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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