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) h = –850 kj what mass of iron is formed when 725 kj of heat are released?

To illustrate clearly, I will rewrite the reaction in a more understandable manner.

2 Al(s) +  Fe₂O₃ (s) ⇒ 2 Fe(s) + Al₂O₃(s)                Δhrxn = –850 kJ

This reaction has a negative sign for the change in enthalpy of reaction. The sign convention only means that the reaction releases energy to the surroundings. In other words, the reaction is exothermic. Focusing on only its magnitude, this means that 850 kJ of energy is needed for this reaction of 2 Aluminum moles and 1 mole of Fe₂O₃  to occur. 

Now, if you only had an energy of 725 kJ, then the reaction is incomplete but it will still form Iron (Fe). We use stoichiometric calculations as follows:

725 kJ * (2 mol Fe/850 kJ) = 1.7 moles of Fe

Knowing that the molar mass of Fe is 55.6 g/mol, then the mass of produced iron is

1.7 mol Fe * 55.6 g/mol = 94.85 g iron