Problem 28 - Chemistry The Molecular Science

Question

Write balanced equations for the recovery of magnesium from seawater. Begin with the precipitation of magnesium hydroxide by addition of calcium hydroxide to seawater.

Step-by-Step Solution

Answer

Precipitate Mg(OH)₂ using Ca(OH)₂; convert to MgCl₂ using HCl, then electrolyze to obtain Mg.

Step 1: Understanding the Context

Seawater contains magnesium ions (Mg²⁺), which we want to recover. We will use calcium hydroxide (Ca(OH)₂) to precipitate out magnesium as magnesium hydroxide (Mg(OH)₂). This forms the basis of recovering magnesium from seawater.

Step 2: Writing the Initial Reaction

The first reaction involves the addition of calcium hydroxide to seawater, which contains magnesium ions. The balanced chemical equation for this precipitation reaction is: \[ \text{Mg}^{2+} (aq) + 2\text{OH}^{-} (aq) \rightarrow \text{Mg(OH)}_2 (s) \]. However, since calcium hydroxide is added to the solution, the full ionic reaction is: \[ \text{Ca(OH)}_2 (aq) + \text{Mg}^{2+} (aq) \rightarrow \text{Ca}^{2+} (aq) + \text{Mg(OH)}_2 (s) \].

Step 3: Separation of Magnesium Hydroxide

Once magnesium hydroxide precipitates, it can be separated from the mixture by filtration since it is insoluble in water. This step doesn't involve a chemical equation, but it's essential for the next steps.

Step 4: Conversion to Pure Magnesium

The magnesium hydroxide precipitate is then typically dissolved in hydrochloric acid (HCl) to form magnesium chloride (MgCl₂): \[ \text{Mg(OH)}_2 (s) + 2\text{HCl} (aq) \rightarrow \text{MgCl}_2 (aq) + 2\text{H}_2\text{O} (l) \].

Step 5: Electrolysis of Magnesium Chloride

Using electrolysis on the magnesium chloride solution, magnesium metal (Mg) is finally obtained. The simplified overall reaction for the electrolysis process is: \[ \text{MgCl}_2 (l) \rightarrow \text{Mg} (s) + \text{Cl}_2 (g) \].

Key Concepts

Chemical Equations

Chemical equations are essential tools in chemistry that describe how substances change into new products in a reaction. In the context of magnesium recovery from seawater, chemical equations allow us to represent processes clearly and accurately. For instance, when writing the equation for the precipitation of magnesium hydroxide, we take into account the reactants and products, ensuring both sides of the equation are balanced. Balancing chemical equations involves making sure that the number of atoms of each element is the same on both sides. This is crucial because it aligns with the law of conservation of mass, which states that matter cannot be created or destroyed in a chemical reaction. Understanding the role of chemical equations is fundamental to grasping more complex processes like precipitation reactions and electrolysis, both of which are involved in magnesium recovery.

Precipitation Reaction

A precipitation reaction occurs when two solutions combine to form an insoluble solid, known as a precipitate. In the case of recovering magnesium from seawater, magnesium ions (\(\text{Mg}^{2+}\)) react with hydroxide ions (\(\text{OH}^{-}\)) to form magnesium hydroxide (\(\text{Mg(OH)}_2\)). This is depicted in the equation:

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

The solid magnesium hydroxide can then be separated from the liquid through filtration. This step is vital because it removes the magnesium ions from the seawater, setting the stage for further chemical processes to refine magnesium into its pure metal form.

Electrolysis

Electrolysis is a key process in extracting and refining metals, including magnesium from its compounds. In the magnesium recovery process, once we have magnesium chloride (\(\text{MgCl}_2\)), electrolysis can be employed to obtain pure magnesium metal. Start by dissolving magnesium chloride in an electrolyte solution. When an electric current is passed through, it splits magnesium chloride into its components:

• \(\text{MgCl}_2 (l) \rightarrow \text{Mg} (s) + \text{Cl}_2 (g) \)

During this process, magnesium ions gain electrons to form magnesium metal, while chloride ions lose electrons to form chlorine gas. This method allows for the efficient extraction of magnesium, making it an important industrial process for metal recovery.

Magnesium Hydroxide

Magnesium hydroxide (\(\text{Mg(OH)}_2\)) is an insoluble compound that precipitates out during the initial stage of magnesium recovery. Its formation is a critical step since it can be easily filtered out from the liquid mixture. This compound serves as an intermediary in the recovery process. Once separated, magnesium hydroxide is dissolved in hydrochloric acid to form magnesium chloride, which is ready for further processing by electrolysis. Although insoluble, magnesium hydroxide is very useful beyond this process. It's commonly known as "milk of magnesia" and is used as an antacid and laxative. This highlights how versatile chemical compounds can have various applications.

Magnesium Chloride

Magnesium chloride (\(\text{MgCl}_2\)) is a key intermediary product in the recovery of magnesium metal from seawater. After magnesium hydroxide is separated, it's reacted with hydrochloric acid to create magnesium chloride in solution:

• \( \text{Mg(OH)}_2 (s) + 2\text{HCl} (aq) \rightarrow \text{MgCl}_2 (aq) + 2\text{H}_2\text{O} (l) \)

This aqueous solution is then subjected to electrolysis, which extracts the magnesium metal. Magnesium chloride's role is pivotal as it transitions from a solid compound to a soluble form, facilitating further processing. Additionally, magnesium chloride is also used for de-icing roads and in the production of textiles, showcasing its industrial relevance beyond just extracting magnesium.