The Two-Cent Colorful Demonstration

Introduction

A myriad of colors is produced by several reactions occurring in one apparatus, and it all starts with two pennies.

Concepts

• Oxidation–reduction
• Acids and bases
• Indicators
• Reactions of metals and acids

Materials

Safety Precautions

Nitric acid is corrosive to eyes, skin and other tissue; strong oxidant; toxic by inhalation; avoid contact with readily oxidized substances. Sodium hydroxide solutions are corrosive to eyes, skin and other tissue. Universal indicator solution is an alcohol-based solution and therefore flammable. Cupric nitrate solution is toxic. Nitrogen dioxide produced in the reaction is an irritant and toxic by inhalation. This demonstration must be done in a hood or well-ventilated area. 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 goverening the disposal of laboratory waste. The resulting product is an acidic soltion of cupric nitrate.  It can be neutralized using sodium carbonate, then flushed down the drain with excess water according to Flinn Suggested Disposal Method #24b.

Prelab Preparation

Assemble the tubing/stopper/glass elbows apparatus as shown in Figure 1. Lubrication of the holes of the stoppers with mineral oil will help with the insertion of the glass tubing elbows. Insert slowly and carefully, with a twisting motion. Wear heavy cloth gloves to help avoid cuts should the glass elbows break. The 12" piece of glass tubing should be pushed down so it is within one inch of the bottom of the flask.

Procedure

1. Fill the boiling flask with approximately 800 mL of distilled water and add 10 to 12 drops of universal indicator solution.
2. Using the medicine dropper, add the 0.1 M NaOH solution dropwise to the flask until the solution is a deep blue color. 
3. Tightly stopper the boiling flask with the #8 2-hole stopper of the assembled apparatus. 
4. Add approximately 50 mL of concentrated (15.8 M) nitric acid to the Erlenmeyer flask. 
5. Holding the 1-hole stopper in one hand, drop the two pennies into the nitric acid and immediately  stopper the Erlenmeyer flask. The red-brown gas given off is nitrogen dioxide and is toxic. Therefore, the stoppers must be inserted quickly and tightly. This reaction should be done only in an operating fume hood or well-ventilated area.
6. Observe the color changes. The indicator solution will change from blue to pink, and the nitric acid  solution will become green, and later a light blue/green (see the Discussion). The reaction will last approximately 10 minutes. 
7. When the reaction is finished, shake the Erlenmeyer flask to dissolve all of the red-brown NO₂ gas in solution before unstoppering the flask.

Teacher Tips

• It is very important that the piece of glass tubing be pushed down to almost the bottom of the Florence flask. This will ensure that the NO₂ gas generated will dissolve in the water. This demonstration must be done in a hood or well-ventilated area. Although the flasks are stoppered tightly and the NO₂ gas is dissolved in water, leaks can occur in the system. Nitrogen dioxide is an irritant and toxic by inhalation. If any NO₂ gas escapes, it should be allowed to dissipate before anyone approaches the apparatus.
• Be sure to use pre-1982 pennies. Pennies minted before 1982 are made entirely of copper. Pennies minted after 1982 have a core of zinc that is plated with a very thin film of copper. Pieces or strips of elemental copper may be used rather than pennies.
• Use a total of approximately 6 grams of copper.

Answers to Questions

1. Draw the set-up for this demonstration. Be sure to include all apparatus, materials and chemicals.

   Exact drawings will vary. See Figure 1 in the Background section.

2. Describe what you observed in this demonstration.

   When the pennies were dropped into the Erlenmeyer flask with the nitric acid, it let off a red-brown gas. The solution in the boiling flask changed from blue to pink, while the nitric acid solution turned green. Toward the end of the reaction, the nitric acid solution turned blue.

3. Write a balanced chemical equation for each of the following reactions.
   1. Copper reacting with nitric acid. Hint: Nitrogen dioxide is one of the products.

      Cu(s) + 4HNO₃(aq) →Cu(NO₃)₂(aq) + 2NO₂(g) + 2H₂O(l)

   2. Nitrogen dioxide dissolving in water

      2NO₂(g) + H₂O(aq) →2HNO₃(aq)

4. Why did the solution in the boiling flask change from blue to a pink/yellow?

   The product of the reaction between nitrogen dioxide and water is nitric acid. The nitric acid lowered the pH of the solution. There was universal indicator already present in the boiling flask, which change from its color in a base, blue, to its color in an acid, pink/yellow.

Discussion

The copper pennies react with the nitric acid to produce nitrogen(II) oxide.

3Cu(s) + 2NO₃^–(aq) + 8H⁺(aq) → 3Cu²⁺(aq) + 4H₂O(l) + 2NO(g)

The nitrogen(II) oxide quickly reacts with oxygen to form nitrogen dioxide, a red-brown gas.

2NO(g) + O₂(g) → 2NO₂(g)

As the NO₂ gas pressure increases, it bubbles through the basic (blue) solution in the boiling flask. Nitrogen dioxide is very soluble in water. As it dissolves, it reacts with the water to form nitric acid, which lowers the pH of the solution as it is formed.

2NO₂(g) + H₂O(aq) → 2HNO₃(aq)

As the solution changes from basic to acidic, the color of the universal indicator changes from blue to pink/yellow.

When the copper pennies are completely dissolved, the reaction stops. As the Erlenmeyer flask cools, the pressure inside the flask decreases. This lower pressure draws the solution from the boiling flask into the Erlenmeyer flask. As the liquid flows into the Erlenmeyer flask, it dilutes the cupric nitrate solution that was formed by the initial reaction of copper with nitric acid. The Cu²⁺ ions color this solution a bright blue.

References

Special thanks to Pat Funk of Watkins Memorial High School in Pataskala, OH, for bringing this demo to our attention.

Shakhashiri, B. Z. Chemical Demonstrations: A Handbook for Teachers in Chemistry; University of Wisconsin: Madison; Vol. 2, pp 165–166, Vol. 3, pp 83–91.