Problem 46 - Chemistry: The Central Science in SI Units, Global Edition

Question

Indicate whether each statement is true or false: (a) Intermetallic compounds have a fixed composition. (b) Copper is the majority component in both brass and bronze. (c) In stainless steel, the chromium atoms occupy interstitial positions.

Step-by-Step Solution

Answer

(a) True: Intermetallic compounds have a fixed composition with well-defined crystal structures. (b) True: Copper is the majority component in both brass (copper and zinc) and bronze (copper and tin). (c) False: In stainless steel, it is the carbon atoms that occupy interstitial positions in the iron lattice, not the chromium atoms.

Step 1: (a) Intermetallic compounds have a fixed composition.

Intermetallic compounds are formed when two or more metals combine in a specific stoichiometry. They exhibit well-defined crystal structures and have a fixed composition. So, this statement is true.

Step 2: (b) Copper is the majority component in both brass and bronze.

Brass is an alloy made primarily of copper and zinc. Bronze is an alloy made primarily of copper and tin. In both cases, copper is the majority component. So, this statement is also true.

Step 3: (c) In stainless steel, the chromium atoms occupy interstitial positions.

Stainless steel is an alloy made primarily of iron, with small amounts of carbon and other elements such as chromium, nickel, or molybdenum. The chromium atoms in stainless steel form a passive oxide layer on the surface that helps prevent corrosion, but they do not occupy interstitial positions. Instead, the carbon atoms occupy interstitial positions in the iron lattice. Thus, this statement is false.

Key Concepts

Intermetallic Compounds

Intermetallic compounds are unique classes of materials that boast a fixed composition. Unlike alloys, whose compositions can vary, intermetallic compounds have a precise stoichiometry. This means the ratio of the metals combined to form these compounds remains constant. These compounds are made from two or more different metals that bind together in a specific atomic arrangement, giving rise to well-defined crystal structures.
Intermetallics have diverse applications due to their distinctive properties, such as high strength and notable resistance to oxidation. For example, materials like NiTi (nickel-titanium) are used in medical devices because of their shape-memory properties, while others are utilized in aerospace applications for their durability and lightness. Understanding the fixed nature of intermetallic compounds can aid in predicting their behavior, which is extremely useful in materials science and engineering.

Alloys

Alloys are created by blending two or more elements, with at least one being a metal, to achieve desired mechanical, chemical, or physical properties. Brass and bronze are classic examples of alloys where copper is the majority component.

• Brass is an alloy where copper is primarily combined with zinc. The exact composition can vary, allowing for changes in properties like color and malleability, making it suitable for musical instruments and decorative items.


• Bronze is another copper-based alloy, but it is primarily combined with tin. It is known for its hardness and resistance to corrosion, which is why it's been used historically in tools and statues.

Alloys are vital in various industries due to their ability to combine the best properties of their constituent elements, customizing material attributes as required.

Stainless Steel

Stainless steel is a remarkable alloy mainly composed of iron, with a mix of carbon and crucial elements like chromium, nickel, or molybdenum. The presence of chromium is particularly important because it forms a passive layer of chromium oxide on the surface, which protects the material from rusting, giving stainless steel its name.
This oxide layer is very thin yet incredibly strong, providing excellent corrosion resistance. It's worth noting that, contrary to some beliefs, chromium atoms do not occupy interstitial positions in the iron lattice. Instead, the smaller carbon atoms fill these spaces, contributing to the steel's strength and hardness.
Stainless steel is widely used, from household utensils to massive architectural structures, due to its durability and low maintenance needs. Understanding its composition and the role of each element can help you appreciate why it's so widely valued in engineering and construction.