Question

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

Short answer

The ions that will always be spectator ions in a precipitation reaction are (b) Nitrate ion (NO₃⁻) and (c) Ammonium ion (NH₄⁺), as they form soluble salts with every cation and anion, respectively, causing no precipitate to form in the reactions they are involved in.

Step-by-step solution

Ion (a): Chloride Ion (Cl-)

The solubility rule tells us that chloride ions generally form soluble salts except silver(I), mercury(I), and lead(II) chloride. Due to these few exceptions, it may sometimes participate in precipitation reactions and, thus, cannot be always considered a spectator ion.

Ion (b): Nitrate Ion (NO₃-)

The solubility rule states that nitrate ions form soluble salts with every cation. This means no precipitate is formed when nitrate is present, indicating that the nitrate ion will always be a spectator ion in precipitation reactions.

Ion (c): Ammonium Ion (NH₄⁺)

The solubility rule informs us that ammonium ions form soluble salts with every anion, which means that ammonium salts will always be dissolved in a solution, and no precipitate is formed. So, the ammonium ion can always be considered a spectator ion in precipitation reactions.

Ion (d): Sulfide Ion (S²-)

The solubility rule tells us that sulfide ions form insoluble salts with many cations such as calcium, barium, lead, and others. This means that the sulfide ion will often participate in precipitation reactions, so it cannot always be considered a spectator ion.

Ion (e): Sulfate Ion (SO₄²-)

The solubility rule states that sulfate ions form soluble salts with most cations, except calcium, barium, lead, mercury(I), and silver(I). Due to these exceptions, the sulfate ion often participates in precipitation reactions and cannot always be considered a spectator ion. Based on the analysis of each ion, we can conclude that: The ions that will always be spectator ions in a precipitation reaction are: (b) Nitrate ion (NO₃⁻) and (c) Ammonium ion (NH₄⁺).

Key concepts

Solubility Rules

Understanding solubility rules is essential when identifying spectator ions in precipitation reactions. These rules, which are based on empirical observations, provide guidelines to predict whether a compound will dissolve in water, thus forming a soluble salt, or form a precipitate, an insoluble substance that separates from the solution. For students grappling with chemistry homework, a solid handle on these rules can simplify the process of solving complex reaction problems.

Soluble salts are those which remain disassociated in ionic form in aqueous solutions. Some ions, like the nitrate ion (o₃⁻) and the ammonium ion (h₄⁺), have the distinctive property of forming soluble salts with virtually any other ion, thereby indicating their roles as spectator ions—ions that do not participate in the actual chemical change but exist in the same form on both the reactant and product sides of a chemical equation. In contrast, ions like the chloride ion (o⁻), while often forming soluble salts, have notable exceptions and can sometimes be involved in the formation of a precipitate. Thus, solubility rules are the key to unlocking the mysteries of which substances will form a new product and which will merely observe the reaction unfold.

Nitrate Ion

The nitrate ion (o₃⁻) is a common anion found in various inorganic compounds. According to solubility rules, it is known for its high solubility in water. The rules state that all nitrate salts are soluble, meaning they can dissolve in water without forming a precipitate. This distinctive chemical behavior makes the nitrate ion a universal spectator ion in precipitation reactions.

For students seeing the term 'spectator ion' in their textbook exercises, it's important to recognize that these ions do not change their oxidation state or form any part of the precipitate. They are essentially the uninvolved bystanders of the reaction. The reliability of the nitrate ion as a spectator ion simplifies the prediction process in stoichiometry and can be a relief during complex problem-solving. Given its role in reactions, if you encounter a nitrate ion in a precipitation reaction, you can confidently assume it will not contribute to the formation of the precipitate, hence its designation as a spectator ion.

Ammonium Ion

Like the nitrate ion, the ammonium ion (h₄⁺) also plays a predictable role in precipitation reactions due to solubility rules. These rules clearly endorse that all salts containing ammonium ions are soluble in water. For learners, this is another piece of chemical trivia that can greatly expedite solving chemistry equations. As the ammonium ion does not participate in the formation of a precipitate, it will always be present in the solution as a spectator ion.

When tackling exercises in chemistry, understanding that the ammonium ion is always a spectator ion lets you streamline your approach to balancing equations and predicting outcomes of reactions. This non-reactive nature is specifically beneficial when identifying the components in a mixture that will remain unchanged throughout the reaction process. The presence of ammonium ions indicates no precipitate formation with this ion, simplifying the analysis of the reaction's net ionic equation.