91Ó°ÊÓ

A precipitation reaction occurs when two solutions are mixed, and an insoluble solid forms and settles out of the solution. In the exercise provided, this happens when sodium hydroxide (\( \text{NaOH} \)) is added to lead(II) nitrate (\( \text{Pb(NO}_3)_2 \)). The chemical reaction that takes place leads to the formation of lead(II) hydroxide (\( \text{Pb(OH)}_2 \)) as a precipitate, a solid that falls out of the liquid mixture.

• Sodium hydroxide (\(\text{NaOH}\)) and lead(II) nitrate (\(\text{Pb(NO}_3 ext{)}_2\)) react in an aqueous solution.
• The product of their reaction is the insoluble \(\text{Pb(OH)}_2\), which is the precipitate.
• An aqueous solution of sodium nitrate (\(\text{NaNO}_3\)) is also formed and remains dissolved in the solution.

The reaction is represented by the equation: \[\text{Pb(NO}_3\text{)}_2(aq) + 2\text{NaOH}(aq) \rightarrow \text{Pb(OH)}_2(s) + 2\text{NaNO}_3(aq)\]
In this context, the precipitate formation is a key example of a precipitation reaction, showing how reactants can create a solid byproduct in a liquid environment.

Complex ion formation involves the rearrangement and combination of ions into a new structure. In this situation, when extra sodium hydroxide is added, the lead(II) hydroxide precipitate interacts further. This leads to the formation of a soluble complex ion \([\text{Pb(OH)}_4]^{2-}\).

• Initially, \(\text{Pb(OH)}_2\) is a precipitate which is less soluble.
• Adding more hydroxide ions causes \(\text{Pb(OH)}_2\) to dissolve again.
• The dissolution forms a soluble complex ion \([\text{Pb(OH)}_4]^{2-}\), in which lead is surrounded by four hydroxide groups.

This step involves transforming an insoluble compound into a soluble complex, portraying how chemical equilibria can shift with varying conditions. The equation for this transformation is:\[\text{Pb(OH)}_2(s) + 2\text{OH}^-(aq) \rightarrow [\text{Pb(OH)}_4]^{2-}(aq)\]
Here, the initial ionic compound becomes a more complex and soluble version through the process of complex ion formation.

A balanced chemical equation is essential because it reflects the law of conservation of mass, indicating that atoms are neither created nor destroyed in a chemical reaction. In the context of the given exercises:

• The initial reaction balances the number of atoms by showing that two hydroxide ions react with one lead ion to create lead hydroxide.
• Both initial and secondary reactions respect this rule, maintaining a balance in terms of reactants and products, ensuring that each element present at the beginning of the reaction is accounted for at the end.
• For example, in the precipitation reaction, the reactants have two sodium ions, two nitrate ions, two hydroxide ions, and one lead ion balancing the products.

The equations given are:\[\text{Pb(NO}_3\text{)}_2(aq) + 2\text{NaOH}(aq) \rightarrow \text{Pb(OH)}_2(s) + 2\text{NaNO}_3(aq)\]and\[\text{Pb(OH)}_2(s) + 2\text{OH}^-(aq) \rightarrow [\text{Pb(OH)}_4]^{2-}(aq)\]
By ensuring that the chemical equations are balanced, it becomes possible to precisely track how each substance transforms during a reaction. Every balanced equation gives an insight into the fundamental changes taking place on a molecular level, ensuring proper representation of all species involved.