Problem 36 - 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

(a) Zn + 2H⁺ → H₂ + Zn²⁺ (b) Mg(OH)₂ + 2H⁺ → Mg²⁺ + 2H₂O (c) 2H⁺ + CaCO₃ → Ca²⁺ + H₂O + CO₂ (d) Fe²⁺ + S²⁻ → FeS

Step 1: Write the Unbalanced Equation for (a) Zn with HCl

The given unbalanced chemical equation is: \ \( \mathrm{Zn}(\mathrm{s})+\mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{H}_{2}(\mathrm{g})+\mathrm{ZnCl}_{2}(\mathrm{aq}) \)

Step 2: Balance Equation (a) Zn with HCl

To balance the equation, ensure each kind of atom has the same number on both sides. Note that there are 2 Cl in \( \mathrm{ZnCl}_2 \), so we need two \( \mathrm{HCl} \): \ \( \mathrm{Zn}(\mathrm{s})+2\mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{H}_{2}(\mathrm{g})+\mathrm{ZnCl}_{2}(\mathrm{aq}) \)

Step 3: Write the Net Ionic Equation for (a) Zn with HCl

Break down aqueous compounds into ions and eliminate spectator ions, resulting in: \ \( \mathrm{Zn}(\mathrm{s}) + 2\mathrm{H}^{+}(\mathrm{aq}) \rightarrow \mathrm{H}_{2}(\mathrm{g}) + \mathrm{Zn}^{2+}(\mathrm{aq}) \)

Step 4: Write the Unbalanced Equation for (b) Mg(OH)2 with HCl

The given unbalanced chemical equation is: \ \( \mathrm{Mg}(\mathrm{OH})_{2}(\mathrm{s})+\mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{MgCl}_{2}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\ell) \)

Step 5: Balance Equation (b) Mg(OH)2 with HCl

There are 2 OH in \( \mathrm{Mg}(\mathrm{OH})_{2} \), so 2 OH will make 2 H2O. Balance HCl to get enough Cl for \( \mathrm{MgCl}_{2} \) and 2 more H atoms: \ \( \mathrm{Mg}(\mathrm{OH})_{2}(\mathrm{s})+2\mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{MgCl}_{2}(\mathrm{aq})+2\mathrm{H}_{2} \mathrm{O}(\ell) \)

Step 6: Write the Net Ionic Equation for (b) Mg(OH)2 with HCl

The net ionic equation, excluding spectator ions, is: \ \( \mathrm{Mg}(\mathrm{OH})_{2}(\mathrm{s}) + 2\mathrm{H}^{+}(\mathrm{aq}) \rightarrow \mathrm{Mg}^{2+}(\mathrm{aq}) + 2\mathrm{H}_{2} \mathrm{O}(\ell) \)

Step 7: Write the Unbalanced Equation for (c) HNO3 with CaCO3

The given unbalanced chemical equation is: \ \( \mathrm{HNO}_{3}(\mathrm{aq})+\mathrm{CaCO}_{3}(\mathrm{s}) \rightarrow \mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\ell)+\mathrm{CO}_{2}(\mathrm{g}) \)

Step 8: Balance Equation (c) HNO3 with CaCO3

Ca is already balanced but notice \( \mathrm{NO}_3 \) affects nitrogen. Start with one \( \mathrm{CaCO}_3 \), you need 2 \( \mathrm{HNO}_3 \) for 2 NO3: \ \( 2\mathrm{HNO}_{3}(\mathrm{aq})+\mathrm{CaCO}_{3}(\mathrm{s}) \rightarrow \mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\ell)+\mathrm{CO}_{2}(\mathrm{g}) \)

Step 9: Write the Net Ionic Equation for (c) HNO3 with CaCO3

The net ionic equation, excluding spectator ions, is: \ \( 2\mathrm{H}^{+}(\mathrm{aq}) + \mathrm{CaCO}_{3}(\mathrm{s}) \rightarrow \mathrm{Ca}^{2+}(\mathrm{aq}) + \mathrm{H}_{2}\mathrm{O}(\ell) + \mathrm{CO}_{2}(\mathrm{g}) \)

Step 10: Write the Unbalanced Equation for (d) (NH4)2S with FeCl2

The given unbalanced chemical equation is: \ \( \left(\mathrm{NH}_{4}\right)_{2}\mathrm{S}(\mathrm{aq})+\mathrm{FeCl}_{2}(\mathrm{aq}) \rightarrow \mathrm{NH}_{4} \mathrm{Cl}(\mathrm{aq})+\mathrm{FeS}(\mathrm{s}) \)

Step 11: Balance Equation (d) (NH4)2S with FeCl2

Notice \( \left(\mathrm{NH}_{4}\right)_{2} \) and the iron compound balances out with one \( \mathrm{FeS} \), requiring only two NH4: \ \( \left(\mathrm{NH}_{4}\right)_{2}\mathrm{S}(\mathrm{aq})+\mathrm{FeCl}_{2}(\mathrm{aq}) \rightarrow 2\mathrm{NH}_{4} \mathrm{Cl}(\mathrm{aq})+\mathrm{FeS}(\mathrm{s}) \)

Step 12: Write the Net Ionic Equation for (d) (NH4)2S with FeCl2

Remove spectator ions, focusing on those involved in forming \( \mathrm{FeS} \): \ \( \mathrm{Fe}^{2+}(\mathrm{aq}) + \mathrm{S}^{2-}(\mathrm{aq}) \rightarrow \mathrm{FeS}(\mathrm{s}) \)

Key Concepts

Net Ionic Equations

Net ionic equations are essential in understanding the essence of chemical reactions taking place in a solution. They focus on the particles that actually participate in the reaction, omitting spectator ions, which do not change during the reaction. When writing a net ionic equation:

• Begin by writing the balanced molecular equation.
• Separate the soluble compounds into their respective ions. Insoluble and liquid compounds are left intact.
• Identify and eliminate the spectator ions, leaving only the ions that participate in forming the reaction's products.

For example, consider the net ionic equation from step 3 for zinc reacting with hydrochloric acid: \[\mathrm{Zn}( ext{s}) + 2\mathrm{H}^{+}( ext{aq}) \rightarrow \mathrm{H}_{2}( ext{g}) + \mathrm{Zn}^{2+}( ext{aq})\] Here, the chloride ions (Cl⁻) were removed because they do not participate in the actual chemical change.

Stoichiometry

Stoichiometry is all about the calculation of reactants and products in chemical reactions. It's the practice of balancing chemical equations to ensure that the law of conservation of mass is observed. Essentially, it reveals the quantitative relationships between substances as they participate in chemical reactions.
To balance a chemical equation like \( \mathrm{Mg}( ext{OH})_{2} + 2\mathrm{HCl} \rightarrow \mathrm{MgCl}_{2} + 2\mathrm{H}_{2} \mathrm{O} \),
follow these steps:

• Count the number of atoms for each element on both sides of the equation.
• Adjust the coefficients before each compound to balance the molecules, ensuring the same number of each type of atom on both sides of the equation.
• Verify that all coefficients are in the lowest possible ratio that balances the equation.

Balancing ensures that no atoms are lost or gained, fulfilling the requirement that mass is conserved during a reaction.

Chemical Reactions

Chemical reactions are the processes by which substances interact to form new products. These reactions involve breaking and creating bonds between atoms, which produce new substances with different properties. They can be classified into several types, such as:

• Single displacement reactions, like the reaction of zinc with hydrochloric acid: \( \mathrm{Zn} + 2\mathrm{HCl} \rightarrow \mathrm{H}_{2} + \mathrm{ZnCl}_{2} \).
• Double displacement reactions, as seen when ammonium sulfide reacts with iron(II) chloride: \( \left(\mathrm{NH}_{4}\right)_{2}\mathrm{S} + \mathrm{FeCl}_{2} \rightarrow 2\mathrm{NH}_{4}\mathrm{Cl} + \mathrm{FeS} \).
• Combustion, synthesis, and decomposition reactions.

Understanding the type of reaction helps predict the products and understand the conditions required for the reaction. It provides a comprehensive view of how substances change and the energy exchange involved.