For 2Al(s) + Fe2O3(s) → Al2O3(s) + 2Fe(s), use ΔH°f values of Fe2O3(s) = -824.2 kJ/mol and Al2O3(s) = -1675.7 kJ/mol. What is ΔH° for the reaction?

Core Concept

Al(s) and Fe(s) are elements in their standard states, so their formation enthalpies are zero and drop out of both sums. Products contribute -1675.7 kJ and reactants contribute -824.2 kJ. Subtracting gives ΔH° = -1675.7 - (-824.2) = -851.5 kJ. The strongly negative result shows why thermite releases enormous heat: the Al-O bonds formed are far stronger than the Fe-O bonds broken.