what about carbon dioxide allows it to fuel the burning of magnesium
The General Chemistry Demo Lab
Reaction Of Magnesium Metal With Carbon Dioxide
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Introduction
- We are familiar with carbon dioxide, COtwo, as a product of combustion. For instance, when we fire glucose, C6H12Osix, the products of the reaction are carbon dioxide and water:
CsixH12Ohalf dozen + half-dozen O2 
If we desire carbon dioxide to participate in a chemic reaction without calculation external energy, we demand to observe a reactant that has a high gratis free energy or a product that has an even lower free energy to brand the reaction favorable.
The reaction we are going to demonstrate here involves the extremely exothermic (heat evolving) and thermodynamically favorable reaction of magnesium metal (Mg) with carbon dioxide:
ii Mg(southward) + CO2
This reaction produces temperatures in excess of 2000 degrees C. If whatever oxygen is nowadays during the reaction, the magnesium will react with information technology to form magnesium oxide, MgO, and the carbon dioxide would not react. Therefore, we need to find a way to exclude oxygen. Nosotros take a perfect way of doing that as you'll see in just a moment.
[Intro] [Comments]
The Demo
This demo involves called-for magnesium and is inappropriate for some audiences and venues (elementary schools, for example). Burning magnesium demos take produced injuries and it is upwards to the lecture demonstrator to ensure that their audience is properly protected past distance, blast shield, experimental modifications and other means that are prudent for the venue selected past the demonstrator. This demo can only be performed indoors if there is adequate ventilation; go along in mind that MgO smoke is generated and tin can be a respiratory irritant and/or trigger smoke alarms. When performing this demo for an audience, one or more test runs should be made without spectators to ensure that all prudent safe considerations accept been properly met. The reader assumes all risks associated with performing this demonstration and recognizes that this document can not anticipate all possible circumstances and does not guarantee that the procedures outlined herein are risk or hazard-free.
The demo starts with a solid block of Dry Ice (frozen carbon dioxide). Retrieve that carbon dioxide sublimes at -78 degrees C, so proper cryogenic gloves must exist used to gear up this demonstration.
| The first step is to cut the block in half. The best way to do this neatly is to brand a thin saw cutting and then use a wide masonry chisel. Alternatively, saw the whole thing in half. Using the chisel method this takes about two minutes. |  |
 | The adjacent pace is to make hemispherical depressions approximately six to 8 cm in diameter in each piece. Yous can try doing this with a spatula, icepick etc., but it takes forever. If you use an electrical drill and a large pigsty saw the process only takes about a minute for each cavity. |
| Important rubber note : When the ii halves of the block fit together "also well", this could impede the venting of evolved gases and pb to a "burp" that could potentially eject burning material. Therefore, it is of import that 1 provide venting of the cavity. The easiest way to do this is to have a hand saw and make two perpendicular cuts nigh 1/4" to 1/two" deep beyond the faces of the blocks that will fit together. When the blocks are assembled, this gives four horizontal vent holes that are approximately the thickness of a pencil. Another benefit of the vent holes is that it makes the escaping vapors expect more spectacular as they jet outward. | |
| Next, we place some magnesium turnings in the hole nosotros made for the bottom slice. These turnings shown here are tarnished, just they worked OK (Mg powder is not recommended equally information technology may burn down too quickly and explode). A typical sit-in uses 6 to 8 grams of turnings for a large lecture hall. Nosotros have done up to 25 grams at a time, merely the reaction produces a fairly significant mist that contains MgO. Be conscientious to practise a pocket-sized test run to encounter if your room's size, ventilation, audience, smoke detectors etc. volition allow this demo to exist run indoors. 25 g is definitely pushing one'due south luck for an indoor demonstration. |  |
 | To get united states of america over the activation bulwark, we need to add a petty heat. The fastest and easiest mode to practise this is to ignite the Mg with a propane torch. Place a condom shield betwixt the block and your audience and and so calorie-free the magnesium. As presently every bit it catches fire, speedily place the other one-half block on elevation. Be sure you orient the acme beforehand then that the top fits correctly. ( Important: provide the vent holes discussed above!). Turn out the room lights if doing this indoors. |
| Click on the image at the right to view a QuickTime flick of the demonstration (you can download this for your platform if it is not already installed on your reckoner). The file is 654 Kb. The actual duration of the demo is longer than the 29 seconds shown hither; we deleted some of the footage in the eye. The camera compensates for the incredible brightness of this demonstration (notice how the background disappears in one case the reaction starts). It is and so brilliant that your audition should be warned non to stare directly at the block during the period of greatest intensity! | |
 | After the reaction has subsided, we are left with a chunk of material that looks like this. It has a coating of water water ice condensation (remember the block is -78 degrees C), and so permit's scoop the residue into a weighing pan and examine it. |
| Now that the water ice condensation has melted and we accept broken up the residuum, we can run across that the cloth consists of a white solid and a black solid. The black solid is the carbon we hoped to produce and, no, information technology does not contain any Cthreescore. The white material is the magnesium oxide. |  |
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Comments
- This reaction produces a searing, white-hot light. A darkened room is lit upward every bit if past spotlights, a considerable feat considering that the flame is contained behind several cm of opaque solid!
- Before and after the demonstration the dry ice block is virtually unchanged. In fact, you can run this demo at to the lowest degree a dozen times with the same block! This raises the question "if the reaction is so intense and it's 2000+ degrees C, why doesn't it melt the block?"
That is a great question so allow's examine it a scrap further. Offset, permit the states calculate how much carbon dioxide is required for the reaction. If we use 16 k of Mg with a molecular weight of 24 yard/mol, then we have (sixteen chiliad)/(24 grand/mol) = 0.66 mol of Mg. Co-ordinate to our balanced reaction above, we need half this number of moles of CO2 = 0.33 mol. Given a molecular weight of 44 for CO2, that is (0.33 mol)(44 thou/mol) = 14.67 grams, roughly a half-ounce.
"Ah!" you lot say. "What well-nigh all the Rut? Doesn't that melt the COii?" Expert question. Let's calculate how much thermal energy is released in this reaction. If we expect at the standard state enthalpy values for the products and reactants we find they are:
Component Hf o
Mg 0 kJ/mol CO2 (g) -393.5 kJ/mol MgO (southward) -601.8 kJ/mol C (southward) 0 kJ/mol Mg and C are zero considering, past definition, the
The
That is a fairly large gratis energy change. For comparing, the combustion of methyl hydride gas is -818 kJ/mol. Nosotros run into that the thermodynamic driving force for this reaction is a) the high stability (low energy) of the MgO that is formed and b) the reaction stoichiometry which forms two moles of this very stable substance.
Now, allow'southward assume that all of the free energy went into heat, even though nosotros know that much of it went into making light. If we consume 0.33 mol of carbon dioxide in this reaction then that liberates (0.33)(810.1 kJ) = 267 kJ of energy that we tin put into melting the carbon dioxide cake.
The heat of vaporization of carbon dioxide is 571 kJ/kg. If nosotros were to use all of our 267 kJ to a) vaporize the carbon dioxide b) non raise the gas temperature above -78 degrees (both of which are very generous assumptions; think how much light we got) so the most we could possibly vaporize is (267 kJ)/(571 kJ/kg) = 0.47 kg or roughly 1 pound. The block weighs approximately 40 pounds, then this isn't a tremendous change in mass.
- Another mutual question is "why doesn't the carbon burn?" The respond to this 1 is straightforward. When the pinnacle block is placed over the burning Mg, the small amount of oxygen in the crenel is speedily consumed. The burning Mg vaporizes large amounts of carbon dioxide, displacing whatever remaining oxygen and creating a positive outward pressure that prevents air from diffusing back into the cavity.
- Discover that it would exist a very bad thought to put out a burning magnesium fire with a carbon dioxide burn extinguisher! Flammable metal fires require specialized extinguishers. If you lot're interested in knowing more, run across this page on fire extinguishers.
A few points worth mention here are:
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This page was last updated Friday, March 27, 2015 and is copyright 2001 past Rob Toreki. All rights reserved. Thanks to the late Terry Todd for his assistance in preparing the demonstration for filming. You are missed, friend.
Source: http://www.ilpi.com/genchem/demo/co2mg/index.html
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