Problem 33 - Chemistry and Chemical Reactivity

Question

Classify each of the following materials as falling into one of the categories listed in Table \(12.2 .\) What particles make up these solids and what are the forces of attraction between particles? Give one physical property of each. (a) gallium arsenide (b) polystyrene (c) silicon carbide (d) perovskite, \(\mathrm{CaTiO}_{3}\)

Step-by-Step Solution

Answer

(a) Covalent network, high electron mobility; (b) Molecular solid, low density; (c) Covalent network, high hardness; (d) Ionic solid, high dielectric constant.

Step 1: Classify Gallium Arsenide

Gallium arsenide (GaAs) is a compound semiconductor, falling into the category of covalent network solids. It is composed of gallium and arsenic atoms bonded covalently. The forces of attraction are covalent bonds. One physical property is that gallium arsenide has high electron mobility, making it useful in electronic applications.

Step 2: Classify Polystyrene

Polystyrene is a polymer, classified as a molecular solid. It is made up of long chains of styrene monomers. The forces of attraction between these particles are Van der Waals forces. A physical property of polystyrene is its low density, making it lightweight and easy to mold.

Step 3: Classify Silicon Carbide

Silicon carbide (SiC) is a covalent network solid. It consists of silicon and carbon atoms arranged in a crystal lattice bonded covalently. The forces of attraction are strong covalent bonds. A physical property of silicon carbide is its high hardness, often used as an abrasive material.

Step 4: Classify Perovskite (CaTiO3)

Perovskite (CaTiO3) is classified as an ionic solid. It is composed of calcium, titanium, and oxygen ions, held together by ionic bonds. One of its physical properties is having a high dielectric constant, useful in capacitors and other electronic devices.

Key Concepts

Covalent Network Solids

Covalent network solids are fascinating types of materials where atoms are bonded in an extensive network through covalent bonds. This structure leads to some interesting characteristics.
These solids are made up of atoms connected by a network of covalent bonds. Examples include diamonds and silicon carbide. Each atom is covalently bonded to several other atoms, creating a rigid and stable structure. The strength of the covalent bonds results in these solids being very hard and having high melting points.
One of the most remarkable features of covalent network solids is their conductivity. Materials like gallium arsenide exhibit high electron mobility, making them highly useful in electronic applications such as semiconductors.

• Very hard and durable.
• High melting points due to strong bondings.
• Typically poor conductors of electricity, but exceptions like semiconductors exist.

Covalent network solids are crucial in many applications, from industrial abrasives to cutting-edge electronics.

Molecular Solids

Molecular solids are composed of discrete molecules held together by relatively weak intermolecular forces. Examples include substances like ice and polystyrene.
These solids have significant diversity because the forces holding them together - such as Van der Waals forces, hydrogen bonds, and dipole-dipole interactions - are much weaker than covalent or ionic bonds.
Because of these weaker forces, molecular solids tend to have lower melting points and may be softer compared to other types of solids. Their properties can vary greatly depending on the specific forces involved. For instance, polystyrene's low density makes it a popular choice for lightweight packaging and insulation materials.

• Lower melting points than covalent network solids.
• Typically softer and more easily deformed.
• Variable properties depending on the molecules involved.

This versatility makes molecular solids highly useful for everyday applications like packaging and consumer goods.

Ionic Solids

Ionic solids are an intriguing type of material known for being composed of ions. In these solids, the positive and negative ions are arranged in a geometric lattice, held together by strong electrostatic forces called ionic bonds.
Common examples include table salt (NaCl) and perovskite. These materials typically have high melting and boiling points due to the strength of the ionic bonds, which require significant energy to break.
The electrical properties of ionic solids are equally interesting. While they do not conduct electricity in their solid state, they can when dissolved in water or melted, as the ions are then free to move.

• High melting and boiling points.
• Do not conduct electricity in solid form.
• Become conductive when in liquid form or dissolved in water.

Ionic solids are essential in various applications, such as in the manufacturing of ceramics and the functioning of many electronic devices.

Physical Properties of Solids

The physical properties of solids vary widely depending on the type of solid and the nature of the bonding between their particles.
Hardness, melting point, conductivity, and other characteristics are influenced by the solid's structure and bonding. For example, covalent network solids like silicon carbide are extremely hard and have high melting points, whereas molecular solids like polystyrene are generally softer and melt at lower temperatures.
Understanding these properties helps in selecting materials for specific applications. For instance, the high dielectric constant of perovskite is ideal for capacitors, while the high hardness of silicon carbide is preferred for abrasives.

• Different solids have unique properties based on their internal structures.
• Physical properties are key in determining a solid's suitability for specific applications.
• From semiconductors to packaging, the right solid choice can greatly enhance product performance.

Knowing about these properties assists in the design and use of materials across technology, construction, and consumer industries.