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Chapter 5: Problem 106

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

Expert verified
CO鈧� and water react with magnesium, making the fire worse.

Step by step solution

01

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.
02

Analyzing Carbon Dioxide Reaction with Magnesium

Carbon dioxide (CO鈧�) is often used in fire extinguishers to smother fires by cutting off the oxygen supply. However, when CO鈧� is applied to a magnesium fire, magnesium can continue to burn by reducing CO鈧� 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.
03

Examining the Water Reaction with Magnesium

Water (H鈧侽) 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 (H鈧�), which is highly flammable. This could result in an explosion if lit, making the situation more dangerous.
04

Conclusion

Both CO鈧� 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.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

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 (CO鈧�) 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 CO鈧� 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 (H鈧侽) 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 (H鈧�).
  • 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, CO鈧� , 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.

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Most popular questions from this chapter

Consider the reaction $$\begin{aligned}2 \mathrm{H}_{2} \mathrm{O}(g) \longrightarrow & 2 \mathrm{H}_{2}(g)+\mathrm{O}_{2}(g) \\ \Delta H=&+483.6 \mathrm{~kJ} / \mathrm{mol}\end{aligned}$$ at a certain temperature. If the increase in volume is 32.7 \(\mathrm{L}\) against an external pressure of \(1.00 \mathrm{~atm},\) calculate \(\Delta U\) for this reaction. \((1 \mathrm{~L} \cdot \mathrm{atm}=101.3 \mathrm{~J})\)

(a) A person drinks four glasses of cold water \(\left(3.0^{\circ} \mathrm{C}\right)\) every day. The volume of each glass is \(2.5 \times 10^{2} \mathrm{~mL}\). How much heat (in kJ) does the body have to supply to raise the temperature of the water to \(37^{\circ} \mathrm{C},\) the body temperature? (b) How much heat would your body lose if you were to ingest \(8.0 \times 10^{2} \mathrm{~g}\) of snow at \(0^{\circ} \mathrm{C}\) to quench your thirst? (The amount of heat necessary to melt snow is \(6.01 \mathrm{~kJ} / \mathrm{mol}\).)

How are the standard enthalpies of an element and of a compound determined?

Glauber's salt, sodium sulfate decahydrate \(\left(\mathrm{Na}_{2} \mathrm{SO}_{4} .\right.\) \(\left.10 \mathrm{H}_{2} \mathrm{O}\right),\) undergoes a phase transition (i.e., melting or freezing) at a convenient temperature of about \(32^{\circ} \mathrm{C}\) : \(\begin{aligned}{\mathrm{Na}_{2} \mathrm{SO}_{4} \cdot 10 \mathrm{H}_{2} \mathrm{O}(s) \longrightarrow \mathrm{Na}_{2} \mathrm{SO}_{4} \cdot 10 \mathrm{H}_{2} \mathrm{O}(l)}{\Delta H^{\circ}} &=74.4 \mathrm{~kJ} / \mathrm{mol} \end{aligned}\) As a result, this compound is used to regulate the temperature in homes. It is placed in plastic bags in the ceiling of a room. During the day, the endothermic melting process absorbs heat from the surroundings, cooling the room. At night, it gives off heat as it freezes. Calculate the mass of Glauber's salt in kilograms needed to lower the temperature of air in a room by \(8.2^{\circ} \mathrm{C}\). The mass of air in the room is \(605.4 \mathrm{~kg} ;\) the specific heat of air is \(1.2 \mathrm{~J} / \mathrm{g} \cdot{ }^{\circ} \mathrm{C}\).

Calculate the heat released when \(2.00 \mathrm{~L}\) of \(\mathrm{Cl}_{2}(g)\) with a density of \(1.88 \mathrm{~g} / \mathrm{L}\) reacts with an excess of sodium metal at \(25^{\circ} \mathrm{C}\) and 1 atm to form sodium chloride.
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