Question

Which of these substances has the highest melting point? The lowest melting point? Explain your choices briefly. (a) \(\mathrm{LiBr}\) (b) \(\mathrm{CaO}\) (c) \(\mathrm{CO}\) (d) \(\mathrm{CH}_{3} \mathrm{OH}\)

Short answer

\(\mathrm{CaO}\) has the highest melting point; \(\mathrm{CO}\) has the lowest.

Step-by-step solution

Identify the Type of Bond

Analyze the types of bonds present in each compound to understand their general strength and relation to melting points. \(\mathrm{LiBr}\) and \(\mathrm{CaO}\) are ionic compounds, which typically have high melting points due to strong ionic bonds. \(\mathrm{CO}\) and \(\mathrm{CH}_{3}\mathrm{OH}\) are molecular compounds, with \(\mathrm{CO}\) having van der Waals forces and \(\mathrm{CH}_{3}\mathrm{OH}\) having hydrogen bonds.

Compare Ionic Compounds

Ionic compounds have stronger bonds due to electrostatic attraction. Compare \(\mathrm{LiBr}\) and \(\mathrm{CaO}\): \(\mathrm{CaO}\) has a higher melting point than \(\mathrm{LiBr}\) because \(\mathrm{CaO}\) is formed from doubly charged ions \(\mathrm{Ca}^{2+}\) and \(\mathrm{O}^{2-}\), leading to stronger electrostatic forces than the singly charged ions in \(\mathrm{LiBr}\).

Compare Molecular Compounds

For molecular compounds, \(\mathrm{CO}\) has weak van der Waals forces compared to \(\mathrm{CH}_{3}\mathrm{OH}\), which has hydrogen bonding, a stronger intermolecular force. Thus, \(\mathrm{CH}_{3}\mathrm{OH}\) has a higher melting point than \(\mathrm{CO}\).

Determine Highest Melting Point

From the comparison, \(\mathrm{CaO}\), an ionic compound with strong electrostatic forces, has the highest melting point among all given substances.

Determine Lowest Melting Point

The molecular compound \(\mathrm{CO}\) has the weakest van der Waals forces and, therefore, the lowest melting point among the substances given.

Key concepts

Ionic Compounds

Ionic compounds form due to the attraction between oppositely charged ions. They result from the transfer of electrons between atoms, typically a metal and a non-metal. These strong ionic bonds are responsible for many of the distinctive properties of ionic compounds, including their high melting points.

Ionic compounds form lattice structures. These structures are stable, as each ion attracts several oppositely charged ions. A great deal of energy is needed to overcome these forces and melt the compound, which explains why substances like calcium oxide (\(\mathrm{CaO}\)) have extremely high melting points.

• Calcium oxide consists of doubly charged ions \(\mathrm{Ca^{2+}}\) and \(\mathrm{O^{2-}}\), resulting in powerful electrostatic forces.
• Lithium bromide (\(\mathrm{LiBr}\)), while still an ionic compound, involves singly charged ions that don't bind as tightly as doubly charged ones.

The differences in charge and ionic interaction impact melting points, with compounds like \(\mathrm{CaO}\) exhibiting higher melting points than those like \(\mathrm{LiBr}\).

Molecular Compounds

Molecular compounds are formed from non-metal atoms and are held together by covalent bonds. These bonds result from the sharing of electron pairs between atoms. Unlike ionic compounds, molecular compounds have relatively low melting points.

• Carbon monoxide (\(\mathrm{CO}\)) is a molecular compound, where atoms are bonded covalently.
• Methanol (\(\mathrm{CH}_{3}\mathrm{OH}\)) is another example, but with the ability to form hydrogen bonds.

Hydrogen bonds occur when a hydrogen atom covalently bonded to a strongly electronegative atom, like oxygen, forms a weak attraction with another electronegative atom.

This additional bonding can increase the melting point of a molecular compound, as seen in \(\mathrm{CH}_{3}\mathrm{OH}\), which has a higher melting point than \(\mathrm{CO}\), due to the presence of these bonds.

Intermolecular Forces

Intermolecular forces, though weaker than intramolecular (covalent or ionic) bonds, significantly influence the physical properties of substances, including their melting points.

• Van der Waals forces: These are weak attractions between all atoms and molecules, most prominent when there are no other stronger forces acting, such as in \(\mathrm{CO}\).
• Hydrogen bonding: A special type of dipole-dipole interaction, present in \(\mathrm{CH}_{3}\mathrm{OH}\), contributes to a higher melting point.
• Dipole-dipole interactions: These forces occur in polar molecules, also playing a role in molecular arrangements and melting points.

Particularly, hydrogen bonds, though only about 5% as strong as covalent bonds, align molecules closely and require more energy to break. This makes substances with hydrogen bonds, like methanol, less easily melted compared to those with only van der Waals forces, such as \(\mathrm{CO}\).

Understanding these interactions helps explain the behavior of molecular compounds in various conditions.