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Name: Jaye
Status: student
Grade: 9-12
Country: Canada
Date: April 2009


Question:
How can you find the molar enthalpy of combustion of magnesium directly? (without using Hess's Law)?



Replies:
Burn a mole of magnesium.

Richard Barrans, Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming


Jaye,

You have identified the beauty of Hess' Law. It is the safe way -- paper, pencil, and calculator.

No high school lab, (and probably very few university labs) should undertake the experimental combustion of magnesium. Your teacher may have demonstrated the combustion of magnesium in air -- lots of bright light, heat and smoke. Not the kind of reaction students should do without more training.

Several years ago a truck containing metallic magnesium travelled through Chicago on one of the expressways. Somehow, the magnesium ignited. The truck driver stopped the truck under a bridge. After the fire department extinguished the flames, observers noted that (1) much of the bridge had melted, and (2) the concrete highway under the bridge had disintegrated.

Warren Young


The combustion of magnesium metal is the reaction: Mg + 1/2 O2 = MgO. Hess's Law states that the enthalpy of a reaction is the sum of the enthalpies of formation of the products minus the sum of the enthalpies of formation of the reactants, using a properly balanced chemical equation. The enthalpy of formation of ELEMENTS in their respective standard states (i.e. the form of the element at 1 atm pressure at the temperature of interest -- usually 298.15 kelvins) is assigned a value of zero . In the case of the combustion of Mg (the reaction above), the enthalpy of combustion the enthalpy of formation of MgO (-601.6 k-J/mol MgO) are identical. It's not clear to me why you want to shy away from Hess's Law, but it is possible to measure the enthalpy of combustion of Mg using a reaction calorimeter (also referred to as a "bomb calorimeter", since the reaction in the calorimeter is run at constant volume rather than constant pressure, which is contained in the definition of the enthalpy of reaction. The conversion from constant volume to constant pressure is straightforward. See the web site for details about Hess's Law, and bomb calorimetry:

http://www.mikeblaber.org/oldwine/chm1045/notes/Energy/HessLaw/
Energy04.htm

http://www.chem.hope.edu/~polik/Chem345-2000/bombcalorimetry.htm

Vince Calder


Jaye,

In essence this should be straightforward, although, if you actually tried to do it, it could prove problematic.

You would need to burn a known mass of Mg in O2, you would need to use pure O2 as Mg reacts with N2 as well as O2 when burnt in air, forming MgO and Mg3N2. You would need to set up this reaction so that the energy released heated up a known mass of a suitable substance (water is generally used) and then you can use Q = m.Cp.dT to calculate the heat released. Then divide the heat by the moles of Mg you burnt. But here are the problems:

How will you make sure all the heat released goes into the water?

How will you make sure all the Mg reacts?

How will you make sure no heat escapes to the environment, or could you allow for heat loss in some way?

Mg + O2 generally takes a lot of energy to start the reaction, how will you put in this energy and not use it to heat the water?

To help you answer these problems you could try looking up a device called a "bomb calorimeter". Good luck!!

Tom Collins



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