How much energy in kJ is evolved during the reaction of

Question

 Fe ₂O ₃(s) + 2 Al(s) → Al ₂O ₃(s) + 2 Fe(s) ΔH° _rxn = -852 kJ

How much energy in kJ is evolved during the reaction of 74.6 g of Al, according to the reaction below? Assume that there is excess Fe ₂O ₃.

Answer 1

Hello, Iris,

This is known as the thermite reaction. It is a highly exothermic process that has found use in many applications, including welding railroad track sections together. Google it for some interesting demonstrations.

I'll venture to say that the unit for the heat should read kJ/mole(Al₂O₃). Otherwise, the unit has no useful meaning. I'll use that assumption in the calculations.

Given that the iron oxide is in excess, we'll assume all of the Al (74.6 grams) is consumed. Convert grams into moles Al by dividing it by the molar mass of Al, 27.0. to give 2.76 moles Al. The equation says we'll get 1/2 that many moles of Al₂O₃ , or 1.38 moles Al₂O₃.

(1.38 moles Al₂O₃)*(-852kJ/mole Al₂O₃) = -1177 kJ (the negative sign means energy was released from the system and into the environment, perhaps to a railroad track section, or your lab partner, if they aren't careful.

Bob