Question

Why is naphthalene \(\left(\mathrm{C}_{10} \mathrm{H}_{8}\right)\) more soluble than \(\mathrm{CsF}\) in benzene?

Short answer

Naphthalene is non-polar like benzene, whereas \( \mathrm{CsF} \) is ionic, and benzene, being non-polar, dissolves non-polar substances better.

Step-by-step solution

Identify the Solvents and Solutes

Benzene is a non-polar solvent. Naphthalene \( (\mathrm{C}_{10} \mathrm{H}_{8}) \) is also non-polar, while \( \mathrm{CsF} \) is an ionic compound, made from Caesium and Fluoride.

Assess Solubility Based on Polarity

Non-polar solvents like benzene are more effective at dissolving non-polar solutes. This is due to the fact that interactions between non-polar molecules (such as van der Waals forces) facilitate the dissolution process. As a result, naphthalene, being non-polar, is more soluble in benzene. In contrast, ionic compounds like \( \mathrm{CsF} \) generally require a polar solvent to dissolve effectively.

Apply 'Like Dissolves Like' Principle

The general principle of solubility is 'like dissolves like,' meaning polar solvents dissolve polar substances, and non-polar solvents dissolve non-polar substances. Since benzene is non-polar, and naphthalene is also non-polar, they have a similar nature, hence naphthalene dissolves more easily in benzene compared to \( \mathrm{CsF} \), which is ionic.

Key concepts

Non-polar Solvents

Non-polar solvents such as benzene have molecules that share electrons equally. This means they do not have poles like a magnet does. As a result, these solvents lack a charge and cannot effectively interact with charged or polar molecules.
Non-polar solvents like benzene excel in dissolving substances with similar non-polar characteristics. This happens because molecules interact through weak van der Waals forces:

• These forces exist between all molecules, but are significant in non-polar interactions.
• They allow non-polar molecules to approach and mix with each other.
• The absence of charge prevents strong electrostatic attractions.

This makes benzene perfect for dissolving non-polar substances such as naphthalene, making it much more soluble than other polar or ionic compounds.

Ionic Compounds

Ionic compounds consist of positively and negatively charged ions. These are formed when atoms lose or gain electrons, creating charged particles that attract each other through strong electrostatic forces.
The structure of ionic compounds, like CsF (Cesium fluoride), involves a crystal lattice of alternating charges:

• The positive and negative ions alternate in a fixed pattern.
• This structure relies heavily on strong ionic bonds for stability.
• Without a polar solvent, these bonds are hard to break.

To dissolve ionic compounds effectively, a polar solvent is usually needed. Polar solvents have molecules with a partial charge, which can interact with the ions and help separate them from the lattice. In a non-polar solvent like benzene, ionic compounds tend to remain undissolved due to the lack of polar interactions.

Like Dissolves Like Principle

The 'like dissolves like' principle helps predict solubility outcomes. This principle means that substances with similar polarity will dissolve in each other. For example, polar substances dissolve well in polar solvents, and non-polar substances noteasily dissolve in each other.
Here’s how it works:

• Polar with Polar: Polar solvents have charges that can interact with those of polar solutes. This leads to solutions like salt in water.
• Non-Polar with Non-Polar: Non-polar solvents don’t have polar ends, so they mix freely with other non-polar substances like oils.
• Mismatch: When polar and non-polar substances are mixed, they do not interact well, remaining separate like oil and water.

This basic rule of chemistry helps explain why naphthalene, non-polar like benzene, dissolves more readily than ionic compounds such as CsF, which need a polar environment to dissolve properly.