Question

For each of the following pairs of substances, predict which will have the higher melting point and indicate why: (a) \(\mathrm{Ar}, \mathrm{Xe} ;\) (b) \(\mathrm{SiO}_{2}, \mathrm{CO}_{2} ;\) (c) \(\mathrm{KBr}, \mathrm{Br}_{2}:\) (d) \(\mathrm{C}_{6} \mathrm{Cl}_{6}, \mathrm{C}_{6} \mathrm{H}_{6}\)

Short answer

(a) Xenon (Xe) will have a higher melting point due to stronger London dispersion forces, as it is a larger atom than Argon (Ar). (b) Silicon dioxide (SiO2) will have a higher melting point, as it is a covalent network solid with stronger bonds compared to the weak intermolecular forces in Carbon dioxide (CO2). (c) Potassium bromide (KBr) will have a higher melting point due to the strong electrostatic forces in its ionic lattice, as opposed to the weaker intermolecular forces in Bromine (Br2). (d) Hexachlorobenzene (C6Cl6) will have a higher melting point than Benzene (C6H6) due to stronger London dispersion forces resulting from the larger size of chlorine atoms compared to hydrogen atoms.

Step-by-step solution

(a) Ar vs Xe

Argon (Ar) and Xenon (Xe) are noble gases. They do not form compounds with other elements and exist as monoatomic gases. The only intermolecular force between them is London dispersion forces, which are stronger for larger atoms. Since Xenon is larger than Argon, it will have stronger London dispersion forces, and therefore, will have a higher melting point. Prediction: Xenon (Xe) will have a higher melting point.

(b) SiO2 vs CO2

Silicon dioxide (SiO2) is a covalent network solid, where Si atoms are bonded to O atoms in a giant lattice structure. This structure is very stable and requires a significant amount of energy to break the bonds. On the other hand, Carbon dioxide (CO2) is a simple molecular compound, where each carbon is bonded to two oxygen atoms. The molecule is held together by covalent bonds, while intermolecular forces between CO2 molecules are comparatively weak van der Waals forces. Since breaking the covalent network of SiO2 requires more energy than overcoming the weak intermolecular forces in CO2, SiO2 will have a higher melting point. Prediction: Silicon dioxide (SiO2) will have a higher melting point.

(c) KBr vs Br2

Potassium bromide (KBr) is an ionic compound. It consists of potassium cations (K+) and bromide anions (Br-), which are held together in a 3D lattice structure by strong electrostatic forces between the positive and negative ions. The melting point of KBr depends on the strength of these electrostatic forces. Bromine (Br2), on the other hand, is a simple diatomic molecule. It is held together by covalent bonds between the two atoms, while the intermolecular forces between Br2 molecules are weak van der Waals forces. As the intermolecular forces in Br2 are much weaker than the ionic bonds in KBr, potassium bromide will have a higher melting point. Prediction: Potassium bromide (KBr) will have a higher melting point.

(d) C6Cl6 vs C6H6

Hexachlorobenzene (C6Cl6) and Benzene (C6H6) are both molecular compounds with covalent bonds. In C6Cl6, chlorine atoms form the substituents, while in C6H6, hydrogen atoms are the substituents. The size of the chlorine atom and its electron cloud is larger than that of the hydrogen atom, which results in stronger London dispersion forces between the C6Cl6 molecules compared to the C6H6 molecules. Therefore, hexachlorobenzene will have a stronger intermolecular force and thus a higher melting point. Prediction: Hexachlorobenzene (C6Cl6) will have a higher melting point.

Key concepts

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion between neighboring molecules. These forces are crucial in determining the physical properties of substances, including melting and boiling points. Unlike covalent or ionic bonds, which are intramolecular forces that hold atoms together within a molecule, intermolecular forces occur between molecules during interactions.
There are several types of intermolecular forces:

• Dipole-dipole interactions: Occur between polar molecules with permanent dipoles.
• London dispersion forces: Present in all molecules, more significant in larger atoms/molecules.
• Hydrogen bonding: A strong force occurring between molecules where hydrogen is bonded to a highly electronegative atom like nitrogen, oxygen, or fluorine.

The strength of these forces greatly influences the melting points of substances. Stronger intermolecular forces lead to higher melting points because more energy is needed to separate the molecules during phase changes.

Ionic Compounds

Ionic compounds are made up of positive and negative ions arranged in a lattice structure. These ions are typically formed when a metal loses electrons to become a cation, and a non-metal gains electrons to become an anion. The forces that hold these ions together are called electrostatic forces or ionic bonds.
Key Characteristics of Ionic Compounds:

• High melting and boiling points: The strong electrostatic forces between ions require significant energy to overcome.
• Electrical conductivity: When melted or dissolved in water, ionic compounds conduct electricity.
• Solubility: Many ionic compounds dissolve well in water due to the polar nature of water molecules.

Potassium bromide (KBr) is an example of an ionic compound, with a high melting point due to the strong bonds between its ions. In comparison to molecular compounds such as bromine ( Br_2 ), which are held together by weaker van der Waals forces, ionic compounds generally have much higher melting points.

Covalent Structures

Covalent structures are formed when atoms share pairs of electrons to achieve stability and complete their valence shells. These structures can be divided into two types: simple molecular and covalent network structures.

• Simple Molecular Structures: Consist of discrete molecules with weak intermolecular forces between them. Substances like water ( H_2O ) or carbon dioxide ( CO_2 ) are examples, characterized by low melting and boiling points.
• Covalent Network Structures: Comprise a continuous, extended network of bonded atoms. Substances such as diamond or silicon dioxide ( SiO_2 ) fall into this category, featuring very high melting points due to the extensive bonding network that requires substantial energy to break.

Understanding these structures helps explain why SiO_2 has a much higher melting point compared to CO_2 , since breaking its network of strong covalent bonds requires far more energy than separating the weakly held molecules of CO_2 .

London Dispersion Forces

London dispersion forces, also known as induced dipole-induced dipole attractions, are the weakest type of intermolecular force. They arise due to temporary fluctuations in the electron density around an atom or molecule, inducing a temporary dipole that can influence a neighboring atom or molecule.
Characteristics of London Dispersion Forces:

• Present in all molecules: Especially significant in nonpolar molecules where other types of intermolecular forces do not exist.
• Depend on size and shape: Larger atoms or molecules with more electrons have stronger dispersion forces because they form larger temporary dipoles.
• Influence boiling and melting points: As seen with Xenon (Xe) and Argon (Ar), where Xe has stronger dispersion forces due to its larger atomic size, leading to a higher melting point.

When comparing substances like hexachlorobenzene ( C_6Cl_6 ) and benzene ( C_6H_6 ), the presence of larger chlorine atoms in C_6Cl_6 results in stronger dispersion forces compared to the hydrogen atoms in C_6H_6 , leading to a higher melting point in C_6Cl_6 .