Atomic Coatings

The Size of an Atom

 

Introduction

 

The size of an atom is too small to imagine.  Counting the number of atoms in even a microscopic amount of material is an impossible task – it would take a billions years.  It turns out, however, that is it possible to apply a thin surface coating of metal atoms onto another metal.  This is done to change the properties of the underlying or base metal.  In this experiment, the thickness of a zinc metal coating on galvanized iron will be determined and use to “count” the number of layers of atoms in the coating.

 

Concepts

 

              Atomic Size                                      Density, mass, and volume

 

Background

 

Galvanized iron is produced by coating iron with a 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 ay, the zinc coating prevents oxygen from reaching the iron.  The greater the reactivity of zinc continues to protect the iron even after the surface of the zinc has been broken or breaches.  Galvanized iron has many applications, including rain gutters, heating ducts, nails, and screws.

 

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

 

Zn(s)  +  2HCl(aq)  ®  ZnCl2(aq)  +  H2(g)                      Equation 1

 

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 irons, 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.

 

 

 

Pre-Lab Questions

 

  1. What hazards are associated with the use of hydrochloric acid?
  2. What safety precautions must be followed to protect against these hazards?
  3. The reaction represented by Equation 1 must be carried out until all of the zinc has reacted.  What visible sign of reaction can be followed to determine when all of the zinc has reacted?

 

Materials

 

            Electronic Balance                                                    Forceps

            Beakers, 250 mL and 400 mL                                Galvanized iron metal

            Hydrochloric acid, HCl, 6 M, 50 mL                        Metric ruler

            Paper towel                                                               Tap Water

 

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.  Wash hands thoroughly with soap and water before leaving the laboratory.

 

Procedure

 

  1. Obtain a piece of galvanized iron and measure/record the mass.
  2. Measure the length and width of the piece of galvanized iron using a metric rule.

or

(Determine the volume of the galvanized iron by using a water displacement method.)

  1. Place the metal piece in a 400 mL beaker and add enough 6 M hydrochloric acid to cover the metal.
  2. Let the beaker stand until rapid bubble stops.  Note: When the reaction is complete, the piece of galvanized iron will begin to discolor and the solution will turn a pale green color.
  3. When the signs of the reaction indicate that all of the zinc has reacted, add about 20 mL of tap water to the reaction beaker.  This will dilute the hydrochloric acid solution and stop the reaction.
  4. Pour off the diluted acid into a waste beaker as directed by your instructor.
  5. Remove the metal with the forceps.  Holding the metal with the forceps, rinse the metal thoroughly with tap water.
  6. Dry the metal on a piece of paper toweling.  When the metal is completely dry, measure it’s mass again and record the value.
  7. Return the unused metal to the instructor.

 

Post-Lab Calculations and Analysis

 

  1. Determine the mass of the zinc that was reacted.
  2. The density of zinc is equal to 7.13 g/cm3.  Calculate the volume of zinc metal corresponding to the mass of the zinc coating on the piece of galvanized iron.
  3. The zinc coating was present on both sides of the piece of galvanized iron.  Divide the total volume of the zinc coating by two to determine the volume of the zinc coating per side of the galvanized iron.
  4. To determine the volume of a rectangular solid, it is length x width x height = volume.  Rearrange this formula to solve for the unknown height (thickness) of the rectangular solid.
  5. The thickness of a ream, 500 sheets of paper, is 5.0 cm.  Compare the thickness of a piece of paper to the thickness of the zinc coating.
  6. 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.

 

Number of layers of zinc atoms  =  Thickness of zinc coating per side (cm)

                                                                        Diameter of zinc atom (cm)