Problem 142 - General Chemistry: Principles and Modern Applications

Question

Would you expect an ionic solid or a network covalent solid to have the higher melting point?

Step-by-Step Solution

Answer

Generally, a network covalent solid is expected to have a higher melting point than an ionic solid.

Step 1: Understanding the Characteristics of Ionic Solids

Ionic solids are composed of positive and negative ions held together by strong electrostatic forces of attraction, known as ionic bonds. Due to these strong bonding forces, ionic solids generally have high melting points.

Step 2: Understanding the Characteristics of Network Covalent Solids

Network covalent solids are formed by covalent bonds, where atoms share electrons to achieve stability. The bonding in these solids is also very strong since the network structure extends throughout the entire solid, leading to substantial melting points.

Step 3: Comparing Ionic and Network Covalent Solids

While both types have quite high melting points due to strong bonds, the comparison can depend on the specific substances being compared. In general, however, it's commonly accepted that network covalent solids oftentimes have higher melting points due to the extensive, strong bonding structure throughout the entire solid.

Key Concepts

Ionic Solids

Ionic solids are fascinating due to the way they are structured and bonded. These solids consist of a lattice framework comprised of charged ions—cations and anions. The electrostatic attraction, or ionic bonding, between these oppositely charged ions is extremely robust. This intense bonding is the primary reason ionic solids usually exhibit high melting points.

A few key aspects to consider include:

• Lattice Strength: The strength of the lattice is reinforced by the multitude of ionic bonds, requiring substantial energy to disrupt and melt the solid.
• Composition: The size and charge of ions can influence the melting point; smaller ions with higher charges form stronger attractions and thus have higher melting points.

Examples of ionic solids with high melting points include table salt (NaCl) and magnesium oxide (MgO). These solids are typically crystalline and brittle, illustrating the rigid nature of the ionic framework.

Network Covalent Solids

Network covalent solids, sometimes called covalent network crystals, offer an intriguing chemistry and physical performance. These solids form when atoms are bonded by covalent bonds into a continuous and extensive structural network. This structure contributes to their remarkably high melting points.

The defining characteristics of network covalent solids include:

• Intense Bonding: Covalent bonds involve the sharing of electrons between atoms, creating a sturdy link that contributes to high thermal stability.
• Uniform Structure: The network is continuous, meaning that the extreme strength is uniform throughout the entire solid, unlike in other substances where molecular interactions vary.

Diamond and quartz (SiO₂) exemplify network covalent solids, known for their incredible hardness and resilience. Because the bonds in these solids extend throughout their entire grid, melting requires breaking numerous strong bonds, leading to very high melting points.

Bonding Structures

The bonding structures in ionic and network covalent solids demonstrate the profound impact that atomic arrangements and interactions have on physical properties like melting point.

Comparing Bonding Structures:

• Ionic Bonding: In ionic solids, the electrostatic forces between charged ions form a rigid and stable structure. Nevertheless, despite high melting points, the actual temperature can vary based on ion types and lattice arrangement.
• Covalent Network Bonding: Often surpassing ionic solids in thermal stability, network covalent solids owe their high melting points to the seamless connectivity of covalent bonds across the entire solid.

Despite both having strong bonds, network covalent solids generally tend to have higher melting points than ionic solids. This is attributed to their extensive and consistent bonding framework which requires immense energy to break. For example, while sodium chloride melts at 801°C, diamond—a network covalent solid—remains solid at temperatures well exceeding 3500°C.