Show the reaction between NaOH and benzoic acid, which produced the benzoate ion. Show the reaction between HCl and the benzoate ion, to produce the benzoic acid.
The Correct Answer and Explanation is:
Reaction Between NaOH and Benzoic Acid:
Equation:C6H5COOH+NaOH→C6H5COO− Na++H2O\text{C}_6\text{H}_5\text{COOH} + \text{NaOH} \rightarrow \text{C}_6\text{H}_5\text{COO}^- \, \text{Na}^+ + \text{H}_2\text{O}C6H5COOH+NaOH→C6H5COO−Na++H2O
Benzoic acid reacts with sodium hydroxide to form sodium benzoate (the benzoate ion and a sodium cation) and water.
Reaction Between HCl and the Benzoate Ion:
Equation:C6H5COO−+HCl→C6H5COOH+Cl−\text{C}_6\text{H}_5\text{COO}^- + \text{HCl} \rightarrow \text{C}_6\text{H}_5\text{COOH} + \text{Cl}^-C6H5COO−+HCl→C6H5COOH+Cl−
The benzoate ion reacts with hydrochloric acid to regenerate benzoic acid and form chloride ions.
Explanation (300 Words Style):
Benzoic acid (C₆H₅COOH) is a weak organic acid containing a carboxylic acid functional group attached to a benzene ring. When it reacts with a strong base such as sodium hydroxide (NaOH), it undergoes a neutralization reaction. In this reaction, the hydroxide ion (OH⁻) from NaOH deprotonates the carboxylic acid group (–COOH) of benzoic acid. This removes a hydrogen ion (H⁺) from the acid, forming water (H₂O) and the conjugate base of benzoic acid, which is the benzoate ion (C₆H₅COO⁻). The sodium ion (Na⁺) from NaOH then pairs with the benzoate ion to form the salt sodium benzoate.
This process is a typical acid–base reaction: the acid (benzoic acid) donates a proton to the base (hydroxide ion), resulting in the formation of a salt and water. The benzoate ion formed is stabilized by resonance across the carboxylate group, making the reaction favorable.
Conversely, if hydrochloric acid (HCl), a strong acid, is added to a solution containing benzoate ions, a reverse reaction occurs. The benzoate ion acts as a base and accepts a proton (H⁺) from HCl to regenerate benzoic acid. This is an example of a conjugate acid-base pair. The benzoate ion (C₆H₅COO⁻) is the conjugate base of benzoic acid, and when it accepts a proton, the original weak acid is formed. Meanwhile, the chloride ion (Cl⁻) remains in solution as a spectator ion.
These reactions demonstrate key acid–base principles and the concept of reversible proton transfer between conjugate acid–base pairs, important in buffer systems and chemical equilibria.
