Problem 33 - Chemistry The Central Science In Si Units

Question

Which of the following in each pair is likely to be more soluble in hexane, \(\mathrm{C}_{6} \mathrm{H}_{14}:\) (a) \(\mathrm{CCl}_{4}\) or \(\mathrm{CaCl}_{2},(\mathbf{b})\) benzene \(\left(\mathrm{C}_{6} \mathrm{H}_{6}\right)\) or glycerol, \(\mathrm{CH}_{2}(\mathrm{OH}) \mathrm{CH}(\mathrm{OH}) \mathrm{CH}_{2} \mathrm{OH},\) (c) Octanoic acid, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{COOH},\) or acetic acid, \(\mathrm{CH}_{3} \mathrm{COOH}\) ? Explain your answer in each case.

Step-by-Step Solution

Answer

(a) CCl4, (b) Benzene, (c) Octanoic acid are more soluble in hexane.

Step 1: Understand Solubility Concept

Solubility in hexane, a nonpolar solvent, is typically higher for nonpolar substances because of similar intermolecular forces. The principle "like dissolves like" suggests that substances with similar polarities will dissolve in each other.

Step 2: Compare CCl4 and CaCl2

Hexane is nonpolar, so \(\mathrm{CCl}_4\), which is also nonpolar, is more likely to be soluble in hexane than the ionic compound \(\mathrm{CaCl}_2\). Ionic compounds tend to be more soluble in polar solvents, such as water.

Step 3: Compare Benzene and Glycerol

Benzene \((\mathrm{C}_6\mathrm{H}_6)\) is nonpolar, making it more compatible with nonpolar hexane compared to glycerol, which is polar due to its hydroxyl \((\mathrm{OH})\) groups. Therefore, benzene is more soluble in hexane.

Step 4: Compare Octanoic Acid and Acetic Acid

Octanoic acid has a longer nonpolar hydrocarbon chain compared to acetic acid, making it more nonpolar overall. Acetic acid is more polar because it has a shorter nonpolar chain and a larger polar \(\mathrm{COOH}\) group relative to its size. Hence, octanoic acid is more soluble in hexane.

Key Concepts

Nonpolar compounds

Nonpolar compounds are molecules that do not have a significant difference in electric charge across their atoms. This means that their electrons are distributed more evenly. Imagine a tug of war where the teams are equally matched. No team moves the other, just like nonpolar molecules do not have areas of charge that attract or repel strongly.

A classic example of a nonpolar compound is carbon tetrachloride \(\mathrm{CCl}_{4}\), where the shape is symmetrical, and the pull of electrons is evenly balanced. Nonpolar substances generally have weaker intermolecular attractions, such as London dispersion forces, compared to polar substances that may exhibit dipole-dipole attractions or hydrogen bonding.

Knowing whether a compound is polar or nonpolar is important for predicting how it will behave or interact with other substances. Nonpolar compounds tend to blend well with other nonpolar solvents due to these weaker, yet similar, types of molecular interactions.

Hexane

Hexane, represented by the formula \(\mathrm{C}_6 \mathrm{H}_{14}\), is a simple alkane, which is a type of hydrocarbon. Alkanes are saturated hydrocarbons, meaning they contain only single bonds between carbon atoms. This makes the chains of molecules fairly uniform in their electron distribution, contributing to their nonpolarity.

Being nonpolar, hexane does not have any charge differentiation across its structure. This is why it is often used as a solvent for nonpolar substances. For example, it can dissolve oils and fats, which are similarly nonpolar. Because hexane itself is nonpolar, mixing it with other nonpolar molecules does not disrupt the existing balance of intermolecular forces.

Hexane's primary use is in laboratories and industries where organic compounds need to be dissolved or extracted. It is a powerful tool because of its ability to interact with and dissolve various nonpolar substances.

Like dissolves like

The phrase "like dissolves like" is a simple way to explain solubility, reflecting that molecules with similar polarity tend to dissolve well in one another. This concept helps us predict and understand the behavior of substances in solutions.

A nonpolar solvent like hexane will best dissolve nonpolar substances because their interaction relies on similar intermolecular forces, largely London dispersion forces. When similar types of molecules mix, minimal energy is required to break interactions, allowing them to intermingle easily.

• If you have a nonpolar solute, opt for a nonpolar solvent for effective dissolution.
• Polar solutes are best tackled with polar solvents, where hydrogen bonds or dipole-dipole interactions can occur.

Picture oil, a nonpolar substance, easily spreading out in hexane but not in water. Water, being polar, won't dissolve the oil due to polar interactions favoring stronger attractions within similar polar molecules.