Question

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

Short answer

In a precipitation reaction, NO₃⁻ (b) will always be a spectator ion since all nitrates are soluble and will not form any precipitates with any counter-ion.

Step-by-step solution

Analyzing the Solubility Rules

To find the spectator ion, let us analyze the solubility rules for each given ion: (a) Cl⁻: Most chloride salts are soluble, except for those containing Ag⁺, Pb²⁺, and Hg₂²⁺ ions. (b) NO₃⁻: All nitrates (NO₃⁻) are soluble. (c) NH₄⁺: All ammonium salts (NH₄⁺) are soluble. (d) S²⁻: Most sulfides are insoluble, except those of group 1 and 2 elements, and those of the ammonium ion (NH₄⁺). (e) SO₄²⁻: Most sulfates are soluble, except for those of Pb²⁺, Sr²⁺, Ba²⁺, and Ca²⁺.

Identifying the Always Soluble Ion

From the analysis of solubility rules, we can observe that: (a) Cl⁻ has a few exceptions in its solubility. (d) S²⁻ is mostly insoluble, with a few exceptions. (e) SO₄²⁻ has some exceptions in its solubility. However, (b) NO₃⁻ and (c) NH₄⁺ are soluble with no exceptions. Thus, these two ions will always be spectator ions as they will not form any precipitate with any ion. Since the question is asking for just one spectator ion, we must choose the best fit from these two options.

Choosing the Best Fit

Between NO₃⁻(b) and NH₄⁺(c), we will choose NO₃⁻ as the always-soluble spectator ion because: - As a monovalent anion, it has the least probability of forming insoluble salts. Thus, the correct answer is (b) NO₃⁻.

Key concepts

Solubility Rules

Understanding the solubility rules is essential for predicting the outcomes of chemical reactions, especially precipitation reactions.

Soluble substances can dissolve in a solvent, such as water, whereas insoluble substances do not dissolve significantly. These rules are guidelines that help determine whether a particular salt (ionic compound) will dissolve in water. Certain patterns exist, such as most sodium, potassium, and ammonium salts being soluble, irrespective of the accompanying anion.

Chlorides, bromides, and iodides are also generally soluble, with notable exceptions like salts of silver, lead, and mercury. Importantly, nitrates (O3 ) and acetates are always soluble, making them common spectator ions in precipitation reactions since they do not contribute to the formation of precipitates.

Conversely, carbonates, phosphates, and sulfides are largely insoluble, except when paired with highly soluble cations like those of group 1 elements. Understanding these rules aids in predicting whether a salt will remain in solution or form a precipitate.

Precipitate Formation

Precipitate formation occurs when two aqueous solutions, containing different ions, react to form an insoluble product. This is a hallmark of precipitation reactions, a type of chemical reaction important in various fields, from environmental science to medicine.

When the ions of two soluble salts interact, and one of the possible products is an insoluble salt, a precipitate will form. The insolubility arises due to the low solubility product (Ksp) of the compound, a measure of the product of the ionic concentrations at saturation.

Predicting Precipitation

To predict whether a precipitation reaction will occur, one must consider the solubility rules and the potential outcomes of ion exchange when solutions are mixed. For instance, when solutions of silver nitrate and sodium chloride are mixed, silver chloride, an insoluble salt, precipitates out of the solution.

It's also crucial to note that temperature can influence the solubility of salts, and therefore, the conditions under which a reaction takes place are important for predicting precipitate formation.

Soluble Salts

Soluble salts are the ionic compounds that readily dissolve in a solvent, forming a homogeneous mixture called a solution. Salts are composed of cations and anions, and when these ions interact with the solvent molecules, specifically water in many cases, the salt dissociates into its component ions.

The dissolved ions are uniformly distributed throughout the solution, which can conduct electricity due to the presence of these mobile charged particles. Solubility is not the same for all salts; it varies widely depending on the specific ions involved. Some salts, such as table salt (sodium chloride), dissolve easily in water because of their strong interactions with water molecules. Others, like calcium carbonate, are not very soluble due to their larger lattice energies, which require more energy to overcome than what water can provide.

The solubility of a salt is crucial in many fields, for example, in water treatment, where certain salts must be dissolved to treat hardness, or in medical applications where electrolyte solutions are needed for patient hydration. Moreover, the concept of soluble salts also plays a prominent role in understanding the ion-exchange processes, such as those used in water softeners and other filtration systems.