Problem 74 - Chemistry and Chemical Reactivity

Question

Identify and name the water-insoluble product in each reaction and write the net ionic equation: (a) \(\mathrm{CuCl}_{2}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{S}(\mathrm{aq}) \rightarrow \mathrm{CuS}+2 \mathrm{HCl}\) (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) Insoluble: CuS, Net Ionic: \( \mathrm{Cu}^{2+} + \mathrm{H}_2\mathrm{S} \rightarrow \mathrm{CuS} + 2\mathrm{H}^+ \); (b) Insoluble: CaCO₃, Net Ionic: \( \mathrm{Ca}^{2+} + \mathrm{CO}_3^{2-} \rightarrow \mathrm{CaCO}_{3} \); (c) Insoluble: AgI, Net Ionic: \( \mathrm{Ag}^+ + \mathrm{I}^- \rightarrow \mathrm{AgI} \)

Step 1: Identify the water-insoluble product for reaction (a)

In the reaction \( \mathrm{CuCl}_{2}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{S}(\mathrm{aq}) \rightarrow \mathrm{CuS}+2 \mathrm{HCl} \), the possible products are \( \mathrm{CuS} \) and \( \mathrm{HCl} \). From solubility rules, sulfides of most metals, including copper (Cu), are insoluble in water, whereas \( \mathrm{HCl} \) is soluble. Thus, the water-insoluble product is \( \mathrm{CuS} \).

Step 2: Write the net ionic equation for reaction (a)

Start with the complete ionic equation: \[ \mathrm{Cu}^{2+}(\mathrm{aq}) + 2\mathrm{Cl}^-(\mathrm{aq}) + \mathrm{H}_2\mathrm{S}(\mathrm{aq}) \rightarrow \mathrm{CuS}(\mathrm{s}) + 2\mathrm{H}^+(\mathrm{aq}) + 2\mathrm{Cl}^-(\mathrm{aq}) \]Cancel out spectator ions (\( \mathrm{Cl}^- \)) to get the net ionic equation: \[ \mathrm{Cu}^{2+}(\mathrm{aq}) + \mathrm{H}_2\mathrm{S}(\mathrm{aq}) \rightarrow \mathrm{CuS}(\mathrm{s}) + 2\mathrm{H}^+(\mathrm{aq}) \]

Step 3: Identify the water-insoluble product for reaction (b)

In the reaction \( \mathrm{CaCl}_{2}(\mathrm{aq})+\mathrm{K}_{2} \mathrm{CO}_{3}(\mathrm{aq}) \rightarrow 2 \mathrm{KCl}+\mathrm{CaCO}_{3} \), the potential products are \( \mathrm{KCl} \) and \( \mathrm{CaCO}_{3} \). According to solubility rules, carbonates are generally insoluble except for those of alkali metals and ammonium. Thus, \( \mathrm{CaCO}_{3} \) is water-insoluble.

Step 4: Write the net ionic equation for reaction (b)

Write the complete ionic equation first: \[ \mathrm{Ca}^{2+}(\mathrm{aq}) + 2\mathrm{Cl}^-(\mathrm{aq}) + 2\mathrm{K}^+(\mathrm{aq}) + \mathrm{CO}_3^{2-}(\mathrm{aq}) \rightarrow 2\mathrm{K}^+(\mathrm{aq}) + 2\mathrm{Cl}^- (\mathrm{aq}) + \mathrm{CaCO}_{3}(\mathrm{s}) \]Cancelling the spectator ions (\( \mathrm{K}^+ \) and \( \mathrm{Cl}^- \)), the net ionic equation is: \[ \mathrm{Ca}^{2+}(\mathrm{aq}) + \mathrm{CO}_3^{2-}(\mathrm{aq}) \rightarrow \mathrm{CaCO}_{3}(\mathrm{s}) \]

Step 5: Identify the water-insoluble product for reaction (c)

For the reaction \( \mathrm{AgNO}_{3}(\mathrm{aq})+\mathrm{NaI}(\mathrm{aq}) \rightarrow \mathrm{AgI}+\mathrm{NaNO}_{3} \), the possible products are \( \mathrm{AgI} \) and \( \mathrm{NaNO}_{3} \). According to solubility guidelines, most iodides are soluble except those of silver, lead, and mercury. Consequently, \( \mathrm{AgI} \) is insoluble.

Step 6: Write the net ionic equation for reaction (c)

Convert the equation to its complete ionic form: \[ \mathrm{Ag}^+(\mathrm{aq}) + \mathrm{NO}_3^-(\mathrm{aq}) + \mathrm{Na}^+(\mathrm{aq}) + \mathrm{I}^-(\mathrm{aq}) \rightarrow \mathrm{AgI}(\mathrm{s}) + \mathrm{Na}^+(\mathrm{aq}) + \mathrm{NO}_3^-(\mathrm{aq}) \]Remove the spectator ions (\( \mathrm{Na}^+ \) and \( \mathrm{NO}_3^- \)) to get the net ionic equation: \[ \mathrm{Ag}^+(\mathrm{aq}) + \mathrm{I}^-(\mathrm{aq}) \rightarrow \mathrm{AgI}(\mathrm{s}) \]

Key Concepts

Water-insoluble products

In chemical reactions, some products do not dissolve in water. These are known as water-insoluble products. Identifying these is crucial because they often form a solid precipitate, marking the completion of a reaction. In the reactions provided:

• The reaction of \( \mathrm{CuCl}_{2}(\mathrm{aq}) + \mathrm{H}_{2} \mathrm{S}(\mathrm{aq}) \) produces \( \mathrm{CuS} \), which is a water-insoluble product.
• The reaction \( \mathrm{CaCl}_{2}(\mathrm{aq}) + \mathrm{K}_{2} \mathrm{CO}_{3}(\mathrm{aq}) \) results in \( \mathrm{CaCO}_{3} \) as the insoluble product.
• The reaction \( \mathrm{AgNO}_{3}(\mathrm{aq}) + \mathrm{NaI}(\mathrm{aq}) \) yields \( \mathrm{AgI} \), another insoluble product.

By identifying these products, you can understand where a chemical reaction will create a precipitate.

Solubility rules

Solubility rules are guidelines that help predict whether specific ionic compounds dissolve in water. This is an essential concept when writing net ionic equations. Here are key rules:

• Most nitrate (\( \mathrm{NO}_3^- \)) salts are soluble.
• Compounds containing alkali metal ions (\( \mathrm{Li}^+, \mathrm{Na}^+, \mathrm{K}^+, \mathrm{Cs}^+, \mathrm{Rb}^+ \)) and ammonium ion (\( \mathrm{NH}_4^+ \)) are soluble.
• Chlorides, bromides, and iodides are generally soluble, except those of \( \mathrm{Ag}^+, \mathrm{Pb}^{2+}, \mathrm{Hg}_2^{2+} \).
• Sulfides and carbonates are generally insoluble, with exceptions for alkali metals and \( \mathrm{NH}_4^+ \).

Applying these rules helps to quickly determine whether the product of a reaction will stay in solution or form a precipitate.

Chemical equations

Chemical equations are symbolic representations of chemical reactions. They show the reactants turning into products. A complete balanced chemical equation includes all reactants and products and respects the law of conservation of mass. For instance:

• For \( \mathrm{CuCl}_2 \) and \( \mathrm{H}_2\mathrm{S} \), the balanced chemical equation is \( \mathrm{CuCl}_{2} + \mathrm{H}_{2} \mathrm{S} \rightarrow \mathrm{CuS} + 2 \mathrm{HCl} \).
• For \( \mathrm{CaCl}_2 \) and \( \mathrm{K}_2\mathrm{CO}_3 \), it is \( \mathrm{CaCl}_{2} + \mathrm{K}_{2} \mathrm{CO}_{3} \rightarrow 2 \mathrm{KCl} + \mathrm{CaCO}_{3} \).

By expressing chemical reactions in equations, you can understand the transformation and stoichiometry of the involved substances. Chemical equations can also be simplified to net ionic equations, focusing only on the species that engage in the reaction.

Spectator ions

In ionic reactions, spectator ions are ions that do not participate in the chemical change. They remain in the solution unchanged. Identifying these ions is crucial when writing net ionic equations as they are omitted to simplify the equation to its most direct form.

• For the reaction \( \mathrm{CuCl}_{2}(\mathrm{aq}) + \mathrm{H}_{2} \mathrm{S}(\mathrm{aq}) \), chloride ions (\( \mathrm{Cl}^- \)) are the spectators.
• In the reaction \( \mathrm{CaCl}_{2}(\mathrm{aq}) + \mathrm{K}_{2} \mathrm{CO}_{3}(\mathrm{aq}) \), both potassium ions (\( \mathrm{K}^+ \)) and chloride ions (\( \mathrm{Cl}^- \)) are spectators.
• For \( \mathrm{AgNO}_{3}(\mathrm{aq}) + \mathrm{NaI}(\mathrm{aq}) \), sodium (\( \mathrm{Na}^+ \)) and nitrate (\( \mathrm{NO}_3^- \)) ions are spectators.

Omitting these ions from the net ionic equation focuses on the chemical species that actually undergo a change. This simplification highlights the core transformation in the reaction.