Question

For each of the following precipitation reactions, complete and balance the equation, indicating clearly which product is the precipitate. a. \(\mathrm{Ba}\left(\mathrm{NO}_{3}\right)_{2}(a q)+\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}(a q) \rightarrow\) b. \(\operatorname{CoCl}_{3}(a q)+\operatorname{NaOH}(a q) \rightarrow\) c. \(\operatorname{FeCl}_{3}(a q)+\left(\mathrm{NH}_{4}\right)_{2} \mathrm{S}(a q) \rightarrow\)

Short answer

a. \(\mathrm{Ba}\left(\mathrm{NO}_{3}\right)_{2}(a q)+\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}(a q) \rightarrow \mathrm{BaSO}_{4}(s)+2(\mathrm{NH}_{4}\mathrm{NO}_{3})(a q)\) b. \(\operatorname{CoCl}_{3}(a q)+3\operatorname{NaOH}(a q) \rightarrow \operatorname{Co(OH)_{3}}(s)+3\operatorname{NaCl}(a q)\) c. \(\operatorname{FeCl}_{3}(a q)+3\left(\mathrm{NH}_{4}\right)_{2} \mathrm{S}(a q) \rightarrow 2\operatorname{FeS}(s)+6\mathrm{NH}_{4} \operatorname{Cl}(a q)\)

Step-by-step solution

Identify Reaction Components

In this reaction, the components are \(\mathrm{Ba}\left(\mathrm{NO}_{3}\right)_{2}\) and \(\left(\mathrm{NH}_{4}\right)_{2}\mathrm{SO}_{4}\).

Exchange Ions in the Reaction

By exchanging the ions, we get Barium sulfate (BaSO4) and Ammonium nitrate (\((\mathrm{NH}_{4})_{2}\mathrm{NO}_{3}\)).

Identify the Precipitate

Barium sulfate (BaSO4) forms a solid precipitate, while Ammonium nitrate remains aqueous.

Complete and Balance the Equation

The balanced equation is: \(\mathrm{Ba}\left(\mathrm{NO}_{3}\right)_{2}(a q)+\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}(a q) \rightarrow \mathrm{BaSO}_{4}(s)+2(\mathrm{NH}_{4}\mathrm{NO}_{3})(a q)\). b. \(\operatorname{CoCl}_{3}(a q)+\operatorname{NaOH}(a q) \rightarrow\)

Identify Reaction Components

In this reaction, the components are \(\operatorname{CoCl}_{3}\) and \(\operatorname{NaOH}\).

Exchange Ions in the Reaction

By exchanging the ions, we get Cobalt hydroxide (Co(OH)3) and Sodium chloride (NaCl).

Identify the Precipitate

Cobalt hydroxide (Co(OH)3) forms a solid precipitate, while Sodium chloride remains aqueous.

Complete and Balance the Equation

The balanced equation is: \(\operatorname{CoCl}_{3}(a q)+3\operatorname{NaOH}(a q) \rightarrow \operatorname{Co(OH)_{3}}(s)+3\operatorname{NaCl}(a q)\). c. \(\operatorname{FeCl}_{3}(a q)+\left(\mathrm{NH}_{4}\right)_{2} \mathrm{S}(a q) \rightarrow\)

Identify Reaction Components

In this reaction, the components are \(\operatorname{FeCl}_{3}\) and \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{S}\).

Exchange Ions in the Reaction

By exchanging the ions, we get Iron sulfide (FeS) and Ammonium chloride (\(\mathrm{NH}_{4} \operatorname{Cl}\)).

Identify the Precipitate

Iron sulfide (FeS) forms a solid precipitate, while Ammonium chloride remains aqueous.

Complete and Balance the Equation

The balanced equation is: \(\operatorname{FeCl}_{3}(a q)+3\left(\mathrm{NH}_{4}\right)_{2} \mathrm{S}(a q) \rightarrow 2\operatorname{FeS}(s)+6\mathrm{NH}_{4} \operatorname{Cl}(a q)\).

Key concepts

Chemical Equations

In chemistry, chemical equations are essential tools that describe chemical reactions. A chemical equation represents the substances involved in a chemical reaction: the reactants (starting materials) and the products (new substances formed). Chemical reactions often occur in the context of a changing environment, where atoms are rearranged. The general form of a chemical equation is:

\[ \text{Reactants} \rightarrow \text{Products} \]
In a precipitation reaction, soluble reactants in an aqueous solution react to form at least one insoluble product, which separates out of the mixture as a solid known as a precipitate. For example, when barium nitrate \(\text{Ba(NO}_3\text{)}_2\) reacts with ammonium sulfate \(\text{(NH}_4)_2\text{SO}_4\), a solid product \(\text{BaSO}_4\) is formed. The equation reflects this as follows:

\[ \text{Ba(NO}_3\text{)}_2(aq) + \text{(NH}_4\text{)}_2\text{SO}_4(aq) \rightarrow \text{BaSO}_4(s) + 2\text{(NH}_4\text{NO}_3\text{)}(aq) \]

Balancing Equations

Balancing chemical equations is a critical skill in chemistry. It ensures that the same number of each type of atom appears on both sides of the equation. This principle is based on the Law of Conservation of Mass, which states that mass cannot be created or destroyed in a chemical reaction. Hence, atoms must be accounted for in the final products as they were in the initial reactants.

To balance an equation:

• Write the unbalanced equation.
• Count the number of atoms of each element on both sides of the equation.
• Add coefficients (numbers in front of molecules) to balance the atoms.

For example, in the reaction between cobalt chloride \(\text{CoCl}_3\) and sodium hydroxide \(\text{NaOH}\), the equation should be balanced as:

\[ \text{CoCl}_3(aq) + 3\text{NaOH}(aq) \rightarrow \text{Co(OH)}_3(s) + 3\text{NaCl}(aq) \]
Here, adding the coefficient 3 in front of \(\text{NaOH}\) and \(\text{NaCl}\) balances the equation by ensuring equal numbers of sodium and chlorine atoms on both sides.

Precipitate Formation

Precipitates are solid products that form when certain ions in an aqueous solution react and are insoluble. Precipitate formation is a key aspect of precipitation reactions. The formation of a precipitate indicates a chemical change and can be used to drive a reaction to completion.

In a chemical equation, a precipitate is indicated by the state symbol \((s)\) for a solid following the compound in the equation. For example, in the reaction:
\[ \text{FeCl}_3(aq) + 3\text{(NH}_4\text{)}_2\text{S}(aq) \rightarrow 2\text{FeS(s)} + 6\text{NH}_4\text{Cl}(aq) \]
Iron sulfide \(\text{FeS}\) is the precipitate, and it separates from the other aqueous components as a solid.

Solubility Rules

Solubility rules help predict whether a precipitate will form when two ionic solutions are mixed. These rules provide guidelines on which ionic compounds are likely to dissolve in water and which are not. Knowing the solubility of substances helps chemists anticipate the outcome of mixing different solutions.

General solubility guidelines include:

• Most nitrates \((\text{NO}_3^-)\), acetates \((\text{C}_2\text{H}_3\text{O}_2^-)\), and chlorates \((\text{ClO}_3^-)\) are soluble.
• Most salts containing Group 1 metal ions \((\text{Li}^+, \text{Na}^+, \text{K}^+)\) and ammonium \((\text{NH}_4^+)\) are soluble.
• Sulfates \((\text{SO}_4^{2-})\) are generally soluble, but exceptions include \(\text{BaSO}_4\), the precipitate in our example reaction, as well as \(\text{PbSO}_4\) and \(\text{CaSO}_4\).

Applying these rules helps determine products' solubility and identify precipitates in reactions.