Question

From a thermochemical point of view, explain why a carbon dioxide fire extinguisher or water should not be used on a magnesium fire.

Short answer

CO₂ and water react with magnesium, making the fire worse.

Step-by-step solution

Understanding Magnesium's Reactivity

Magnesium is a highly reactive metal, especially at higher temperatures. When magnesium is burning, it reacts with various other substances, including oxygen, with a vigorous exothermic reaction that releases a significant amount of heat.

Analyzing Carbon Dioxide Reaction with Magnesium

Carbon dioxide (CO₂) is often used in fire extinguishers to smother fires by cutting off the oxygen supply. However, when CO₂ is applied to a magnesium fire, magnesium can continue to burn by reducing CO₂ to carbon (C) and generating an even more exothermic reaction: \( 2Mg + CO_2 \rightarrow 2MgO + C \). This reaction releases heat and intensifies the fire rather than extinguishing it.

Examining the Water Reaction with Magnesium

Water (H₂O) should not be used to extinguish a magnesium fire because magnesium reacts vigorously with water. The reaction \( Mg + 2H_2O \rightarrow Mg(OH)_2 + H_2 \) releases hydrogen gas (H₂), which is highly flammable. This could result in an explosion if lit, making the situation more dangerous.

Conclusion

Both CO₂ and water react with burning magnesium in a way that can exacerbate the fire. The use of either results in additional reactions that release energy or create flammable gases, hence they are not effective means to extinguish a magnesium fire.

Key concepts

Magnesium Reactivity

Magnesium is known for its high reactivity, especially when exposed to high temperatures. This metal is a powerhouse in generating enthusiastic and rapid reactions with various elements or compounds. When magnesium burns, it reacts with oxygen in the air, resulting in a vibrant and intense flame. This reaction is called exothermic, meaning it releases heat. Because of this property, magnesium can react explosively with other materials while burning. Understanding this reactivity is crucial, especially in situations where controlling or extinguishing the fire is required.

Carbon Dioxide Reaction

Carbon dioxide (CO₂) is commonly used in fire extinguishers due to its ability to suppress fires by removing oxygen, which is essential for combustion. However, the story changes when it comes to magnesium fires. Instead of putting out the fire, carbon dioxide can participate in a reaction with burning magnesium.

• The chemical reaction that takes place: \( 2Mg + CO_2 \rightarrow 2MgO + C \)
• This reaction results in the production of magnesium oxide (MgO) and carbon (C).
• It releases additional heat, making the situation more explosive.

Using CO₂ will make the fire more intense and difficult to control.

Water Reaction

While water is typically an effective extinguishing agent, it is notably unsuitable for use on magnesium fires. The reaction between water (H₂O) and magnesium is highly vigorous:

• The reaction follows: \( Mg + 2H_2O \rightarrow Mg(OH)_2 + H_2 \)
• This produces magnesium hydroxide (Mg(OH)_2) and hydrogen gas (H₂).
• The release of hydrogen gas is hazardous because it is extremely flammable.

Attempting to extinguish a magnesium fire with water can potentially cause an explosion, significantly increasing the danger of the situation.

Exothermic Reactions

Exothermic reactions are characterized by the release of energy in the form of heat. This principle is at the core of many chemical processes.

• When a substance burns, there are reactions that release heat and sometimes light.
• Magnesium, when it reacts with other substances like oxygen, CO₂ , or water, results in exothermic reactions.
• The energy from these reactions can intensify fires and complicate firefighting efforts.

Understanding exothermic reactions is essential, especially for safety. In magnesium fires, the release of energy not only fuels the ongoing combustion but also catalyzes secondary reactions, which complicate extinguishing efforts.