[OneWayTicket]

The actual question is:

Thermite is a mixture of aluminum and iron(III) oxide that can be ignited with a magnesium ribbon fuse to form iron and aluminum oxide. The thermite reaction is so exothermic that the iron produced is in the molten state. At one time, the reaction was used to weld railroad tracks. How much heat would be liberated from a reaction in which 5.0g of Al and 20.0g of iron (III) oxide is available? (Assume reactants and products are at 25 degrees Celsius.)

I was super confused about how to do this, but I tried finding the # of moles of each and then finding the limiting reagent (which I got Al as the limiting reagent). I then multiplied the number of moles and the heat of formation (which I found online) to get 310.0 kJ. Since I wasn't quite sure how this worked, I'm not confident at all with my answer.

Any help/direction/correct answer would be greatly appreciated, thanks!

[Aegis6]

So the first thing you want to do is to write out your chemical reaction:

2Al + Fe₂O₃  2Fe + Al₂O₃

Then, find the heats of formation for all substances involved:

Since Al and Fe are elements, they have a heat of formation of 0 kJ/mol and we can ignore them.
The ΔHf of Fe₂O₃ is -826kJ/mol and the ΔHf of Al₂O₃ is -1675.7kJ/mol.

Since Hess' Law tells us that Heat of Reaction = Σ(moles product*ΔHf product) - Σ(moles reactant*ΔHf reactant), we can thusly set up the problem like this:

For the reaction of the equation 2Al + Fe₂O₃  2Fe + Al₂O₃:
 Heat of Reaction =
          (1{mols Fe₂O₃}*-826kJ/mol{Heat of formation for Fe₂O₃})-(1{mols Al₂O₃}*-1675.7kJ/mol{Heat of formation for Al₂O₃})

From this equation, we can see that the Heat of Reaction is 849.7kJ/mol Rxn

As you said that there are 5.0g Al and Aluminum is the limiting reagent of the equation, we can therefore set up our equation like so to find the final amount of energy produced:

5g Al * (1 mol Rxn / 53.963g Al) * (849.7 kJ / 1 mol Rxn)

Following this equation through, we get a released energy of 78.73kJ.

Hope that helps!!

[OneWayTicket]

Thanks a ton! That actually makes sense. I was pretty far off, I hadn't even found the heat of formation for Iron Oxide for some reason.. just a scatterbrained moment. This makes sense though, thanks!