Problem 11 - Chemistry and Chemical Reactivity

Question

Predict the products of each precipitation reaction. Balance the completed equation, and then write the net ionic equation. (a) \(\mathrm{NiCl}_{2}(\mathrm{aq})+\left(\mathrm{NH}_{1}\right)_{2} \mathrm{S}(\mathrm{aq}) \longrightarrow ?\) (b) \(\operatorname{Mn}\left(\mathrm{NO}_{3}\right)_{2}(\mathrm{aq})+\mathrm{Na}_{3} \mathrm{PO}_{4}(\mathrm{aq}) \longrightarrow ?\)

Step-by-Step Solution

Step 1: Predict the Products for Reaction (a)

The reactants are \(\mathrm{NiCl}_{2}(\mathrm{aq})\) and \((\mathrm{NH}_{4})_{2}\mathrm{S}(\mathrm{aq})\). Nickel chloride and ammonium sulfide can exchange ions to form \(\mathrm{NiS}(\mathrm{s})\) and \(\mathrm{NH}_{4}\mathrm{Cl}(\mathrm{aq})\).

Key Concepts

Chemical Equations

Chemical equations are used to represent what happens during a chemical reaction. They show the reactants and the products formed. A reactant is a substance present before the reaction, while a product is formed as a result of the reaction.

An equation starts with the reactants on the left, an arrow pointing to the right, and the products on the right. For example, the reaction between nickel chloride and ammonium sulfide can be depicted as follows:

\[\mathrm{NiCl}_{2}(\mathrm{aq}) + (\mathrm{NH}_{4})_{2}\mathrm{S}(\mathrm{aq}) \longrightarrow \mathrm{NiS}(\mathrm{s}) + 2\mathrm{NH}_{4}\mathrm{Cl}(\mathrm{aq})\]

This equation conveys that nickel sulfide (\mathrm{NiS}(\mathrm{s})) is a solid, a precipitate formed from the reaction, and ammonium chloride (\mathrm{NH}_{4}\mathrm{Cl}(\mathrm{aq})) remains in solution. The state symbols such as \((\mathrm{aq})\) for aqueous and \((\mathrm{s})\) for solid are crucial for understanding the nature of each substance. This helps in visualizing and predicting the outcome of the reaction.

Net Ionic Equation

Net ionic equations provide a simplified version of chemical reactions by including only the ions and molecules directly involved in forming the precipitate. This allows a clearer view of the actual chemical change.

To write a net ionic equation, firstly, break down the aqueous substances into their constituent ions. Consider the full ionic equation for the reaction:

• \(\mathrm{Ni}^{2+}(\mathrm{aq}) + 2\mathrm{Cl}^{-}(\mathrm{aq}) + 2\mathrm{NH}_{4}^{+}(\mathrm{aq}) + \mathrm{S}^{2-}(\mathrm{aq}) \rightarrow \mathrm{NiS}(\mathrm{s}) + 2\mathrm{NH}_{4}^{+}(\mathrm{aq}) + 2\mathrm{Cl}^{-}(\mathrm{aq})\)

Notice that \(\mathrm{NH}_{4}^{+}\) and \(\mathrm{Cl}^{-}\) appear unchanged on both sides. These are spectator ions, which don’t participate directly in the formation of the precipitate.

Eliminating the spectator ions leaves:

• \(\mathrm{Ni}^{2+}(\mathrm{aq}) + \mathrm{S}^{2-}(\mathrm{aq}) \rightarrow \mathrm{NiS}(\mathrm{s})\)

This net ionic equation shows only the formation of the solid nickel sulfide where the ions combine.

Balancing Equations

Balancing equations ensures that the same number of each type of atom appears on both sides of a chemical equation according to the Law of Conservation of Mass. Each side of the equation must reflect the same total number of atoms for each element involved.

Take the unbalanced equation from our example again: \[\mathrm{NiCl}_{2}(\mathrm{aq}) + (\mathrm{NH}_{4})_{2}\mathrm{S}(\mathrm{aq}) \longrightarrow \mathrm{NiS}(\mathrm{s}) + \mathrm{NH}_{4}\mathrm{Cl}(\mathrm{aq})\]

Begin by identifying each element in the equation:
A nickel atom (\(\mathrm{Ni}\)) and a sulfur atom (\(\mathrm{S}\)) each appear once, and since they form a compound, they are balanced.

Now look at chlorine and ammonium ions:

• Initially, you have 2 chlorine (\(\mathrm{Cl}^{-}\)) ions with nickel chloride and 2 ammonium (\(\mathrm{NH}_4^{+}\)) ions per unit of ammonium sulfide.
• This will yield 2 formula units of \(\mathrm{NH}_{4}\mathrm{Cl}(\mathrm{aq})\) when all ions are involved, rendering the equation balanced: \[\mathrm{NiCl}_{2}(\mathrm{aq}) + (\mathrm{NH}_{4})_{2}\mathrm{S}(\mathrm{aq}) \longrightarrow \mathrm{NiS}(\mathrm{s}) + 2\mathrm{NH}_{4}\mathrm{Cl}(\mathrm{aq})\]

Balancing is a fundamental skill in chemistry that not only meets essential requirements but also aids in comprehending both reactants and products in a chemical reaction inventory.