Problem 33 - Chemistry The Central Science

Question

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

Step-by-Step Solution

Answer

In hexane, the more soluble compounds in each pair are: (a) CCl4, (b) Benzene, and (c) Octanoic acid. This is because hexane is a nonpolar solvent, and the selected compounds have nonpolar characteristics or longer nonpolar chains, adhering to the "like dissolves like" principle.

Step 1: Analyzing Polarity of CCl4 and CaCl2

In pair (a), we have Carbon Tetrachloride (CCl4) and Calcium Chloride (CaCl2). CCl4 is a nonpolar molecule, as it has a symmetric distribution of the electron cloud. On the other hand, CaCl2 is an ionic compound, which is highly polar.

Step 2: Comparing Solubilities of CCl4 and CaCl2 in Hexane

Since hexane is a nonpolar solvent and CCl4 is a nonpolar molecule, CCl4 is more likely to be soluble in hexane than the polar CaCl2.

Step 3: Analyzing Polarity of Benzene and Glycerol

In pair (b), we have benzene (C6H6) and glycerol (CH2(OH)CH(OH)CH2OH). Benzene is a nonpolar molecule, with a symmetrical structure and a uniform distribution of the electron cloud. Glycerol, however, has three hydroxyl groups (-OH), which makes it a polar molecule.

Step 4: Comparing Solubilities of Benzene and Glycerol in Hexane

Since hexane is a nonpolar solvent, benzene, which is also nonpolar, is more likely to be soluble in hexane compared to the polar glycerol.

Step 5: Analyzing Polarity of Octanoic acid and Acetic acid

In pair (c), we have octanoic acid (CH3(CH2)6COOH) and acetic acid (CH3COOH). Both of these molecules are carboxylic acids, and they have polar regions due to the presence of the carboxyl group (-COOH). However, octanoic acid also has a long nonpolar chain of hydrocarbons.

Step 6: Comparing Solubilities of Octanoic acid and Acetic acid in Hexane

Due to the longer nonpolar chain in octanoic acid, it is more likely to be soluble in the nonpolar solvent hexane compared to the smaller acetic acid molecule, which has a higher proportion of polar regions. In conclusion, the compounds that are more likely to be soluble in hexane are: (a) CCl4 (b) Benzene (c) Octanoic acid

Key Concepts

Polarity

Polarity refers to the distribution of electrical charge across a molecule. Molecules are considered polar when they have a difference in electronegativity between atoms leading to an uneven charge distribution. In simpler terms, one end of the molecule may be slightly positively charged, while the other end is negatively charged. This phenomenon creates what is known as a dipole moment. In nonpolar molecules, the distribution of charges is even, and there's no permanent dipole moment, meaning the molecule does not have distinct positive and negative ends.

Understanding polarity is crucial in determining the solubility of substances. The principle of 'like dissolves like' explains that polar solvents, like water, dissolve polar substances well. Conversely, nonpolar solvents are better at dissolving nonpolar substances. This is important when assessing solubility in situations involving polar and nonpolar compounds.

Nonpolar Solvents

Nonpolar solvents are liquids that have molecules with no significant charge separation, making them ideal for dissolving nonpolar substances. Hexane, for example, is a nonpolar solvent due to its symmetric structure and uniform distribution of electrons. In nonpolar solvents, molecular interactions occur through dispersion forces, which are weak but sufficient to dissolve similar nonpolar compounds.

When considering solubility, it's essential to remember that nonpolar solvents do not attract polar or ionic solutes strongly, which leads to poor solubility of these substances. In the case of hexane, its nonpolar nature allows it to dissolve nonpolar substances such as Carbon Tetrachloride (CCl4) and benzene, while being inefficient at dissolving polar substances like Calcium Chloride (CaCl2) or Glycerol.

Hexane Solubility

Hexane is a commonly used organic solvent with a characteristic nonpolar nature. It is structurally composed of a simple chain of carbon and hydrogen atoms [\[ C_6H_{14} \]]. Its nonpolar characteristics make it highly effective at dissolving other nonpolar compounds.

When assessing solubility in hexane, the key factor is the match between the polarities of solute and solvent. For example, hexane dissolves nonpolar substances because they interact through weak van der Waals forces. Hence, compounds like benzene (\[ C_6H_6 \]), which are also nonpolar, will dissolve in hexane, while polar or ionic molecules, such as glycerol or Calcium Chloride, remain undissolved.

Ionic and Covalent Compounds

Ionic and covalent compounds differ fundamentally in their bonding and structural properties, which influence their solubility characteristics. Ionic compounds are composed of positively and negatively charged ions held together by strong electrostatic forces. These compounds, such as Calcium Chloride (\[ CaCl_2 \]), are typically highly soluble in polar solvents, due to the solvent's ability to stabilize the charged ions.

On the other hand, covalent compounds consist of atoms sharing electrons within a molecule. These can be nonpolar, like Carbon Tetrachloride (\[ CCl_4 \]), or polar, like glycerol, depending on the electronegativity differences between the bonded atoms. Nonpolar covalent compounds dissolve well in nonpolar solvents like hexane. Understanding the nature of a compound's bonding can help predict its solubility behavior in different solvents.