Problem 98 - Chemistry and Chemical Reactivity

Question

Describe how to prepare zinc chloride by (a) an acid-base reaction, (b) a gas- forming reaction, and (c) an oxidation-reduction reaction. The available starting materials are \(\mathrm{ZnCO}_{3}, \mathrm{HCl}, \mathrm{Cl}_{2}, \mathrm{HNO}_{3,} \mathrm{Zn}(\mathrm{OH})_{2}, \mathrm{NaCl}, \mathrm{Zn}\left(\mathrm{NO}_{3}\right)_{2},\) and Zn. Write complete, balanced equations for the reactions chosen.

Step-by-Step Solution

Answer

1. Acid-base: \( \mathrm{ZnCO}_{3} + 2\mathrm{HCl} \rightarrow \mathrm{ZnCl}_{2} + \mathrm{CO}_{2} + \mathrm{H}_{2}\mathrm{O} \). 2. Gas-forming: \( \mathrm{Zn} + 2\mathrm{HCl} \rightarrow \mathrm{ZnCl}_{2} + \mathrm{H}_{2} \). 3. Redox: \( \mathrm{Zn} + \mathrm{Cl}_{2} \rightarrow \mathrm{ZnCl}_{2} \).

Step 1: Acid-Base Reaction: Selecting Reactants

For an acid-base reaction to form zinc chloride, choose zinc carbonate \( \mathrm{ZnCO}_{3} \) as the base and hydrochloric acid \( \mathrm{HCl} \) as the acid.

Step 2: Acid-Base Reaction: Writing the Equation

The reaction of zinc carbonate with hydrochloric acid produces zinc chloride, carbon dioxide, and water. The balanced equation is:\[ \mathrm{ZnCO}_{3} + 2\mathrm{HCl} \rightarrow \mathrm{ZnCl}_{2} + \mathrm{CO}_{2} + \mathrm{H}_{2}\mathrm{O} \]

Step 3: Gas-Forming Reaction: Selecting Reactants

For a gas-forming reaction, use zinc \( \mathrm{Zn} \) and hydrochloric acid \( \mathrm{HCl} \). This reaction will also liberate hydrogen gas.

Step 4: Gas-Forming Reaction: Writing the Equation

Zinc reacts with hydrochloric acid to produce zinc chloride and hydrogen gas. The balanced equation is:\[ \mathrm{Zn} + 2\mathrm{HCl} \rightarrow \mathrm{ZnCl}_{2} + \mathrm{H}_{2} \]

Step 5: Oxidation-Reduction Reaction: Selecting Reactants

For an oxidation-reduction reaction, use zinc \( \mathrm{Zn} \) and chlorine gas \( \mathrm{Cl}_{2} \). Here, zinc is oxidized and chlorine is reduced.

Step 6: Oxidation-Reduction Reaction: Writing the Equation

Zinc reacts with chlorine gas to produce zinc chloride. The balanced equation is:\[ \mathrm{Zn} + \mathrm{Cl}_{2} \rightarrow \mathrm{ZnCl}_{2} \]

Key Concepts

Acid-Base Reaction

In chemistry, an acid-base reaction involves the transfer of protons (H⁺ ions) between reactants. Here, zinc carbonate (ZnCO₃) reacts with hydrochloric acid (HCl). This type of reaction is characterized by the neutralization process, where an acid reacts with a base to produce a salt and water.

When ZnCO₃ is introduced to HCl, the carbonates react with the HCl to form zinc chloride (ZnCl₂), carbon dioxide (CO₂), and water (H₂O). The process can be understood in these steps:

• ZnCO₃ acts as the base that reacts with the acid HCl.
• This reaction produces ZnCl₂, a salt, alongside CO₂ gas, and water (H₂O).

The balanced chemical equation for this reaction is:
\[ \mathrm{ZnCO}_{3} + 2\mathrm{HCl} \rightarrow \mathrm{ZnCl}_{2} + \mathrm{CO}_{2} + \mathrm{H}_{2}\mathrm{O} \]This balanced equation indicates that two moles of hydrochloric acid are required to fully react with one mole of zinc carbonate.

The evolution of carbon dioxide gas is a clear indicator of the reaction's progression as it effervesces out of the solution.

Gas-Forming Reaction

A gas-forming reaction often involves the production of a gas as one of the products. Here, zinc (Zn) reacts with hydrochloric acid (HCl) to produce zinc chloride (ZnCl₂) and hydrogen gas (H₂). This reaction is typical of a metal-acid reaction.

In this specific context, the process is simple to understand:

• Zinc (Zn) metal reacts with HCl to form soluble ZnCl₂.
• The release of hydrogen gas (H₂) is observable as bubbles forming in the solution.

The reaction equation is succinctly represented as:
\[ \mathrm{Zn} + 2\mathrm{HCl} \rightarrow \mathrm{ZnCl}_{2} + \mathrm{H}_{2} \]
Here, one mole of zinc reacts with two moles of hydrochloric acid. The visible formation of hydrogen gas bubbly is a straightforward way to confirm this reaction is occurring. This type of reaction showcases a single exchange, where the zinc substitutes hydrogen from hydrochloric acid, forming hydrogen gas.

This gas-forming reaction is important in industrial processes where the generation of hydrogen could be utilized.

Oxidation-Reduction Reaction

Oxidation-reduction reactions (often called redox reactions) involve the transfer of electrons between molecules. In the zinc and chlorine gas reaction, zinc (Zn) loses electrons (is oxidized), and chlorine (Cl₂) gains electrons (is reduced), forming zinc chloride (ZnCl₂).

These steps explain the process:

• Zinc (Zn) loses two electrons to form Zn²⁺. This is oxidation.
• Chlorine gas (Cl₂) gains these two electrons to form two chloride ions (Cl⁻). This is reduction.

Writing the balanced chemical equation:
\[ \mathrm{Zn} + \mathrm{Cl}_{2} \rightarrow \mathrm{ZnCl}_{2} \]
This equation shows that one mole of zinc reacts with one mole of chlorine gas to produce zinc chloride. The entire process revolves around electron transfer.

This redox reaction is significant in industries where it's used to produce ZnCl₂, which acts as a catalyst in organic synthesis, or as a galvanizing agent. Understanding the transformation of the underlying oxidation states (zinc from zero to +2, chlorine from 0 to -1) is crucial in mastering redox chemistry.