When copper reacts with moist carbon dioxide (CO 2) in air, it forms a green coating of which one of the following compounds?
Cupric carbonate
Understanding Copper's Reaction with Moist Carbon Dioxide
When copper metal is exposed to moist air containing carbon dioxide for a long time, it undergoes a chemical reaction known as corrosion or tarnishing. This reaction results in the formation of a green coating on the surface of the copper object. Let's break down what causes this.
The Process of Copper Tarnishing
Air contains several components that can react with copper:
- Oxygen (\(\text{O}_2\)): Copper reacts with oxygen in the air, especially in the presence of moisture, to form copper oxides.
- Water (\(\text{H}_2\text{O}\)): Moisture is crucial for the reaction to occur at room temperature.
- Carbon dioxide (\(\text{CO}_2\)): This gas is present in the atmosphere and plays a key role in forming the green coating.
The green coating formed is not just simple copper carbonate (\(\text{CuCO}_3\)), but rather basic copper carbonate, which is a mixture of copper carbonate and copper hydroxide (\(\text{Cu(OH)}_2\)). Its chemical formula is typically represented as \(\text{CuCO}_3 \cdot \text{Cu(OH)}_2\). This compound is also known as verdigris when formed naturally by weathering of copper.
Why the Green Coating is Basic Copper Carbonate
The reaction is complex, but it can be simplified into a process where copper first reacts with oxygen and water to form copper hydroxide, which then reacts with carbon dioxide to form basic copper carbonate. A simplified overall reaction can be represented as:
\( \text{2Cu(s)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)} + \text{O}_2\text{(g)} \rightarrow \text{CuCO}_3 \cdot \text{Cu(OH)}_2\text{(s)} \) (Basic Copper Carbonate)
Looking at the options provided, "Cupric carbonate" is listed. While the actual green coating is basic copper carbonate (\(\text{CuCO}_3 \cdot \text{Cu(OH)}_2\)), the term "cupric carbonate" (\(\text{CuCO}_3\)) is often used in a general sense in introductory contexts, and basic copper carbonate is a derivative containing the carbonate group and cupric ions (\(\text{Cu}^{2+}\)). Among the given choices, "Cupric carbonate" is the compound most closely associated with the green layer formed on copper by exposure to moist air and carbon dioxide.
Analyzing the Options
Let's examine why the other options are incorrect:
- Cuprous oxide (\(\text{Cu}_2\text{O}\)): This is a reddish-brown compound formed when copper reacts with oxygen at high temperatures or under certain conditions. It is not the green coating observed when copper is exposed to moist air and \(\text{CO}_2\).
- Cupric oxide (\(\text{CuO}\)): This is a black compound formed when copper is strongly heated in air (oxygen). It is also not the green coating.
- Copper sulphate (\(\text{CuSO}_4\)): Copper sulphate is a blue (when hydrated) or white (when anhydrous) crystalline solid. It is typically formed by the reaction of copper or copper compounds with sulfuric acid. It does not form as a result of copper reacting with moist air and carbon dioxide.
Therefore, based on the chemical reaction and the color of the resulting compound, the green coating formed when copper reacts with moist carbon dioxide in air is basic copper carbonate, which is best represented by the option "Cupric carbonate" in this context.
Summary of the Reaction
Exposure of copper to moist air containing carbon dioxide leads to the formation of a protective green layer. This layer is a basic copper carbonate, effectively referred to as cupric carbonate among the given choices.
| Reactants | Conditions | Product (Green Coating) | Chemical Formula |
|---|---|---|---|
| Copper (\(\text{Cu}\)) | Moist air (contains \(\text{O}_2\), \(\text{H}_2\text{O}\)) + Carbon Dioxide (\(\text{CO}_2\)) | Basic Copper Carbonate (often referred to as Cupric Carbonate) | \(\text{CuCO}_3 \cdot \text{Cu(OH)}_2\) |
| Components of air (\(\text{O}_2\), \(\text{H}_2\text{O}\)) + \(\text{CO}_2\) |
Revision Table: Copper Reactions and Coatings
| Reactants / Conditions | Resulting Coating/Compound | Color |
|---|---|---|
| Copper + Moist air (\(\text{O}_2\), \(\text{H}_2\text{O}\)) + \(\text{CO}_2\) | Basic Copper Carbonate (\(\text{CuCO}_3 \cdot \text{Cu(OH)}_2\)) | Green |
| Copper + \(\text{O}_2\) (heated) | Cuprous Oxide (\(\text{Cu}_2\text{O}\)) | Reddish-brown |
| Copper + \(\text{O}_2\) (strongly heated) | Cupric Oxide (\(\text{CuO}\)) | Black |
| Copper + Sulfuric Acid (\(\text{H}_2\text{SO}_4\)) | Copper Sulphate (\(\text{CuSO}_4\)) | Blue (hydrated) / White (anhydrous) |
Additional Information on Copper Tarnishing
The green coating on copper, often seen on old copper roofs, statues (like the Statue of Liberty), or cookware, is commonly called patina. This patina layer is quite stable and actually protects the underlying copper from further corrosion. The formation of this protective layer is a beneficial aspect of copper's reaction with the environment.
The exact composition of the patina can vary depending on the pollutants present in the air. In urban or industrial areas with high levels of sulfur dioxide (\(\text{SO}_2\)), copper sulfates (\(\text{CuSO}_4 \cdot 3\text{Cu(OH)}_2\)) can also form, contributing to the green layer.
However, in a typical clean environment exposed to moist air and carbon dioxide, the primary component of the green patina is basic copper carbonate (\(\text{CuCO}_3 \cdot \text{Cu(OH)}_2\)). The question specifically mentions reaction with moist carbon dioxide, which directly points to the formation of this carbonate-containing compound.
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