Problem 13 - Chemistry and Chemical Reactivity

Question

Although there are a number of magnesiumcontaining minerals, the commercial source of this metal is seawater. Treatment of seawater with \(\mathrm{Ca}(\mathrm{OH})_{2}\) gives insoluble \(\mathrm{Mg}(\mathrm{OH})_{2} .\) This reacts with HCl to produce \(\mathrm{MgCl}_{2}\), which is dried, and the metal is obtained by electrolysis of the molten salt. Write balanced net ionic equations for the three reactions described here.

Step-by-Step Solution

Answer

Net ionic equations: \( \text{Mg}^{2+} + 2\text{OH}^- \rightarrow \text{Mg(OH)}_2 \); \( \text{Mg(OH)}_2 + 2\text{H}^+ \rightarrow \text{Mg}^{2+} + 2\text{H}_2\text{O} \); \( \text{Mg}^{2+} + 2\text{e}^- \rightarrow \text{Mg} \).

Step 1: Precipitation of Magnesium Hydroxide

When seawater is treated with calcium hydroxide, an exchange reaction takes place. The soluble Mg²⁺ ions in the seawater react with Ca(OH)₂ to form insoluble Mg(OH)₂. The balanced net ionic equation is:\[ \text{Mg}^{2+} (aq) + 2 \text{OH}^- (aq) \rightarrow \text{Mg(OH)}_2 (s) \]

Step 2: Formation of Magnesium Chloride

Magnesium hydroxide reacts with hydrochloric acid to form magnesium chloride and water. The reaction removes the hydroxide ions. The balanced net ionic equation is:\[ \text{Mg(OH)}_2 (s) + 2 \text{H}^+ (aq) \rightarrow \text{Mg}^{2+} (aq) + 2\text{H}_2\text{O} (l) \]

Step 3: Electrolysis of Magnesium Chloride

The magnesium chloride produced is melted and subjected to electrolysis to extract magnesium metal. The overall reaction involves the reduction of magnesium ions to magnesium metal. The balanced net ionic equation is:\[ \text{Mg}^{2+} (l) + 2\text{e}^- \rightarrow \text{Mg} (s) \]

Key Concepts

Precipitation Reaction

In chemistry, a precipitation reaction occurs when two solutions react and form an insoluble solid known as a precipitate. This happens due to the exchange of ions between the reactants, which results in a solid compound being formed from aqueous ions. Let's explore the example of magnesium hydroxide.
When seawater, containing dissolved magnesium ions (\(\text{Mg}^{2+}\)) is treated with calcium hydroxide (\(\text{Ca(OH)}_2\)), a classic precipitation reaction takes place. The soluble magnesium ions ((\(\text{Mg}^{2+}\))) combine with hydroxide ions (\(\text{OH}^-\)) to form an insoluble compound, magnesium hydroxide \(\text{Mg(OH)}_2\).
The net ionic equation for this reaction is:

• \[ \text{Mg}^{2+} (aq) + 2 \text{OH}^- (aq) \rightarrow \text{Mg(OH)}_2 (s) \]

This equation shows the key species involved, omitting the spectator ions not directly participating in the formation of the precipitate. Such reactions are important in various industrial processes, including water treatment and metal extraction.

Electrolysis

Electrolysis is a chemical process that uses electricity to cause a chemical change, usually the breakdown of compounds into their elements. In this process, ions migrate towards electrodes of opposite charges, where they gain or lose electrons, transforming into neutral atoms or molecules.
The role of electrolysis in magnesium extraction is crucial. Once magnesium chloride (\(\text{MgCl}_2\)) is formed and melted, it undergoes electrolysis. During this process, magnesium ions (\(\text{Mg}^{2+}\)) in the molten state are reduced at the cathode, gaining electrons to form magnesium metal:

• \[ \text{Mg}^{2+} (l) + 2\text{e}^- \rightarrow \text{Mg} (s) \]

This step is a key aspect of converting magnesium ions into usable metal. Electrolysis is broadly employed in industries to purify metals like aluminum, copper, and magnesium, making it essential for metal extraction and purification.

Magnesium Extraction

Magnesium extraction involves various stages to convert magnesium-containing minerals into usable metal. A significant process in magnesium extraction is the reaction of magnesium hydroxide with hydrochloric acid to form magnesium chloride.
In this step, the insoluble magnesium hydroxide \(\text{Mg(OH)}_2\), obtained from the precipitation reaction, reacts with hydrochloric acid (\(\text{HCl}\)). The hydroxide ions are consumed to form water, converting magnesium hydroxide to soluble magnesium ions and producing magnesium chloride:

• \[ \text{Mg(OH)}_2 (s) + 2 \text{H}^+ (aq) \rightarrow \text{Mg}^{2+} (aq) + 2\text{H}_2\text{O} (l) \]

This is a crucial step in obtaining magnesium chloride in an appropriate form for the electrolysis process. Following this, the melted \(\text{MgCl}_2\) undergoes electrolysis to yield pure magnesium metal. This sequence highlights the transition of magnesium from dissolved ions in seawater to solid metal, showcasing the intricate steps involved in its extraction.