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Thermite Lab Writeup

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  • June 7, 2008
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Thermite
Background:
Thermite is a powder made from aluminum powder and a metal oxide [usually iron oxide (Fe2O3, known as rust)]. The thermite reaction is a redox reaction, where Aluminum reduces the oxide of another metal. For example, when using iron oxide (as I did) the equation would be Fe2O3(s) + 2 Al(s) -> Al2O3(s) + 2 Fe(l). Black or blue iron oxide (Fe3O4) could also be used. Other examples of possible oxides are manganese thermite (MnO2), Cr2O3, and copper thermite (CuO). Aluminum can also be replaced by any reactive metal. Aluminum, however, is the safest material and the cheapest to use. Thermite, if you look at the equation, supplies it own oxygen, which means that it can burn under water or in oxygen depleted areas. This also means, however, that the thermite reaction cannot be smothered out and cannot be put out by throwing water onto it. What happens in a thermite reaction:

In my experiment, I used a 1:3 weight ratio of aluminum powder to iron oxide. If we look at the standard enthalpy values for the products and reactants, we find: Component Hfo (kJ/mol)
Fe2O3(s)-822.2
Al(s)0
Al2O3(s)-1,669.8
Fe (s)0
Fe and Al are zero because the Hfo of elements in their standard states are zero. The ∆H for this reaction is the sum of the ∆Hfo's of the products - the sum of the ∆Hfo's of the reactants (multiplying each by their stoichiometric coefficient in the balanced reaction equation), i.e.: ∆Horxn = (1 mol)( HfoAl2O3) + (2 mol)( HfoFe) - (1 mol)( HfoFe2O3) - (2 mol)( HfoAl) ∆Horxn = (1 mol)(-1,669.8 kJ/mol) + (2 mol)(0) - (1 mol)(-822.2 kJ/mol) - (2mol)(0 kJ/mol) ∆Horxn = -847.6 kJ

That energy change is very large (to put into perspective, the combustion of methane gas, the gas used in Bunsen burners, is -818 kJ/mol). The actual reaction for thermite is: Fe2O3(s) + 2 Al(s) -> Al2O3(s) + 2 Fe(l). solid iron oxide powder, mixed with aluminum powder, goes to aluminum oxide (alumina) and liquid iron. If we look at this equation, we can see...