Problem 5 - Chemistry: Introducing Inorganic, Organic and Physical Chemistry

Question

The UK \(£ 1\) coin is made from a nickel-brass alloy, containing \(70 \%\) copper, \(5.5 \%\) nickel, and \(24.5 \%\) zinc. Is this likely to be a substitutional alloy or an interstitial alloy? (Section 6.3 )

Step-by-Step Solution

Answer

The UK £1 coin is most likely a substitutional alloy.

Step 1: Identify the Main Components

The UK £1 coin's nickel-brass alloy contains three metals: 70% copper, 5.5% nickel, and 24.5% zinc. Observe that all three components are metals.

Step 2: Understand Types of Alloys

Substitutional alloys are formed when the atoms of the components can replace each other in the crystal lattice. Interstitial alloys are formed when smaller atoms fit into the spaces between the larger metal atoms in the crystal lattice.

Step 3: Compare Atomic Sizes

Copper, nickel, and zinc are all similar in atomic size. Copper has an atomic radius of 128 pm, nickel 124 pm, and zinc 134 pm. This similarity in size suggests that these metals can substitute for one another in the crystal structure.

Step 4: Determine the Type of Alloy

Given the similar atomic sizes, the alloy is more suitable to be termed a substitutional alloy because the metallic components can replace each other in the metal lattice rather than fitting into the interstices.

Key Concepts

Substitutional alloy

A substitutional alloy is a type of alloy where the different atoms are able to substitute for one another within the metal's crystal structure.
This occurs when the atoms of the various components are of similar size. The ability of atoms to replace each other in the crystal lattice is due to this similarity in their atomic radii.
Such alloys are usually formed between metals with atomic radii that do not differ significantly.

• Examples of substitutional alloys include brass and bronze, where the constituent atoms, such as copper and zinc or copper and tin, have relatively similar atomic sizes.
• This substitution leads to a solid solution where the properties of the alloy can be significantly different from the constituent metals, often leading to enhanced durability, corrosion resistance, or mechanical strength.

Substitutional alloys are essential in many industries because they allow various metals to be combined to create materials with desired properties for different applications, ranging from coins to construction materials.

Interstitial alloy

Interstitial alloys occur when smaller atoms fit into the spaces or interstices between the larger metal atoms in a crystal lattice. This generally happens when there is a significant size difference between the host metal atoms and those being added.
The host metal atoms form the main crystal structure, while the smaller atoms, often non-metals such as carbon or hydrogen, occupy the spaces in between.

• A classic example of interstitial alloy is steel, where carbon atoms are small enough to fit into the interstices of iron atoms.
• This insertion of smaller atoms can drastically change the properties of the metal, often increasing hardness and strength, as seen in carbon steel.

When considering whether an alloy is interstitial, it's essential to examine the atomic sizes and determine if the added atoms could fit into the spaces between the host metal atoms. Interstitial alloys are crucial in producing strong and durable materials used extensively in various industries, including construction and automotive engineering.

Nickel-brass alloy

Nickel-brass is a specific type of substitutional alloy, typically composed of copper, zinc, and nickel. In this alloy, copper serves as the primary metal, and zinc and nickel substitute within its crystal lattice.
Due to the relatively similar atomic sizes of copper (128 pm), nickel (124 pm), and zinc (134 pm), these metals can replace each other effectively, forming a stable crystal structure called a solid solution.

• The composition of nickel-brass gives it a unique combination of strength, corrosion resistance, and an appealing yellowish color.
• These properties make it particularly suitable for manufacturing durable objects, such as coins. The UK £1 coin is made from a nickel-brass alloy, designed to withstand circulation and harsh conditions.

Throughout history, nickel-brass has been widely employed in minting coins and various decorative applications due to its appealing aesthetics and physical properties. Its composition allows it to be both functional and decorative, making it an important material in various industries.