Problem 66 - Chemistry and Chemical Reactivity

Question

Identify and name the water-insoluble product in each reaction and write the net ionic equation: (a) \(\operatorname{CuCl}_{2}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{S}(\mathrm{aq}) \rightarrow \mathrm{CuS}+2 \mathrm{HCl}\) (b) \(\mathrm{CaCl}_{2}(\mathrm{aq})+\mathrm{K}_{2} \mathrm{CO}_{3}(\mathrm{aq}) \rightarrow 2 \mathrm{KCl}+\mathrm{CaCO}_{3}\) (c) \(\mathrm{AgNO}_{3}(\mathrm{aq})+\mathrm{NaI}(\mathrm{aq}) \rightarrow \mathrm{AgI}+\mathrm{NaNO}_{3}\)

Step-by-Step Solution

Answer

(a) CuS, (b) CaCO3, (c) AgI are the insoluble products; net ionic equations show precipitation formation.

Step 1: Rewriting Full Equations as Ionic Equations

For each reaction, write the ionic form by separating aqueous compounds into their respective ions.(a) \[\text{Cu}^{2+}(aq) + 2\text{Cl}^-(aq) + \text{H}_2\text{S}(aq) \rightarrow \text{CuS}(s) + 2\text{H}^+(aq) + 2\text{Cl}^-(aq)\](b) \[\text{Ca}^{2+}(aq) + 2\text{Cl}^-(aq) + 2\text{K}^+(aq) + \text{CO}_3^{2-}(aq) \rightarrow 2\text{K}^+(aq) + 2\text{Cl}^-(aq) + \text{CaCO}_3(s)\](c) \[\text{Ag}^+(aq) + \text{NO}_3^-(aq) + \text{Na}^+(aq) + \text{I}^-(aq) \rightarrow \text{AgI}(s) + \text{Na}^+(aq) + \text{NO}_3^-(aq)\]

Step 2: Identifying and Naming the Water-Insoluble Products

Recognize the solid precipitate formed (insoluble products) in each reaction.(a) The water-insoluble compound is \(\text{CuS}(s)\), Copper(II) sulfide.(b) The water-insoluble compound is \(\text{CaCO}_3(s)\), Calcium carbonate.(c) The water-insoluble compound is \(\text{AgI}(s)\), Silver iodide.

Step 3: Writing the Net Ionic Equations

In net ionic equations, only include the ions that participate in forming the precipitate.(a) \[\text{Cu}^{2+}(aq) + \text{S}^{2-}(aq) \rightarrow \text{CuS}(s)\](b) \[\text{Ca}^{2+}(aq) + \text{CO}_3^{2-}(aq) \rightarrow \text{CaCO}_3(s)\](c) \[\text{Ag}^+(aq) + \text{I}^-(aq) \rightarrow \text{AgI}(s)\]

Key Concepts

Precipitation Reactions

Precipitation reactions occur when two aqueous solutions combine and form an insoluble solid. This solid is known as a precipitate. When the ions in solution reach a concentration that exceeds the solubility product, the excess forms a solid. For example, when aqueous solutions of calcium chloride and potassium carbonate are mixed, calcium carbonate (\(CaCO_3\)) precipitates out of the solution.
Precipitation reactions are crucial for understanding the behavior of ionic compounds in solutions. These reactions are commonly used in chemistry to separate ions in a mixture. The formation of a precipitate signals that a chemical change has occurred, as new substances are formed from the original solutes.

• The reaction must result in a product that is insoluble in water.
• The insoluble product formed is often white or otherwise visibly different from the solution.
• These reactions help identify ions in analytical chemistry.

Recognizing precipitation reactions helps predict when a solution will form a solid upon mixing and how that process can be controlled in practical applications.

Water-Insoluble Compounds

Water-insoluble compounds are those that do not dissolve in water to a significant extent. These compounds often form in precipitation reactions when two soluble salts interact. Compounds like copper(II) sulfide (\(CuS\)), calcium carbonate (\(CaCO_3\)), and silver iodide (\(AgI\)) are examples of such insoluble compounds that can be identified easily.
The solubility of a compound depends largely on the nature of the ions involved. Insolubility is due to the strong ionic bonds in the lattice structure of the solid that water molecules are unable to break. Therefore, even in water, these compounds will remain as distinct solids rather than dissolving.
Understanding which compounds are insoluble in water is vital in various fields, including environmental science and medicine.

• Solubility rules provide a rough guide to predicting the solubility of various compounds.
• Compounds containing certain anions, like carbonates, phosphates, sulfides, are often insoluble except with alkali metals or ammonium cations.
• Precipitates can have industrial applications, like removing pollutants from water.

Recognizing water-insoluble compounds is key in predicting and explaining chemical behaviors and reactions in aqueous systems.

Ionic Equations

Ionic equations provide a detailed depiction of reactions, showing the actual species involved. These equations separate the reactants and products into their respective ionic forms when they are dissolved in water. Only the ions that take part in forming the insoluble compound (the precipitate) are included in the net ionic equation.
The process begins with the 'full ionic equation,' which shows all ions present in the solution. From this, the 'net ionic equation' is derived by removing spectator ions. Spectator ions are ions that do not participate in the formation of the precipitate and remain in solution as they are.
Understanding ionic equations is fundamental for mastering reactions in aqueous solutions:

• Shows which ions directly engage in forming the new chemical species.
• Helps chemists visualize and predict reaction processes.
• Facilitates balance and conservation of charge in chemical reactions.

Creating ionic equations allows chemists to focus on the chemical changes that occur, providing a simpler and clearer representation of complex reactions.