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DETERMINATION OF THE THICKNESS OF ZINC ON A PIECE OF GALVANIZED METAL

* PURPOSE

In this experiment, students will determine the thickness of zinc on a piece of galvanized Iron. They will be introduced to the reactivates of metals with acids and will develop some feeling for the size of an atom.

* DESCRIPTION

This experiment is appropriate for a general or first-year college-prep course. Knowing the density of zinc, students can determine the thickness of the zinc coating on the metal by determining the area of the metal covered with zinc and the mass of zinc removed when the metal is reacted with hydrochloric acid. If students are also given the radius of a zinc atom, they can calculate the thickness of the coating in terms of number of zinc atoms.

* TIME REQUIRED

Approximately 25 minutes.

* MATERIALS

Chemicals:
6 M HCI (500 mL concentrated HCl solution diluted to 1.00 liter with distilled or deionized water)*
3 to 5-cm rectangular piece of galvanized metal *
Equipment:
centimeter ruler
250-mL beaker*
centigram balance
*See Modifications/Substitutions

* HAZARDS

Concentrated solutions of HCl will burn skin and damage clothing. Fumes from acid can be caustic and/or irritating; do this experiment In a well-ventilated area (hood, if available). Goggles must be worn throughout this experiment.

* MODIFICATIONS/SUBSTITUTIONS

  1. HCl solution is available from a hardware store as muriatic acid, 28% HCl; this solution is approximately 8 M and may be substituted for 6 M HCl solution.
  2. Scraps of galvanized metal are available from contractors or a piece of galvanized metal can be purchased from a hardware store.
  3. A Styrofoam cup or small glass jar may be substituted for the beaker.

* PROCEDURE

  1. Determine the mass of the metal and record.
  2. Use the ruler to measure the length and width of the metal and record.
  3. Place the metal in the beaker and cover with 6 M HCl solution. When the gas bubbles stop appearing rapidly, add water; pour off the diluted acid, and rinse and dry the metal that remains.
  4. Determine the mass of the remaining metal and record.
  5. Calculate the area of the metal rectangle covered by the zinc. Remember the zinc covers two surfaces of the metal.
  6. Given that the density of zinc is 7.14 g/cm3, calculate the volume of zinc on the metal.
  7. Determine the thickness of the zinc coating.
  8. Given that the radius of the Zn atom is 2.66 × 10-8 cm, calculate the thickness of the zinc coating in terms of zinc atoms.

* DISPOSAL

Diluted acid solution may be flushed down the drain with water. Remaining metal may be disposed of with solid waste.

* DISCUSSION

Active metals such as zinc react with acids to release hydrogen gas according to the following equation:

2 HCl (aq) + Zn(s)----> H2 (g) + Zn2+(aq) + 2 Cl-(aq)

Iron is much less reactive than zinc and so it reacts with acid only slowly. It is therefore possible to use the rate of bubble formation to signal when removal of the zinc coating is complete.

* TIP

Have students write down the formulas for the area and volume of the zinc coating and the density of the zinc. By manipulating the equations, students can solve for the thickness of the zinc coating.
* Remind students that the coating covers both sides of the galvanized metal.

* REFERENCES

Toon and Ellis, Laboratory Experiments for Foundations of Chemistry ; Holt / Rinehart & Winston, 1973. A similar experiment is described in this work.

Submitted by Annis Hapkiewicz


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