Question

The highly exothermic thermite reaction, in which aluminum reduces iron(III) oxide to elemental iron, has been used by railroad repair crews to weld rails together. 

2Al(s) + Fe₂O₃(s) - 2Fe(s) + Al₂O₃(s)  AH=-850 kJ 

What mass of iron is formed when 725 kJ of heat are released?

Answer 1

2 Al(s) +  Fe₂O₃ (s) ⇒ 2 Fe(s) + Al₂O₃(s) ΔH_rxn = –850 kJ
as you see that negative sign show the reaction is exothermic but we have to take only magnitude for our answer.

from reaction, we see that 2 mole Al and 1 mole Fe₂O₃ gives 2 mole of iron in which 850KJ energy is released to the surrounding. So,
850 KJ energy released when 2 mole of iron is formed.
1 KJ energy released when 2/850 mole of iron is formed.
725 KJ energy released = 2/850 *725 mole of iron formed.
= 1.7 mole iron
molar mass of Fe is 55.6 g/mol

1 mole of iron = 55.6 g of iron
So, 1.7 mole of iron = 55.6 g * 1.7
= 94.85 g of iron is formed.