Question

Write balanced net ionic equations for the reactions that occur in each of the following cases. Identify the spectator ion or ions in each reaction. (a) \(\mathrm{Ba}(\mathrm{OH})_{2}(a q)+\mathrm{FeCl}_{3}(a q) \longrightarrow\) (b) \(\mathrm{ZnCl}_{2}(a q)+\mathrm{Cs}_{2} \mathrm{CO}_{3}(a q) \longrightarrow\) (c) \(\mathrm{Na}_{2} \mathrm{~S}(a q)+\operatorname{CoSO}_{4}(a q) \longrightarrow\)

Short answer

(a) Spectator ions: \(\mathrm{Ba^{2+}}\), \(\mathrm{Cl^-}\); (b) Spectator ions: \(\mathrm{Cs^+}\), \(\mathrm{Cl^-}\); (c) Spectator ions: \(\mathrm{Na^+}\), \(\mathrm{SO_4^{2-}}\).

Step-by-step solution

Write Complete Ionic Equations

Start by breaking down each compound in the reactions into its constituent ions since they are in aqueous forms.(a) For \( \mathrm{Ba(OH)}_2(aq) + \mathrm{FeCl}_3(aq) \), the complete ionic equation is:\[ \mathrm{Ba^{2+}}(aq) + 2\mathrm{OH^-}(aq) + \mathrm{Fe^{3+}}(aq) + 3\mathrm{Cl^-}(aq) \longrightarrow \mathrm{Ba^{2+}}(aq) + \mathrm{Fe(OH)}_3(s) + 3\mathrm{Cl^-}(aq) \](b) For \( \mathrm{ZnCl}_2(aq) + \mathrm{Cs}_2 \mathrm{CO}_3(aq) \), the complete ionic equation is:\[ \mathrm{Zn^{2+}}(aq) + 2\mathrm{Cl^-}(aq) + 2\mathrm{Cs^+}(aq) + \mathrm{CO_3^{2-}}(aq) \longrightarrow \mathrm{ZnCO}_3(s) + 2\mathrm{Cs^+}(aq) + 2\mathrm{Cl^-}(aq) \](c) For \( \mathrm{Na}_2 \mathrm{S}(aq) + \mathrm{CoSO}_4(aq) \), the complete ionic equation is:\[ 2\mathrm{Na^+}(aq) + \mathrm{S^{2-}}(aq) + \mathrm{Co^{2+}}(aq) + \mathrm{SO}_4^{2-}(aq) \longrightarrow \mathrm{CoS}(s) + 2\mathrm{Na^+}(aq) + \mathrm{SO_4^{2-}}(aq) \]

Key concepts

Spectator Ions

Spectator ions are the ions that do not participate in the actual chemical reaction and remain unchanged in the solution. They appear on both sides of a complete ionic equation but do not affect the outcome of the reaction. When identifying spectator ions, it's important to check both the reactants and products in a reaction.

• For example, in the reaction between barium hydroxide \( \mathrm{Ba(OH)}_2 \text{(aq)} \) and iron(III) chloride \( \mathrm{FeCl}_3 \text{(aq)} \), the \( \mathrm{Ba^{2+}} \) and \( \mathrm{Cl^-} \) ions are spectators.
• Similarly, in the reaction between zinc chloride \( \mathrm{ZnCl}_2 \text{(aq)} \) and cesium carbonate \( \mathrm{Cs}_2 \mathrm{CO}_3 \text{(aq)} \), the \( \mathrm{Cs^+} \) and \( \mathrm{Cl^-} \) ions are spectators.
• In the reaction involving sodium sulfide \( \mathrm{Na}_2 \mathrm{S} \text{(aq)} \) and cobalt(II) sulfate \( \mathrm{CoSO}_4 \text{(aq)} \), the \( \mathrm{Na^+} \) and \( \mathrm{SO_4^{2-}} \) ions are spectators.

Recognizing spectator ions is crucial for simplifying reactions to their net ionic forms, which show only the species actually involved in the reaction.

Chemical Reactions

Chemical reactions are processes where substances, known as reactants, change into new substances, called products. An understanding of chemical reactions involves knowing the different types, such as synthesis, decomposition, single-displacement, and double-displacement reactions. In our given cases, we are primarily dealing with double-displacement reactions, where cations and anions switch partners to form new compounds.
In a double-displacement reaction, typically two ionic compounds in aqueous solution will exchange ions to create two new compounds. If one of these product compounds becomes insoluble in water, it will precipitate out as a solid. This is known as a precipitation reaction:

• For instance, when \( \mathrm{Ba(OH)}_2 \) and \( \mathrm{FeCl}_3 \) react, \( \mathrm{Fe(OH)}_3 \) forms and precipitates as a solid.
• With \( \mathrm{ZnCl}_2 \) and \( \mathrm{Cs}_2 \mathrm{CO}_3 \), \( \mathrm{ZnCO}_3 \) precipitates out of the solution.
• The reaction between \( \mathrm{Na}_2 \mathrm{S} \) and \( \mathrm{CoSO}_4 \) produces \( \mathrm{CoS} \) as a solid.

Understanding the formation of products and identifying precipitation can help predict the outcomes of chemical reactions.

Aqueous Solutions

An aqueous solution is a solution in which the solvent is water. It is denoted by the symbol \( \text{aq} \) next to the chemical formula and is a common context for many chemical reactions. In aqueous solutions, ionic compounds dissociate into free ions, facilitating reactions between these ions.
In our examples, the reactants are provided in aqueous forms, meaning they are dissolved in water, which allows them to interact more freely. For instance:

• \( \mathrm{Ba(OH)}_2 \text{(aq)} \) dissociates into \( \mathrm{Ba^{2+}} \) and \( \mathrm{OH^-} \) ions in water.
• Similarly, \( \mathrm{ZnCl}_2 \text{(aq)} \) breaks down into \( \mathrm{Zn^{2+}} \) and \( \mathrm{Cl^-} \) ions.
• The water-soluble \( \mathrm{Na}_2 \mathrm{S} \text{(aq)} \) dissociates into \( \mathrm{Na^{+}} \) and \( \mathrm{S^{2-}} \) ions.

This dispersion of ions is essential for ionic reactions and the formation of new compounds through ionic bonds. Thus, recognizing that a solution is aqueous helps in understanding how and why reactions proceed.

Balancing Equations

Balancing chemical equations is the process of ensuring that there are equal numbers of each type of atom on both sides of the equation. This reflects the law of conservation of mass, where matter cannot be created or destroyed in a chemical reaction.In practice, balancing involves adjusting the coefficients—the numbers in front of compounds or elements in a reaction:

• Examine each element in the reactants and match them with the products to maintain the same quantity.
• For the reactions given, it's crucial to confirm that each element appears with the same total counts on both sides after breaking them down into ions.
• For instance, in the decomposition of zinc carbonate in water, both \( \mathrm{Zn} \) and \( \mathrm{CO_3} \) groups balance between reactants and precipitate products.

This ensures that the net ionic equation, representing only those ions that participate in the formation of the product, accurately portrays the real substance conversion. Balancing is fundamental to understanding and predicting the results of chemical reactions.