Question

4.51 The following reactions occur in aqueous solution. Complete and balance the molecular equations using phase labels. Then write the net ionic equations. $$ \begin{array}{l} (a)\mathrm{CaS}+\mathrm{HBr} \longrightarrow \\ \mathrm{(b)} \quad \mathrm{MgCO}_{3}+\mathrm{HNO}_{3} \longrightarrow \\ \mathrm{(c)} \quad \mathrm{K}_{2} \mathrm{SO}_{3}+\mathrm{H}_{2} \mathrm{SO}_{4} \longrightarrow \end{array} $$

Short answer

(a) CaS(s) + 2H^+(aq) → Ca^{2+}(aq) + H_2S(g); (b) MgCO_3(s) + 2H^+(aq) → Mg^{2+}(aq) + CO_2(g) + H_2O(l); (c) SO_3^{2-}(aq) + 2H^+(aq) → H_2SO_3(aq).

Step-by-step solution

Balance the Molecular Equations

First, write the complete molecular equation for each reaction and balance it. (a) Reaction: Calcium sulfide reacts with hydrobromic acid:\[ \text{CaS (s) + 2 HBr (aq)} \rightarrow \text{CaBr}_2 \text{(aq) + H}_2\text{S (g)} \](b) Reaction: Magnesium carbonate reacts with nitric acid:\[ \text{MgCO}_3 \text{(s) + 2 HNO}_3 \text{(aq)} \rightarrow \text{Mg(NO}_3\text{)}_2 \text{(aq) + CO}_2 \text{(g) + H}_2\text{O (l)} \](c) Reaction: Potassium sulfite reacts with sulfuric acid:\[ \text{K}_2\text{SO}_3 \text{(aq) + H}_2\text{SO}_4 \text{(aq)} \rightarrow \text{2 KHSO}_3 \text{(aq)} \]

Determine the Ionic Equations

Convert the balanced molecular equations to full ionic equations, showing all aqueous strong electrolytes as ions. (a) \[ \text{CaS (s) + 2 H}^+ \text{(aq) + 2 Br}^- \text{(aq)} \rightarrow \text{Ca}^{2+} \text{(aq) + 2 Br}^- \text{(aq) + H}_2\text{S (g)} \](b) \[ \text{MgCO}_3 \text{(s) + 2 H}^+ \text{(aq) + 2 NO}_3^- \text{(aq)} \rightarrow \text{Mg}^{2+} \text{(aq) + 2 NO}_3^- \text{(aq) + CO}_2 \text{(g) + H}_2\text{O (l)} \](c) \[ \text{2 K}^+ \text{(aq) + SO}_3^{2-} \text{(aq) + 2 H}^+ \text{(aq) + SO}_4^{2-} \text{(aq)} \rightarrow \text{2 K}^+ \text{(aq) + 2 HSO}_3^- \text{(aq)} \]

Write the Net Ionic Equations

Identify the spectator ions and remove them to write the net ionic equations, which show the actual chemical change.(a) The net ionic equation removes the spectator ions:\[ \text{CaS (s) + 2 H}^+ \text{(aq)} \rightarrow \text{Ca}^{2+} \text{(aq) + H}_2\text{S (g)} \](b) The net ionic equation removes the spectator ions:\[ \text{MgCO}_3 \text{(s) + 2 H}^+ \text{(aq)} \rightarrow \text{Mg}^{2+} \text{(aq) + CO}_2 \text{(g) + H}_2\text{O (l)} \](c) The net ionic equation removes the spectator ions:\[ \text{SO}_3^{2-} \text{(aq) + 2 H}^+ \text{(aq)} \rightarrow \text{H}_2\text{SO}_3 \text{(aq)} \]

Key concepts

Aqueous Solution Reactions

When substances dissolve in water to form an aqueous solution, they can interact in a chemical reaction. These reactions occur when the dissolved substances, known as solutes, interact with water and each other to form products. Aqueous solutions are crucial because many chemical reactions transpire in this medium, making it essential for processes in biological, environmental, and industrial fields.
Water, being a polar solvent, effectively separates ionic compounds into individual ions. For example, when table salt (NaCl) is added to water, it splits into Na⁺ and Cl^-. These ions can then participate in further reactions, leading to new products.

• Aqueous solutions provide an environment where ionic bonds break and new bonds form.
• Reactions in aqueous solutions include precipitation, acid-base, and redox reactions.
• Understanding how substances behave in water is key to predicting reaction outcomes.

Exploring how reactions occur in water is the first step to mastering chemistry in solution.

Molecular Equations

Molecular equations are written to show the complete chemical formulas of the reactants and products in a reaction. They present an overview of the reaction as it appears in nature. These equations include phase labels to denote the physical states of compounds, such as solid (s), liquid (l), gas (g), or aqueous (aq) for substances dissolved in water.
For instance, the reaction of calcium sulfide (CaS) with hydrobromic acid (HBr) in aqueous solution is expressed as a molecular equation:\[\text{CaS (s) + 2 HBr (aq)} \rightarrow \text{CaBr}_2 \text{(aq) + H}_2\text{S (g)}\]

• Molecular equations help visualize the reactants transforming into products.
• It is important to include correct coefficients to keep the equation balanced.
• The phase labels offer insight into the reaction's conditions and how substances interact.

These equations serve as an essential blueprint in understanding chemical reactions, offering a broad view before diving into more detailed ionic interactions.

Balancing Chemical Equations

Balancing chemical equations ensures the law of conservation of mass is upheld, meaning matter is neither created nor destroyed during a reaction. Each element must have the same number of atoms on both sides of the equation. This step is crucial because it provides the precise stoichiometric ratios needed for predicting the quantities of products formed from given reactants.
To balance a chemical equation:

• Identify all reactants and products.
• Count the number of each type of atom in reactants and products.
• Use coefficients to balance atoms, ensuring both sides reflect equal amounts.

For example, in the reaction between magnesium carbonate and nitric acid:\[\text{MgCO}_3 \text{(s) + 2 HNO}_3 \text{(aq)} \rightarrow \text{Mg(NO}_3\text{)}_2 \text{(aq) + CO}_2 \text{(g) + H}_2\text{O (l)}\]
Here, coefficients adjust to balance atom counts for magnesium, carbon, oxygen, hydrogen, and nitrogen. Balancing is a foundational skill in chemistry, laying the groundwork for more advanced topics like reaction rates and equilibrium.

Ionic and Net Ionic Equations

Ionic and net ionic equations zoom in on the actual chemical changes happening during a reaction in aqueous solutions. These equations break down soluble ionic compounds into their separate ions, highlighting the activities and interactions of ions.
In an ionic equation, all strong electrolytes are represented as ions. For instance, the reaction between calcium sulfide and hydrobromic acid can be expressed in ionic form:\[\text{CaS (s) + 2 H}^+ \text{(aq) + 2 Br}^- \text{(aq)} \rightarrow \text{Ca}^{2+} \text{(aq) + 2 Br}^- \text{(aq) + H}_2\text{S (g)}\]
A net ionic equation strips away the spectator ions, those not directly involved in the reaction, focusing solely on the ions that change:\[\text{CaS (s) + 2 H}^+ \text{(aq)} \rightarrow \text{Ca}^{2+} \text{(aq) + H}_2\text{S (g)}\]

• Ionic equations give insight into the role of ions during reactions.
• Net ionic equations clarify the core chemical transformation occurring.
• Understanding these concepts aids in predicting reaction behavior and outcomes.

By analyzing these types of equations, one gains a deeper understanding of the processes driving chemical reactions.