Problem 53 - A Complete Resource Book in Chemistry for JEE Main

Question

When metallic copper comes in contact with moisture, a green powdery/pasty coating can be seen over it. This is chemically known as (a) copper carbonate-copper sulphate (b) copper sulphate-copper sulphide (c) copper carbonate-copper hydroxide (d) copper sulphide-copper carbonate

Step-by-Step Solution

Answer

The coating is copper carbonate-copper hydroxide (c).

Step 1: Understanding the Problem

When metallic copper is exposed to moisture, it reacts with elements in the atmosphere, leading to the formation of a green coating known as patina. This is a chemical process where copper reacts mainly with carbon dioxide (CO2), oxygen (O2), and water (H2O) to form specific compounds.

Step 2: Analyze the Reaction Compounds

The green patina is primarily composed of copper carbonate (CuCO3) and copper hydroxide (Cu(OH)2). This means that the copper reacts with both carbon dioxide and water to form the respective carbonate and hydroxide compounds.

Step 3: Choosing the Correct Option

From the given options, we need to choose the compounds that match the description of the patina. Since the patina is copper carbonate and copper hydroxide, we can eliminate options based on other compounds not fitting this description.

Step 4: Validate the Correct Answer

The correct composition, copper carbonate-copper hydroxide, matches option (c). This confirms that when copper tarnishes with moisture, it forms these two compounds, giving the characteristic green color.

Key Concepts

Copper Patina

When exposed to the elements, copper does not remain shiny and brown for long. Instead, it develops a distinctive green layer known as patina. This is a natural process and occurs over time due to chemical reactions with the environment.
Patina serves several purposes:

• It provides a protective layer that prevents further corrosion of the copper underneath.
• The aesthetic value of patina is cherished for its historical and artistic significance.

Patina formation involves the reaction of copper with carbon dioxide ( CO_2 ), oxygen ( O_2 ), and water ( H_2O ) in the environment. The result is primarily a mix of copper carbonate ( CuCO_3 ) and copper hydroxide ( Cu(OH)_2 ), which gives the copper surface its greenish tint. This is a great example of how natural processes can create protective barriers while also adding beauty.

Copper Reactions

Copper reactions are essential for understanding how copper transforms when exposed to atmospheric conditions. The green patina on copper is a product of such intricate reactions. Let’s break down the process:
Copper oxidizes in the presence of oxygen ( O_2 ) and moisture ( H_2O ). This leads to the formation of copper(I) oxide ( Cu_2O ), which is initially red. As exposure continues, the copper(II) hydroxide ( Cu(OH)_2 ) forms. This is further combined with carbon dioxide ( CO_2 ) from the air to create copper carbonate ( CuCO_3 ).

• Initial copper exposure forms red copper(I) oxide.
• Prolonged exposure results in the green patina of copper carbonate and hydroxide.

Understanding these reactions helps explain why copper artifacts, especially those used outdoors, develop their characteristic look over time.

Metal Corrosion

Corrosion is a natural process that occurs when metals undergo chemical reactions with their environment, leading to deterioration. For copper, corrosion takes the form of the beautiful green patina we observe.
However, corrosion in metals is not always desirable:

• In certain metals, corrosion can lead to structural weakness, hence not all metals have beneficial outcomes like copper does.
• Corrosion can result in unsightly appearances and the need for maintenance or replacement.

In copper, the patina acts as a protective layer. It slows down further corrosion and thus prolongs the life of the copper item. While patina is a form of corrosion, it showcases how sometimes, corrosion can be both protective and aesthetically pleasing. Understanding metal corrosion is key to preserving metal artifacts and infrastructure.