Chapter 12: Problem 22
Describe the difference between a molecular solid and a covalent network solid.
Short Answer
Expert verified
Molecular solids consist of individual molecules held together by weak intermolecular forces, resulting in generally soft substances with low melting and boiling points and low electrical conductivity. Examples include ice, dry ice, and sugar. On the other hand, covalent network solids consist of atoms bonded together by strong covalent bonds in a continuous 3D network, making them hard substances with high melting and boiling points and varying electrical conductivity depending on their structure. Examples include diamond, graphite, and silicon dioxide.
Step by step solution
01
Define Molecular Solids
Molecular solids are substances made up of distinct molecules held together by weak intermolecular forces, such as dispersion forces, dipole-dipole forces, or hydrogen bonding. These solids are generally soft with low melting and boiling points because the weak intermolecular forces can be easily overcome by the addition of heat or pressure. Examples of molecular solids include ice (water), carbon dioxide in its solid form (dry ice), and sugar.
02
Define Covalent Network Solids
Covalent network solids are substances whose atoms are linked together by strong covalent bonds, forming a continuous three-dimensional network throughout the entire solid. This makes them extremely strong and hard, with high melting and boiling points because a great amount of energy is required to break the strong covalent bonds. Examples of covalent network solids include diamond, graphite (both allotropes of carbon), and silicon dioxide (quartz).
03
Comparing Molecular Solids and Covalent Network Solids
The primary difference between molecular solids and covalent network solids lies in their structures and the types of bonds holding them together. Molecular solids consist of individual molecules held together by weak intermolecular forces, while covalent network solids consist of atoms bonded together by strong covalent bonds in a continuous network.
04
Comparing their Properties
Due to the differences in bonding, molecular solids and covalent network solids exhibit different properties:
1. Molecular solids are generally soft with low melting and boiling points, while covalent network solids are hard with high melting and boiling points.
2. Molecular solids typically have low electrical conductivity due to the presence of weak intermolecular forces, while covalent network solids may have varying electrical conductivity (e.g., diamond is an insulator, and graphite is a conductor) based on their covalent bonding structure.
3. Molecular solids are often soluble in polar solvents because of possible interactions between the solvent molecules and the weak intermolecular forces between the solid's molecules, while covalent network solids are generally insoluble due to their strong covalent bonds.
In summary, molecular solids and covalent network solids differ primarily in their structure, bonding, and properties. Molecular solids consist of individual molecules held together by weak intermolecular forces and have low melting points, low boiling points, and low electrical conductivity. Covalent network solids consist of atoms bonded together by strong covalent bonds, forming a continuous 3D network, and are hard, have high melting and boiling points, and varying electrical conductivity depending on their structure.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Intermolecular Forces
In the realm of chemistry, understanding the forces that hold substances together is crucial. Intermolecular forces are the forces that exist between molecules. They include various types of interactions: dispersion forces, dipole-dipole forces, and hydrogen bonds.
These forces are significantly weaker compared to covalent bonds, which makes them easier to overcome. This characteristic affects the physical properties of molecular solids. For instance, because molecular solids like ice or sugar are held together by these weak forces, they tend to be softer and have lower melting and boiling points.
These forces are significantly weaker compared to covalent bonds, which makes them easier to overcome. This characteristic affects the physical properties of molecular solids. For instance, because molecular solids like ice or sugar are held together by these weak forces, they tend to be softer and have lower melting and boiling points.
- Dispersion forces: Present in all molecular interactions, stronger in larger molecules.
- Dipole-dipole forces: Only present in polar molecules.
- Hydrogen bonds: A special, strong type of dipole attraction, particularly in molecules with O, N, or F bonded to H.
Covalent Bonds
Covalent bonds form when atoms share electrons, creating a strong bond between them. This type of bond is responsible for the structure of covalent network solids, like diamond or quartz, where atoms are connected in a vast three-dimensional network.
The strength of these covalent bonds means that these substances require significant energy to break apart, leading to their characteristically high melting and boiling points. Unlike molecular solids, these bonds do not simply break under normal temperatures or pressures, which contributes to their hardness and rigidity.
The strength of these covalent bonds means that these substances require significant energy to break apart, leading to their characteristically high melting and boiling points. Unlike molecular solids, these bonds do not simply break under normal temperatures or pressures, which contributes to their hardness and rigidity.
- Shared electron pairs create stable bonds.
- Results in strong, durable structures.
- Energy-intensive to disrupt, explaining high thermal stability.
Melting and Boiling Points
Melting and boiling points are fundamental properties that provide insights into the strength of forces holding a substance together. In molecular solids, low melting and boiling points indicate that only weak intermolecular forces must be overcome.
This is evident in substances like ice or dry ice, which readily melt or sublimate at relatively low temperatures. On the other hand, covalent network solids showcase extremely high melting and boiling points. For instance, diamond can withstand very high temperatures without melting due to its strong covalent bonds.
This is evident in substances like ice or dry ice, which readily melt or sublimate at relatively low temperatures. On the other hand, covalent network solids showcase extremely high melting and boiling points. For instance, diamond can withstand very high temperatures without melting due to its strong covalent bonds.
- Molecular solids: Low temperatures suffice to overcome weak intermolecular forces.
- Covalent network solids: Require much higher energy due to strong covalent bonding.
Electrical Conductivity
Electrical conductivity is influenced by a material's structure and the freedom of charge carriers within it. Molecular solids generally have low electrical conductivity because their electrons are tightly bound in covalent bonds and there are no free charge carriers.
In contrast, covalent network solids may exhibit different levels of conductivity based on their specific structures. For example, while diamond is an electrical insulator as its electrons are localized, graphite conducts electricity due to delocalized electrons along its planes.
In contrast, covalent network solids may exhibit different levels of conductivity based on their specific structures. For example, while diamond is an electrical insulator as its electrons are localized, graphite conducts electricity due to delocalized electrons along its planes.
- Molecular solids: Low conductivity due to fixed electrons.
- Covalent network solids: Varying conductivity, dependent on electron mobility.
Solubility
Solubility refers to the ability of a substance to dissolve in a solvent. Molecular solids tend to be soluble in polar solvents. This is because similar weak forces can exist between the solute and solvent, which assists in dissolving the solute.
Examples include sugar dissolving in water due to the interactions between polar molecules and hydrogen bonds. On the other hand, covalent network solids generally show low solubility, primarily due to their strong covalent bonds which are hard to disrupt.
Examples include sugar dissolving in water due to the interactions between polar molecules and hydrogen bonds. On the other hand, covalent network solids generally show low solubility, primarily due to their strong covalent bonds which are hard to disrupt.
- Molecular solids: Soluble in polar solvents due to similar intermolecular interactions.
- Covalent network solids: Generally insoluble due to strong internal bonds.