Which of the following ions will always be a spectator ion in a precipitation reaction? (a) \(\mathrm{Cl}^{-},(\mathrm{b}) \mathrm{NO}_{3}^{-},(\mathrm{c}) \mathrm{NH}_{4}^{+},(\mathrm{d}) \mathrm{S}^{2-},\) (e) \(\mathrm{SO}_{4}^{2-}\). Explain briefly. [Section \(\left.4.2\right]\)

A precipitation reaction is a type of chemical reaction in which two soluble salts in an aqueous solution form an insoluble solid called the precipitate. There are generally four types of ions present in the reaction: two cations (positive ions) and two anions (negative ions). One of the cation-anion pairs will form the precipitate, while the other pair will remain soluble in the solution as spectator ions.

We will individually check each given ion to see whether or not they will be a spectator ion in a precipitation reaction. (a) Cl⁻: Chloride ions (Cl⁻) can form soluble and insoluble salts. For example, silver chloride (AgCl) is insoluble, while sodium chloride (NaCl) is soluble. Therefore, Cl⁻ is not always a spectator ion. (b) NO₃⁻: Nitrate ions (NO₃⁻) form soluble salts with most cations. It is very rare for a nitrate salt to be insoluble. Therefore, NO₃⁻ is most likely to be a spectator ion in a precipitation reaction. (c) NH₄⁺: Ammonium ions (NH₄⁺) typically form soluble salts with most anions. It is very rare for an ammonium salt to be insoluble. Therefore, NH₄⁺ is most likely to be a spectator ion in a precipitation reaction. (d) S²⁻: Sulfide ions (S²⁻) can form both soluble and insoluble salts. For example, sodium sulfide (Na₂S) is soluble, while lead sulfide (PbS) is insoluble. Therefore, S²⁻ is not always a spectator ion. (e) SO₄²⁻: Sulfate ions (SO₄²⁻) can form soluble and insoluble salts depending on the cation it is paired with. For example, barium sulfate (BaSO₄) is insoluble, while sodium sulfate (Na₂SO₄) is soluble. Therefore, SO₄²⁻ is not always a spectator ion.

Based on our analysis in Step 2, we can conclude that ions (b) NO₃⁻ and (c) NH₄⁺ are the ions that will always be spectator ions in a precipitation reaction. This is because these ions almost always form soluble salts with other ions and do not participate in the formation of the precipitate.