Problem 44 - Chemistry and Chemical Reactivity

Question

Balance the following equations, and then write the net ionic equation: (a) \(\mathrm{Zn}(\mathrm{s})+\mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{H}_{2}(\mathrm{g})+\mathrm{ZnCl}_{2}(\mathrm{aq})\) (b) \(\mathrm{Mg}(\mathrm{OH})_{2}(\mathrm{s})+\mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{MgCl}_{2}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\ell)\) (c) \(\mathrm{HNO}_{3}(\mathrm{aq})+\mathrm{CaCO}_{3}(\mathrm{s}) \rightarrow \mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\ell)+\mathrm{CO}_{2}(\mathrm{g})\) (d) \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{S}(\mathrm{aq})+\mathrm{FeCl}_{2}(\mathrm{aq}) \rightarrow \mathrm{NH}_{4} \mathrm{Cl}(\mathrm{aq})+\mathrm{FeS}(\mathrm{s})\)

Step-by-Step Solution

Answer

Balanced equations for (a)-(d) provided, net ionic equations highlight ions involved.

Step 1: Balance equation (a)

For the reaction \( \mathrm{Zn} + \mathrm{HCl} \rightarrow \mathrm{H}_{2} + \mathrm{ZnCl}_{2} \), balance the elements. Zinc already appears balanced. Zinc and hydrogen need to be balanced in the ionic forms: \( \mathrm{Zn} + 2\mathrm{HCl} \rightarrow \mathrm{H}_2 + \mathrm{ZnCl}_2 \). Now the equation is balanced with 1 Zn, 2 H, and 2 Cl atoms on each side.

Step 2: Write net ionic equation for (a)

Zn is solid and HCl dissociates in solution: \( \mathrm{HCl} \rightarrow \mathrm{H}^+ + \mathrm{Cl}^- \). The net ionic equation focuses on the ions that change: \( \mathrm{Zn}(\mathrm{s}) + 2\mathrm{H}^+(\mathrm{aq}) \rightarrow \mathrm{H}_2(\mathrm{g}) + \mathrm{Zn}^{2+}(\mathrm{aq}) \). Cl^- are spectator ions and omitted.

Step 3: Balance equation (b)

Reaction \( \mathrm{Mg(OH)}_2 + \mathrm{HCl} \rightarrow \mathrm{MgCl}_2 + \mathrm{H}_2\mathrm{O} \) needs to balance \( \mathrm{H} \) from \( \mathrm{HCl} \) to \( \mathrm{H}_2\mathrm{O} \). The appropriately balanced equation is: \( \mathrm{Mg(OH)}_2 + 2\mathrm{HCl} \rightarrow \mathrm{MgCl}_2 + 2\,\mathrm{H}_2\mathrm{O} \), now with balanced Mg, Cl, and H atoms.

Step 4: Write net ionic equation for (b)

Dissolved ions for soluble species \( \mathrm{Mg(OH)}_2(s) \) and \( \mathrm{HCl} \rightarrow \mathrm{H}^+ + \mathrm{Cl}^- \): the net ionic focuses on \( \mathrm{Mg} \) and \( \mathrm{OH}^- \) forming water: \( \mathrm{Mg(OH)}_2(s) + 2\mathrm{H}^+ \rightarrow 2\,\mathrm{H_2O}(\ell) + \mathrm{Mg}^{2+}(\mathrm{aq}) \). \( \mathrm{Cl}^- \) are spectator ions.

Step 5: Balance equation (c)

Start with \( \mathrm{HNO}_3 + \mathrm{CaCO}_3 \rightarrow \mathrm{Ca(NO}_3)_2 + \mathrm{H}_2\mathrm{O} + \mathrm{CO}_2 \). Balance first the \( \mathrm{Ca} \) and \( \mathrm{NO}_3 \) parts: \( 2\mathrm{HNO}_3 + \mathrm{CaCO}_3 \rightarrow \mathrm{Ca(NO}_3)_2 + \mathrm{H}_2\mathrm{O} + \mathrm{CO}_2 \). Now it is balanced on both sides.

Step 6: Write net ionic equation for (c)

Consider ions in solution: \( \mathrm{CaCO}_3 \) as solid and \( \mathrm{HNO}_3 \rightarrow \mathrm{H}^+ + \mathrm{NO}_3^- \). Real net ionic: \( 2\mathrm{H}^+ + \mathrm{CaCO}_3(s) \rightarrow \mathrm{Ca}^{2+}(\mathrm{aq}) + \mathrm{H}_2\mathrm{O} + \mathrm{CO}_2 \). Nitates are spectator ions.

Step 7: Balance equation (d)

The equation \( \mathrm{(NH}_4)_2\mathrm{S} + \mathrm{FeCl}_2 \rightarrow \mathrm{NH}_4\mathrm{Cl} + \mathrm{FeS} \) involves 2 amine on the left so requires 2 \( \mathrm{NH}_4\mathrm{Cl} \) on the right: \( \mathrm{(NH}_4)_2\mathrm{S} + \mathrm{FeCl}_2 \rightarrow 2\mathrm{NH}_4\mathrm{Cl} + \mathrm{FeS} \). Now balanced for all elements.

Step 8: Write net ionic equation for (d)

Soluble ions \( \mathrm{Fe}^{2+} + \mathrm{Cl}^- \) and \( \mathrm{NH}_4^+ + \mathrm{S}^{2-} \) lead net ionic: \( \mathrm{Fe}^{2+}(\mathrm{aq}) + \mathrm{S}^{2-}(\mathrm{aq}) \rightarrow \mathrm{FeS}(\mathrm{s}) \). \( \mathrm{Cl}^- \) and \( \mathrm{NH}_4^+ \) are spectators.

Key Concepts

Net Ionic Equations

Understanding net ionic equations can seem challenging, but they're vital for comprehending chemical reactions. These equations display only the substances that actively participate in the reaction. For example, consider the reaction in equation (a) where zinc (Zn) reacts with hydrochloric acid (HCl). When dissolved in water, HCl separates into ions, but zinc remains solid. The net ionic equation isolates only the reactive elements:

• Zinc, a solid, reacts with hydrogen ions in solution.
• This equation illustrates zinc becoming zinc ions upon reaction completion.

By focusing only on the substances participating in the reaction, we simplify our understanding and avoid clutter from spectator ions. This streamlined approach helps students to focus on the chemical change happening.

Spectator Ions

Spectator ions are simply ions that do not change during the course of a chemical reaction. They are present in both the reactants and products without undergoing any transformation. Take equation (a) again where hydrochloric acid disassociates into hydrogen ions and chloride ions:

• Chloride ions ( Cl^- ) do not engage in the key reaction with zinc.
• Thus, they remain unchanged and are not included in the net ionic equation.

Recognizing spectator ions is crucial as it highlights the active participants of the reaction. This clarity aids in better understanding the essence of the chemical transformation taking place.

Aqueous Solutions

In chemistry, when a substance is described as "aqueous" it means it is dissolved in water. Aqueous solutions are significant because many reactions occur in this medium. In our given exercises, hydrochloric acid ( HCl ( aq) ) serves as a common example:

• HCl, when dissolved, disassociates into its constituent ions—H^+ and Cl^-.
• These ions become mobile and participate directly in reactions.

Understanding aqueous solutions allows one to appreciate how substances transform in liquid mediums. Since water is a universal solvent, exploring these solutions opens a window to a myriad of reactions involving dissolved ions.

Chemical Reactions

Chemical reactions describe the process where substances, known as reactants, change to form new substances called products. Each exercise above involves a unique chemical reaction. For instance:

• In equation (b), magnesium hydroxide reacts with hydrochloric acid to form magnesium chloride and water.
• This chemical change illustrates the diversity of transformations - solid and aqueous ingredients turn into a liquid product.

Understanding chemical reactions entails recognizing reactants and products, balancing equations, and sometimes writing net ionic equations. Such exercises build foundational knowledge essential for more advanced studies in chemistry.