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Name:  Jules
Status: other
Grade: other
Location:IA
Country: N/A
Date: 4/5/2005


Question:
In one of our programs here we are burning magnesium shavings in a block of dry ice. The question is - What is the temperature of the magnesium shavings fire? How hot is it?


Replies:
WARNING: THIS IS A DANGEROUS EXPERIMENT

I am not sure of your experimental apparatus for this demonstration, that is, how you are going to initiate and contain the reaction; however, be aware that this experiment is potentially EXTREMELY HAZARDOUS. First, the reaction (see below) is highly exothermic so the reaction may begin as Mg (solid) and CO2 (solid) but will undoubtedly quickly end up being Mg (liquid / vapor) and CO2 (gas). Second, the Group 1 and Group 2 elements reduce CO2 to elemental carbon so there is no way to extinguish the fire / explosion that is going to ensue except to dump sand on it (that assumes you will be able to get close enough to do anything). Third, unless the design of the experiment is careful, there will be no way to prevent the reaction of hot Mg with oxygen and / or water vapor. The reaction with water vapor will produce H2 (gas) which in an uncontrolled arrangement is going to react explosively with O2. Fourth, if not carefully contained, the reaction is likely detonate scattering molten Mg and "white hot" MgO and C dust.

The nominal reaction is: 2Mg(solid) + CO2(gas) = 2MgO(solid) + C(solid) and the heat of reaction is -193.5 kcal (reaction as written). Unless you are using very small quantities of reactants and are also prepared to deal with a detonation, DO NOT TRY THIS REACTION. As any fire safety person will tell you, most metal fires once started, are impossible to extinguish.

I would recommend you examine what chemical principle you are intending to demonstrate, and consider a safer alternative than the proposed Mg / CO2 reaction.

Vince Calder



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