Question

Write balanced complete ionic and net ionic equations for each reaction. (a) \(\mathrm{AgNO}_{3}(a q)+\mathrm{KCl}(a q) \longrightarrow \mathrm{AgCl}(s)+\mathrm{KNO}_{3}(a q)\) (b) \(\mathrm{CaS}(a q)+\mathrm{CuCl}_{2}(a q) \longrightarrow \mathrm{CuS}(s)+\mathrm{CaCl}_{2}(a q)\) (c) \(\mathrm{NaOH}(a q)+\mathrm{HNO}_{3}(a q) \longrightarrow \mathrm{H}_{2} \mathrm{O}(l)+\mathrm{NaNO}_{3}(a q)\) (d) \(2 \mathrm{~K}_{3} \mathrm{PO}_{4}(a q)+3 \mathrm{NiCl}_{2}(a q) \longrightarrow \mathrm{Ni}_{3}\left(\mathrm{PO}_{4}\right)_{2}(s)+6 \mathrm{KCl}(a q)\)

Short answer

The net ionic equations are: (a) \(\mathrm{Ag}^+ + \mathrm{Cl}^- \longrightarrow \mathrm{AgCl}(s)\), (b) \(\mathrm{Ca}^{2+} + \mathrm{CuS}(s) \longrightarrow \mathrm{CuS}(s) + \mathrm{Ca}^{2+}\), (c) \(\mathrm{OH}^- + \mathrm{H}^+ \longrightarrow \mathrm{H}_2\mathrm{O}(l)\), (d) \(3\mathrm{Ni}^{2+} + 2\mathrm{PO}_4^{3-} \longrightarrow \mathrm{Ni}_3(\mathrm{PO}_4)_2(s)\).

Step-by-step solution

- Write the complete ionic equations for each reaction

Break down all the aqueous compounds into their respective ions. Leave solids, liquids, and gases in their molecular form.

- Identify and remove spectator ions

Spectator ions are ions that appear on both sides of the reaction and do not participate in the chemical change. They can be canceled out.

- Write the net ionic equations

The net ionic equation only includes the ions and molecules directly involved in the chemical change, excluding the spectator ions.

- Balance the net ionic equations

Make sure that the number of atoms of each element and total charge is the same on both sides of the equation.

Key concepts

Balanced Chemical Equations

Understanding balanced chemical equations is essential for studying chemical reactions. A balanced equation ensures that the same number of atoms for each element is present on both sides of the equation, adhering to the law of conservation of mass. This means that matter is neither created nor destroyed during a chemical reaction.

In the context of our exercise, where we have several reactions, each equation needs to be balanced to accurately represent the reaction. To achieve this, coefficients are used in front of compounds to adjust the number of atoms. For instance, in reaction (d), the coefficient 2 in front of \(K_3PO_4\) and 3 in front of \(NiCl_2\) ensure that there are equal numbers of each atom on both sides of the reaction. This step is crucial before proceeding to write complete ionic or net ionic equations, as it provides a correct starting point.

Spectator Ions

Spectator ions are ions in an aqueous solution that do not partake in the actual chemical reaction. They essentially 'watch' the reaction happen without being changed or consumed. These ions are present in the same form on both the reactant and product sides of a full ionic equation.

In step 2 of our exercise solution, the identification and elimination of spectator ions are important because they simplify the equation to its essence, which is what happens during the net reaction. For example, in reaction (b), the \(Ca^{2+}\) and \(Cl^-\) ions remain unchanged and are present on both sides of the equation, thus considered spectator ions. Removing them helps to focus on the main event of the reaction, which in this case is the formation of \(CuS\) solid.

Aqueous Solution Reactions

Aqueous solution reactions involve substances that are dissolved in water to form ions. These reactions are commonplace in chemistry and can include precipitation reactions, acid-base neutralizations, and redox reactions.

In our textbook exercise, we deal with aqueous solutions where compounds like \(AgNO_3\) and \(KCl\) dissolve and dissociate into ions when mixed. When these ions interact, solid precipitates or new substances can form, as seen in reaction (a), where \(AgCl\) precipitates out of the solution. In step 1, we write complete ionic equations for the reactions to show all the ionic species involved. This process lays the groundwork for understanding how the compounds dissociate in water, which is a critical component of the chemical reaction. It's these changes that we want to focus on when determining the net ionic equation, as they represent the true chemical change occurring in the aqueous solution.