Download Lab: size of the atom

Page 1 –Atomic Coatings
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 billion years! It turns out,
however, that it is 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 used
to "count" the number of layers of atoms in the coating.
Concepts
• Density, mass, and volume
• Atomic size
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 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, including rain gutters, heating ducts, nails
and screws, etc.
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 equation.
Zn(s) + 2HCI(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 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.
Pre-Lab 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 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?
Atomic Coatings
tomic Coatings - Page 2
Balance, centigram (0.01-g precision)
Beakers, 250- and 400-mL, 1 each
Forceps
Galvanized iron sheets, about 3 cm x 3 cm square, 2
Hydrochloric acid solution, HCI, 6 M, 50 mL
Metric ruler, marked in mm
Paper towels
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 and chemical-resistant gloves and apron. Wash hands thoroughly with soap and
water before leaving the laboratory.
Procedure
1. Obtain a piece of galvanized iron and measure and record its mass to the nearest 0.01 g
using the centigram balance.
2. Measure the length and width of the piece of galvanized iron using a metric ruler. Record
the length and width of the iron to the nearest 0.1 cm.
3. Place the metal piece 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 bubbling 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.
5. When the signs of reaction indicate that all of the zinc has reacted, add about 200 mL of
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 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 with a second piece of galvanized iron.
10. Return the used metal pieces to the instructor for disposal.
Flinn ChemTopic— Labs — Atomic and Electron Structure
Page 3 - Atomic Coatings
Name:
Class/Lab Period:
Atomic Coatings
Data and Results Table
Trial 1
Trial 2
Mass of galvanized iron, initial
Length of galvanized iron
Width of galvanized iron
Mass of galvanized iron, final
Mass of zinc removed
Volume of zinc coating
Volume of zinc coating per side
Thickness of coating per side
Number of layers of zinc atoms per side
Post-Lab Calculations and Analysis
(Show all work. Enter the results of the calculations in the Data and Results Table.)
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.
2. The density of zinc is equal to 7.13 g/cm 3 . 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 unknown volume.
Density = Mass
Volume
Atomic Coatings
Coatings — Page 4
The zinc coating was present on both sides of the piece of galvanized iron. Divide the total
volume of the zinc coating (Calculation #2) by two to determine the volume of the zinc
coating per side of the galvanized iron.
4. What is the formula for the volume of a rectangular solid? Rearrange this formula to
solve for the unknown height (thickness) of a rectangular solid if the volume, length, and
width of the solid are known. Check with the teacher before solving.
5. Solve the above equation for the thickness of the zinc coating per side of the galvanized
iron: Substitute the known values for the volume (per side) and the length and width of
the galvanized iron into the formula.
•
6. 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.
7. The diameter of a single zinc atom is 2.7 x 10 --s 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 coatinz.c.
Number of layers of zinc atoms =
Thickness of zinc coating per side (cm)
Diameter of zinc atom (cm)
•
Flinn ChemTopic"' Labs — Atomic and Electron Structure
The dial
atom wr
www.wc
The vah.
distanc,
reporter
64