Finding the Amount of Copper in Pennies Samantha Stretar Partners: Alexandra Swartz and Brenton Sullivan Michelle Veite, Professor Yasmin Jessa, CHM144D, 11/13/14 Introduction Strong oxidizing agents are needed to oxidize copper. Nitric acid is an example of a strong oxidizing agent that has the ability to oxidize less active metals including Cu. The balanced equation of the oxidization reaction between Cu and HNO 3 is: Cu ( s ) + 4HNO 3 ( aq ) + 2H 2 O ( l ) Cu(H 2 O) 4 2+ ( aq ) + 2NO 2 ( g ) + 2NO 3 - ( aq ) Copper is present in pennies along with zinc. When Zn is oxidized by HNO 3 the following reaction occurs: Zn ( s ) + 4HNO 3 ( aq ) + 2H 2 O ( l ) Zn(H 2 O) 4 2+ ( aq ) + 2NO 2 ( g ) + 2NO 3 - ( aq ) The oxidation reactions of Cu and Zn produce complex ions of Cu(H 2 O) 4 2+ and Zn(H 2 O) 4 2+ . The central metal ions, Cu 2+ and Zn 2+ , are covalently bonded to the neutral molecular ligands H 2 O. These H 2 O ligands can be displaced by ammonia to create two new complex ions: Cu(H 2 O) 4 2+ ( aq ) + 4NH 3 (aq) Cu(NH 3 ) 4 2+ (aq) + 4H 2 O (l) Zn(H 2 O) 4 2+ ( aq ) + 4NH 3 (aq) Zn(NH 3 ) 4 2+ (aq) + 4H 2 O (l) Formation of the new Cu(NH 3 ) 4 2+ ion creates a deep blue color because the ion absorbs red light. The blue color allows for the determination of the concentration of Cu without conflict of the concentration of Zn. Therefore, aqueous NH 3 is an effective additive to determine the concentration of Cu in a penny. The concentration of a compound in a solution is directly proportional to the amount of light energy absorbed by that compound in the solution at a particular wavelength. A UV-visible spectrometer is able to measure the amount of light energy absorbed at a particular wavelength for the Cu(NH 3 ) 4 2+ ion. The main objective of the experiment was to determine the amount of copper present in a penny. Another objective was to determine if there was a relationship between the Cu content and the age and condition of the coin. 1
The hypothesis tested was that the percentage of copper present in a penny is around 2.5% by mass based off of Congressional mandate. Various factors could affect the percentage of copper present in a penny such as the age and condition of the penny sample. Most of the copper in a penny is present on the outer surface, so wear and tear over multiple years could lead to erosion on the surface and therefore a decrease in the % of Cu present in a penny. Methods I. Preparation of the Standard Solutions and Penny Samples Forty milliliters of 5.00 g/L stock Cu 2+ solution was measured using a clean beaker. Six, 50- mL volumetric flasks were obtained and cleaned. The stock Cu 2+ solution, in volumes of 0.00 mL, 2.00 mL, 4.00 mL, 6.00 mL, 8.00 mL, and 10.00 mL, was pipetted into the separate volumetric flasks.
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- Fall '08
- Chemistry, Penny Samples