Problem 59 - Chemistry The Molecular Science

Question

Which is most soluble in cyclohexane, \(\mathrm{C}_{6} \mathrm{H}_{12}\) ? Least soluble? Explain your reasoning. (a) \(\mathrm{NaCl}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) (c) \(\mathrm{C}_{3} \mathrm{H}_{8}\)

Step-by-Step Solution

Answer

\(\mathrm{C}_{3} \mathrm{H}_{8}\) is most soluble and \(\mathrm{NaCl}\) is least soluble in cyclohexane.

Step 1: Understand Solubility Principles

Solubility is influenced by the principle that 'like dissolves like.' Substances with similar intermolecular forces tend to dissolve in each other. Cyclohexane (\(\mathrm{C}_{6} \mathrm{H}_{12}\)) is nonpolar, so it dissolves nonpolar substances better.

Step 2: Identify the Nature of the Compounds

Analyze the polarity of each compound: (a) \(\mathrm{NaCl}\) is ionic, (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) (alcohol) is polar due to the \(-\mathrm{OH}\) group, (c) \(\mathrm{C}_{3} \mathrm{H}_{8}\) is nonpolar (alkane).

Step 3: Determine Most Soluble

Since cyclohexane is nonpolar, \(\mathrm{C}_{3} \mathrm{H}_{8}\) (a nonpolar molecule) is most soluble due to similar nonpolar characteristics.

Step 4: Determine Least Soluble

\(\mathrm{NaCl}\), being an ionic compound, is least soluble in cyclohexane, as ionic compounds dissolve better in polar solvents.

Key Concepts

Nonpolar Solvent

Cyclohexane is an example of a nonpolar solvent due to its molecular structure. It consists only of carbon and hydrogen atoms with symmetric distribution of electrons, resulting in no permanent dipole moment. This characteristic makes it excel at dissolving nonpolar substances, following the principle of "like dissolves like."

In general, nonpolar solvents are:

• Symmetrical in structure, leading to even electron sharing.
• Lack distinct positive or negative ends, unlike polar molecules.
• Effective at dissolving other nonpolar compounds such as oils, fats, and nonpolar gases.

Understanding the nature of solvent interactions can help predict solubility trends and behaviors in various chemical reactions and processes.

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion between neighboring molecules. They play a crucial role in determining solubility. Cyclohexane predominantly interacts through London dispersion forces, a type of van der Waals force, which are weak interactions present in all molecules but are the only type existing between nonpolar molecules.

Key types of intermolecular forces include:

• London dispersion forces: Weak and temporary, due to momentary dipoles.
• Dipole-dipole interactions: Occur in polar molecules with permanent dipoles.
• Hydrogen bonds: Strong type of dipole-dipole interaction found in molecules like water and alcohols.
• Ionic bonds: Very strong, between charged ions, typically found in ionic compounds like NaCl.

For cyclohexane, the lack of significant dipole-dipole interactions or hydrogen bonding means it is ineffective at dissolving polar substances or ionic compounds. It favors other materials that exhibit similar London dispersion forces.

Polarity Analysis

Analyzing polarity is essential in predicting the solubility of substances. It involves examining the molecular structure to determine the distribution of electrical charge across the molecule. Polarity results from significant differences in electronegativity between bonded atoms, leading to partial charges.

For the analyzed compounds:

• NaCl: Completely ionic and highly polar, made of a sodium cation and chloride anion. Dissolves better in polar solvents like water.
• CH₃CH₂OH: Has a polar -OH group, which introduces hydrogen bonding capabilities, making it more soluble in polar solvents.
• C₃H₈: Nonpolar like cyclohexane due to its symmetrical arrangement and lack of significant electronegativity differences, making it highly soluble in cyclohexane.

Understanding these polarity differences is fundamental in predicting interactions and determining the solvent suitability for various compounds.