Thermite Lab Writeup

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 в?†H for this reaction is the sum of the в?†Hfo's of the products - the sum of the в?†Hfo's of the reactants (multiplying each by their stoichiometric coefficient in the balanced reaction equation), i.e.:

в?†Horxn = (1 mol)( HfoAl2O3) + (2 mol)( HfoFe) - (1 mol)( HfoFe2O3) - (2 mol)( HfoAl)

в?†Horxn = (1 mol)(-1,669.8 kJ/mol) + (2 mol)(0) - (1 mol)(-822.2 kJ/mol) - (2mol)(0 kJ/mol)

в?†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 that the type of reaction is redox. The thermodynamic driving force for this reaction is the high stability (low free energy) of the aluminia (Al2O3). Another factor is how Al3+ has a greater affinity for oxygen than the Fe3+. This property is called oxophilicity (literally, �oxygen loving’).

Alternative procedures

For igniting the thermite, I used a strip of magnesium. This is the most practical and cheapest method. This method, however, does have its disadvantages. While it burns without releasing cooling gases, it is hard to ignite, and in windy/wet conditions, it may get extinguished. Another problem is that magnesium is a very good conductor of heat, meaning that heating one side of the ribbon can cause the other end of the ribbon to get hot enough to cause premature ignition. Another potential problem is that magnesium requires an outside source of oxygen to burn.

One way we could have lighted the thermite is with a potassium permanganate (KMnO4) and glycerine or ethylene glycol. This would have created a reaction that would slowly increase the temperature until flames are produced. This method is also unreliable, however, because factors like particle size and ambient temperature comes into play.

Another fun way to ignite thermite is with your everyday sparkler. This can be a dangerous method because the sparkler sends out iron sparks, which are hot enough to prematurely ignite the thermite (even if they don’t come into direct contact). This is especially dangerous with finely ground thermite.

The grade of the thermite plays a role into this reaction. The more finely powdered the thermite is, the faster and easier it is to light. For example, fine powdered thermite can be lighted with regular lighters, kitchen matches, and even flint spark lighters (the burning of rare earth metals like lanthanum and cerium).

Some experiments call for the molten iron to drip into a bucket of water. This however, is VERY dangerous. Mixing water with thermite is also dangerous. When water is added into thermite, a phreatomagmatic explosion may occur, sending hot materials into all directions. A phreatomagmatic explotion is normally the result of interaction between water and magma. The explotion is composed of magmatic gases and steam from a very hot material (normally magma, but in this case, molten iron).

Other forms of �thermite’

There exist different forms of �thermite’. One variety is Thermate. Thermate burns faster and hotter. Thermate is created by combining thermite, sulfur, and barium nitrate. The thermate weight ratio is 69% thermite, 29% barium nitrate, 2% sulfur. There are many other variations of thermite, but thermate is the most common alternate form of thermite.

List of different �thermites’:

Iron (III) oxide (Red):

Fe2O3(s) + 2Al(s) в†' Al2O3(s) + 2Fe(l); О"H = -847.6 kJ/mol

The ratio of Iron (III) oxide to Aluminum powder by mass is about 3:1. This is the reaction I chose to do.

Iron (II, III) oxide (Black):

3Fe3O4(s) + 8Al(s) в†' 4Al2O3(s) + 9Fe(l); О"H = -3347.6 kJ/mol

The reaction using Fe3O4 (-3347.6kJ/mol) produces a substantially larger amount of energy than the reaction using Fe2O3 (-847.6kJ/mol).