Question

If an aqueous solution of iron(III) sulfate (a compound used in dyeing textiles and also for etching aluminum) is mixed with a solution of barium chloride, a precipitate of barium sulfate forms and the solution that remains contains iron(III) chloride. Write the molecular, ionic, and net ionic equations for this reaction.

Short answer

Molecular Equation: Fe2(SO4)3 (aq) + 3BaCl2 (aq) → 3BaSO4 (s) + 2FeCl3 (aq). Ionic Equation: 2Fe^{3+} (aq) + 6SO4^{2-} (aq) + 3Ba^{2+} (aq) + 6Cl^- (aq) → 3BaSO4 (s) + 2Fe^{3+} (aq) + 6Cl^- (aq). Net Ionic Equation: 3Ba^{2+} (aq) + 3SO4^{2-} (aq) → 3BaSO4 (s).

Step-by-step solution

Write the Molecular Equation

First, it's important to write down the chemical formulas for each reactant and product. For iron(III) sulfate, the chemical formula is Fe2(SO4)3, and for barium chloride, it is BaCl2. When mixed, they produce a precipitate of barium sulfate (BaSO4) and an aqueous solution of iron(III) chloride (FeCl3). The balanced molecular equation is: Fe2(SO4)3 (aq) + 3BaCl2 (aq) → 3BaSO4 (s) + 2FeCl3 (aq).

Write the Ionic Equation

The ionic equation shows all of the ions that are present in the solution. First, break down all of the soluble compounds into their ions (the compounds that dissociate completely in water): 2Fe^{3+} (aq) + 6SO4^{2-} (aq) + 3Ba^{2+} (aq) + 6Cl^- (aq) → 3BaSO4 (s) + 2Fe^{3+} (aq) + 6Cl^- (aq).

Write the Net Ionic Equation

The net ionic equation includes only those compounds and ions that undergo a chemical change. Spectator ions (ions that do not participate in the precipitation reaction) should be omitted. Iron(III) and chloride ions are spectator ions in this reaction. The net ionic equation is: 3Ba^{2+} (aq) + 3SO4^{2-} (aq) → 3BaSO4 (s).

Key concepts

Molecular Equation

Understanding the molecular equation involves recognizing how reactants combine to form products in a chemical reaction. In the context of our textbook example, the reactants are iron(III) sulfate and barium chloride, while the products are barium sulfate and iron(III) chloride. A molecular equation provides a concise representation of the reaction, showcasing the compounds in their undissociated form.

It's crucial to ensure that the equation is balanced, meaning that the same number of each type of atom is present on both sides. For our exercise, the balanced molecular equation is:
\[ \text{Fe}_2(\text{SO}_4)_3 (\text{aq}) + 3\text{BaCl}_2 (\text{aq}) \rightarrow 3\text{BaSO}_4 (\text{s}) + 2\text{FeCl}_3 (\text{aq}) \]

This equation is essential because it conveys the stoichiometry of the reaction, instructing us on the proportion of reactants needed to form the specified amount of products.

Ionic Equation

The ionic equation, a step further from the molecular equation, provides deeper insight into the behavior of compounds that dissociate into ions in aqueous solutions. When we dissect our example's molecular equation into ions, we need to identify the soluble compounds (those that dissociate in water) and represent them as their constituent ions.

Ultimately, we generate the full ionic equation showing all the ions present during the reaction:
\[ 2\text{Fe}^{3+} (\text{aq}) + 6\text{SO}_4^{2-} (\text{aq}) + 3\text{Ba}^{2+} (\text{aq}) + 6\text{Cl}^- (\text{aq}) \rightarrow 3\text{BaSO}_4 (\text{s}) + 2\text{Fe}^{3+} (\text{aq}) + 6\text{Cl}^- (\text{aq}) \]

This expanded view of the reaction enables us to observe the role of each ion before and after the formation of the precipitate. It also sets the stage for identifying spectator ions, which are ions that remain unchanged and do not participate in the actual chemical reaction.

Net Ionic Equation

The net ionic equation strips down the ionic equation to its essence, spotlighting only the species that partake in the chemical change. In our precipitation reaction, the key players are the ions that combine to form the precipitate, namely barium and sulfate ions. The net ionic equation thus excludes spectator ions, which in this case are the iron(III) and chloride ions that remain unaltered in solution.

Our net ionic equation succinctly describes the formation of the precipitate:
\[ 3\text{Ba}^{2+} (\text{aq}) + 3\text{SO}_4^{2-} (\text{aq}) \rightarrow 3\text{BaSO}_4 (\text{s}) \]

This equation is the heart of the reaction and it helps in identifying the actual chemical events taking place, providing a streamlined narrative of the reaction that focuses solely on the formation of the solid precipitate.

Precipitation Reaction

A precipitation reaction occurs when certain ions in solution combine to form an insoluble compound, resulting in the formation of a solid that settles out of the solution, known as a precipitate. Our example features such a reaction, where barium ions and sulfate ions meet and bond to create barium sulfate, the precipitate.

Such reactions are vital in various industries, such as in waste-water treatment to remove unwanted ions, or in the aforementioned dyeing textiles and etching aluminum processes. Understanding the intricacies of precipitation reactions, not only conceptually but also by writing and balancing the associated equations, is key to manipulating and applying these reactions in practical scenarios.

To predict if a precipitation reaction will occur, one must be familiar with the solubility rules for ionic compounds in water. These rules are based on empirical data and can often foresee the formation of a precipitate when two aqueous solutions are combined.