Problem 32 - Chemistry: The Central Science in SI Units, Global Edition

Question

Copper exposed to air and water may be oxidized. The green oxidized product is referred to as "patina". (a) Write a balanced chemical equation to show the reaction of copper to copper (II) ions with oxygen and protons from acid rain. (b) Would you expect some kind of "patina" on a silver surface? Explain.

Step-by-Step Solution

Answer

The balanced chemical equation for the oxidation of copper to copper (II) ions is: \(2Cu + O_2 + 4H^+ \rightarrow 2Cu^{2+} + 2H_2O\). A green "patina" forms on copper surfaces due to the formation of copper(II) carbonate and copper(II) hydroxide. However, silver undergoes a different oxidation process, resulting in a blackish tarnish (silver sulfide) rather than a green patina.

Step 1: (a) Balancing the Chemical Equation

To write a balanced chemical equation for the reaction of copper (Cu) to form copper (II) ions (Cu²⁺), we need to consider the involvement of oxygen (O₂) and protons (H⁺) from acid rain. The reaction can be divided into two half-reactions: oxidation of copper and reduction of oxygen. For oxidation of copper: Cu → Cu²⁺ + 2e⁻ For the reduction of oxygen, considering the protons (H⁺) from acid rain: O₂ + 4e⁻ + 4H⁺ → 2H₂O Now, we will balance the electrons by multiplying the first half-reaction by 2: 2Cu → 2Cu²⁺ + 4e⁻ O₂ + 4e⁻ + 4H⁺ → 2H₂O Next, we will add these half-reactions together to create the overall balanced equation: 2Cu + O₂ + 4H⁺ → 2Cu²⁺ + 2H₂O So the balanced chemical equation is: 2Cu + O₂ + 4H⁺ → 2Cu²⁺ + 2H₂O

Step 2: (b) Patina on a Silver Surface

The greenish "patina" that forms on copper surfaces is primarily due to the formation of copper(II) carbonate (CuCO₃) and copper(II) hydroxide (Cu(OH)₂). Silver undergoes a different oxidation process in which it reacts with sulfur compounds, such as hydrogen sulfide (H₂S) that might be present in the atmosphere. This results in a blackish tarnish, silver sulfide (Ag₂S), on the surface rather than a green patina like copper. Therefore, we can expect a tarnished, black surface on silver, but not a green "patina" similar to that on copper surfaces.

Key Concepts

Chemical Reaction Balancing

Balancing chemical equations is essential in chemistry to ensure that the number of atoms for each element is conserved through the reaction. In the reaction of copper oxidizing to form copper (II) ions, we need to balance the input and output of electrons and elements.

When copper reacts with oxygen and protons from acid rain, the process can be broken down into two half-reactions:

• The oxidation half-reaction for copper: Copper loses two electrons as it becomes copper (II) ions: \(2\mathrm{Cu} \rightarrow 2\mathrm{Cu}^{2+} + 4e^-\).
• The reduction half-reaction for oxygen: Oxygen gains electrons, with protons assisting in the formation of water: \(\mathrm{O}_2 + 4e^- + 4\mathrm{H}^+ \rightarrow 2\mathrm{H}_2\mathrm{O}\).

To balance these reactions, we need to ensure the number of electrons lost by copper is equal to those gained by oxygen. This results in the balanced equation:
\[2\mathrm{Cu} + \mathrm{O}_2 + 4\mathrm{H}^+ \rightarrow 2\mathrm{Cu}^{2+} + 2\mathrm{H}_2\mathrm{O} \]
Balancing is crucial for understanding the stoichiometry of a reaction, helping chemists predict the quantities of reactants and products involved.

Patina Formation

Patina formation is a fascinating process that happens when copper is exposed to elements like air, water, and pollutants over time. This natural process results in a greenish layer known as patina, which is primarily composed of compounds like copper(II) carbonate \(\mathrm{CuCO}_3\) and copper(II) hydroxide \(\mathrm{Cu(OH)}_2\).

The transformation involves the oxidation of copper by atmospheric oxygen, often accelerated by acidic conditions such as acid rain. This creates a protective layer that prevents further corrosion and gives aged copper structures, like the Statue of Liberty, their iconic green color.

The patina layer serves more than just an aesthetic purpose:

• It helps preserve the copper underneath by acting as a barrier against further environmental exposure.
• In architecture and art, the unique look of patina is often aesthetically desirable for its historical and rustic appearance.

Understanding patina formation is essential for fields like material conservation, where protecting or replicating these effects can be vital.

Silver Tarnish

Unlike copper, silver does not form a green patina. Instead, it tarnishes by forming a blackish layer known as silver sulfide \(\mathrm{Ag}_2\mathrm{S}\). This occurs when silver reacts with sulfur compounds present in the atmosphere, such as hydrogen sulfide \(\mathrm{H}_2\mathrm{S}\).

Silver tarnishing involves a straightforward chemical reaction:

• Silver surfaces react with sulfur-rich compounds to generate silver sulfide.
• This tarnish appears black or dark grey and is commonly seen on silverware or jewelry.

The process begins as bright silver is exposed to air, progressively forming a thin, dark layer that masks its shine. Unlike patina, silver tarnish doesn't generally provide any protective benefits.
Removal is relatively simple; however, it's not permanent. Regular maintenance or protective coatings can help in prolonging the lustrous appearance of silver.
Whether in chemistry or craftsmanship, knowing the tarnishing mechanisms aids in better preservation and care of silver objects.