Problem 91 - Chemistry and Chemical Reactivity

Question

Suppose you wish to prepare a sample of magnesium chloride. One way to do this is to use an acid-base reaction, the reaction of magnesium hydroxide with hydrochloric acid. $$\mathrm{Mg}(\mathrm{OH})_{2}(\mathrm{s})+2 \mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{MgCl}_{2}(\mathrm{aq})+2 \mathrm{H}_{2} \mathrm{O}(\ell)$$ When the reaction is complete, evaporating the water will give solid magnesium chloride. Suggest another way to prepare \(\mathrm{MgCl}_{2}.\)

Step-by-Step Solution

Answer

Use magnesium metal and hydrochloric acid to produce magnesium chloride and hydrogen gas.

Step 1: Identifying a possible reaction

To prepare magnesium chloride (\(\mathrm{MgCl}_{2}\)) through another chemical reaction, we can consider using magnesium metal with hydrochloric acid. This is an oxidation-reduction reaction.

Step 2: Writing the chemical reaction

The reaction between magnesium metal and hydrochloric acid can be represented by the equation:\[\mathrm{Mg}( ext{solid}) + 2\mathrm{HCl}( ext{aqueous}) \rightarrow \mathrm{MgCl}_{2}(\text{aqueous}) + \mathrm{H}_{2}(\text{gas})\]In this reaction, the magnesium metal reacts directly with hydrochloric acid to form magnesium chloride and hydrogen gas.

Step 3: Consideration of reaction conditions

This reaction is typically performed by adding a piece of magnesium metal to an aqueous solution of hydrochloric acid. The magnesium metal reacts exothermically with the acid, producing magnesium chloride in solution and hydrogen gas which will bubble out of the mixture.

Step 4: Collection and drying

After the reaction completes, evaporate the water from the magnesium chloride solution in order to obtain solid magnesium chloride. This involves heating the solution until all the water has evaporated, leaving the solid magnesium chloride behind.

Key Concepts

Acid-Base Reaction

An acid-base reaction is a process in which an acid and a base react to produce water and a salt. In our case, the reaction involves magnesium hydroxide \[\mathrm{Mg}(\mathrm{OH})_{2}(\mathrm{s})\]as the base and hydrochloric acid \[\mathrm{HCl}(\mathrm{aq})\]as the acid. When these two substances mix, they undergo a neutralization reaction, producing magnesium chloride, \[\mathrm{MgCl}_{2}(\mathrm{aq}),\]and water, \[\mathrm{H}_{2}\mathrm{O}(\ell)\]. Here's how it works:

• The hydroxide ions (OH⁻) from the magnesium hydroxide react with the hydrogen ions (H⁺) from the hydrochloric acid to form water.
• Simultaneously, the remaining magnesium ions (Mg²⁺) combine with the chloride ions (Cl⁻) to form magnesium chloride.

This is a typical double displacement reaction, and it's crucial in both laboratory and industrial chemical processes. Such reactions are essential in chemistry due to their simplicity and ease of completion, often requiring only the mixing of the reactants.

Oxidation-Reduction Reaction

An oxidation-reduction reaction, also known as a redox reaction, involves the transfer of electrons between substances. In the preparation of magnesium chloride by using magnesium metal and hydrochloric acid, this principle is at work. Consider the following reaction:\[\mathrm{Mg}(\text{solid}) + 2\mathrm{HCl}(\text{aqueous}) \rightarrow \mathrm{MgCl}_{2}(\text{aqueous}) + \mathrm{H}_{2}(\text{gas})\]

• Here, magnesium metal is oxidized, meaning it loses electrons and forms magnesium ions (Mg²⁺).
• The hydrogen ions (H⁺) from hydrochloric acid gain these electrons, reducing them to form hydrogen gas (H₂).

This exchange of electrons results in the formation of magnesium chloride in solution. Redox reactions are vital in multiple applications, ranging from energy storage systems like batteries to natural biological processes. Understanding them can provide insights into a broad range of chemical behaviors.

Evaporation Process

After conducting either an acid-base or oxidation-reduction reaction to obtain magnesium chloride in aqueous form, the next step is to extract the solid compound. To do this, the evaporation process is employed. This method relies on heating the solution until the solvent (usually water) transitions into vapor, leaving behind the solute - in this case, solid magnesium chloride. Key points in the process:

• Evaporation separates substances based on their volatility. The less volatile components remain as solid residues.
• The solution is heated until all the water evaporates, ensuring only the solid magnesium chloride remains.

This simple yet effective method is widely used in the purification and recovery of substances from solutions. It's an essential technique in various fields, including chemistry, cooking, and environmental management, allowing for the recovery of dissolved solids efficiently.