict themselves!

For this lab, students are provided with a table of half-reactions (see last page of this handout for a sample).

The set-up for the lab is fairly simple—solution to be made are

100 mL each of

0.15 M NaBr (or KBr)

0.15 M NaI (or KI)

0.25 M NaCl (or KCl)

0.15 M Na2SO3

Solutions placed in color-coded 2 x 50 mL portions polystyrene widemouth jars (with color coded matching pipets). Available on the buffet table

ABCD salt solutions can be refrigerated overnight—important for preservation of the sulfite, which is susceptible to air oxidation. If the sulfite is left unrefrigerated overnight, it may be best to make a fresh solution. Longer storage of the sulfite solution is not recommended.

Buffet Table also provides:

• 0.015 M KMnO4provided in 5 reagent bottles with pipets in bottles. Do not reuse the same bottles provided in the Color by Number lab, as they may contain traces of sulfuric acid, which will allow partial oxidation of Br- in the Shadow of Doubt lab.
• 3 M H2SO4provided in 5 or 6 reagent bottles (with pipets in sidecar test tubes)
• Heptaneprovided in 3 reagent bottles with pipets in sidecar test tubes
• 1-liter WASTE beaker (add a scoop of sodium bisulfite to decolorize this solution (reduces I2, Br2, Cl2, MnO4-))

Each student lab station requires:

• 6x small test tubes (13 x 100 mm)
• 1x test tube rack
• Vortex mixer
• centrifuge may be helpful

In past years, students have misidentified bromide and chloride because they believe that the yellow reaction with the big smell is chlorine. They are unwilling to admit that Br2 might have an odor similar to that of chlorine. I’m hoping that with the “Shadow of a Doubt” title, more students will be willing to admit that they have conflicting evidence (or incomplete evidence).

Assignment #142: A Shadow of Doubt?

(LAB)

Overview: In this lab, you will perform redox reactions with a variety of common anions (anion = a negatively charged ion). Each reaction can be understood using half-reactions to identify the products of oxidation and reduction. At the start of the lab, you will NOT KNOW the identities of the various ions, but you may be able to deduce them based on the experimental evidence you collect. However, it is quite likely that (as the title of the lab implies) you may be faced with some doubt as to which anion is which.

Write-up instructions: Please make a multi-paragraph write-up that does NOT copy any phrases or sentences from the instruction page. Be sure to write balanced chemical equations for all reactions that occur (that means 6 total equations!) and explain WHY things worked the way they did. Include pictures that show the colors present in the test tubes, and explain what molecules/ions create these colors. Present your evidence in as convincing a manner as possible. If you are left with a shadow of doubt, please explain the reasons for your doubt.

Procedures: As always, start by equipping yourself with proper safety gear!

1. To do this lab, each individual will need a half-reaction chart and some scratch paper.

2. At your lab station, you should find 4 clean test tubes in the test tube rack.

3. Your instructor will show you bottles containing the 4 salt solutions you will be using in this lab. Each bottle is filled with a solution of a sodium or potassium salt containing a particular anion. It is the anion that is of importance in today's experiments. The salts are labeled A, B, C, & D, and the four anions (in random order) are chloride, bromide, iodide, and sulfite.

4. Use your half-rxn chart to find an oxidationhalf-reaction for chloride, bromide and iodide. Copy these onto your note page. Also copy the following half-reaction for the oxidation of sulfite ion onto your note page:

SO32-(aq) + H2O (l)  SO42-(aq) + 2 H+(aq) + 2 e-E° = -0.20 Volts

5. Use the pipets provided in the stock jars to accurately measure 1 mL of Salt A into a labeled test tube. Then measure out 1 mL of Salt B into a second tube. Continue with salts C and D in your two remaining test tubes. After collecting all four salt solutions, check your test tubes to see that they all contain the same volume of liquid. If one seems to be “off”, send it back for a refill.

6. Use a graduated pipet to accurately add 1 mL of 0.015-molar potassium permanganate (KMnO4) solution to each of your four test tubes. Note: please “borrow” a bottle of permanganate from the buffet table.

7. Look for signs of reaction. Describe (sketch?) the changes that you see. You should have two tubes that have reacted (and two tubes that have not reacted). At this point, you have added KMnO4without any acid (H+). In the absence of acid, permanganate undergoes the following half-reaction (note the electrode potential).

MnO4-(aq) + 2 H2O (l) + 3 e-  MnO2(s) + 4 OH-(aq)E = +.59 volts

Use this half-reaction to determine why you have two tubes that have reacted and two tubes that have not reacted. Write balanced equations for the two reactions that have occurred.

8. Thoughtfully consider the chemical equations you have written, looking for evidence of reaction products in your test tubes. You may find it helpful to test the pH of a drop of liquid from the tubes that have reacted!

Assignment #142: A Shadow of Doubt? (continued)

9. In the presence of acid, MnO4- undergoes the following half-reaction, with a potential of +1.50 volts:

MnO4-(aq) + 8 H+(aq) + 5 e-  Mn2+(aq) + 4 H2O (l)

Question: If the reaction mixture is acidified to increase permanganate’s potential to +1.50 volts, which of the anions (Cl-, Br-, I-, SO32-) should become oxidized?

10. Write four balanced redox equations for the reaction of acidified MnO4- with all the anions in this lab (include electrode potentials). Hint: this is not much work if you realize the similarities between the oxidation half-reactions!

11. Based on your balanced equations and the hints provided below, make predictions of what you should expect to see (and smell) in your 4 test tubes.

Cl-, Br-, I-, SO32-: these are the ions your started with--you already know that all of these ions are colorless with no particular odors.

Mn2+: you saw this ion last week in your Color by Number lab!

Chlorine (Cl2) from the Greek chloros, meaning “yellow-green”: Chlorine gas dissolves well in water to produce a pale yellow-green color. As you know, chlorine has a powerful odor.

Bromine (Br2) from the Greek bromos, meaning “stench”: Orange-red when concentrated, yellow when dilute. Some swimming pools utilize bromine instead of chlorine as a disinfectant.

Iodine (I2) from the Greek iodes, meaning “violet”: Yellow, orange, brown or red when in aqueous solution. Violet when dissolved in non-polar solvent. Also can produce violet vapors.

Sulfate (SO42-) The color of sulfate ion is evident in the bottle of 3-M sulfuric acid.

12. Use a plastic pipet to add 1 mL of 3-molar H2SO4 to all four tubes. Agitate and look for color changes. Please borrow a bottle of sulfuric acid from the buffet table to perform this step.

13. Sketch the appearance of your test tubes at this stage. If you still have a purple color at this stage, it means that the permanganate has not yet been reduced.

14. Carefully smell the contents of each tube. Wafting recommended!

15. Compare the results observed with the results you expected based on your balanced equations. You should have some discrepancies between expected and observed results, which will lead to doubt, but you should be able to identify some of the anions.

16. Remove 1 mL of solution from your purple test tube and transfer to a clean test tube. This tube should remain heptane-free and will be used in step #19.

17. To gather further evidence, you will use heptane as a non-polar organic solvent to “extract” the non-polar chemicals present in your test tubes. Heptane is a hydrocarbon with the formula C7H16 that will form a layer on top of the water.

Use a pipet to add about 1 mL of heptane to each of your 4 original tubes and vortex vigorously. Heptane (although flammable) is rather inert in most situations. It is NOT reacting with anything in this experiment. Its only purpose here is to act as a solvent. The heptane will extract any non-polar molecules produced in the reactions. Look for signs of these non-polar molecules in the heptanes layers

18. Identify the molecules that are producing colors in your test tubes. Try to label the contents of each tube in both the top and bottom layers (i.e. what non-polar molecules are in the heptane layer? What ions and molecules are in the aqueous layer??).

19. At this point, you still have one tube that has undergone very little reaction. Therefore, it hasn’t progressed to make products that would give it a unique color or smell. In order to speed the rate of this reaction, you should put 60 mL of tap water in a beaker, microwave the water for 1 minute, and immerse the tube containing the purple solution that you reserved in step #16 in the hot water bath. Look for signs of reaction. A smell test may help identify products that are forming.

20. When you are finished, pour the contents of all your tubes into the waste beaker on the buffet table. Rinse your tubes with water and set them upside-down in your test tube rack.

Notes for #146: Let There Be Light!

Instructional video from ACR92651:

This lab is an exciting day, in which students discover the power of electrochemistry. Students must make some intelligent decisions in the design of their galvanic cells in order to produce a battery capable of lighting up an LED. For those students who successfully achieve this task with time left on the clock, a second challenge is to wire two student-made cells together in series to create a higher voltage.

The initialhands-on challenge is to create a standard copper/zinc galvanic cell, which will be easy for students who have done their homework. This standard cell utilizes a salt bridge. Students measure the functionality of their galvanic cell using a volt/ammeter (both voltage and current output are measured). Typically, the voltage is very close to the standard potential of 1.10 volts, but the current is very low (less than 1 milliamp). In order to light an LED, a voltage of 1.7 volts and at least 10 mA are required.