Problem 44 - Chemistry: The Central Science in SI Units, Global Edition

Question

For each of the following alloy compositions, indicate whether you would expect it to be a substitutional alloy, an interstitial alloy, or an intermetallic compound: (a) \(\mathrm{Cu}_{0.66} \mathrm{Zn}_{0.34}\) (b) \(\mathrm{Ag}_{3} \mathrm{Sn}\) (c) \(\mathrm{Ti}_{0.99} \mathrm{O}_{0.01}\)

Step-by-Step Solution

Answer

Composition (a): Cu0.66 Zn0.34 is likely a substitutional alloy due to the similar atomic radii and both elements being metals. Composition (b): Ag3 Sn is likely an intermetallic compound due to the stoichiometric ratio and different element types. Composition (c): Ti0.99 O0.01 is likely an interstitial alloy, with Oxygen occupying interstitial positions between Titanium atoms.

Step 1: Composition (a): Cu0.66 Zn0.34

For the given alloy composition Cu0.66 Zn0.34, we need to look at the properties of the elements Copper (Cu) and Zinc (Zn). Both elements are metals and have similar atomic radii. They are also neighbors in the periodic table which increases the likelihood of forming a solid solution. Based on these properties, we can infer this is likely a substitutional alloy.

Step 2: Composition (b): Ag3 Sn

For the given alloy composition Ag3 Sn, we have Silver (Ag) and Tin (Sn) elements. Silver is a noble metal, while Tin is a post-transition metal. The ratio between the elements is 3 to 1, with a definite stoichiometric ratio. These factors indicate this is likely an intermetallic compound.

Step 3: Composition (c): Ti0.99 O0.01

For the given alloy composition Ti0.99 O0.01, we have Titanium (Ti) and Oxygen (O) elements. Titanium is a metal, and Oxygen is a nonmetal. Also, the concentration of Oxygen is significantly lower than the concentration of Titanium, which means the nonmetal atoms most likely occupy the interstitial positions between the metal atoms. Based on these properties, we can infer this is likely an interstitial alloy.

Key Concepts

Substitutional Alloy

A substitutional alloy is formed when atoms of a solute element replace some of the host atoms in the crystal lattice of the solvent element. This typically occurs when the atomic radii and electro-negativity of the elements are similar.
This similarity ensures that the solute atoms can seamlessly occupy the spaces meant for the original atoms without causing structural instability in the lattice.

• Common Examples: Bronze and Brass.
• Characteristics: Homogeneous structure, often improved ductility and strength.

In substitutional alloys, the degree of solubility of the solute in the solvent will depend on factors like the relative sizes of the atoms, crystal structure, and electron valency. When considering an example like \( ext{Cu}_{0.66} ext{Zn}_{0.34}\), both copper and zinc are able to substitute for each other because of their similar properties. Their sizes are close, and they fit well within each other's atomic arrangement, forming a stable alloy.

Interstitial Alloy

Interstitial alloys occur when small atoms, usually non-metals, occupy the spaces or interstices between larger metal atoms in a crystal lattice. Unlike substitutional alloys, the added atoms do not replace the host metal atoms, but instead fit into the spaces within the lattice.
This often leads to increased hardness and strength due to the inhibition of dislocation movement in the metal lattice.

• Commonly Used Non-Metals: Carbon, Nitrogen.
• Key Properties: Stronger and harder than their base metals, but may become less ductile.

Because interstitial spaces can accommodate atoms significantly smaller than those of the host metal, alloys such as \( ext{Ti}_{0.99} ext{O}_{0.01}\) form as the small oxygen atoms fit neatly into the gaps among the larger titanium atoms, giving rise to increased rigidity and strength.

Intermetallic Compound

Intermetallic compounds are alloys with a stoichiometric or fixed ratio between different metallic elements, forming a new phase with distinct properties. They often possess unique crystal structures and properties, differing from both of the constituent elements. These structures can create highly ordered arrangements resulting in increased stability and high melting points.

• Features: Well-defined compositions, often with complex lattice structures.
• Advantages: High thermal and electrical conductivity, magnetic properties.

Typically, intermetallic compounds like \( ext{Ag}_3 ext{Sn}\) have a definite stoichiometry, and the resulting compound exhibits properties not found in either pure metal constituent. The precise ratio and arrangement lead to a stability that enhances characteristics such as hardness, corrosion resistance, and in some cases, superconductivity.