Problem 40 - Chemistry Matter and Change

Question

Copper and air Copper statues, such as the Statue of Liberty, begin to appear green after they have been exposed to air. In this redox process, copper metal reacts with oxygen to form solid copper oxide, which forms the green coating. Write the reaction for this redox process, and identify what is oxidized and what is reduced in the process.

Step-by-Step Solution

Answer

The balanced redox reaction between copper and oxygen is: 2Cu + O2 -> 2CuO In this reaction, copper (Cu) is oxidized from an oxidation state of 0 to +2 (in CuO), and oxygen (O₂) is reduced from an oxidation state of 0 to -2 (in CuO).

Step 1: Write down the reactants and products of the reaction

We need to set up the balanced chemical equation for the redox reaction that occurs between copper (Cu) and oxygen (O) to form copper oxide (CuO). Copper is a metal and oxygen is in the atmosphere. Copper gets oxidized to copper oxide, which is responsible for the green color. Cu + O -> CuO

Step 2: Balance the chemical equation

In order to balance the equation, we need to make sure that the number of atoms are equal on both sides. There are 2 oxygen atoms on the right side, so we need to have 2 oxygen atoms on the left side as well. Cu + O2 -> 2CuO Now the number of copper atoms is also equal on both sides (1 on the left and 2 on the right). So, we should multiply the Cu by 2. 2Cu + O2 -> 2CuO Now the equation is balanced with the same number of atoms on both sides.

Step 3: Determine the oxidation states and identify the redox process

Let's identify what is being reduced and what is being oxidized in this process. To do this, we'll determine the oxidation states of each atom before and after the reaction: (before) Cu: 0 | O₂: 0 (after) Cu in CuO: +2 | O in CuO: -2 Copper undergoes a change from 0 to +2, meaning it loses two electrons; this is the oxidation process. Oxygen undergoes a change from 0 to -2, meaning it gains two electrons; this is the reduction process. Therefore, in this reaction: - Copper is oxidized. - Oxygen is reduced.

Key Concepts

Oxidation

Oxidation is a fascinating concept in the study of chemistry. As you may have learned, oxidation involves the loss of electrons from a substance during a chemical reaction. In everyday terms, this often involves a substance combining with oxygen. A classic example is when metal rusts or tarnishes.

The term comes from the process's most common example—the reaction of substances with oxygen. However, oxidation can occur without oxygen's presence. It's all about losing electrons, regardless of the chemical partners involved. This concept is a key part of redox reactions, where oxidation and reduction occur simultaneously. In redox, what one substance loses, another gains as they exchange electrons.

Reduction

Reduction is the counterpart to oxidation and completes the redox cycle. During reduction, a substance gains electrons. If oxidation is like losing weight (electrons), reduction is like bulking up (with electrons). This process happens in tandem with oxidation; they are like two sides of the same coin.

In our example of copper and air, when copper becomes oxidized, oxygen is reduced. The electrons that copper loses are gained by oxygen. This results in the formation of copper oxide. Understanding reduction is as crucial as understanding oxidation since both processes usually occur together. A practical way to remember this is with the mnemonic OIL RIG, which stands for "Oxidation Is Loss, Reduction Is Gain."

Copper Oxidation

Copper oxidation is a specific type of oxidation reaction we often observe in our everyday environment. One of the most famous examples of copper oxidation is the transformation of copper surfaces to a green hue over time when exposed to air.

The copper metal loses electrons to form copper oxide. You might be familiar with the greenish patina that develops on copper structures—a result of this oxidation process. The chemical reaction for copper oxidation can be simplified as:

• Reaction: 2Cu + O₂ -> 2CuO

This reaction balances the atoms on each side, making it clear how copper loses electrons. The loss of electrons in copper is the core of the oxidation process.

Statue of Liberty

The Statue of Liberty is an iconic example of copper oxidation in action. Gifted to the United States from France, this colossal statue is covered in roughly 30 tons of copper, which has undergone oxidation over the years.

Initially, the statue was a shiny brown when it was first constructed. However, as it stood exposed to the elements, it reacted with the oxygen and moisture in the atmosphere. This resulted in a chemical reaction that created a patina of copper oxide, forming that familiar green color we all recognize today.

Interestingly, this green layer serves a protective function. It shields the underlying copper from further corrosion, which is an unintended yet beneficial outcome of its oxidation process.

Copper Oxide

Copper oxide is the product of copper undergoing oxidation, forming a compound with interesting properties and applications. There are two primary forms of copper oxide: copper(I) oxide, or Cu₂O, which is red, and copper(II) oxide, CuO, which is black or greenish-black, depending on its state.

In the context of the Statue of Liberty and other copper-containing monuments, copper(II) oxide is prevalent. This oxide results from the copper elements in the statue reacting with oxygen over time. The equation for forming copper oxide might appear like this:

• Reduction of Oxygen by Copper: O₂ + 4Cu -> 2CuO

Copper oxide's formation is not just a matter of aesthetics; it's a pivotal process that explains many historical and everyday chemical phenomena.