Magnesium & Its Compounds: Chemical & Physical Properties

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Magnesium is a chemical element with an atomic number of 12 and a chemical symbol of Mg. Magnesium metal is a silver-white metal with a metallic luster. However, when exposed to air, its surface gradually oxidizes and takes on a light gray color. Magnesium has a relatively low density of about 1.74 g/cm³. This makes it a lightweight metal, and its alloys with aluminum are widely used in the aerospace industry. The melting point of magnesium is approximately 650 degrees Celsius, and its boiling point is around 1090 degrees Celsius. Magnesium metal is soft and can be easily scratched with tools like keys. Magnesium exhibits good conductivity, heat conductivity, and malleability.

Chemical Properties of Magnesium

Magnesium has two outer electrons, which are easily lost in chemical reactions. Magnesium is not as reactive as alkali metals, but it is slightly more reactive than other metals.

Reactivity of Magnesium with Oxygen

Magnesium is a fairly soft metal with a shiny silver surface when first cut. The surface quickly becomes dull as magnesium reacts with oxygen at room temperature to form a coating of white gray magnesium oxide. The dense magnesium oxide coating prevents further oxidation, allowing magnesium metal to be stored in the air. When ignited in the air, magnesium burns intensely to emit a bright white light that contains a significant amount of harmful ultraviolet radiation. Additionally, white smoke containing magnesium oxide particles is produced during combustion. Magnesium metal is commonly used in the production of fireworks, illuminating projectiles, and special effects in war movies.

2Mg(s) + O₂(g) → 2MgO(s)

Reactivity of Magnesium with Carbon Dioxide

Due to its strong reductive properties, magnesium can react not only with oxygen but also remove oxygen atoms from certain compounds. When a burning magnesium strip is placed in carbon dioxide, it continues to burn. White, granular magnesium oxide is formed, and black carbon is displaced from the carbon dioxide.

2Mg(s) + CO₂(g) → 2MgO(s) + C(s)

Reactivity of Magnesium with Halogens and Nitrogen

Magnesium can react with chlorine gas at room temperature to form magnesium chloride. Nitrogen gas and bromine require heating to react with magnesium.

Mg(s) + Cl₂(g) → MgCl₂(s) Mg(s) + Br₂(g) → MgBr₂(s)

3Mg(s) + N₂(g) → Mg₃N₂(s)

Reactivity of Magnesium with Water and Acids

Magnesium can displace hydrogen from acids and water. Magnesium reacts slowly with cold water due to the poorly soluble magnesium hydroxide layer formed on its surface. However, hot water dissolves more magnesium hydroxide and accelerates the chemical reaction rate, causing magnesium to react quickly with hot water and producing bubbles. Although magnesium hydroxide is slightly soluble, the pink phenolphthalein shows the solution to be alkaline. Acidic solution is rich in hydrogen ions, magnesium rapidly displaces hydrogen to produce hydrogen gas. This reaction is highly vigorous.

Mg(s) + 2H₂O(g) → Mg(OH)₂(s) + H₂(g)

Mg(s) + 2HCl(g) → MgCl₂(aq) + H₂(g)

Reaction of Magnesium with Ammonium Chloride

Fundamentally, the reaction of magnesium with ammonium chloride is a reaction between magnesium and an acid. The ammonium ions absorb a small amount of hydroxide ions to make the solution acidic. The magnesium reacts with the hydrogen ions in the solution to release hydrogen gas and a significant amount of heat. The hydrolysis product of the ammonium ion, ammonia water, decomposes into ammonia gas upon heating.

Mg(s) + 2NH₄Cl(aq) → MgCl₂(aq) + 2NH₃(g) + H₂(g)

Compounds of Magnesium

Common compounds of magnesium are white magnesium oxide and white magnesium hydroxide. Magnesium oxide has a dense structure and is not easily reactive with cold water, but it can react with hot water to form magnesium hydroxide. Magnesium hydroxide is a sparingly soluble base and can react with acids.

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