Even after twenty years of use, most quarters have the same shiny appearance that they did when they were minted. Most pennies, however, have dulled considerably. What does this suggest about the position of the metal used to make quarters and the metal used to make pennies on the activity series?
Yes, copper metal is easier to oxidize than silver metal. In regard to the position in the activity series, Cu is above Ag in the series. The rate of reaction, however, is not a function of a metal's position in the table.
The activity series is a list of metals arranged in order of their reactivity. When a metal is more reactive, it tends to corrode more easily and lose its luster. In this case, the fact that most quarters maintain their shiny appearance after twenty years suggests that the metal used to make quarters is less reactive and lower on the activity series compared to the metal used to make pennies. The metal used to make pennies is more likely to be higher on the activity series, making it more prone to corrosion and the dulling of its appearance over time.
The difference in appearance between quarters and pennies after extended use suggests a difference in the reactivity of the metals used to make these coins. To understand this, we can consider the activity series of metals.
The activity series of metals is a list that ranks metals in order of their reactivity. The higher a metal is in the activity series, the more reactive it is. Reactivity refers to a metal's tendency to form compounds by losing electrons, which can result in oxidation or tarnishing.
In the case of the US coins, quarters are primarily made of an alloy called cupronickel, which consists of 75% copper and 25% nickel. Copper is relatively low on the reactivity scale, meaning it is less reactive compared to other metals. As a result, it doesn't readily oxidize or react with substances in the environment. Nickel also has a low reactivity, further contributing to the shiny appearance of quarters over time.
On the other hand, pennies, or one-cent coins, are made of copper-plated zinc. The core of the penny is zinc, while the outer layer is copper. While copper, as mentioned earlier, is relatively low in reactivity, zinc is more reactive and tends to lose electrons more easily. Consequently, the zinc in the core of the penny will react with oxygen and other substances in the environment, gradually tarnishing the appearance of the coin.
Therefore, the difference in the reactivity of metals used to make quarters (cupronickel) and pennies (copper-plated zinc) on the activity series explains why most quarters retain their shiny appearance even after decades, whereas pennies tend to dull considerably over time.