Question

The solubility of benzoic acid \(\left(\mathrm{HC}_{7} \mathrm{H}_{5} \mathrm{O}_{2}\right)\) is \(0.34 \mathrm{g} / 100 \mathrm{mL}\) in water at \(25^{\circ} \mathrm{C}\) and is \(10.0 \mathrm{g} / 100 \mathrm{mL}\) in benzene \(\left(\mathrm{C}_{6} \mathrm{H}_{6}\right)\) at \(25^{\circ} \mathrm{C}\). Rationalize this solubility behavior. (Hint: Benzoic acid forms a dimer in benzene.) Would benzoic acid be more or less soluble in a 0.1-M NaOH solution than it is in water? Explain.

Short answer

In conclusion, benzoic acid has higher solubility in benzene due to its non-polar character and dimer formation, while its solubility in water is limited by the presence of its polar -COOH and non-polar -C6H5 functional groups. The solubility of benzoic acid in a 0.1-M NaOH solution is more than in water because it reacts with NaOH to form more soluble sodium benzoate salts.

Step-by-step solution

Understanding Solubility Behavior of Benzoic Acid in Water and Benzene

To rationalize the solubility behavior of benzoic acid in water and benzene, we need to understand the characteristics of each solvent and how they influence the solubility of benzoic acid. Water is a polar solvent, and benzene is a non-polar solvent. According to the rule "like dissolves like," polar solutes are more soluble in polar solvents, and non-polar solutes are more soluble in non-polar solvents. Benzoic acid has polar -COOH and non-polar -C6H5 functional groups, so it has both polar and non-polar character. In water, the polar -COOH group forms hydrogen bonds with water molecules, which increases its solubility. However, the non-polar -C6H5 group does not interact well with water molecules and decreases the solubility of benzoic acid. In benzene, the non-polar -C6H5 group strongly interacts with benzene molecules, making the solubility of benzoic acid much higher in benzene than in water. The dimer formation (molecules forming pairs) in benzene also contributes to the higher solubility, as it is a non-polar interaction.

Comparing Solubility of Benzoic Acid in 0.1-M NaOH Solution and Water

To determine if benzoic acid will be more or less soluble in a 0.1-M NaOH solution than it is in water, we need to consider the chemical reaction that occurs when benzoic acid reacts with NaOH. When benzoic acid is added to a NaOH solution, it reacts with NaOH to form sodium benzoate (C6H5COONa) and water: \[ HC_7H_5O_2 + NaOH \rightarrow C_6H_5COONa + H_2O \] Sodium benzoate is the salt of benzoic acid and is more soluble in water than benzoic acid itself due to the ionic nature of the sodium benzoate. The presence of NaOH in solution promotes the formation of sodium benzoate, which increases the solubility of benzoic acid in the NaOH solution compared to water alone.

Conclusion

Benzoic acid is more soluble in benzene than in water due to the non-polar character of benzene, which favors the solubility of benzoic acid's non-polar -C6H5 group. The formation of dimers also contributes to its high solubility in benzene. Additionally, benzoic acid is more soluble in a 0.1-M NaOH solution than in water because it reacts with NaOH, increasing the solubility by forming the more soluble sodium benzoate salts.

Key concepts

Benzoic Acid

Benzoic acid, known chemically as \( \text{C}_7 \text{H}_6 \text{O}_2\), is an important compound in chemistry. It has a unique structure that consists of both a polar carboxylic acid group (\( -\text{COOH} \)) and a non-polar phenyl group (\( -\text{C}_6\text{H}_5 \)).
This dual nature gives it properties of both polar and non-polar compounds. Because of this dual character, benzoic acid's solubility varies across different solvents.
The extent to which it dissolves in a solvent is governed by the interactions between the solvent molecules and the functional groups of benzoic acid.
Understanding these interactions can help us rationalize its varying solubility in different environments.

Polar and Non-Polar Solvents

In chemistry, solvents are broadly categorized into polar and non-polar types based on their molecular composition and structure.
Polar solvents, like water, have molecules with significant positive and negative ends (dipoles). Non-polar solvents, like benzene, have molecules without these distinct poles.

• Polar solutes tend to be more soluble in polar solvents due to the "like dissolves like" principle.
• Similarly, non-polar solutes are more soluble in non-polar solvents because there are no intermolecular forces repelling them.
• Benzoic acid can interact with both types of solvents, but solubility is influenced largely by which part of the molecule dominates in the interaction.

Understanding this principle allows us to explain why benzoic acid is more soluble in benzene than in water.

Hydrogen Bonding

Hydrogen bonding is a vital concept when discussing solubility, especially for polar compounds like benzoic acid.
It occurs when hydrogen atoms are attracted to electronegative atoms such as oxygen or nitrogen in nearby molecules.

• In water, the presence of -COOH groups in benzoic acid allows for hydrogen bonding with water molecules.
• This results in increased solubility in water as these hydrogen bonds help dissolve the polar groups of benzoic acid.

However, despite hydrogen bonding, the overall solubility is limited due to the acid's non-polar phenyl group, showing the balanced effect of the molecule's polar and non-polar segments.

Dimer Formation

When in non-polar solvents like benzene, benzoic acid molecules can form dimers. These are pairs of molecules connected by interactions such as hydrogen bonds or van der Waals forces.
In benzene, the dimers formed from benzoic acid are predominantly through hydrogen bonds but in a way that suits the non-polar environment.

• These dimers are stable and dissolve more easily in benzene than separate molecules would.
• This dimer formation enhances the solubility of benzoic acid in benzene compared to polar solvents like water.

Dimerization is a crucial concept to grasp as it explains the unexpected high solubility of polar compounds in non-polar solvents.

Sodium Benzoate

Sodium benzoate is the sodium salt of benzoic acid. It forms when benzoic acid reacts with a base, such as sodium hydroxide (NaOH). The reaction is as follows:
\[ \text{HC}_7\text{H}_5\text{O}_2 + \text{NaOH} \rightarrow \text{C}_6\text{H}_5\text{COONa} + \text{H}_2\text{O} \]
The transformation from benzoic acid to sodium benzoate significantly alters its solubility properties. Sodium benzoate is much more soluble in water compared to benzoic acid.

• This is because sodium benzoate is ionic, and ions generally dissolve well in polar solvents like water.
• The overwhelming solubility of sodium benzoate in water compared to benzoic acid is because of the ionic interactions between the sodium ion and water molecules.

This transformation provides an excellent example of how chemical reactions can be used to modify solubility properties for practical applications.