Atomic Coatings – The Size of an Atom Lab
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 (it is called electroplating). In this experiment, the thickness of a zinc metal coating on galvanized iron will be determine and used to “count” the number of layers of atoms in the coating. The number of atoms of zinc will also be determined.
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 and 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, ect.
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.
Equation 1
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
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.
Purpose of this experiment is to:
a. determine the number of layers of zinc atoms in the protective coating on a sheet of galvanized iron;
b. determine the number of moles of zinc and the number of atoms of zinc in the protective coating;
c. observe a chemical reaction.
Safety: 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. Wear goggles and aprons. Wash hands with soap and water after lab.
Pre-Lab Questions:
1. What is the purpose of coating the galvanized iron with zinc?
2. The reaction represented by Equation 1 must be carried out until all of the zinc has reacted. What visible sign of the reaction can be followed to determine when all of the zinc has reacted? (Hint read procedure step 4)
3. How many moles of zinc are present in a 2.00 gram sample? How many atoms are present?
Procedure:
1. Obtain a piece of galvanized iron and measure and record its mass using a 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.
3. Place the metal piece in a beaker and add enough 6M hydrochloric acid to cover the metal (about 25mL).
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 in the data table. Write the name of your favorite TV show, movie, or book by your name on your Prelab.
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.
Data and Calculations:
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
Refer to Conclusion Questions to complete the following:-- Trial 1-- Trial 2
1. Mass of zinc coating
2. Moles of zinc coating
3. Atoms of zinc coating
4. Volume of zinc coating
5. Volume of zinc coating per side
6. Thickness of zinc coating per side
7. Number of layers of zinc atoms per side
Conclusion Questions: Show all work. Enter the results of the calculations in the Data and Results Table.
1. To calculate the mass of the zinc coating on the galvanized iron subtract the final mass of galvanized iron from the initial mass of the galvanized iron.
2. Convert the mass of zinc (determined in question 1) to moles of zinc using the molar mass of zinc.
3. Convert moles of zinc (determined in question 2) to atoms of zinc using Avogadro’s number.
4. To determine the volume of the zinc coating on the galvanized iron use the density of zinc (7.13 g/cm^3) and the mass of zinc from question 1.
5. The zinc coating was present on both sides of the piece of galvanized iron. Divide the total volume of zinc coating, determined in question 4, by two to determine the volume of the zinc coating per side of the galvanized iron.
6. The formula for the volume of a rectangular solid is V=lwh. Rearrange this formula to solve for the unknown height (thickness) and substitute in the known volume per side (determined in question 5) and the length and width of the galvanized iron.
7. The diameter of a single zinc atom is 2.7X10^-8cm. Divide the thickness of the zinc coating per side (determined in question 6) of the galvanized iron by the diameter of a singe 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)