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Subject

Math, Science & Technology, Science (NYS P-12)

The History of the Penny

The first penny was minted in 1787, designed by Benjamin Franklin, and named after the British term 'Pence'. The original penny was 100% copper and was more than five times heavier almost 50% larger than the penny we have come to know today. In 1864, the composition of the penny was changed to contain 95% copper and 5% zinc. In 1909, the former president Lincoln appeared on the penny to commemorate his 100th birthday. In 1943 only, the copper of the penny was averted for the war effort at which time the penny was made of zinc-coated steel, a very limited number of these pennies were minted. In 1982, the rising price of copper prompted the U.S. government to change the composition of the penny, and to make the penny chiefly of a much cheaper metal. In 1982, the internal composition of the penny changed to 99.2% Zinc and 0.8% copper, and the outside coated with a thin layer of pure copper. Most people can not visibly see a difference between a penny minted before 1982 or after. Since 1982 was a transitional year for the penny, there are literally seven different mintings of the 1982 penny. Today's pennies are less dense and less expensive to make.

Explanation

The chemical process occurring during this lab experience is based on the composition of the post-1983 penny, explained above. The mixture of powdered zinc and hot sodium hydroxide forms a film of zinc over the outside surface of the penny, turning it silver. When the silver penny is heated in a weak flame, the zinc and copper atoms gain energy and shift around creating a solution of solids. This solution is referred to as "brass", an alloy that bears resemblance to gold because of its color. An alloy is a homogeneous mixture of metals, they are solutions of solids.

Teacher Preparation

A solution of 2M Sodium Hydroxide should be prepared and kept under the fume hood. Students should be given enough notice to bring in pennies dated after 1983, or you can bring in yours, one per student is recommended. Have Powdered Zinc on hand, a finer particle size works best

Materials

Safety goggles, 2 - 150 ml Beakers, Graduated Cylinder, 50 ml of 2M NaOH, 2.0 g of zinc powder, Pennies (post-1982), Tongs, Hot Plate, Cold tap water, paper towel, bunsen Burner

Procedure

• 1. Measure 50 ml of NaOH in your graduated cylinder, and pour into one of your beakers.
• 2. Weigh out 2.0 g of Zinc powder and add to the beaker containing NaOH
• 3. Half fill the other beaker with cold tap water, and set aside.
• 4. Place the beaker containing NaOH and Zinc solution on the hot plate and turn it on to medium heat.
• 5. Using the tongs, carefully add the pennies to the solution
• 6. DO NOT allow your Sodium Hydroxide solution to reach a boil. If the solution begins to boil, turn down the heat on the hot plate.
• 7. Watch for any changes in the pennies, use the tongs to turn the pennies over periodically.
• 8. After they have turned a silver color, remove the pennies using the tongs and place them into the beaker of cold-water prepared earlier. Let them sit for only a few seconds.
• 9. Using the tongs, remove the pennies from the cold water and dry them using a paper towel.
• 10. Carefully, light the Bunsen burner and adjust the flame so that you can clearly see the inner blue cone.
• 11. Using the tongs, hold one penny in the upper portion of the flame until you see another color change, this time from silver to gold. If you heat the penny too intensely, you will not see the desired results. Hold it well above the blue cone, the heat should be mild.
• 12. Place the gold penny in the cold-water beaker to cool for a few minutes.
• 13.Remove the penny from the water, and dry.
• 14.You may make as many golden pennies as time allows – amaze your friends and relatives –sell them to a freshman!!

Description

During this lab investigation, students will witness a chemical change as they create a brass alloy using a post-1982 penny, sodium hydroxide, and zinc metal. The penny will undergo a chemical change and turn from copper to silver and when exposed to heat, change to gold. The outcome will be a "Golden Penny".

Safety Concerns

MUST WEAR SAFETY GOGGLES AT ALL TIMES!! Sodium Hydroxide is Caustic and can be dangerous if used without proper supervision. Use caution when heating the Sodium Hydroxide. DO NOT allow it to boil for any longer than a few seconds at a time. The vapors can cause irritation to the respiratory tract. In case of an accidental spill, or contact with skin, immediately wash with soap and water until the "greasy" feel is gone.

Related Academic Standards

New York State

S.HS.PS.1.2 Students who demonstrate understanding can construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical propertie

S.HS.PS.1.5 Students who demonstrate understanding can apply scientific principles and evidence to explain how the rate of a physical or chemical change is affected when conditions are varied.

S.HS.PS.1.12  Use evidence to illustrate that some chemical reactions involve the transfer of electrons as an energy conversion occurs within a system.

MST4.C.C.PS3.1 Students explain the properties of materials in terms of the arrangement and properties of the atoms that compose them.

MST4.C.C.PS3.2 Students use atomic and molecular models to explain common chemical reactions.

MST4.C.C.PS5.2 Students explain chemical bonding in terms of the behavior of electrons.

MST4.C.P.5.3 Students compare energy relationships within an atom’s nucleus to those outside the nucleus.