Write two complete chemical reactions, one that describes the solid p-Toluic acid becoming soluble, and the other that describes the soluble compound becoming insoluble once again.
The correct answer and explanation is:
Here are the two chemical reactions describing the solubility change of p-Toluic acid:
Reaction 1: Solid p-Toluic acid becomes soluble
CH₃C₆H₄COOH(s) + NaOH(aq) → CH₃C₆H₄COONa(aq) + H₂O(l)
(Solid p-Toluic acid reacts with aqueous sodium hydroxide to form aqueous sodium p-toluate and liquid water)
Reaction 2: Soluble compound becomes insoluble again
CH₃C₆H₄COONa(aq) + HCl(aq) → CH₃C₆H₄COOH(s) + NaCl(aq)
(Aqueous sodium p-toluate reacts with aqueous hydrochloric acid to form solid p-Toluic acid and aqueous sodium chloride)
Explanation:
p-Toluic acid is an organic compound classified as a carboxylic acid. In its neutral form, it is a solid at room temperature and is only sparingly soluble in water. This limited solubility is because while the carboxylic acid group (-COOH) can form hydrogen bonds with water, the large, non-polar aromatic ring (the benzene ring with a methyl group) disrupts the hydrogen-bonding network of water, making dissolution energetically unfavorable.
To make p-Toluic acid soluble in water, we utilize its acidic property through an acid-base reaction. As shown in Reaction 1, when solid p-Toluic acid is treated with a strong base like sodium hydroxide (NaOH) in water, it undergoes deprotonation. The acidic hydrogen from the -COOH group is removed, forming the carboxylate anion (CH₃C₆H₄COO⁻). This anion combines with the sodium cation (Na⁺) from the base to form the salt, sodium p-toluate (CH₃C₆H₄COONa). Unlike the neutral p-Toluic acid molecule, this salt is an ionic compound. Ionic compounds are generally very soluble in polar solvents like water because the charged ions are strongly attracted to the partial charges of the water molecules via strong ion-dipole interactions, effectively pulling the salt into solution. Thus, the initially solid p-Toluic acid is converted into a soluble species in the aqueous phase.
To revert the compound back to its insoluble state, we reverse the acid-base reaction, as shown in Reaction 2. By adding a strong acid, such as hydrochloric acid (HCl), to the aqueous solution of sodium p-toluate, we introduce a high concentration of hydrogen ions (H⁺). These protons react with the soluble carboxylate anions (CH₃C₆H₄COO⁻), adding a proton back to reform the neutral carboxylic acid, p-Toluic acid (CH₃C₆H₄COOH). Since p-Toluic acid is poorly soluble in water in its neutral form, especially in the presence of the dissolved salt byproduct (NaCl), it precipitates out of the solution as a solid, effecti