Question

(a) Would you expect stearic acid, \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{16} \mathrm{COOH},\) to be more soluble in water or in carbon tetrachloride? Explain.

Short answer

Stearic acid would be more soluble in carbon tetrachloride than in water, as it is predominantly nonpolar, making it more compatible with the nonpolar solvent according to the principle "like dissolves like". The intermolecular forces between stearic acid and carbon tetrachloride are stronger compared to those with water, leading to better solubility.

Step-by-step solution

Identify the polarity of stearic acid

Stearic acid has a polar carboxylic acid group (-COOH) and a long nonpolar hydrocarbon chain. The majority of the molecule is nonpolar since the long hydrocarbon chain greatly outnumbers the single polar carboxylic acid group. Therefore, we can consider stearic acid to be predominantly nonpolar.

Identify the polarity of the solvents

Water is a polar solvent because of the difference in electronegativities between hydrogen and oxygen atoms. The presence of hydrogen bonding in water results in a strong polar nature. Carbon tetrachloride, \(\mathrm{CCl}_{4}\), is a nonpolar solvent since it has a symmetrical tetrahedral structure with equal distribution of electronegativity among the chlorine atoms.

Apply the principle "like dissolves like"

Since stearic acid is predominantly nonpolar and the principle "like dissolves like" states that nonpolar substances are more likely to dissolve in nonpolar solvents, we can conclude that stearic acid will be more soluble in carbon tetrachloride than in water.

Explain the solubility in terms of intermolecular forces

In water, stearic acid would experience weak dispersion forces with the water molecules due to the nonpolar hydrocarbon chain, which is insufficient to overcome the strong hydrogen bonding in water. Thus, stearic acid is not very soluble in water. However, in carbon tetrachloride, the dispersion forces between stearic acid and the solvent molecules are much stronger, which allows stearic acid to dissolve more readily in carbon tetrachloride. (a) Stearic acid will be more soluble in carbon tetrachloride than in water because the majority of its structure is nonpolar, which makes it more compatible with the nonpolar solvent. This is due to the principle "like dissolves like" and the intermolecular forces present between stearic acid and the solvents.

Key concepts

Stearic Acid

Stearic acid, represented by the chemical formula \( \mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{16} \mathrm{COOH} \), is an interesting molecule because it features both polar and nonpolar characteristics.
It consists of a carboxylic acid group \( -\mathrm{COOH} \), which is polar, and a long hydrocarbon chain made up of 16 carbon atoms, which is nonpolar.
The hydrocarbon chain is significantly larger than the carboxylic acid group, making stearic acid predominantly nonpolar. This composition affects how stearic acid interacts with different solvents when it comes to solubility.

• The nonpolar hydrocarbon chain doesn't mix well with polar substances like water.
• The small polar part (\( -\mathrm{COOH} \)) can't override the effects of the long chain's nonpolarity.

This combination of properties makes stearic acid more likely to dissolve in nonpolar environments.

Intermolecular Forces

Intermolecular forces are the forces that hold molecules together. They play a vital role when evaluating the solubility of compounds like stearic acid. There are several types of intermolecular forces, but in the context of stearic acid and its solvents, two are particularly relevant: hydrogen bonding and dispersion forces.
Water molecules exhibit strong hydrogen bonding because of the significant electronegativity difference between hydrogen and oxygen atoms. This makes water a highly polar solvent.
In contrast, carbon tetrachloride (\( \mathrm{CCl}_{4} \)) is nonpolar and relies primarily on dispersion forces.

• Hydrogen bonds: Very strong and occur between molecules with \( \mathrm{H} \) bonded to electronegative atoms like \( \mathrm{O} \).
• Dispersion forces: Weaker and present in all molecular interactions, more significant in larger, nonpolar molecules.

For stearic acid, the dominant intermolecular interaction in \( \mathrm{CCl}_{4} \) is the dispersion force, which sufficiently holds the nonpolar parts of the acid together with the solvent.

Polar and Nonpolar Substances

The concept "like dissolves like" helps us understand why substances dissolve in one another based on their polarity. Polar substances have dipoles, meaning they have positive and negative charges on separate parts of the molecule. Nonpolar substances have even charge distribution.
In polar substances, interactions like hydrogen bonding are strong due to attractive forces between positive and negative dipoles. Nonpolar substances typically rely on weaker dispersion forces.
Stearic acid, despite having a polar carboxyl group, behaves predominantly as a nonpolar substance due to its large hydrocarbon chain.

• Water (a polar substance): Strong hydrogen bonds; poor at dissolving nonpolar substances.
• Carbon tetrachloride (a nonpolar substance): Relies on dispersion forces; dissolves nonpolar substances like stearic acid well.

Understanding these properties helps predict solubility behavior, where nonpolar stearic acid dissolves better in nonpolar solvents like carbon tetrachloride.