Materials Included In Kit

Additional Materials Required

Prelab Preparation

Use the heavy-duty scissors provided with the kit to cut each 6 cm x 6 cm piece of galvanized iron into 3 cm x 3 cm quarters for student use. For safety reasons, we recommend that the teacher rather than the students cut the metal pieces.

Safety Precautions

Hydrochloric acid solution is toxic by ingestion or inhalation and is severely corrosive to skin and eyes. Avoid contact with skin and eyes. The pieces of galvanized iron may have sharp edges that can cut skin. Exercise caution when cutting the metal for students. Handle the metal pieces with forceps. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Remind students to wash hands thoroughly with soap and water before leaving the laboratory. 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. Excess hydrochloric acid may be neutralized with base and washed down the drain with excess water according to Flinn Suggested Disposal Method #24b. The used metal pieces may be disposed of in the solid waste (trash) according to Flinn Suggested Disposal Method #26a.

Lab Hints

• The laboratory work for this experiment can easily be completed within a typical 50-minute class period. It is recommended that the teacher schedule some time during the class period to review the post-lab calculations.
• Teachers may want students to measure the width and length of their galvanized iron pieces in two or three places along each edge and then average the results. This will reduce the experimental error resulting from pieces of metal that are not perfectly square.
• The calculations ignore any zinc that may be present along the edges of the galvanized iron pieces. If we assume that the 3-cm squares were cut from a large sheet of galvanized iron, then this simplification is valid.

Teacher Tips

• As an optional exercise, students may use the data obtained in this experiment to verify the diameter of a zinc atom.
  {13958_Tips_Equation_1}
  Comparing the calculated diameter of a zinc atom with the literature value of 2.7 x 10^–8 cm gives an error of 11%.
• Galvanized iron was first introduced in France in the early 1800s. Modern methods for the preparation of galvanized iron utilize the electrochemical plating of zinc onto iron. Iron is placed into a solution of a zinc salt, and electricity is passed through the solution to reduce the zinc ions to metallic zinc. The term “galvanized” iron honors Luigi Galvani, a pioneer in the study of electricity.

Answers to Prelab Questions

1. Read the Procedure and the Safety Precautions. What hazards are associated with the use of hydrochloric acid? What safety precautions must be followed to protect against these hazards?

   Hydrochloric acid is toxic and corrosive and can cause severe skin burns. Wear chemical splash goggles, chemical resistant gloves and a chemical-resistant apron to prevent contact of hydrochloric acid with skin, eyes and clothing.

2. The reaction represented by Equation 1 in the Background section must be carried out until all of the zinc has reacted. What visible signs of reaction will be used to determine when all of the zinc has reacted?

   The reaction of the zinc metal is complete when the bubbling due to the formation of hydrogen gas stops (or slows down). Note: Students may need some guidance in deciding when the reaction has stopped. There may be some bubbling at the end, but it will be very slow. The color of the solution changes to pale green when the iron begins to react.

Sample Data

Answers to Questions

1. Subtract the final mass of galvanized iron from the initial mass of galvanized iron to calculate the mass of the zinc coating on the piece of galvanized iron.

   Trial 1: Mass of zinc coating = 3.28 g – 3.08 g = 0.20 g
   Trial 2: Mass of zinc coating = 3.18 g – 2.99 g = 0.19 g

2. The density of zinc is equal to 7.14 g/cm³. Calculate the volume of zinc metal corresponding to the mass of the zinc coating on the piece of galvanized iron. Hint: Rearrange the formula for density to solve for the volume of zinc.
   {13958_Answers_Equation_1}
3. The zinc coating was present on both sides of the piece of galvanized iron. Divide the volume of the zinc coating by two to determine the volume of the zinc coating per side of the galvanized iron.

   Trial 1: Volume of zinc coating per side = 0.028 cm³/2 = 0.014 cm³
   Trial 2: Volume of zinc coating per side = 0.027 cm³/2 = 0.014 cm³

4. 1. The formula for the volume (V) of a rectangular solid is V = L x W x H. Rearrange this formula to solve for the height (thickness) of a rectangular solid if the volume (V), length (L) and width (W) of the solid are known.
      {13958_Answers_Equation_4}
   2. Solve this equation to determine the thickness of the zinc coating per side of the galvanized iron. Hint: Substitute the known values for the volume (per side) and the length and width of the galvanized iron into the formula.
      {13958_Answers_Equation_5}
5. The diameter of a single zinc atom is 2.7 x 10^–8 cm. Divide the thickness of the zinc coating per side of the galvanized iron by the diameter of a single zinc atom to calculate the number of layers of atoms in the zinc coating.
   {13958_Answers_Equation_7}
6. The thickness of a ream (500 sheets) of paper is approximately 5.0 cm. Compare the thickness of a piece of paper to the thickness of the zinc coating (per side). This gives a “mental picture” of the thickness of a layer of atoms.

   The average thickness of a piece of paper (0.010 cm) is about 7–8 times thicker than that of the galvanized coating!

References

This activity was adapted from Flinn ChemTopic^™ Labs, Volume 3, Atomic and Electron Structure; Cesa, I; Ed; Flinn Scientific: Batavia IL, 2003.

Introduction

The size of an atom is too small to imagine. Counting the number of atoms in even a microscopic amount of material might take a billion years. We can get some idea of the size of an atom by investigating the mass of the invisible zinc coating on galvanized iron. In this experiment, the thickness of the zinc coating on galvanized iron will be determined and used to “count” the number of layers of zinc atoms on the surface.

Concepts

• Atomic size
• Density
• Mass
• Volume

Background

Galvanized iron is produced by coating iron with a very thin layer of metallic zinc. The zinc coating protects the underlying iron metal against rusting or corrosion. Zinc is more reactive than iron and thus reacts with oxygen in the air and with water before the iron does. In this way, the zinc coating prevents oxygen from reaching the iron. The greater reactivity of zinc continues to protect the iron even after the surface of the zinc has been broken or breached. Galvanized iron has many applications (e.g., rain gutters, heating ducts, nails and screws).

The amount of zinc on the surface of galvanized iron can be determined by reacting the zinc with hydrochloric acid, according to the following equation.

The products of the reaction are zinc chloride, which dissolves in the hydrochloric acid solution, and hydrogen gas, which bubbles out of the solution. By measuring the mass of a piece of galvanized iron before and after its reaction with hydrochloric acid, the mass of zinc that reacted can be calculated. The mass of zinc can be related, in turn, to the number of layers of zinc atoms in the zinc coating by considering the density of the metal, the surface area of the galvanized iron, and the size of a zinc atom.

Experiment Overview

The purpose of this experiment is to determine the number of layers of zinc atoms in the protective coating on a sheet of galvanized iron.

Materials

Prelab Questions

1. Read the Procedure and the Safety Precautions. What hazards are associated with the use of hydrochloric acid? What safety precautions must be followed to protect against these hazards?
2. The reaction represented by Equation 1 in the Background section must be carried out until all of the zinc has reacted. What visible signs of reaction will be used to determine when all of the zinc has reacted?

Safety Precautions

Hydrochloric acid solution is toxic by ingestion or inhalation and is severely corrosive to skin and eyes. Avoid contact with skin and eyes. The pieces of galvanized iron may have sharp edges that can cut skin. Handle the metal pieces with forceps. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Procedure

1. Obtain a piece of galvanized iron and measure its length and width using a metric ruler. Record the length and width of the iron to the nearest 0.1 cm.
2. Measure and record the mass of the galvanized iron to the nearest 0.01 g using a centigram balance.
3. Place the galvanized iron in a 400-mL beaker and add enough 6 M hydrochloric acid to cover the metal (about 25 mL).
4. Let the beaker stand until the rapid gas bubbling stops. Note: When all of the zinc has reacted, the piece of galvanized iron will begin to discolor and the solution will turn a pale green color as the iron starts to react.
5. When the signs of reaction indicate that all of the zinc has reacted, carefully add about 200 mL of cold tap water to the reaction beaker. This will dilute the hydrochloric acid solution and stop the reaction.
6. Pour off the diluted acid into a waste beaker as directed by your instructor.
7. Remove the metal from the beaker with a forceps. Holding the metal with the forceps, rinse the metal thoroughly with tap water.
8. Dry the metal on a piece of paper toweling. When the metal is completely dry, measure its mass again and record the value to the nearest 0.01 g in the data table.
9. If time permits, repeat steps 1–8 using a second piece of galvanized iron (Trial 2).
10. Return the used metal pieces to the instructor for disposal.

Student Worksheet PDF

13958_Student1.pdf