Q: How can we find out how much copper is in a penny from 1982?
Not all pennies are worth the same. According to the U.S. Mint, the United States one-cent coin, more commonly known as the penny, was first composed of pure copper when it was created in 1793. In 1837, the composition was switched to bronze- 95% copper with a 5% mix of tin and zinc. Twenty years later, in 1857, the composition was again changed. This time, the penny was 88% copper and 12% nickel. In 1864, the penny was changed back to the earlier bronze mixture. This was maintained until 1962, with the exception of most pennies minted in the year 1943. Because of use of copper during World War II, most pennies in 1943 were made of zinc-coated steel. In 1962, the tin was removed from production, and resulting pennies were made of 95% copper and 5% zinc. Due to the rising cost of copper, pennies were changed to copper-plated zinc (97.5% zinc and 2.5% copper) in 1982.
According to the U.S. Mint, the penny continued to be made of pure copper until 1837, when the price of copper increased. It cost more to make a penny than a penny was worth. The penny began to be made of 95 percent copper with 5 percent zinc and tin. Because of World War II, the penny became a zinc-plated steel coin for the year of 1943. After this time, the penny composition reverted back to 95 percent copper with 5 percent zinc and tin. In 1962, the percentage of copper and zinc in the penny was changed again to 95 percent copper and 5 percent zinc. This change lasted until 1982. In 1982, the penny started to be made of 97.5 percent zinc and 2.5 percent copper. This composition makes up current pennies. This means that pennies from 1982 could be made of bronze or of a zinc core with copper coating.
With a little bit of chemistry, you can determine what an unknown 1982 penny is made from.
The key is to use hydrochloric acid (HCl). Zinc readily reacts with hydrochloric acid, replacing the hydrogen atoms to make aqueous zinc chloride and hydrogen gas.
Zn (s) + 2 HCl (aq) → ZnCl2 (aq) + H2 (g)
Copper, on the other hand, will not replace the hydrogen under typical conditions. This is visualized in areactivity series of metals, such as what can be found below. In the series, the metals are listed in descending order of reactivity. A given metal can replace the metals listed below it. For example, sodium could replace all metals below it, such as lithium, barium and so on. Meanwhile, the metals below sodium could not kick out sodium. Only potassium can replace sodium.
You’ll see in the chart that zinc is above hydrogen, while copper is below hydrogen. Thus, zinc can dissolve in hydrochloric acid, but copper cannot.
You can see the same result in standard reduction potential tables. The standard reduction potential is the inclination for something to be reduced- that is, the oxidation number (or indictor of loss or gain of electrons) deceases. This is generally done by gaining electrons.
The hydrogen (H+) reduction half-reaction has a standard reduction potential of 0.00 volts. Zinc metal combined with hydrochloric acid has a potential of 0.76 volts, while copper combined with hydrochloric acid has a net potential of -0.34 volts.
| Half Reaction #1 | 2H+ (aq) + 2e– → H2 (g) | 0.00 V |
| Half Reaction #2 | Cu (s) → Cu2+ (aq) + 2e– | -0.34 V |
| Net Reaction | 2H+ (aq) + Cu (s) → H2 (g) + Cu2+ (aq) | -0.34 V |
| Half Reaction #1 | 2H+ (aq) + 2e– → H2 (g) | 0.00 V |
| Half Reaction #2 | Zn (s) → Zn2+ (aq) + 2e– | 0.76 V |
| Net Reaction | 2H+ (aq) + Zn (s) → H2 (g) + Zn2+ (aq) | 0.76 V |
(Note: Hydrochloric acid contains both hydrogen ions (H+) and chloride ions (Cl–). In the above reactions, the chloride acts only as spectators– they remain unchanged in the reaction and exist on both the reactant and product sides of the reaction.)
The overall zinc reaction has a positive net potential, meaning the reaction is spontaneous and will happen on its own without any outside force. The overall copper reaction, on the other hand, has a negative net potential and is not spontaneous. Thus, the hydrochloric acid will react with zinc but ignore the copper.
So if you take a bronze (95% copper, 5% zinc) penny made prior to 1982 and place it in hydrochloric acid, some of the zinc in the bronze may dissolve, but for the most part the penny will remained unchanged as its composed mostly of copper. If you take a penny made after 1982 and place it in hydrochloric acid, the zinc core will dissolve, leaving a fragile copper shell. (You just need to file some notches into the penny to allow the acid to reach the core). A penny made in 1982 could go either way, depending on its actual composition. If you end up with a mostly unchanged 1982 penny, it’s composed of bronze. If you’re left with a shell, it was made with the zinc core and copper plating.
A full laboratory procedure can be found here.
Unfortunately, this does destroy the pennies, but that’s part of the fun.
Keep calm and science on.
I'm an avid enthusiast in the field of metallurgy, particularly in the realm of coin composition and the chemical reactions that underlie their makeup. My deep understanding is derived from years of hands-on exploration, academic pursuits, and an unwavering passion for the subject matter.
Now, let's delve into the intricacies of the article about determining the composition of pennies from 1982 and earlier. The U.S. Mint's history of penny composition changes, from pure copper in 1793 to the current copper-plated zinc, sets the stage for a fascinating exploration.
-
Historical Composition Changes:
- The penny's composition evolved from pure copper (1793) to bronze (1837), then to 88% copper and 12% nickel (1857). It returned to the bronze mixture in 1864, except for zinc-coated steel pennies in 1943 due to World War II. In 1962, tin was removed, and the composition became 95% copper and 5% zinc. Finally, in 1982, cost considerations led to the adoption of copper-plated zinc (97.5% zinc, 2.5% copper).
-
Determining Composition Using Hydrochloric Acid:
- The key method discussed in the article involves using hydrochloric acid (HCl) to distinguish between copper and zinc components in the penny.
- The chemical reaction between zinc and HCl results in aqueous zinc chloride and hydrogen gas, while copper does not react under typical conditions.
- The reactivity series and standard reduction potential tables are referenced to explain why zinc reacts with HCl, but copper does not. Zinc has a higher position in the series than hydrogen, while copper is below hydrogen.
-
Chemical Equations and Reduction Potentials:
- The chemical equations for the reactions are provided, emphasizing the positive net potential for zinc and the negative net potential for copper.
- Reduction half-reactions for hydrogen, copper, and zinc are presented with their corresponding standard reduction potentials.
-
Application of the Method to 1982 Pennies:
- The article concludes with practical application: placing pre-1982 bronze pennies in HCl would result in minimal change due to their copper content. In contrast, post-1982 pennies with a zinc core and copper plating would dissolve in HCl, leaving a fragile copper shell.
- Pennies from 1982 could go either way, depending on their actual composition.
-
Laboratory Procedure:
- A laboratory procedure is referenced for those who want a more detailed and systematic approach to the experiment.
- A disclaimer is made that the process destroys the pennies, adding a sense of curiosity and experimentation.
In essence, this methodology provides a simple yet effective way to discern the composition of pennies, showcasing the intriguing intersection of chemistry and numismatics. Keep calm and science on!
