Question

Write a balanced chemical equation for each of the following double- replacement reactions: (a) Aqueous solutions of chromium(III) iodide and sodium hydroxide react to give a chromium(III) hydroxide precipitate and aqueous sodium iodide. (b) Aqueous solutions of nickel(II) sulfate and mercury(I) nitrate react to give a mercury(I) sulfate precipitate and aqueous nickel(II) nitrate.

Short answer

(a) \(\text{CrI}_3 (aq) + 3 \text{NaOH} (aq) \rightarrow \text{Cr(OH)}_3 (s) + 3 \text{NaI} (aq)\) (b) \(\text{NiSO}_4 (aq) + \text{Hg}_2(\text{NO}_3)_2 (aq) \rightarrow \text{Hg}_2\text{SO}_4 (s) + \text{Ni(NO}_3)_2 (aq)\)

Step-by-step solution

Write the Reactants for Reaction (a)

For reaction (a), the reactants are chromium(III) iodide \((\text{CrI}_3)\) and sodium hydroxide \((\text{NaOH})\). Both are in aqueous solution, so we represent them as \(\text{CrI}_3 (aq)\) and \(\text{NaOH} (aq)\).

Identify the Products for Reaction (a)

The products of this reaction are chromium(III) hydroxide \((\text{Cr(OH)}_3)\) as a precipitate and sodium iodide \((\text{NaI})\), which remains in aqueous form. This gives us \(\text{Cr(OH)}_3 (s)\) and \(\text{NaI} (aq)\).

Balance the Chemical Equation for Reaction (a)

The unbalanced equation is \(\text{CrI}_3 + \text{NaOH} \rightarrow \text{Cr(OH)}_3 + \text{NaI}\). To balance it, ensure the number of each type of atom on both sides is equal. The balanced equation is: \[ \text{CrI}_3 (aq) + 3 \text{NaOH} (aq) \rightarrow \text{Cr(OH)}_3 (s) + 3 \text{NaI} (aq) \].

Write the Reactants for Reaction (b)

For reaction (b), the reactants are nickel(II) sulfate \((\text{NiSO}_4)\) and mercury(I) nitrate \((\text{Hg}_2(\text{NO}_3)_2)\), both in aqueous form. Thus, we write \(\text{NiSO}_4 (aq)\) and \(\text{Hg}_2(\text{NO}_3)_2 (aq)\).

Identify the Products for Reaction (b)

The products of this reaction are mercury(I) sulfate \((\text{Hg}_2\text{SO}_4)\) which forms a precipitate, and nickel(II) nitrate \((\text{Ni(NO}_3)_2)\) which stays aqueous. We represent these as \(\text{Hg}_2\text{SO}_4 (s)\) and \(\text{Ni(NO}_3)_2 (aq)\).

Balance the Chemical Equation for Reaction (b)

The unbalanced equation is \(\text{NiSO}_4 + \text{Hg}_2(\text{NO}_3)_2 \rightarrow \text{Hg}_2\text{SO}_4 + \text{Ni(NO}_3)_2\). Balance it by ensuring equal numbers of each atom on both sides: \[ \text{NiSO}_4 (aq) + \text{Hg}_2(\text{NO}_3)_2 (aq) \rightarrow \text{Hg}_2\text{SO}_4 (s) + \text{Ni(NO}_3)_2 (aq) \].

Key concepts

Double Replacement Reactions

Double replacement reactions, sometimes called metathesis reactions, occur when parts of two compounds are exchanged to form two new compounds. These reactions are common in aqueous solutions where ions can freely move and interact.

Here's how it typically works:

• Two ionic compounds in solution are mixed.
• The cations (positive ions) and anions (negative ions) of the compounds switch places.
• This exchange forms two new products, one of which is often a precipitate (a solid that forms out of the solution), a gas, or a weak electrolyte.

To identify a double replacement reaction, look for:

• Two compounds as reactants.
• Formation of a solid precipitate, gas, or water in the products.

In our example exercises, we saw reactions such as those between aqueous chromium(III) iodide and sodium hydroxide, which resulted in chromium(III) hydroxide as a precipitate, illustrating this principle well.

Balancing Chemical Equations

Balancing chemical equations is crucial to represent chemical reactions accurately. It ensures that the same number of each type of atom is present on both sides of the equation, reflecting the law of conservation of mass.

To balance an equation:

• Write the unbalanced equation using correct formulas for all reactants and products.
• Count the number of atoms of each element in the reactants and products.
• Add coefficients to balance the atoms of each element on both sides of the equation.
• Ensure that the coefficients are in the simplest possible ratio.

Take, for example, the reaction between chromium(III) iodide and sodium hydroxide:- Start with the unbalanced equation \( \text{CrI}_3 + \text{NaOH} \rightarrow \text{Cr(OH)}_3 + \text{NaI} \).- By balancing it, we adjust coefficients to obtain: \( \text{CrI}_3 + 3 \text{NaOH} \rightarrow \text{Cr(OH)}_3 + 3 \text{NaI} \) This highlights how balancing equations ensures each kind of atom is conserved during the reaction.

Aqueous Solutions

Aqueous solutions play a pivotal role in chemical reactions, especially double replacement reactions. When a compound is dissolved in water, it dissociates into ions, which allows them to interact more easily and form new compounds.

Key points about aqueous solutions:

• They are solutions where water is the solvent, capable of dissolving various substances.
• Ionic compounds typically disintegrate into their constituent ions in an aqueous solution.
• These ions are responsible for conducting electricity, making aqueous solutions good conductors.

In our examples, compounds like chromium(III) iodide and sodium hydroxide are dissolved in water before reacting. This dissolution is crucial as it allows the ions to interact and form products like chromium(III) hydroxide precipitate, clearly illustrating how aqueous environments facilitate chemical interactions.