Magnesium Chloride (MgCl₂): Structure, Properties, Preparation & Uses

Magnesium Chloride (MgCl₂) is an ionic compound formed by magnesium (Mg²⁺) and chlorine (Cl⁻) ions. It forms hydrates MgCl₂·nH₂O, where n can range from 1 to 12. These salts are colorless or white solids that are highly soluble in water. These compounds and their solutions, both of which occur in nature, have a variety of practical uses. Anhydrous magnesium chloride is the principal precursor to magnesium metal, which is produced on a large scale. Hydrated magnesium chloride is the form most readily available.

It can be extracted from brine or seawater, or synthesized by treating magnesium hydroxide or magnesium oxide with hydrochloric acid. It’s commonly used as a de-icing agent on roads, where it works by lowering the freezing point of water, thus preventing ice from bonding to the pavement.

Structure of Magnesium Chloride (MgCl₂)

Molecular Structure

• Ionic Bonding: Magnesium chloride consists of magnesium cations (Mg²⁺) and chloride anions (Cl⁻). Each magnesium ion is surrounded by two chloride ions due to the 1:2 ratio of magnesium to chlorine in the compound. This forms an ionic bond as magnesium donates two electrons (one to each chloride ion), achieving a stable electron configuration.

Crystal Structure

• Lattice Structure: At room temperature, magnesium chloride adopts a crystal structure known as the cadmium chloride (CdCl₂) structure. This is a type of layered structure:
  • Layer Arrangement: In this structure, magnesium ions are octahedrally coordinated by six chloride ions. Each chloride ion is shared between three octahedra, creating layers of edge-sharing MgCl₆ octahedra.
  • Unit Cell: The unit cell of MgCl₂ is rhombohedral, but it’s often described in terms of a larger hexagonal unit cell for simplicity. Here, magnesium ions occupy the octahedral sites within layers of chloride ions, which are arranged in a close-packed manner but not in direct contact with each other due to the magnesium ions in between.
• Packing: The layers stack in a way where chloride ions from one layer sit above and below the magnesium ions of adjacent layers, forming a 3D network. This stacking leads to a structure where each magnesium ion has a coordination number of 6 (surrounded by 6 chloride ions), and each chloride ion is coordinated to 3 magnesium ions.
• Interlayer Forces: The interaction between these layers is primarily electrostatic (ionic bonding), which accounts for the relatively high melting and boiling points of magnesium chloride. However, these forces are not as strong as covalent bonds within a layer, which explains why MgCl₂ can deliquesce or absorb moisture from the air; the water molecules can insinuate themselves between these layers.

Hydrates

• Hydrated Forms: Magnesium chloride can form several hydrates, like MgCl₂·6H₂O (hexahydrate), where water molecules are incorporated into the crystal structure. In the hexahydrate form, magnesium ions are surrounded by a mixture of chloride ions and water molecules, altering the coordination environment and the overall crystal structure.

Physical properties of Magnesium Chloride (MgCl₂)

• Appearance: Magnesium chloride typically appears as a white or colorless crystalline solid.
• Odor: It is odorless.
• Molecular Weight: 95.211 g/mol for the anhydrous form.
• Density:
  • Anhydrous MgCl₂ has a density of about 2.32 g/cm³ at room temperature.
  • The hexahydrate (MgCl₂·6H₂O) has a lower density, approximately 1.569 g/cm³.
• Melting Point:
  • Anhydrous magnesium chloride melts at around 714°C (1,317°F).
  • The hexahydrate decomposes upon heating rather than melting, losing water to form lower hydrates or the anhydrous form.
• Boiling Point: The boiling point of anhydrous MgCl₂ is approximately 1,412°C (2,574°F).
• Solubility:
  • Highly soluble in water. At 20°C, about 54.3 g of anhydrous MgCl₂ can dissolve in 100 g of water, increasing with temperature.
  • It’s also soluble in ethanol but less so than in water.
• Hygroscopic Nature: Magnesium chloride is highly hygroscopic, meaning it readily absorbs moisture from the air. This property makes it effective for applications like dust control and as a drying agent.
• Crystal Structure:
  • As mentioned, it adopts a cadmium chloride-type crystal structure where magnesium is octahedrally coordinated by chloride ions.
• Hardness: On the Mohs scale, it would not typically be measured for hardness as it’s more relevant for minerals, but ionic crystals like MgCl₂ are generally not very hard compared to covalent or metallic crystals.
• Refractive Index: Not commonly cited for salts like MgCl₂ in solid form, but solutions of magnesium chloride will affect light passage due to dissolved ions.
• Thermal Conductivity: As with many ionic solids, magnesium chloride has relatively low thermal conductivity compared to metals or some covalent compounds.
• Electrical Conductivity:
  • Solid magnesium chloride does not conduct electricity well because the ions are fixed in the lattice. However, when dissolved in water or melted, it conducts electricity because the ions can move freely.
• Deliquescence: Because of its hygroscopic nature, magnesium chloride will deliquesce in humid environments, absorbing enough water from the air to form a solution.

Chemical properties of Magnesium chloride

Reactivity:

• Ionic Nature: As an ionic compound, MgCl₂ dissociates in water to form Mg²⁺ and Cl⁻ ions. This dissociation is what makes it highly soluble and allows it to conduct electricity when in solution or molten state.

Reactions:

• With Water:
  • When dissolved in water, magnesium chloride dissociates:MgCl2(s)→Mg2+(aq)+2Cl−(aq)
  • The hydration of these ions releases heat (exothermic process), which is why magnesium chloride solutions can feel warm when prepared.
• Acid-Base Properties:
  • The magnesium ion can form complexes with water molecules, leading to a slightly acidic solution due to the hydrolysis of the hydrated magnesium ion:[Mg(H2O)6]2+→[Mg(H2O)5(OH)]++H+
• Formation of Magnesium Hydroxide:
  • With strong bases, magnesium chloride can react to form magnesium hydroxide:MgCl2+2NaOH→Mg(OH)2↓+2NaClThis reaction is used in various industrial processes and can be observed as a white precipitate.
• Redox Reactions:
  • Magnesium chloride can be involved in redox reactions where magnesium might be reduced from its +2 oxidation state, although this is less common in typical uses. However, in molten state or under specific electrochemical conditions, magnesium can be produced:Mg2++2e−→Mg
• Complex Formation:
  • Magnesium ions can form complexes with various ligands due to their small size and double positive charge. This is relevant in biological systems and in chemical synthesis:Mg2++6NH3→[Mg(NH3)6]2+
• Dehydration:
  • When heated, the hydrated forms of magnesium chloride lose water. For instance:MgCl2⋅6H2O→MgCl2+6H2O
• Use in Organic Synthesis:
  • It can act as a Lewis acid in some reactions, facilitating various organic transformations by coordinating with electron-rich species.
• Electrolysis:
  • Molten magnesium chloride can be electrolyzed to produce magnesium metal at the cathode and chlorine gas at the anode:MgCl2(l)→Mg(l)+Cl2(g)

Stability:

• In Air: Anhydrous magnesium chloride is stable in dry air but will absorb moisture to form hydrates.
• Thermal Decomposition: While the anhydrous form has a high decomposition temperature, the hydrates will lose water upon heating before the magnesium chloride itself decomposes at much higher temperatures.

Environmental Chemistry:

• Impact on Water: When used for de-icing or dust control, runoff containing MgCl₂ can affect water bodies, potentially increasing salinity and magnesium content which might impact aquatic life.

Preparation of Magnesium chloride

Magnesium chloride can be prepared through several methods, depending on whether one starts with raw materials or if it’s a laboratory or industrial scale synthesis:

From Magnesium Hydroxide or Oxide:

• Using Hydrochloric Acid:
  • This is one of the most straightforward methods, especially in a laboratory setting. When magnesium hydroxide (Mg(OH)₂) or magnesium oxide (MgO) reacts with hydrochloric acid (HCl), magnesium chloride is formed along with water:
    Mg(OH)2+2HCl→MgCl2+2H2OMgO+2HCl→MgCl2+H2O
  • After the reaction, the solution can be evaporated to crystallize magnesium chloride.

From Magnesium Metal:

• Direct Reaction with Hydrochloric Acid:
  • Magnesium metal can react directly with hydrochloric acid to produce magnesium chloride and hydrogen gas:
    Mg+2HCl→MgCl2+H2
  • This method is highly exothermic and should be done with caution due to the rapid evolution of hydrogen gas.

From Seawater or Brine:

• Industrial Extraction:
  • Magnesium chloride can be extracted from seawater or natural brines, which contain a significant amount of magnesium ions. The process involves:
    1. Precipitation of Magnesium Hydroxide: Lime (calcium hydroxide) or dolomite is added to seawater to precipitate magnesium hydroxide:
       Mg2++Ca(OH)2→Mg(OH)2↓+Ca2+
    2. Conversion to Magnesium Chloride: The magnesium hydroxide is then treated with hydrochloric acid:
       Mg(OH)2+2HCl→MgCl2+2H2O
    3. Crystallization: The solution is concentrated by evaporation to crystallize out magnesium chloride hexahydrate (MgCl₂·6H₂O), which can be further processed to obtain anhydrous magnesium chloride if required.

From Carnallite:

• Carnallite (KCl·MgCl₂·6H₂O), a mineral, can be processed to obtain magnesium chloride:
  • Dissolution and Crystallization: Carnallite is dissolved in water, and through fractional crystallization, magnesium chloride can be separated from potassium chloride.

Electrolysis:

• Industrial Production of Magnesium:
  • While this process primarily produces magnesium metal, magnesium chloride is an essential intermediate. Molten magnesium chloride (derived from dehydration of hydrated magnesium chloride or directly from carnallite) undergoes electrolysis:
    MgCl2(l)→Mg(l)+Cl2(g)
  • Here, magnesium chloride is not the final product but is used in the process where chlorine gas is also generated as a byproduct.

Uses of Magnesium chloride

• One of the most common uses of magnesium chloride is for de-icing roads, sidewalks, and parking lots during winter. It’s less corrosive than sodium chloride (table salt) and works at lower temperatures.
• Applied to unpaved roads, mining sites, and construction areas to control dust. Its hygroscopic nature helps to keep the surface moist, reducing airborne dust.
• Used as an admixture in concrete to accelerate the setting time and improve the strength of concrete, especially in cold weather.
• Employed in the processing of cotton and in the manufacture of paper for its role in dye fixation and as a fire retardant.
• In some medical treatments, magnesium chloride can be used for magnesium infusion or in baths for transdermal magnesium therapy, believed to help with muscle cramps, stress relief, and other health issues.
• Utilized in the production of fire-resistant materials due to its ability to resist high temperatures.
• In some regions, magnesium chloride (known as nigari in Japanese cuisine) is used as a coagulant in the production of tofu from soy milk.
• In the ceramic industry, it can be used to control the viscosity and provide strength to the ceramic bodies.