Problem 90 - 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 \(\mathrm{Zn} .\) Write complete balanced equations for the reactions chosen.

Step-by-Step Solution

Answer

Zinc chloride can be prepared by: (a) reacting \( \mathrm{ZnCO}_3 \) with \( \mathrm{HCl} \), (b) also involves \( \mathrm{ZnCO}_3 \) with \( \mathrm{HCl} \) forming \( \mathrm{CO}_2 \) gas, and (c) reacting zinc with chlorine.

Step 1: Acid-Base Reaction

To prepare zinc chloride via an acid-base reaction, we can use zinc carbonate \( \mathrm{ZnCO}_3 \) and hydrochloric acid \( \mathrm{HCl} \).The reaction is:\[ \mathrm{ZnCO}_3 + 2 \mathrm{HCl} \rightarrow \mathrm{ZnCl}_2 + \mathrm{H}_2\mathrm{O} + \mathrm{CO}_2 \uparrow \]This reaction involves zinc carbonate reacting with hydrochloric acid to produce zinc chloride, water, and carbon dioxide gas.

Step 2: Gas-Forming Reaction

For a gas-forming reaction, zinc carbonate \( \mathrm{ZnCO}_3 \) can be used with hydrochloric acid \( \mathrm{HCl} \) as well.The reaction is already given in the acid-base reaction, where \( \mathrm{CO}_2 \) gas is formed:\[ \mathrm{ZnCO}_3 + 2 \mathrm{HCl} \rightarrow \mathrm{ZnCl}_2 + \mathrm{H}_2\mathrm{O} + \mathrm{CO}_2 \uparrow \]Here, the formation of carbon dioxide gas represents the gas-forming aspect of the reaction.

Step 3: Oxidation-Reduction Reaction

To prepare zinc chloride using an oxidation-reduction reaction, we can react zinc \( \mathrm{Zn} \) with chlorine \( \mathrm{Cl}_2 \).The balanced chemical equation is:\[ \mathrm{Zn} + \mathrm{Cl}_2 \rightarrow \mathrm{ZnCl}_2 \]In this reaction, zinc is oxidized from 0 to +2 oxidation state, and chlorine is reduced from 0 to -1 oxidation state, forming zinc chloride.

Key Concepts

Acid-Base Reaction

An acid-base reaction involves an acid reacting with a base to produce a salt and water. When preparing zinc chloride via this method, we use zinc carbonate \( \mathrm{ZnCO}_3 \) and hydrochloric acid \( \mathrm{HCl} \). The reaction is as follows:
\[ \mathrm{ZnCO}_3 + 2 \mathrm{HCl} \rightarrow \mathrm{ZnCl}_2 + \mathrm{H}_2\mathrm{O} + \mathrm{CO}_2 \uparrow \]
This equation shows \( \mathrm{ZnCO}_3 \), acting as the base, reacting with \( \mathrm{HCl} \), the acid. The products include zinc chloride \( \mathrm{ZnCl}_2 \), water, and carbon dioxide gas. This type of reaction is fundamental in inorganic chemistry.

• The carbonate ion \( \mathrm{CO}_3^{2-} \) from zinc carbonate reacts with the hydrogen ion \( \mathrm{H}^+ \) from hydrochloric acid.
• The hydrogen ions combine with the carbonate ions to form water and carbon dioxide gas.

Such reactions are common in everyday chemical experiences, such as baking soda reacting with vinegar.

Gas-Forming Reaction

Gas-forming reactions are reactions that result in the production of a gas. The reaction between zinc carbonate \( \mathrm{ZnCO}_3 \) and hydrochloric acid \( \mathrm{HCl} \) doesn't just yield zinc chloride, it also produces carbon dioxide gas, highlighting the gas-forming aspect:
\[ \mathrm{ZnCO}_3 + 2 \mathrm{HCl} \rightarrow \mathrm{ZnCl}_2 + \mathrm{H}_2\mathrm{O} + \mathrm{CO}_2 \uparrow \]
This process is a neat demonstration of how a solid and a liquid can generate a gas.

• The carbon dioxide gas \( \mathrm{CO}_2 \) is formed as the carbonate compound breaks down in the presence of the acid, releasing gas that can be visibly observed as bubbles.
• This type of reaction not only changes the chemical composition but often leads to noticeable physical changes, making it exciting for visual demonstrations.

Gas-forming reactions are often used in various industrial processes and experimental demonstrations.

Oxidation-Reduction Reaction

Oxidation-reduction reactions, or redox reactions, involve the transfer of electrons between substances. Preparing zinc chloride utilizing a redox reaction involves zinc \( \mathrm{Zn} \) and chlorine gas \( \mathrm{Cl}_2 \). The balanced equation for this reaction is:
\[ \mathrm{Zn} + \mathrm{Cl}_2 \rightarrow \mathrm{ZnCl}_2 \]
In this reaction, you'll see a clear exchange of electrons.

• Zinc \( \mathrm{Zn} \) is oxidized, meaning it loses electrons, going from an oxidation state of 0 to +2.
• Chlorine \( \mathrm{Cl}_2 \) is reduced, as it gains electrons, its oxidation state changes from 0 to -1.

Redox reactions are foundational in chemistry because they explain reactions involved in energy transfer processes, such as those occurring in batteries and metabolic pathways.