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Coin (Penny) Colors—Electroplating

  • Page ID
    128799
  • Required Training

    Required PPE

    UC Lab Safety Fundamentals

    Lab coat, safety glasses/goggles,

    nitrile gloves

    Equipment

    Chemicals

    Hot plate

    20-30 Clean pennies (cleaned with steel wool)

    Tongs

    25 g Zinc metal

    DI squirt bottle

    100 mL 3 M NaOH

    250 mL Beaker

    Procedure:

    1. Put 25 g granular zinc metal and 100 mL 3 M NaOH into a 250 mL beaker.
    2. Heat the beaker on a hot plate until it begins to boil.
    3. Add cleaned pennies to the bath. Stir with a stirring rod.
    4. After about 1 minute, the pennies will be silver colored (zinc plated). Do not leave the pennies in longer than 3 minutes, as they will have clumps of zinc stuck to them.
    5. Remove the pennies and rinse with DI water.
    6. Heat the on hot plate, and they will form a “gold” (brass) amalgam. Immediately remove once you see a shiny “gold” appearance.

    Discussion:

    The solution may fizz as it is being heated, this is hydrogen gas given off as sodium zincate is formed. The reaction below explains this:

    Zn(s) + 2NaOH(aq) + 2H2O(l) → Na2Zn(OH)4(aq) + H2(g)

    The plating will only occur if the zinc and the copper are in contact with each other. The two half reactions are shown below with their chemical potentials.

    Zn(OH)42− + 2 e Zn(s) + 4 OH

    −1.199

    Zn2+ + 2 e Zn

    −0.7628

    The free zinc ions are complexed from Zn2+ (formed at the zinc electrode (anode)) to Zn(OH)42- and are reduced at the copper electrode (cathode) to form zinc metal. Upon heating, the two metals amalgamate as the zinc migrates into the copper, forming brass (undetermined % composition).

    Hazards:

    Use extreme caution when dealing with boiling hot base. Sodium hydroxide is corrosive. Wear all appropriate PPE. Use caution while using the Bunsen burner—hydrogen gas (explosive) is produced by the formation of sodium zincate.

    SOP: clipboard_e0497c8aaffe121699242f1f7048b42a8.png

    Corrosive – Sodium Hydroxide

    Disposal (by Storeroom)

    Return all materials to the storeroom for proper disposal. Storeroom treats zinc solution with dilute H2SO4 and flushes down the drain with copious water.

    Acknowledgements

    http://www.rsc.org/images/alchemy_tcm18-188862.pdf

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