X854: Prep Notes

Introduction

A copper penny appears to turn to silver after heating in an evaporating dish with a mixture.  The penny then appears to turn to gold when heated in a flame.

In evaporating dish:

Zn + NaOH → Na2ZnO2

 sodium zincate

Cu in penny reduces Na2ZnO2 to metallic zinc:

Cu + Zn2+ --à Cu2+ + Zn

Heat causes a fusion of zinc and copper to form brass (gold-colored).  Brass is 60-82% Cu and 18-40% Zn.

Supplies

Short List

  • Goggle and gloves (heat resistant)
  • gloves
  • Pre-1982 penny
  • Video camera, tripod and video cables or document camera
  • Bunsen burner or propane torch
  • Tongs
  • Hot plate
  • Paper towels
  • Distilled water
  • 250 mL beaker
  • Crucible, 3.5 in. diameter with handle

Chemicals

  • 6.0 M NaOH
  • Zinc, fine powder
  • Conc. HCl

Prior to Lecture  (prep time ~ 5 min.)

  1. In the hood, transfer 5-10g zinc dust or fine granular zinc in a 3.5 inch crucible with handle. Pour in enough 6M NaOH to cover the zinc but do not fill the dish more than one-third full.  In the hood, heat to a boil before lecture.  Remove from heat.  Place crucible on hot plate on lecture bench near an active vent.  Tell instructor to heat mixture to boiling before performing the demo in class.
  2. Clean several pennies with concentrated HCl until they shine.  Newer pennies work best, but those produced after 1983 have a zinc core and can melt if heated too strongly in a burner or torch flame.  Do not hold them in a flame more than 3-5 seconds.
  3. Assemble the following equipment:
    • hot plate by vent
    • tongs
    • gloves: a heat tolerant pair and a latex pair
    • distilled water in wash bottle and a 250 ml beaker for waste
    • paper towels
    • goggles
    • gas lighter

To Conduct Demonstration

  1.  Heat NaOH/Zn mixture to near boiling in a crucible (3.5 in in diameter with handle)  on a hot plate.
  2. Place one or more cleaned pennies into the mixture and continue to warm for 3 or 4 minutes.
  3. Remove the pennies when they are completely and evenly coated with zinc.  Wash them and blot them dry.
  4. Using tongs, hold each penny in a Bunsen burner flame (3-4 sec.). Students in first few rows report being able to see the zinc turn grey while in the flame.
  5. Wash and dry the "gold" penny.

 Demo Time: ~ 5 minutes

Notes:

  1. Clean the pennies in concentrated HCl.  Watch zinc filled pennies (after 1983) to make sure the HCl doesn’t get to the zinc and dissolve it.
  2. Stir NaOH/Zn mixture with a glass rod.
  3. Zn will clump up on the bottom of the crucible in the NaOH.  The Zn compound can stick to the pennies, so experiment with the placement of the pennies to minimize this occurrence.
  4. This demonstration is not recommended for large audience viewing unless a video camera or document camera can be used to project the demonstration on the screen.

 Safety and Disposal

Safety:  Weigh the zinc dust in a hood.  Heat sodium hydroxide and zinc dust using the ventilation grating in Chem 140 or in a chemical fume hood. Use tongs to handle the pennies.  Use caution with hot sodium hydroxide and hot pennies.  Brass pennies are safe to handle (when cool) and may be given away to students.

Disposal: 

Decant the NaOH and neutralize with dilute HCl. Pour the remaining zinc into a 250 mL beaker and place it in the hood. Slowly add 1M HCl until all the solid zinc dissolves and when the solution is cool, dispose in the metals waste carboy.