Old Glory

Introduction

Place three clear and colorless solutions all in a row, put on some patriotic march music in the background and add a yellow solution to each. Cheers, cheers for the red, white and blue—the colors of Old Glory emerge in time to the music!

Concepts

• Complex ion formation
• Double replacement reaction
• Prussian blue

Materials

Safety Precautions

Potassium ferrocyanide solution may liberate poisonous hydrogen cyanide upon contact with concentrated acids and heat. Do not heat or allow to come in contact with strong acids. Silver nitrate is moderately toxic by ingestion and irritating to skin, eyes and mucous membranes; it will stain skin and clothing. Iron(III) chloride solution is a corrosive liquid and a body tissue irritant; it is slightly toxic by ingestion. Avoid contact of all chemicals with skin and clothing. Wear chemical splash goggles, chemical resistant gloves and a chemical resistant apron. Please consult 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. The red, white, and blue reaction mixtures that are produced may be combined and washed down the drain with excess water according to Flinn Suggested Disposal Method #26b.

Procedure

1. Place three 150-mL beakers in a row on the demonstration table.
2. Add approximately 20 mL of the appropriate solution to each beaker, as follows:
   {14093_Procedure_Table_1}
3. Measure out 120 mL of iron(III) chloride solution into a fourth 150-mL beaker.
4. Slowly and dramatically, add equal amounts (about 40 mL) of iron(III) chloride solution to each of the prepared beakers 1–3. (Note: For ease in measurement, fill the beakers to the 60-mL mark.) The solution in the first beaker will turn red, in the second beaker a white precipitate will appear, and in the third beaker a blue solution will form.

Student Worksheet PDF

14093_Student1.pdf

Teacher Tips

• This kit contains enough materials to perform the demonstration as written seven times: 140 mL each of potassium thiocyanate, potassium ferrocyanide and silver nitrate solutions and 1 L of iron(III) chloride solution.
• Add drama to the demonstration by playing patriotic march music in the background.
• Use this demonstration as a colorful introduction to the nature and types of chemical reactions. Discuss the types of observations that can be used as evidence that a chemical reaction has occurred.
• This demonstration can be safely upscaled to larger volumes for bigger audiences.

Answers to Questions

1. Describe the contents of each of the beakers. What solution was added to the beakers? What color changes occurred?

   Iron(III) chloride was added to the contents already in the beakers to produce the color changes.

   Beaker 1—contained potassium thiocyanate. Solution turned red.

   Beaker 2—contained silver nitrate. Solution turned a milky white.

   Beaker 3—contained potassium ferrocyanide. Solution turned blue.

2. Write a balanced chemical equation for the reaction that occurred in each beaker.
   1. Iron(III) ions reacting with thiocyanate ions.

      Fe³⁺(aq) + SCN^–(aq) → Fe(SCN)n^3–n (aq)

   2. Iron(III) chloride reacting with silver nitrate.

      FeCl₃(aq) + 3AgNO₃(aq) → Fe(NO₃)₃(aq) + 3AgCl(s)

   3. Iron(III) ions reacting with the iron(II) complex ion K₄Fe(CN)₆.

      4Fe³⁺(aq) + 3Fe(CN)₆^4–(aq) → Fe₄[Fe(CN)₆]₃

3. Explain how the solution in Beaker 2 became milky white.

   The reaction between iron(III) chloride and silver nitrate produced silver chloride, which is insoluble, thereby generating a milky white precipitate.

4. In Beaker 4, a compound informally known as “Prussian blue” is generated. Based on the equations you wrote above, what is the chemical name for Prussian blue?

   The chemical name for Prussian blue is iron(III) ferrocyanide, which is the compound produced in the reaction between potassium ferrocyanide and iron(III) chloride in the third beaker.

Discussion

The red, white and blue color changes illustrate three types of chemical reactions:

Reaction 1—Complex Ion Formation. Iron(III) ions (Fe³⁺) react with thiocyanate ions (SCN^–) to form dark red-orange complex ions, as shown in Equation 1. The value of n can vary from 1–3. This reaction is the basis of a classic experiment used to demonstrate reversible chemical reactions, equilibrium and Le Chatelier’s Principle.

Reaction 2—Double Replacement Reaction. Iron(III) chloride reacts with silver nitrate to form insoluble silver chloride and iron(III) nitrate, as shown in Equation 2. The driving force for the reaction is the formation of insoluble silver chloride, which precipitates out of the reaction mixture to give a milky white suspension. Reaction 3—Formation of Prussian Blue. Iron(III) ions in iron(III) nitrate react with the iron(II) complex ion in K₄Fe(CN)₆ to form a mixed iron(III)/iron(II) compound called iron(III) ferrocyanide, Fe₄[Fe(CN)₆]₃. The historical name for this compound is Prussian blue—discovered in 1710, it was one of the first dye pigments to be manufactured for commercial use.