A. Write a balanced equation for the reaction between benzoic acid and sodium bicarbonate (acting as a base). This will make it very soluble in water. B. Write a balanced equation for the reaction between sodium benzoate and HCl. Why does the organic product have a lowered solubility in water?
The Correct Answer and Explanation is:
A. Balanced Equation (Benzoic acid + Sodium bicarbonate):C6H5COOH+NaHCO3→C6H5COONa+H2O+CO2\text{C}_6\text{H}_5\text{COOH} + \text{NaHCO}_3 \rightarrow \text{C}_6\text{H}_5\text{COONa} + \text{H}_2\text{O} + \text{CO}_2C6H5COOH+NaHCO3→C6H5COONa+H2O+CO2
B. Balanced Equation (Sodium benzoate + Hydrochloric acid):C6H5COONa+HCl→C6H5COOH+NaCl\text{C}_6\text{H}_5\text{COONa} + \text{HCl} \rightarrow \text{C}_6\text{H}_5\text{COOH} + \text{NaCl}C6H5COONa+HCl→C6H5COOH+NaCl
Explanation:
Benzoic acid is a weak organic acid with limited solubility in water due to its nonpolar aromatic ring and only one polar carboxylic group. When it reacts with sodium bicarbonate, a weak base, it undergoes an acid-base reaction forming sodium benzoate, water, and carbon dioxide. Sodium benzoate is the salt of benzoic acid and possesses a negatively charged carboxylate group (−COO−-COO^-−COO−) which significantly increases its solubility in water. The ionic nature of this salt enhances interactions with polar water molecules, making it more soluble than the parent benzoic acid.
On the other hand, when sodium benzoate reacts with hydrochloric acid, a strong acid, it undergoes a reverse acid-base reaction. The hydrogen ion (H⁺) from HCl protonates the benzoate ion, converting it back to benzoic acid, which is neutral and less polar. As a result, the solubility of benzoic acid in water decreases. The aromatic ring and the reformation of the neutral carboxylic acid reduce its capacity to interact with polar solvents like water, making it more likely to precipitate out of solution.
This shift between soluble and insoluble forms is often used in organic chemistry to separate benzoic acid from mixtures or to purify it by precipitation. The change in solubility is primarily governed by the compound’s polarity and ionization state in aqueous environments.
