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) \(S^{2-}\), (e) \(\mathrm{SO}_{4}^{2-}\). Explain briefly. [Section 4.2]

Short answer

The spectator ions in a precipitation reaction are (b) Nitrate ion (\(\mathrm{NO}_{3}^{-}\)), as it forms soluble compounds with all cations, and (c) Ammonium ion (\(\mathrm{NH}_{4}{ }^{+}\)), as it forms soluble compounds with all anions. This is because they don't change or participate in the reaction and are part of soluble compounds when mixed with other ions.

Step-by-step solution

Review solubility rules

According to solubility rules, the following are generally soluble: 1. All compounds of Group 1 elements (alkali metals) and ammonium ion (\(\mathrm{NH}_{4}{ }^{+}\)) are soluble. 2. All nitrates (\(\mathrm{NO}_{3}^{-}\)), acetates (\(\mathrm{CH}_{3}\mathrm{COO}^{-}\)), and chlorates (\(\mathrm{ClO}_{3}^{-}\)) are soluble. 3. All common halides (except those of silver, lead, and mercury) are soluble. Analyzing the ions given in the exercise: (a) Chloride ion (\(\mathrm{Cl}^{-}\)) forms soluble compounds with most cations except those of silver, lead, and mercury, so it might not always be a spectator ion. (b) Nitrate ion (\(\mathrm{NO}_{3}^{-}\)) forms soluble compounds with all cations. (c) Ammonium ion (\(\mathrm{NH}_{4}{ }^{+}\)) forms soluble compounds with all anions. (d) Sulfide ion (\(S^{2-}\)) forms insoluble compounds with most cations except those of Group 1 elements and ammonium ion. (e) Sulfate ion (\(\mathrm{SO}_{4}^{2-}\)) forms soluble compounds with most cations, except those of barium, calcium, and lead.

Identify the spectator ion

Based on our analysis, we can conclude that the spectator ions in a precipitation reaction are: (b) Nitrate ion (\(\mathrm{NO}_{3}^{-}\)) because it forms soluble compounds with all cations. (c) Ammonium ion (\(\mathrm{NH}_{4}{ }^{+}\)) because it forms soluble compounds with all anions. The ions \(\mathrm{Cl}^{-}\), \(S^{2-}\), and \(\mathrm{SO}_{4}^{2-}\) do not always form soluble compounds, so they are not always spectator ions.

Key concepts

Solubility Rules

Understanding the solubility rules is vital for predicting the outcomes of various chemical reactions, especially when dealing with ionic compounds in aqueous solutions. Solubility refers to the ability of a substance to dissolve in a solvent, and it plays a key role in precipitation reactions.

Solubility rules are guidelines that help predict whether an ionic compound will dissolve in water. These rules are not absolute, but they give a good approximation for most cases. Some of the key points include:

• Most compounds containing alkali metal ions (a^+, K^+, etc.) and the ammonium ion (NH_{4}^+) are soluble.
• Nitrates (NO_{3}^-), acetates (CH_3COO^-), and perchlorates (ClO_{4}^-) are also soluble, without exceptions.
• Common halides (chlorides, bromides, and iodides) are soluble, except when paired with lead (II), silver, or mercury (I).
• Most sulfates (SO_{4}^{2-}) are soluble, with the exception of those containing barium, strontium, calcium, and lead (II).

There are more detailed solubility rules, but these are the broad strokes that are useful in anticipating the formation of a precipitate. By knowing these rules, students can often predict the products of a reaction and whether a precipitation reaction will occur.

Precipitation Reaction Chemistry

A precipitation reaction is a type of chemical reaction that occurs when two soluble ionic compounds in solution are combined and result in the formation of an insoluble compound, or precipitate. This process can be visually recognized as the formation of a solid within the previously clear solution.

To determine if a precipitation reaction will occur, it is important to look at the solubility of the possible products that can be formed from the reactant ions. When the product of a reaction is insoluble, or less soluble than the reactants, it will fall out of the solution as a solid. The key in such reactions is to apply the solubility rules to figure out the solubility of the products. Moreover, understanding these reactions is crucial as they are used in various applications, from water purification to qualitative analysis in laboratories.

To represent these reactions, chemists use ionic equations. In such equations, substances that are strong electrolytes (such as soluble ionic compounds) are written as individual ions. The substances that remain in their molecular form are the ones that are insoluble and form the precipitate. An interesting aspect of these reactions is the presence of spectator ions which do not participate in the reaction and remain in the solution in their ionic form.

Ionic Compounds Solubility

The solubility of ionic compounds is a measure of how well an ionic substance can be dissolved in a solvent, such as water. The degree of solubility can vary greatly; some compounds dissociate completely in water, becoming completely solvated by water molecules, while others have very limited solubility, which can lead to the formation of a precipitate during a reaction.

Solubility is influenced by various factors such as temperature, the nature of the solvent and solute, and the presence of other ions in solution. In the context of precipitation reactions, the solubility of ionic compounds is critical because it determines whether a substance will remain in the aqueous phase or form a solid precipitate. Understanding the solubility of ionic compounds also helps chemists control the direction and extent of a chemical reaction.

As an important practical application, the solubility principles are utilized in pharmaceuticals to control the rate at which medication is released into the body and in environmental science to understand the mobility and bioavailability of minerals and pollutants.