ALCHEMY

Copper  Silver Gold

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    This is a PlayChem experiment in which you place yourself in the shoes of the alchemist, and convert a US penny from a coppery to a silvery and finally a golden coin.
    This is not magic or sleigh-of-hand.  This is real chemistry.
   
To do this you will mix "earth", "water" and "fire".
       
Earth is the solid (Zn) melted from the inside of a  penny.
       
Water is "Butter of Zinc" solution; 1.0 M ZnCl2.
       
Fire is heat to create "silver" and convert "silver" to "gold"
 

To create "EARTH", you will melt a U.S., 1983 or later,  penny.
Make sure the molten zinc falls on the bench top!
Shake gently if necessary to get melt to drop.
Be careful not to burn yourself.
Molten Zn is hot and remains hot for a minute or so!

The reaction consists of ...
                Zn (
EARTH), pennies and 1 M ZnCl2 (WATER).
The mixture is heated to a gentle boil (FIRE).

The pennies turn silver in 10-15 min.
Turn pennies over if necessary.
Boil very gently so reaction does not go to dryness.

 BUT have you achieved the alchemist's dream... converting  copper, a base metal,  to gold?
    Have you really nudged copper two rows down its column in the Periodic Table?
    How can you tell if you can retire now on this newfound source of riches?
    And if it isn't gold, what is it and why did it happen?

Be Prepared to:
  
Calculate surface area of a coin. Assume a simple cylinder.
   
Area cylinder = (2 p r2) + (p d t)   r = radius  d = diameter  t =thickness
   
Volume occupied by atom in metal = (MW)/(Density)(6.02X1023)
   
Thickness of a mass of metal deposited on a surface
   
Measure potentials using Logger Pro

How many layers of atoms plated out?

  • Assume changes in penny are due to a thin plating of metal

  • Assume each metal atom plated is a cube. (simple cubic lattice)

  • Mass, MW and Avogadro's Number gives Number of Atoms

  • MW, Density and Avogadro's Number gives the cube's Volume

  • Cube root of the  cube's Volume gives Length of Side.

  • The cube's Side squared gives the Area of  a Face.

  • Area of Face X Number of Atoms gives Total Area covered if atoms are lined up next to each other one layer thick.

  • Total Area Covered / Surface Area of three coins = Layers

  • Length of Side X Number of Layers gives Thickness.

  • In doing the above calculations, it is advisable to use units!

  • There is an easier way to calculate thickness of the plated coating:

  • Mass of metal plated and density gives volume of metal plated.

  • Dividing Volume of metal plated by surface area plated gives thickness.


WHAT is the REACTION POTENTIAL?

Use LoggerPro
Measure zinc vs pennies
Follow directions on polarity
Make sure you have good connections
Put in Statistic Boxes to show values
Hand in your Graph.
Alternative Printout when not using LoggerPro

 

Aspects of General Chemistry

  •  Solids and lattice structure

  •  Diffusion of atoms through a solid

  •  Redox Reactions and Potentiometry

  •  Kinetics: Difusion speeds up at higher temperatures

Cleaning Copper Pennies with Salt and Vinegar
You will clean your pennies with salt and vinegar.
This is a common household cleaner for tarnished copper.
The increase in ionic strength brought on by the dissolved NaCl lowers the pH of the vinegar solution making it more effective in cleaning the oxides and carbonates from the copper surface.
The chloride ions also complex with the copper further shifting the equilibrium.
Although the pennies cleaned in this manner look shiny and pretty, there is a problem. The presence of Cu metal and Cu+2 ions  in an acid chloride solution leads to the reduction of the Cu+2 and the deposition of a thin film of insoluble CuCl on the surface of the copper.  This surface coating prevents the  reaction from taking place and the penny does not turn silvery.
To remove this CuCl coating, use a piece of steel-wool and buff up the penny.  You should see a small change in the color of the copper surface.  The reddish color of the salt and vinegar cleaned penny becomes more yellow-orange after buffing with steel-wool.

R.W. Kluiber April 18, 2006; May 3, 2008, January 22, 2009