Brass Penny

Summary: 

A copper penny is coated with a layer of zinc by heating in a zincate solution.  The penny is then heated in a flame reacting zinc with copper, creating a brass coating on the penny.

Hazards: 

Sodium hydroxide is caustic.

Chemicals and Solutions: 

6 M sodium hydroxide

Granular zinc

Materials: 

pennies

porcelain evaporating dish

ringstand, ring and clay triangle

bunsen burner and matches

forceps

beaker of water

Procedure: 

Place about 5 g of granulated zinc in the evaporating dish.

Cover the zinc granules with 6M sodium hydroxide solution.

Set-up a ringstand with ring and clay triangle, adjusting height to accommodate a Bunsen burner.  Place the evaporating dish on the triangle. 

Place a bunsen burner under the evaporating dish and begin heating.

Use forceps to place a penny in the hot zinc/sodium hydroxide mixture.  Leave the penny in the hot mixture for several minutes.  The penny will become coated with zinc and turn a silverish color.

Using forceps to place the now "silver " penny in the beaker of water.

Remove the "silver" penny from the water and blot it dry.

Using forceps, hold the "silver" penny in the flame of a bunsen burner.  Brass is formed turning the penny a gold color.

 

Discussion: 

In the first reaction between the penny and the zinc hydroxide solution, zinc is deposited on the surface of the copper penny.  Granulated zinc reacts with the sodium hydroxide forming zincate ions: [Zn(OH)4]2-.  The zincate ions are then reduced to zinc on the surface of the penny.  The reducing agent is believed to be zinc itself (not copper!) and the driving force is related to the difference in the reduction potentials of zincate ion on different surfaces due to low temperature alloy formation. 

In the second reaction copper and zinc are heated forming an alloy of brass.  (It is worth noting that the color of brass changes depending on the zinc content.  a-brass, with the zinc content less than 35% is golden while g-brass, with a zinc content more than 45% is silvery grey.)

 

 

 

To schedule a demonstration, please send an email to the demonstration lab.

 

Contact:

Eric Camp

Lecture Demonstration Technician

Bagley Hall 171

(206) 543-1606

ericcamp@uw.edu

 

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