End of Year Finale

Introduction

Put an exclamation point on your year-end review of chemistry concepts with this series of colorful and engaging demonstrations. Set of five demonstrations includes:

1. The Silver Surfer—Pour a solution into a Petri dish on the overhead projector, place electrodes into the solution, and watch as the “Silver Surfer” streaks across the surface. Beautiful tin crystals are deposited at the negative electrode in this electrochemical cell, then redissolve when the positive and negative electrodes are switched. Simply reverse the polarity to watch the silver surfer return home!
2. Sour Stomach? Pour Me a Rainbow of Relief—Too much stomach acid? Not to worry—mix milk of magnesia with universal indicator and observe the dramatic spectrum of color changes as the antacid dissolves in simulated stomach acid!
3. Sparks Are Flying!—Bring to life single replacement and exothermic reactions in this loud and fiery variation of the classic thermit pyrotechnics. Take two rusty iron balls, wrap one in aluminum foil, then bang them together and watch the fireworks!
4. In Honor of Flag Day—Mix three sets of two colorless solutions and in half a minute, the three solutions flash to red, white, and blue. A colorful salute to that grand old flag.
5. Uncle Sam’s Splashy Finale—Set up three beakers with hydrogen peroxide, soap and food coloring in each, add a catalyst, and quickly cover the beakers with Uncle Sam’s topper. Hold on to your hats as the ensuing reaction causes patriotic, red, white and blue foam to erupt from the stars and stripes adorned stovepipe.

Optional worksheets are provided for each demonstration. All five demonstrations can be performed during one class period, or one demonstration can be performed each day of the final week.

Concepts

• Electrolysis
• Cathode
• Anode
• Oxidation–reduction
• Acid–base neutralization
• Solubility and equilibrium
• Antacids
• Thermit reaction
• Exothermic reaction
• Activation energy
• Clock reaction
• Acid–base indicators
• Buffers
• Catalysts
• Decomposition reaction

Background

The Silver Surfer
Electrolysis is the process of using an electric current to decompose a compound. An electrolytic cell requires several components, including a power source, electrodes and an electrolytic conducting solution. Oxidation occurs at the anode and reduction occurs at the cathode. Typically in an electrolytic cell, the positive electrode is the anode and the negative electrode is the cathode. In this demonstration, an electric current is passed through a solution of tin(II) chloride.

Experiment Overview

The Silver Surfer
The “Silver Surfer” blazes in all directions as an electric current passes through a solution of tin(II) chloride. Students will enjoy watching the growth of tin crystals in this “electric” oxidation–reduction reaction. Reversing the direction of the current causes the tin crystals to redissolve and grow in the opposite direction.

Sour Stomach? Pour Me a Rainbow of Relief
Mix milk of magnesia (MOM) with universal indicator and observe the dramatic rainbow of colors as the antacid dissolves in the simulated stomach acid! This is a great demonstration to review acid–base chemistry, solubility, equilibrium and consumer chemistry.

Sparks Are Flying!
Take two rusted iron balls, wrap one in aluminum foil, then bang them together and watch the sparks fly!

In Honor of Flag Day
Salute the Stars and Stripes! Mix three sets of two colorless solutions and, in half a minute, a red, white and blue tribute to Old Glory appears.

Uncle Sam’s Splashy Finale
Place a beaker of solution inside a plastic Uncle Sam top hat, add a yellow liquid, and then stand back! A cascade of red, white and blue foam billows out the top of the stovepipe hat.

Materials

Safety Precautions

The acidic tin(II) chloride solution is a 1 M hydrochloric acid solution that is corrosive to body tissue and moderately toxic by ingestion. Avoid contact of all chemicals with eyes and skin. Milk of magnesia is intended for laboratory use only; it has been stored with other non–food-grade laboratory chemicals and is not meant for human consumption. Hydrochloric acid solution is toxic by ingestion and inhalation and is corrosive to skin and eyes. Universal indicator solution is a flammable alcohol-based solution. The Sparks Are Flying! demonstration will produce sparks that may shoot several feet. Wear protective goggles or safety glasses and gloves when performing this demonstration. The balls are heavy. Make sure to have a tight grip on the balls before striking them together. Keep fingers to the side of the balls so they are not pinched. Take care to avoid causing hand, arm or shoulder pain from repeated strikes. A teacher demonstration only. Do not allow students to perform this demonstration. Formaldehyde is an alleged carcinogen, however, recent studies indicate no significant risk of cancer from low level exposure to formaldehyde. Formaldehyde is a strong irritant; avoid breathing vapor. Formaldehyde is highly toxic by ingestion, inhalation and skin absorption. The use of formaldehyde in this demonstration does not present an unnecessary risk. Use a fume hood to prepare the diluted formaldehyde solution. Sodium sulfite is moderately toxic; possible skin irritant. Sodium bisulfite is slightly toxic; severe irritant to skin and tissue as an aqueous solution. The magnesium chloride solution is slightly toxic by ingestion. Phenolphthalein solution is a flammable liquid and a dangerous fire risk; it is moderately toxic by ingestion. Avoid contact of all chemicals with eyes and skin. Hydrogen peroxide solution, 30%, is severely corrosive to the skin, eyes and respiratory tract; it is a very strong oxidant and a dangerous fire and explosion risk. Do not heat the solution. Sodium iodide is slightly toxic by ingestion. Do not stand over the reaction; steam and oxygen are produced quickly. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. In The Silver Surfer demonstration, Tin(II) chloride solution may be neutralized and disposed of according to Flinn Suggested Disposal Method #24b. All solid waste may be disposed of in the trash according to Flinn Suggested Disposal Method #26a. Neutralize the final solution in the Sour Stomach? Pour Me a Rainbow of Relief demonstration according to Flinn Suggested Disposal Method #24b. Excess milk of magnesia can be disposed of according to Flinn Suggested Disposal Method #26a.The iron balls in the Sparks Are Flying! demonstration may be reused many times and ultimately be disposed of in the trash according to Flinn Suggested Disposal Method #26a. See Teaching Tips for a procedure to clean and recharge balls with rust. For the In Honor of Flag Day demonstration, the formaldehyde solutions may be disposed of according to Flinn Suggested Disposal Method #2. The magnesium chloride solution, sodium bisulfite solution, and reaction products may all be disposed of according to Flinn Suggested Disposal Method #26b. Sodium sulfite and its solution may be disposed of according to Flinn Suggested Disposal Method #12b. The foam and solution left in the beaker and on the hat in Uncle Sam’s Splashy Finale may be washed down the drain with plenty of excess water according to Flinn Suggested Disposal Method #26b.

Prelab Preparation

In Honor of Flag Day

0.3 M Formaldehyde solution: This solution must be prepared at least 2 hours before use. In a 1-L Erlenmeyer flask, dilute 10 mL of the 37% formaldehyde solution to the 600 mL mark with deionized or distilled water. Mix thoroughly. Keep the flask covered.

Sulfite/bisulfite solution: Prepare this solution within 24 hours of use. Dissolve 3.8 g of sodium sulfite and 12.6 g of sodium bisulfite in another 1-L Erlenmeyer flask containing about 400 mL of distilled or deionized water. Mix thoroughly to dissolve and dilute to the 600-mL mark with deionized or distilled water.

Procedure

The Silver Surfer

1. Place a clean Petri dish on an overhead projector.
2. Fill the dish approximately one-half full (15–25 mL) with 1 M tin(II) chloride solution.
3. Attach paper clips to opposite sides of the Petri dish.
4. Attach the alligator clips from the 9-V battery cap to each paper clip.
5. Hook the battery cap to the 9-V battery and observe the changes at the anode and the cathode. A milky white precipitate of tin(IV) chloride appears at the anode and metallic tin(0) crystals form at the cathode.
6. Allow the current to run for approximately one minute to see the continued growth. The tin(0) crystals form feather-like projections and grow across the dish (see Figure 1).
   {12595_Procedure_Figure_1}
7. Remove the alligator clip leads from the paper clips and switch the polarity of the electrodes by changing which paper clip the leads are attached too. The previous cathode will now be the anode and vice versa.
8. The crystal formation will reverse—tin crystals will grow at the “new” cathode and the existing crystals at the “new” anode will dissolve back into solution.

Sour Stomach? Pour Me a Rainbow of Relief

1. Measure 20 mL of milk of magnesia using a graduated cylinder and pour it into a 1-L beaker.
2. Place the 1-L beaker on a magnetic stir plate. Add a magnetic stir bar to the beaker.
3. Add water and crushed ice (or ice cubes) to give a total volume of approximately 800 mL. Turn on the stir plate to create a vortex in the mixture.
4. Add about 4–5 mL (about 2 pipets-full) of universal indicator solution. Watch as the white suspension of milk of magnesia turns a deep purple color. The color indicates that the solution is basic.
5. Add 2–3 mL (1 pipet full) of 3 M HCl. The mixture quickly turns red and then goes through the entire universal indicator color range back to purple.
6. Repeat this process, adding HCl one pipet-full at a time, waiting after each addition until the mixture turns back to blue or purple.
7. The process can be repeated a number of times before all of the Mg(OH)₂ has dissolved and has reacted with the HCl. As more acid is added, the color changes will occur more rapidly and eventually the suspension will be completely dissolved. This will be evidenced by a clear, red solution.

Sparks Are Flying!

1. Wrap one of the rusted iron balls with a single layer of aluminum foil.
2. Place the rusted iron ball in one hand and the aluminum foil-wrapped one in the other hand.
3. Making sure the fingers are out of the way, strike down on the aluminum foiled-wrapped ball with the rusted one. Try to strike a glancing blow with the two surfaces (see Figure 2).
   {12595_Procedure_Figure_2}
4. As the balls strike and slide past one another, a loud crack and white sparks are produced and the aluminum is partially welded to the iron ball.
5. Rotate the rusted iron ball to expose a fresh surface of iron oxide. Repeat the process for further sparks and cracks! With a little practice, a loud and flashy scene will be created.

In Honor of Flag Day

1. Set up the demonstration by placing three 600-mL beakers, each with a stirring rod, on display, with a 250-mL beaker behind each.
2. Using a 250-mL graduated cylinder, transfer 200 mL of the 0.3 M formaldehyde solution into each 600-mL beaker.
3. Rinse the graduated cylinder with distilled or deionized water and use the cylinder to measure and add 200 mL of the sulfite–bisulfite solution to each 250-mL beaker.
4. Using a clean 10-mL graduated cylinder, add 3 mL of the phenolphthalein solution to the first 250-mL beaker, 10 mL of the magnesium chloride solution to the second 250-mL beaker and 3 mL of the thymolphthalein solution to the third 250-mL beaker. Rinse the graduated cylinder with distilled water before adding each new solution.
5. Stir the resulting solution in each 250-mL beaker.
6. Quickly pour the contents of each 250-mL beaker to the 600-mL beaker in front of it and stir each mixture.
7. In 20 to 30 seconds, the first beaker will turn bright red, the second will form a white precipitate, and the third beaker will turn brilliant blue.

Uncle Sam’s Splashy Finale

1. Place the plastic hat and the 150-mL beaker on a large plastic demonstration tray or dishpan that is several inches deep.
2. Place the three plastic vials together in the 150-mL beaker. The vials should be at the same height and fit snuggly in the beaker.
3. Measure 25 mL of the 30% hydrogen peroxide into each plastic vial. Caution: Wear chemical-resistant gloves, chemical-resistant apron and chemical splash goggles when handling 30% hydrogen peroxide. Contact with skin will cause burns.
4. Measure 10 mL of dishwashing liquid into a 10-mL graduated cylinder and add approximately 3 mL of dishwashing liquid to each vial containing the hydrogen peroxide. Add a few drops of red food coloring to one vial in the beaker and a few drops of blue food coloring in another vial. Mix each solution and place the hat over the beaker. Make sure that the hat openings are over the vials.
5. Measure 8 mL of 2 M sodium iodide solution into the 10-mL syringe.
6. Tell students you will now “call forth” the spirit of 1776.
7. While holding down and placing pressure on the top of the hat with the meter stick, quickly inject about 2–3 mL of sodium iodide through the hat openings into each vial.
8. Watch as the “spirit of 1776” erupts from the top of the hat!

Student Worksheet PDF

12595_Student.pdf

Teacher Tips

• In The Silver Surfer demonstration, the paper clips may turn black in the tin(II) chloride solution due to a reaction between the metal paper clip and the tin(II) ions. This will not affect the growth of tin crystals.
• Place a small piece of copper wire in the center of the Petri dish so that the ends are pointing at both of the alligator clips. When the electric current starts, tin chloride will precipitate at the anode and crystals will grow at the cathode. The copper wire functions as a second electrode, and the crystal growth pattern will repeat itself at the end of the copper wire.
• Place the Petri dish on top of a sheet of clear acetate transparency and place (+) and (–) signs on the sheet to show the polarity of the electrodes.
• This kit contains enough chemicals to perform the Sour Stomach? Pour Me a Rainbow of Relief demonstration seven times: 150 mL of milk of magnesia, 250 mL of 3‑M hydrochloric acid solution, 50 mL of universal indicator solution and 15 Beral-type pipets.
• If a 1-L beaker is not available, use a 600- or 400-mL beaker. Adjust chemical amounts accordingly. Note: The actual milk of magnesia concentration does not have to be exact in order for the demonstration to work.
• If a magnetic stir plate is not available, the mixture can be stirred with a stirring rod.
• The acid used in the demonstration is 3 M hydrochloric acid (HCl). Actual stomach acid ranges from 0.1 to 1.0 M HCl. However, 3 M HCl is used in this demonstration in order to limit the total acid volume and allow the reaction to go to completion with a reasonable volume of acid. If desired, dilute the 3 M acid to 1 M and perform the experiment as written. The volume of acid needed will be three times greater.
• The reaction is performed on ice in order to slow down the color changes so that all colors in the universal indicator color range can be viewed. The reaction may be performed without the use of ice.
• Consider performing this demonstration at different temperatures—5 °C, 25 °C and 60 °C—to compare the effect of temperature on the rate of reaction.
• An excellent follow-up to this antacid demonstration is Flinn’s Antacid Testing Lab Kit—How Powerful Is Your Antacid? (Catalog No. AP5932).
• For the Sparks Are Flying! demonstration, practice practice, practice! A repeatable motion tends to enhance the effects. Strike the balls at very rusted spots, not clean areas. Be careful, however, because too much practice may lead to soreness in the shoulder and arm muscles.
• Make sure your fingers are out of the way when striking the balls together.
• The use of gloves is recommended. This will prevent the hands from being coated in iron oxide and lessen the likelihood of developing blisters when repeatedly striking the balls together.
• The iron balls can be recharged with rust by soaking them in a salt-water solution overnight and allowing them to air dry. Storing them exposed to the normal atmosphere in a chemical storeroom should also keep them well rusted.
• The aluminum coating, formed at the points of impact, can be removed by cleaning the ball with a scouring pad.
• Don’t expect sparks every time. The activation energy is reached through friction and pressure. A glancing blow is necessary to generate a large amount of concentrated friction and the heat needed to initiate the thermit reaction.
• For the In Honor of Flag Day demonstration, only a small fraction of formaldehyde exists in solution as the formaldehyde molecule, CH₂O. Most of it exists as the hydrate, CH₂(OH)₂
  {12595_Tips_Equation_1}
  In a 37% solution of formaldehyde, the methylene glycol polymerizes forming polyoxymethylene glycols.
  {12595_Tips_Equation_2}
  When this solution is diluted, depolymerization occurs slowly. Waiting two hours before use allows the solution to build up a sufficient concentration of free formaldehyde, which then reacts with the sulfite ion in the clock reaction.
• The sulfite–bisulfite solution must be fresh to avoid having both ions oxidized by oxygen in the air to sulfate ions. This reduces the amount of each ion in solution and also produces sulfuric acid, which changes the pH of the initial solution and can interfere with the clock time and color changes.
• The kit contains enough chemicals and materials to perform the Uncle Sam’s Splashy Finale demonstration as written seven times. The Uncle Sam top hat may be washed and rinsed and reused many times.
• It is best to do this demonstration in a large demonstration tray since there is a lot of foam produced. Cleanup, however, is easy due to the presence of extremely safe final products and the generous amount of detergent.
• The vials and beaker will get very hot—allow to cool before handling.
• The white foam may have a slight brown tinge at the beginning due to free iodine produced by the oxidation of iodide ions by hydrogen peroxide.
• Another catalyst that will catalyze this reaction is manganese(IV) oxide, MnO₂.
• Place a glowing splint in the foam and it will relight, indicating that oxygen is produced in the reaction.
• A drop cloth may be necessary to prevent the foam from landing on carpeting or floors. The iodine may stain the material.

Answers to Questions

The Silver Surfer

The original conducting solution contains tin(II) chloride (SnCl₂). The products of the electrolysis reaction are tin(0) and tin(IV) chloride.

1. Draw two sketches representing your observations during the first and second parts of this demonstration.
   {12595_Answers_Figure_3}

   Accept any reasonable student sketches.

2. How did the two products differ in appearance?

   One of the products [tin(0)] has a metallic, crystaline appearance, growing in feather-like projections outward from the negative electrode (cathode) and expand across the Petri dish. The other product [tin(IV) chloride] is a milky white precipitate which remained localized near the electrode (anode). Student responses will vary.

3. Name the products that were obtained at the anode and at the cathode.

   Anode—tin(IV) chloride
   Cathode—tin(0), tin metal

4. The electric current causes an oxidation–reduction reaction within the conducting solution.
   1. Which product results from reduction of tin(II) ions?

      Tin(0)

   2. Which product results from oxidation of tin(II) ions?

      Tin(IV) chloride

5. What was observed when the “sign” or polarity of the electrodes was switched?

   The products seemed to “reverse” their position. The original tin crystals and tin(IV) precipitate slowly redissolved and disappear. Once the products from the first demonstration completely dissolved back into solution, new products formed at both electrodes. Since the placement of the anode and cathode was switched, the location of each product was also reversed.

Sour Stomach? Pour Me a Rainbow of Relief

1. Describe what happened in this demonstration.

   Universal indicator was added to a mixture of milk of magnesia and ice water, turning the solution purple, indicating a pH of around 10. Hydrochloric acid was added one pipet-full at a time. Each time the solution flashed red before going through a series of color changes, from red to orange to yellow to green to blue to purple again. This process became more and more rapid, until finally the milky suspension turned into a clear, red solution.

2. Write the balanced chemical equation for each of the following reactions.
   1. Neutralization reaction between magnesium hydroxide and hydrochloric acid

      Mg(OH)₂(s) + 2H⁺(aq) → 2H₂O(l) + Mg²⁺(aq)

   2. Dissociation of magnesium hydroxide

      Mg(OH)₂(s) → Mg²⁺(aq) + 2OH^–(aq)

   3. Reaction between hydrogen ions from the acid and hydroxide ions from the base

      H⁺(aq) + OH^–(aq) → H₂O(l)

3. Using Le Chatelier’s Principle, explain why adding HCl caused more magnesium hydroxide to dissolve in solution.

   The reaction in the equation in c uses up hydroxide ions from the equation in b. To reestablish equilibrium the equation in b therefore shifts to the right, causing more magnesium hydroxide to dissolve and react with the acid. As more acid is added, more magnesium hydroxide dissolves until eventually there is none left.

4. Explain why the solution is red and clear at the end of the demonstration.

   All the solid magnesium hydroxide, which was responsible for the milky appearance of the solution, has dissolved, resulting in the solution’s clarity. The red color is due to the universal indicator, which is red in the presence of an acid, in this case the excess hydrochloric acid.

Sparks Are Flying!

1. Describe what happened in this demonstration.

   Two iron balls, one rusted and the other one coated in aluminum foil, were struck sharply together. There was a loud crack and a lot of sparks. Where the balls collided, aluminum was welded to the iron ball.

2. What is the difference between an endothermic and an exothermic reaction? Was this reaction endothermic or exothermic? How do you know?

   An endothermic reaction is a reaction that takes in heat, using heat as a reactant. An exothermic reaction, on the other hand, is a reaction that gives off heat as a product. The reaction was exothermic, as evidenced by the loud crack and the sparks that were produced.

3. In this demonstration, rust (iron oxide) reacted with aluminum foil. Write a balanced chemical equation for this reaction. Include heat on the correct side of the equation.

   Fe₂O₃(s) + 2Al(s) → Al₂O₃(s) + 2Fe(s) + Heat

4. How was the activation energy needed for this reaction supplied?

   Because the iron balls were struck together quickly and forcefully, enough kinetic energy was present to pass the activation energy threshold.

In Honor of Flag Day

1. List the contents of the three 600-mL beakers and the solutions added to each beaker. Describe the color changes in each mixture.

   Each of the three 600-mL beakers contained 200 mL of formaldehyde solution and 200 mL of the sulfite-bisulfite solution. 3 mL of phenolphthalein solution was added to the first beaker, 10 mL of magnesium chloride solution was added to the second, and 3 mL of thymolphthalein was added to the third. After about 30 seconds, the first beaker turned bright red, the second became a cloudy white, and the third turned bright blue.

2. Write a balanced chemical equation for each of the following reactions.
   1. Bisulfite ions reacting with water (Hint: This reaction is reversible.)
      {12595_Answers_Equation_3}
   2. Sulfite ions reacting with water (Hint: This reaction is reversible.)
      {12595_Answers_Equation_4}
   3. Formaldehyde (H₂CO) reacting with sulfite to form hydroxymethyl sulfonate ions (HOCH₂SO₃^–) and hydroxide ions

      H₂O(l) + CH₂O(aq) + SO₃^2–(aq) → HOCH₂SO₃^–(aq) + OH^–(aq)

3. The third reaction listed above consumes sulfite ions and produces hydroxide ions. How does this affect the equilibrium in the first two reversible reactions?

   The consumption of sulfite ions and the production of hydroxide ions causes the first equation to shift to the right and the second equation to shift to the left.

4. The sulfite/bisulfite solution acts as a buffer. What happens when the bisulfite ions are used up? How are the color changes produced?

   When the bisulfite ions have all been used up, the hydroxide ions can no longer be consumed as part of the second reaction. This excess of hydroxide ions causes the pH of the solutions to rise. Phenolphthalein and thymolphthalein are acid–base indicators and are red and blue, respectively, in basic conditions. Magnesium hydroxide forms a white precipitate in solutions with a pH exceeding 9.2.

Uncle Sam’s Splashy Finale

1. Describe what happened in this demonstration.

   Hydrogen peroxide and dishwashing liquid were added to a 150-mL beaker. Then a small amount of sodium iodide was added, the hat was put over the beaker, and thick foam erupted from the hat.

2. Write the chemical equation for the decomposition of hydrogen peroxide.

   2H₂O₂(aq) → 2H₂O(g) + O₂(g)

3. Why does the dishwashing liquid foam?

   The water vapor and oxygen gas get trapped in the dishwashing liquid, causing it to foam.

4. What was the purpose of the sodium iodide? Did it get used up during the reaction?

   The sodium iodide served as a catalyst, which is a substance that speeds up a reaction but is not a reactant itself. It was not used up during the reaction.

Discussion

The Silver Surfer
This demonstration utilizes an electric current to cause an oxidation–reduction reaction within a solution of tin(II) chloride. Tin(II) ions are oxidized to an insoluble precipitate of tin(IV) chloride at the anode and are reduced to metallic tin [tin(0)] at the cathode.

The overall reaction is called a disproportionation reaction—a chemical reaction in which one reactant acts as both oxidizing and reducing agent. As a result, Sn²⁺ ions are converted into both a more oxidized species (SnCl₄) and a more reduced product (Sn).

Sour Stomach? Pour Me a Rainbow of Relief
The active ingredient in milk of magnesia is magnesium hydroxide, Mg(OH)₂. Magnesium hydroxide forms a suspension in water since it has a very low solubility—0.0009 g/100 mL in cold water and 0.004 g/100 mL in hot water.

Initially in the demonstration, the solution is basic due to the small amount of Mg(OH)₂ that goes into solution. The universal indicator gives the entire solution a violet color, indicating a pH of about 10. (See Universal Indicator Color Chart.) Upon adding hydrochloric acid (the simulated “stomach acid”), the mixture quickly turns red because the acid disperses throughout the beaker, first neutralizing the small amount of dissolved Mg(OH)₂, and then turning the solution acidic from the excess acid that is present. The excess acid causes more Mg(OH)₂ from the suspension to gradually dissolve. As more of the Mg(OH)₂ goes into solution, the acid is neutralized and eventually the solution becomes basic again from the excess Mg(OH)₂ that is present. The addition of universal indicator allows this process to be observed. During the process, the color of the mixture goes through the entire universal indicator color range—from red to orange to yellow to green to blue and finally back to violet. By adding more “stomach acid,” the process can be repeated several times before all of the Mg(OH)₂ is dissolved and eventually neutralized.

Magnesium hydroxide is classified as a weak base (in most textbooks) due to its very limited solubility in water. This limited solubility makes it an ideal compound to use in commercial antacids since it slowly dissolves as it neutralizes stomach acid rather than dissolving all at once. The neutralization reaction is the reaction between Mg(OH)₂ (a weak base) and HCl (a strong acid). The net ionic equation for the reaction is shown in Equation 1. The reacting species for the strong acid, HCl, is the hydrogen ion, H⁺. In contrast, since Mg(OH)₂ is a weak base, the principal reacting species is the undissociated Mg(OH)₂ compound. The acid–base reaction involves Mg(OH)₂ and H⁺ ions as reactants. The products are water molecules and Mg²⁺ ions in solution. Because the chloride ion, Cl^–, from HCl is a spectator ion, it is not included in the net ionic equation. While Mg(OH)₂ is practically insoluble, a certain amount of Mg(OH)₂ dissociates into ions when put in water. The extent of dissociation of Mg(OH)₂ is indicated by its solubility product constant, K_sp. The K_sp at 25 °C for Mg(OH)₂ is 6 x 10^–12, indicating that only a small amount of Mg(OH)₂ dissociates into ions and the reaction equilibrium lies far to the left, according to Equation 2. In the demonstration, the initial milk of magnesia suspension in water contains very few Mg²⁺ and OH^– ions before the acid is added. As HCl is added to the beaker containing milk of magnesia, the H⁺ ions from the HCl react with the OH^– ions (those that are actually in solution from the Mg(OH)₂) according to Equation 3. The reaction between H⁺ (stomach acid) and OH^– (antacid) to form water uses up some of the OH^– ions and drives Equation 2 to the right, causing more Mg(OH)₂ to dissolve and dissociate into ions. As OH^– ions are removed from solution by the H⁺ ions, more and more Mg(OH)₂ is forced to dissociate to replace those ions, according to Le Chatelier’s Principle. As more acid is added, the Mg(OH)₂ continues to dissociate until all of it is dissolved and Equation 2 lies all the way to the right. The final solution in the milk of magnesia demonstration will thus be clear and acidic (red in color from the universal indicator), indicating that the Mg(OH)₂ is fully dissolved and excess acid is present. At this point, the “antacid power” or “acid-neutralizing ability” of the milk of magnesia is depleted.

Sparks Are Flying!
The reaction is the same as the classic thermit reaction but much safer. When the balls are struck, the rust (Fe₂O₃) reacts with the aluminum foil (Al) to produce aluminum oxide (Al₂O₃), elemental iron (Fe) and heat. This reaction is a highly exothermic, single replacement reaction. Aluminum is oxidized and iron is reduced. The melting point of iron is 1530 °C and the reaction temperature reaches approximately 2200 °C (ΔH° = –849 kJ/mole).

Fe₂O₃(s) + 2Al(s) → Al₂O₃(s) + 2Fe(s) + heat

The reaction coordinate diagram for this reaction is shown in Figure 4. The activation energy (E_act) needed for the reaction to occur is provided by the mechanical (kinetic) energy of the iron balls being struck against one another and the aluminum foil. Once the activation energy is reached, the reaction proceeds very rapidly to produce products and release heat. The loud noise and the sparks result from the large amount of thermal energy (ΔH) released by the reaction.

In Honor of Flag Day
The same clock reaction occurs in all three beakers. The only difference is the color of the acid–base indicators used. The pH of the solutions, and the indicator colors, remain constant throughout the clock period, about 30 seconds. After 30 seconds, the pH of each solution quickly increases, causing a sudden change in the indicator colors.

The “clock reaction” involves the reaction of formaldehyde with sulfite ions. Sulfite (SO₃^2–) and bisulfite (HSO₃^–) ions initially produce a buffer system, where the bisulfite is the weak acid and the sulfite is its conjugate base. The initial sulfite/bisulfite buffer solution is slightly acidic, with a pH of approximately 6.4.

Formaldehyde reacts with sulfite ions to form hydroxymethyl sulfonate ions and hydroxide ions, according to Equation 6. As the reaction proceeds, the sulfite ions are consumed and hydroxide ions are produced. This causes a shift to the right of Equation 4 and a shift to the left of Equation 5. This buffering keeps the pH of the solution essentially constant until all the bisulfite ions (HSO₃^–) are consumed. Without the bisulfite ion, no buffering occurs. Hydroxide ions produced in Equation 6 are therefore not consumed in reaction 5 and the pH rapidly rises to approximately 10.5.

The color change in each solution occurs as the solution pH changes from 7 to 10. In the first beaker, phenolphthalein changes from colorless to bright red in the pH range of 8.5–9.6.

In the second beaker, the white solid produced is magnesium hydroxide [Mg(OH)₂], which precipitates under the reaction conditions when the solution pH exceeds 9.2.

In the third beaker, thymolphthalein changes from colorless to blue in the pH range of 9.3–10.6.

Uncle Sam’s Splashy Finale
This demonstration evolves a good deal of heat as shown by the steam coming off the foam as it is produced. The reaction, therefore, is exothermic. The action of a catalyst is demonstrated. The catalyst is the I^–(aq) ion which speeds up the decomposition of the hydrogen peroxide. The decomposition of hydrogen peroxide produces steam and oxygen gas. The oxygen gas and water vapor cause the dishwashing liquid to foam.

References

The Silver Surfer activity was adapted from Electrochemistry, Flinn ChemTopic^™ Labs, Volume 17, Cesa, I., Ed.; Flinn Scientific: Batavia, IL, 2003.

Special thanks to Annis Hapkiewicz, Okemos High School, Okemos, MI, Bette Bridges, Bridgewater–Rayhnam High School, Bridgewater, MA, and Penney Sconzo, Westminster School, Atlanta, GA, for separately bringing the Sour Stomach? Pour Me a Rainbow of Relief demonstration to our attention.

Shakhashiri, B. Z. Chemical Demonstrations: A Handbook for Teachers of Chemistry; University of Wisconson Press: Madison; 1985; Vol. 4, pp. 70–74.

Summerlin, L. R.; Borgford, C. L.; Ealy, J. B. Chemical Demonstrations: A Sourcebook for Teachers, Vol. 2; American Chemical Society: Washington, DC. 1988; p 173.

The Sparks Are Flying! demonstration originated from the work of Troy Lilly, Western Texas College, Snyder, TX, and was first presented to Flinn Scientific by Larry Peck, Texas A&M University, at the 16th Biennial Conference on Chemical Education. Special thanks to Alan Slater, retired, Stratford Central Secondary School, Stratford, Ontario, for providing Flinn Scientific with instructions for this activity.