Pennies, those ubiquitous copper-colored coins, are more than just pocket change. They’re fascinating objects of chemistry, constantly interacting with their environment. Understanding what reacts with pennies can reveal a surprising amount about material science, corrosion, and even history. While we see them as commonplace, pennies are susceptible to a variety of chemical reactions that alter their appearance and composition over time. This article explores the common substances that react with pennies, the science behind these reactions, and what factors influence these processes.
The Composition of a Penny: More Than Meets the Eye
The composition of a penny has changed over time, significantly impacting its reactivity. Knowing what a penny is made of is crucial to understanding its chemical behavior. Pennies weren’t always the solid copper coins we might imagine.
Pre-1982 Pennies: Almost Pure Copper
Before 1982, United States pennies were primarily composed of copper. Specifically, they consisted of 95% copper and 5% zinc. This high copper content made them relatively resistant to corrosion in many environments, although not immune. Copper, while a reactive metal in certain situations, forms a protective oxide layer that slows down further reactions.
Post-1982 Pennies: A Copper-Plated Zinc Core
In 1982, due to rising copper prices, the composition of the penny changed dramatically. Pennies made after this date consist of a 97.5% zinc core plated with a thin layer of 2.5% copper. This change made the penny significantly lighter and more susceptible to corrosion. Zinc is far more reactive than copper, and while the copper plating provides some protection, it’s easily breached, leading to potentially dramatic reactions. The thin copper layer is why the appearance of a penny still seems like pure copper, even though the underlying metal is different.
Common Substances That React With Pennies
Many everyday substances can react with pennies, altering their appearance or causing corrosion. These reactions are often driven by the principles of electrochemistry and oxidation-reduction.
Acids: A Penny’s Nemesis
Acids are among the most potent substances that react with pennies. Acids, even relatively weak ones, can dissolve the copper or zinc, leading to corrosion.
Vinegar (Acetic Acid)
Vinegar, containing acetic acid, is a common household item that readily reacts with pennies. The acetic acid dissolves the copper oxide layer and then attacks the copper or zinc metal itself. The reaction produces copper acetate, a blue-green compound that’s often visible on pennies exposed to vinegar. Furthermore, if the penny is post-1982 and the copper plating is breached, the zinc core will react vigorously with the acid, producing hydrogen gas. The equation for the reaction of copper with acetic acid and oxygen is complex, but the overall effect is the gradual dissolution of the copper.
Lemon Juice (Citric Acid)
Similar to vinegar, lemon juice contains citric acid, another weak acid that reacts with pennies. The reaction is similar to that of vinegar, dissolving the copper oxide and attacking the underlying metal. The citric acid also helps to chelate (bind) the copper ions, preventing them from re-depositing on the penny’s surface and keeping the solution clear. This chelation process accelerates the dissolution of the copper.
Strong Acids (Hydrochloric Acid, Sulfuric Acid)
Strong acids like hydrochloric acid (HCl) and sulfuric acid (H2SO4) react much more aggressively with pennies than weak acids. They quickly dissolve the copper and, in the case of post-1982 pennies, rapidly corrode the zinc core. These reactions generate heat and potentially harmful fumes, so they should only be performed with proper safety precautions. The reaction of zinc with hydrochloric acid produces zinc chloride and hydrogen gas, a flammable mixture.
Salts: Corrosion Catalysts
Salts, especially in the presence of moisture, can also accelerate the corrosion of pennies. Salt solutions act as electrolytes, facilitating the electrochemical reactions that lead to corrosion.
Sodium Chloride (Table Salt)
Sodium chloride, or table salt, itself doesn’t directly react with copper or zinc. However, when dissolved in water, it creates an electrolytic solution that enhances the corrosion process. The chloride ions help to break down the protective oxide layer on the copper, allowing oxygen to attack the metal more readily. This is why pennies left in saltwater environments corrode much faster than those in dry conditions.
Other Salts
Other salts, such as calcium chloride (often used for de-icing roads) and various industrial salts, can also promote penny corrosion in a similar manner. The specific type of salt and its concentration will influence the rate of corrosion.
Ammonia: A Surprising Reaction
Ammonia, a common cleaning agent, reacts with copper to form a deep blue complex ion called tetraamminecopper(II). This reaction is not a simple dissolution; instead, the ammonia molecules bond with the copper ions, forming a stable, soluble complex. This process effectively removes copper from the penny’s surface, revealing a cleaner surface initially, but ultimately leading to corrosion. The deep blue color of the solution is a telltale sign of this reaction.
Hydrogen Peroxide: An Oxidizing Agent
Hydrogen peroxide (H2O2) is a strong oxidizing agent that can react with copper, especially in the presence of an acid catalyst. The hydrogen peroxide oxidizes the copper, forming copper ions that can then react with the acid to form soluble copper salts. This reaction is often used in cleaning solutions to remove tarnish and corrosion from copper surfaces.
Sulfur Compounds: Tarnishing the Penny
Sulfur compounds, such as hydrogen sulfide (H2S) found in polluted air or decaying organic matter, react with copper to form copper sulfide, a black tarnish. This is the primary reason why pennies darken over time. The reaction is relatively slow but can significantly alter the penny’s appearance over years of exposure. The tarnishing process is an example of corrosion, where the metal is chemically altered by its environment.
Factors Influencing the Reaction Rate
Several factors influence the rate at which substances react with pennies. Understanding these factors can help predict how pennies will behave in different environments.
Temperature
Temperature plays a crucial role in chemical reaction rates. Generally, higher temperatures increase the rate of reaction. This is because higher temperatures provide more energy to the reactant molecules, allowing them to overcome the activation energy barrier of the reaction. Therefore, pennies exposed to warm environments will corrode faster than those in cold environments, all other factors being equal.
Concentration
The concentration of the reacting substance also affects the reaction rate. Higher concentrations mean more reactant molecules are available to react with the penny’s surface, leading to a faster reaction. For example, a penny immersed in concentrated hydrochloric acid will corrode much faster than one immersed in dilute hydrochloric acid.
Surface Area
The surface area of the penny exposed to the reacting substance is another important factor. A penny that is partially submerged in a solution will corrode more slowly than one that is fully submerged. Similarly, if the penny has scratches or other surface imperfections, these areas will be more susceptible to corrosion due to the increased surface area available for reaction.
Presence of Other Metals
The presence of other metals in contact with the penny can also influence the reaction rate, especially in the presence of an electrolyte. This phenomenon, known as galvanic corrosion, occurs when two different metals are in electrical contact in an electrolytic environment. The more reactive metal will corrode preferentially, protecting the less reactive metal. In the case of post-1982 pennies, if the copper plating is damaged and the zinc core is exposed, the zinc will corrode preferentially, protecting the copper to some extent.
pH Level
The pH level of the surrounding environment is a critical factor. Acidic environments (low pH) tend to accelerate corrosion, while alkaline environments (high pH) can sometimes inhibit it. However, extreme alkalinity can also lead to corrosion of certain metals, such as aluminum. For pennies, acidic conditions are generally more corrosive due to the ability of acids to dissolve copper and zinc.
The Electrochemistry of Penny Reactions
Many of the reactions involving pennies are electrochemical in nature. Understanding the principles of electrochemistry helps to explain why certain substances react with pennies and how the corrosion process occurs.
Oxidation and Reduction
Electrochemical reactions involve the transfer of electrons between reactants. Oxidation is the loss of electrons, while reduction is the gain of electrons. In the case of penny corrosion, the copper or zinc atoms in the penny lose electrons (are oxidized) to form metal ions, which then dissolve into the surrounding solution. The substance that causes the oxidation gains electrons (is reduced).
Electrolytic Solutions
Electrolytic solutions, such as salt water or acidic solutions, are essential for electrochemical reactions to occur. These solutions contain ions that can conduct electricity, allowing the flow of electrons from the metal to the oxidizing agent. Without an electrolytic solution, the reaction rate is significantly reduced.
Electrode Potential
Each metal has a characteristic electrode potential, which is a measure of its tendency to lose electrons. Metals with more negative electrode potentials are more easily oxidized and corrode more readily. Zinc has a more negative electrode potential than copper, which is why it corrodes preferentially in post-1982 pennies when the copper plating is breached.
Practical Applications and Demonstrations
Understanding the reactivity of pennies has several practical applications and can be used to perform interesting demonstrations.
Cleaning Pennies
The reactivity of pennies with acids can be used to clean tarnished coins. Soaking pennies in vinegar or lemon juice can remove the copper oxide layer, revealing a shiny surface. However, it’s important to rinse the pennies thoroughly after cleaning to remove any residual acid that could continue to corrode the metal.
Penny Battery
A simple battery can be constructed using pennies, cardboard, vinegar, and aluminum foil. The vinegar-soaked cardboard acts as an electrolyte, and the different metals (copper in the penny and aluminum) create a potential difference that can generate a small amount of electricity.
Testing Acidity of Solutions
The reaction of pennies with acids can be used as a rough indicator of the acidity of a solution. A solution that quickly corrodes a penny is likely to be more acidic than one that has little effect.
Long-Term Effects of Reactions on Pennies
The long-term effects of reactions on pennies can range from minor tarnishing to complete disintegration of the coin. The extent of the damage depends on the environmental conditions and the specific substances the penny is exposed to.
Tarnishing and Patina
Over time, pennies exposed to air and moisture will develop a tarnish layer, primarily composed of copper oxide and copper sulfide. This tarnish can range in color from light brown to black. In some cases, a green patina, composed of copper carbonates and sulfates, may also form. This patina is more common in humid environments and can protect the underlying metal from further corrosion.
Corrosion and Degradation
In more aggressive environments, such as those containing acids or salts, pennies can undergo significant corrosion. This can lead to pitting, cracking, and even complete disintegration of the coin. Post-1982 pennies, with their zinc cores, are particularly vulnerable to corrosion in these environments. The zinc core corrodes rapidly, causing the copper plating to flake off and eventually leaving behind a powdery residue.
Conclusion
The seemingly simple penny is a fascinating object of chemical reactions. Its composition, whether primarily copper or a copper-plated zinc core, dictates its reactivity to a wide range of substances. From acids and salts to ammonia and sulfur compounds, various chemicals can alter the appearance and integrity of a penny. Understanding the factors that influence these reactions, such as temperature, concentration, and pH, provides insight into the processes of corrosion and electrochemistry. By exploring the reactivity of pennies, we gain a deeper appreciation for the chemical world around us and the constant interactions that shape the materials we encounter every day. The next time you see a penny, remember that it’s not just a coin; it’s a tiny testament to the power of chemistry.
Why do pennies turn green sometimes?
The green color seen on old pennies is primarily due to the formation of copper carbonates and oxides. Pennies minted before 1982 are almost entirely copper. When copper reacts with oxygen, carbon dioxide, and moisture in the air, it undergoes a chemical reaction called oxidation, forming copper oxide. This copper oxide can then react further with carbon dioxide and water to form copper carbonate, also known as malachite, which is typically green.
The process is accelerated in environments with high humidity, pollutants, and acidic substances. The presence of salt or acids, for instance, can further promote the corrosion of copper, leading to the more rapid formation of the green patina. The exact shade of green can vary depending on the specific composition of the carbonates and oxides formed, as well as the environmental conditions.
What acids react with pennies and what happens?
Various acids can react with pennies, with hydrochloric acid (HCl) and nitric acid (HNO3) being particularly effective. Hydrochloric acid will slowly dissolve the copper, forming copper(II) chloride, a blue-green solution. Nitric acid reacts more vigorously, dissolving the copper and releasing nitrogen dioxide gas, which is a reddish-brown toxic gas. The resulting solution contains copper(II) nitrate.
The reaction with acids results in the copper atoms in the penny being oxidized and becoming copper ions in the solution. The rate of the reaction is dependent on the concentration and temperature of the acid. Stronger acids and higher temperatures will lead to a faster reaction rate. It’s important to note that the zinc core in pennies minted after 1982 will also react with acids, further contributing to the dissolution of the coin.
Do pennies react with bases like baking soda?
While pennies are reactive with acids, they generally don’t react significantly with weak bases like baking soda (sodium bicarbonate). Baking soda solutions are mildly alkaline, and the reaction between copper and such a weak base is very slow and minimal at room temperature. However, baking soda can be useful for cleaning pennies by removing dirt and tarnish.
The cleaning action of baking soda relies on its mild abrasive properties and its ability to neutralize weak acids present in tarnish. When combined with water, baking soda forms a paste that can gently scrub away surface impurities and some of the less tightly bonded corrosion products. However, it will not dissolve the copper metal itself.
Can vinegar clean pennies? How does it work?
Yes, vinegar (acetic acid) is commonly used to clean pennies. Vinegar works by dissolving the copper oxide and copper carbonate that cause the tarnish on the penny’s surface. Acetic acid, a weak acid, reacts with these compounds, converting them into copper acetate, which is soluble in water and can be washed away.
The cleaning process is relatively simple. Soaking pennies in vinegar for a short period, often with a small amount of salt (sodium chloride) to enhance the reaction, is usually sufficient to remove the tarnish. The salt provides chloride ions which can further react with the copper. After soaking, rinsing the pennies with water and drying them is crucial to prevent further oxidation.
What about strong oxidizers like bleach; how do pennies react?
Pennies can react with strong oxidizers like bleach (sodium hypochlorite), but the reaction is generally slow under normal conditions. Bleach will oxidize the copper surface, leading to the formation of copper oxides and potentially copper chlorides. However, the reaction is not as vigorous as with strong acids.
The extent of the reaction depends on the concentration of the bleach, the exposure time, and the presence of other catalysts. Prolonged exposure to concentrated bleach can eventually lead to significant corrosion and discoloration of the penny. Safety precautions, such as wearing gloves and eye protection, should be taken when handling bleach due to its corrosive nature.
Do pennies react with salt water?
Yes, pennies react with salt water, though the reaction is slow. The presence of salt (sodium chloride) in water accelerates the corrosion of copper. Chloride ions from the salt act as a catalyst, promoting the oxidation of copper and the formation of copper chlorides. These chlorides are then further converted into other copper compounds, contributing to the green patina often observed on pennies near coastal areas.
This process is particularly relevant in marine environments where salt water is prevalent. The combination of oxygen, water, and chloride ions creates a highly corrosive environment for copper. Over time, this reaction can lead to significant degradation of the penny’s surface and a noticeable change in its appearance.
Why do pennies have different reactions based on their mint year?
Pennies minted before 1982 are primarily composed of copper (95% copper, 5% zinc), whereas pennies minted after 1982 are made of zinc (97.5% zinc, 2.5% copper plating). This difference in composition significantly affects how they react with various substances. Copper is more resistant to corrosion than zinc, and the copper coating on newer pennies is very thin.
Because zinc is a more reactive metal than copper, post-1982 pennies react more readily with acids and other corrosive substances. If the copper plating is damaged, the zinc core is exposed and quickly corrodes, leading to a more dramatic and faster reaction compared to the pre-1982 copper pennies. This difference in composition and reactivity makes them behave differently in chemical experiments.