Question

Predict whether each of the following substances is likely to have a relatively high or low melting point. (a) \(\mathrm{P}_{4}\) (c) \(\mathrm{CCl}_{4}\) (e) \(\mathrm{CaC}_{2}\) (b) \(\mathrm{CaCl}_{2}\) (d) AlN

Short answer

Based on the analysis, it is predicted that P4 and CCl4 will have low melting points due to the weak intermolecular forces in these structures. CaC2, CaCl2, and AlN are predicted to have high melting points because of the strong ionic bonds in CaC2 and CaCl2 and the covalent network structure in AlN.

Step-by-step solution

Determine the type of bond in P4

P4 consists of phosphorous atoms. Hence, it is a molecular structure with van der Waals forces between the molecules. These forces are relatively weak.

Determine the type of bond in CCl4

CCl4 is made up of covalently bonded carbon and chlorine atoms. It has London dispersion forces, which are considered the weakest intermolecular forces.

Determine the type of bond in CaC2

CaC2 is an ionic compound with calcium ions and carbide ions. Ionic compounds generally have high melting points due to the strong electrostatic attraction between the ions.

Determine the type of bond in CaCl2

CaCl2 is also an ionic compound with calcium ions and chloride ions. It is also likely to have a high melting point due to the strong forces of attraction between the ions.

Determine the type of bond in AlN

AlN is a covalent network solid. In such structures, atoms are bonded by strong covalent bonds in a continuous network. As such, AlN is likely to have a high melting point due to the strength of these covalent bonds.

Key concepts

Melting Points

The melting point of a substance is the temperature at which it changes from a solid to a liquid. A key factor influencing melting points is the nature of the bonds between the atoms or molecules in a substance. Substances with strong bonds between their constituent particles generally exhibit high melting points. When these bonds are strong, more energy is required to break them, which means the substance will melt at a higher temperature.

For example, ionic compounds and covalent network solids tend to have high melting points because they are held together by very strong attractions. In contrast, molecular substances with weaker forces such as van der Waals interactions usually have lower melting points, as these intermolecular interactions require less energy to overcome.

Ionic Compounds

Ionic compounds are formed when electrons are transferred from one atom to another, usually between metals and non-metals, resulting in the formation of positive and negative ions. These ions are held together by strong electrostatic forces known as ionic bonds. One of the hallmark characteristics of ionic compounds is their high melting points.

This is because the electrostatic attraction between the ions is strong, meaning that a substantial amount of energy is required to separate them during melting. Examples of ionic compounds discussed include calcium carbide ( CaC2 ) and calcium chloride ( CaCl2 ), both of which have high melting points due to these strong ionic bonds. Ionic compounds also tend to be brittle and can conduct electricity when molten or dissolved in water.

Covalent Network Solids

Covalent network solids are a class of materials wherein atoms are connected by covalent bonds throughout a continuous network, forming a solid. This solid structure is highly stable and robust because of the strength of covalent bonds, which are some of the strongest chemical bonds known.

In these solids, every atom is bonded to several others, creating a very rigid structure. Because of this, covalent network solids generally have very high melting points. For example, aluminum nitride (AlN) is a covalent network solid and requires significant energy to break the covalent bonds during melting. These materials are typically very hard, have high thermal stability, and often serve as excellent insulators.

Intermolecular Forces

Intermolecular forces are forces of attraction or repulsion which act between neighboring particles (atoms, molecules, or ions). These forces are responsible for many physical properties of materials, including boiling and melting points, vapor pressure, and solubility.

The strength of intermolecular forces varies between different substances, influencing their phase behavior and structural characteristics. For example:

• London Dispersion Forces: These are the weakest intermolecular forces and occur due to temporary dipoles in molecules. Substances like carbon tetrachloride (CCl4) that only have dispersion forces usually have lower melting points.
• Van der Waals Forces: These include London dispersion forces and dipole-dipole interactions. They are generally weaker than covalent, ionic, and metallic bonds. Substances like P4, held together by van der Waals forces, have relatively low melting points.

Understanding the role of intermolecular forces helps in predicting and explaining the melting point trends seen in various substances.