Question

which substance would you expect to have the lowest melting point?
\\(\text{ch}_3\text{oh}\\)
\\(\text{ch}_3\text{f}\\)
\\(\text{ni}\\)
\\(\text{licl}\\)

Explanation:

Identify the types of solids

To compare melting points, we first classify each substance by its chemical bonding and solid-state structure.

• \(\text{CH}_3\text{OH}\) (methanol): Molecular solid.
• \(\text{CH}_3\text{F}\) (fluoromethane): Molecular solid.
• \(\text{Ni}\) (nickel): Metallic solid.
• \(\text{LiCl}\) (lithium chloride): Ionic solid.

Compare bonding strengths

We evaluate the relative strength of the forces holding each solid lattice together.

• Ionic bonds in \(\text{LiCl}\) and metallic bonds in \(\text{Ni}\) are extremely strong primary chemical bonds, resulting in very high melting points.
• Molecular solids are held together by much weaker Intermolecular Forces, giving them significantly lower melting points.

Compare molecular solids

We compare the intermolecular forces between the two molecular candidates, \(\text{CH}_3\text{OH}\) and \(\text{CH}_3\text{F}\).

• \(\text{CH}_3\text{OH}\) exhibits Hydrogen Bonding because it has a hydrogen atom directly bonded to a highly electronegative oxygen atom (\(\text{O-H}\) bond).
• \(\text{CH}_3\text{F}\) experiences dipole-dipole interactions and London dispersion forces, but cannot form hydrogen bonds because its hydrogen atoms are bonded to carbon, not fluorine.
• Since hydrogen bonding is significantly stronger than dipole-dipole interactions, \(\text{CH}_3\text{OH}\) has stronger cohesive forces than \(\text{CH}_3\text{F}\).

Determine the lowest melting point

Because \(\text{CH}_3\text{F}\) has the weakest overall intermolecular forces among all the options, it requires the least thermal energy to overcome these forces and melt. Therefore, \(\text{CH}_3\text{F}\) has the lowest melting point.

Answer:

• (A) \(\text{CH}_3\text{OH}\)
• (B) \(\text{CH}_3\text{F}\) (Correct answer)
• (C) \(\text{Ni}\)
• (D) \(\text{LiCl}\)