Question

Selected chlorides have the following melting points: \(\mathrm{NaCl}\) \(\left(801^{\circ} \mathrm{C}\right), \mathrm{MgCl}_{2}\left(714^{\circ} \mathrm{C}\right), \mathrm{PCl}_{3}\left(-94^{\circ} \mathrm{C}\right), \mathrm{SCl}_{2}\left(-121^{\circ} \mathrm{C}\right)\) (a) For each compound, indicate what type its solid form is (molecular, metallic, ionic, or covalent-network). (b) Predict which of the following compounds has a higher melting point: \(\mathrm{CaCl}_{2}\) or \(\mathrm{SiCl}_{4}\).

Short answer

The solid forms of the given chlorides are: - NaCl: Ionic - MgCl_2: Ionic - PCl_3: Molecular - SCl_2: Molecular Based on the bonding characteristics, CaCl_2 (ionic solid) is predicted to have a higher melting point than SiCl_4 (molecular solid).

Step-by-step solution

a) Determining the solid types

To determine the type of solid for each compound, let's analyze their bonding characteristics and melting points: 1. \(\mathrm{NaCl}\): Sodium chloride has a high melting point (801°C) and is formed by the ionic bond between Na+ and Cl-. Thus, its solid form is ionic. 2. \(\mathrm{MgCl_2}\): Magnesium chloride also has a high melting point (714°C) and is formed by the ionic bond between Mg2+ and Cl-. Its solid form is also ionic. 3. \(\mathrm{PCl_3}\): Phosphorous trichloride has a low melting point (-94°C) and is formed by covalent bonds between P and Cl atoms. The solid form consists of discrete molecules, so its solid form is molecular. 4. \(\mathrm{SCl_2}\): Sulfur dichloride has a low melting point (-121°C) and is formed by covalent bonds between S and Cl atoms. Similar to PCl3, its solid form is molecular. Thus, the solid forms of the given chlorides are: - NaCl: Ionic - MgCl_2: Ionic - PCl_3: Molecular - SCl_2: Molecular

b) Predicting the higher melting point

To predict which compound has a higher melting point between CaCl2 and SiCl4, let's analyze their bonding characteristics: 1. \(\mathrm{CaCl_2}\): Calcium chloride is formed by the ionic bond between Ca2+ and Cl-. Typically, ionic solids have higher melting points due to the strong electrostatic forces between ions. 2. \(\mathrm{SiCl_4}\): Silicon tetrachloride is formed by covalent bonds between Si and Cl atoms. The solid form of SiCl4 consists of discrete molecules, making its solid form molecular. Molecular solids generally have lower melting points due to weaker intermolecular forces. Based on this analysis, \(\mathrm{CaCl_2}\) (ionic solid) is predicted to have a higher melting point than \(\mathrm{SiCl_4}\) (molecular solid).

Key concepts

Ionic Solids

Ionic solids are a type of crystalline solid where the ions are held together by strong electrostatic forces known as ionic bonds. These bonds occur between positively charged cations and negatively charged anions. A classic example of an ionic solid is sodium chloride (NaCl), where sodium ( ext{Na}^+) and chloride ( ext{Cl}^-) ions alternate in a lattice structure.
- **Characteristics**: They are typically hard and brittle. They tend to fracture easily when struck. - **Conductivity**: In solid form, ionic compounds do not conduct electricity, but when melted or dissolved in water, they become conductive due to the movement of ions. - **Solubility**: Ionic solids are usually soluble in polar solvents like water.
Overall, due to the strong bonding between ions, ionic solids exhibit high melting points, as seen in the examples of ext{NaCl} and ext{MgCl}_2.

Molecular Solids

Molecular solids consist of molecules held together by intermolecular forces, such as London dispersion forces, dipole-dipole interactions, or hydrogen bonds. These forces are much weaker compared to ionic or covalent bonds. Examples include phosphorus trichloride (PCl_3) and sulfur dichloride (SCl_2).
- **Characteristics**: They are usually soft and have low melting points. Because the forces between molecules are weak, a small amount of heat is enough to separate the molecules. - **Conductivity**: Molecular solids are usually poor conductors of electricity. This is because their molecules are not charged, and there are no free electrons or ions. - **Solubility**: They can be soluble or insoluble in solvents depending on the polarity of the molecules and the solvent.
Due to the presence of weak intermolecular forces, molecular solids like ext{PCl}_3 and ext{SCl}_2 tend to have significantly lower melting points.

Melting Points

The melting point of a solid is the temperature at which it transforms from a solid phase to a liquid phase. This is a crucial property for identifying and classifying different types of solids.
- **Ionic Solids**: Tend to have high melting points due to the strong ionic bonds. For example, ext{NaCl} melts at ext{801°C}, as large amounts of energy are needed to overcome the electrostatic forces holding the ions together. - **Molecular Solids**: Have much lower melting points since they are held together by weaker intermolecular forces. Consider ext{PCl}_3, which melts at about ext{-94°C}.
The melting point can help differentiate between types of solids and determine their bond strength. Ionic solids like ext{CaCl}_2 would be expected to have a higher melting point than molecular solids like ext{SiCl}_4.

Bonding Characteristics

Bonding characteristics of solids are determined by the types of forces that hold the particles together in the solid state. This significantly affects their physical properties such as hardness, conductivity, and melting points.
- **Ionic Solids**: Characterized by ionic bonds, they exhibit high melting points and are typically brittle. - **Molecular Solids**: Defined by covalent bonds within the molecules but held together by weaker intermolecular forces externally, resulting in low melting points.
Understanding these bonding characteristics is essential for predicting the behavior of solids under various conditions. For instance, knowing that ext{CaCl}_2 is ionic helps predict that it will have a higher melting point compared to ext{SiCl}_4, which is held together by covalent bonds and weaker forces in its solid state.