Problem 43 - Chemistry and Chemical Reactivity

Question

Silicon Describe the structures of \(\mathrm{SiO}_{2}\) and \(\mathrm{CO}_{2}\). Explain why \(\mathrm{SiO}_{2}\) has a very high melting point, whereas \(\mathrm{CO}_{2}\) is a gas.

Step-by-Step Solution

Answer

\(\mathrm{SiO}_{2}\) has a high melting point due to its network covalent structure, while \(\mathrm{CO}_{2}\) is a gas because of its discrete molecular structure.

Step 1: Understanding \\(\mathrm{SiO}_{2}\\) Structure

Silicon dioxide, or \(\mathrm{SiO}_{2}\), has a network covalent structure. Each silicon atom forms four covalent bonds with oxygen atoms, creating a tetrahedral arrangement. These oxygen atoms then bond with other silicon atoms, forming an extensive three-dimensional network.

Step 2: Understanding \\(\mathrm{CO}_{2}\\) Structure

Carbon dioxide, or \(\mathrm{CO}_{2}\), consists of discrete molecules with a linear structure. Each molecule has one carbon atom double-bonded to two oxygen atoms. These molecules do not form a network structure but rather exist as individual units.

Step 3: Explaining \\(\mathrm{SiO}_{2}\\)'s High Melting Point

The high melting point of \(\mathrm{SiO}_{2}\) is due to its robust network covalent structure. Breaking the countless covalent bonds in the vast lattice requires substantial energy, contributing to its high melting point.

Step 4: Explaining \\(\mathrm{CO}_{2}\\)'s Gaseous State

In contrast, \(\mathrm{CO}_{2}\) is a gas at room temperature because it consists of discrete molecular units. The intermolecular forces (van der Waals forces) between these molecules are weak, requiring minimal energy to separate them, resulting in a gaseous state.

Key Concepts

Network Covalent Bonds

Network covalent bonds are a type of chemical bonding in which atoms are interconnected in a large and continuous network. This network is created as each atom shares electrons with multiple neighbors to form covalent bonds, crafting an extensive, interconnected lattice.
Unlike molecular structures that are composed of independent molecules, network covalent compounds exhibit a lattice that is not easily separated, leading to unique physical properties.
Hardness, high melting points, and electrical insulation are common traits associated with network covalent bonds.

• Durability: The strong directional bonds within the network require a significant amount of force to break, making such materials extremely durable.
• Melting Point: The vast number of covalent bonds in the lattice contributes to a high melting point, as seen in silicon dioxide (SiO₂).

Materials with network covalent bonds, like diamonds and quartz, demonstrate considerable strength and resistance to physical harm, due to this interconnected bonding network.

Molecular Structure of CO2

Carbon dioxide ( CO₂ ) has a molecular structure that is both simple and effective. The molecule is formed by one carbon atom double-bonded to two oxygen atoms, creating a linear structure due to the 180-degree angle between the bonded atoms.
The simplicity of the structure helps in understanding its physical state and behavior at room temperature.

• Discrete Molecules: Unlike silicon dioxide, CO₂ is not a network solid. Instead, individual CO₂ molecules exist separately, with each consisting of a single linear arrangement of carbon and oxygen atoms.
• Double Bonds: The double carbon-oxygen bonds are strong but do not extend into larger structures, allowing the force needed to break them compared to network covalent structures to be much lower.

The independent nature of these molecules allows them to move freely, contributing to CO₂ being a gas under standard conditions of temperature and pressure.

Melting Points of SiO2 and CO2

The melting points of substances give insight into the strength and type of bonds between their constituent atoms or molecules. Silicon dioxide ( SiO₂ ) has a notably high melting point, whereas carbon dioxide ( CO₂ ) is typically found as a gas at room temperature.

• SiO₂ High Melting Point: The network covalent bonds in SiO₂ form a robust three-dimensional lattice that requires a significant amount of energy to break. As a result, SiO₂ remains solid until it reaches very high temperatures.
• CO₂ Gaseous State: In contrast, CO₂ consists of discrete molecules. The weak van der Waals forces between these molecules are easily overcome with minimal energy input, making CO₂ a gas under normal conditions.

This stark difference highlights the relationship between intermolecular forces and the physical state of a compound, further emphasizing the uniqueness of network covalent versus molecular structures.