Part C. Calculate the enthalpy change for the thermite reaction:
2Al(s) + Fe2O3(s) → 2Fe(s) + Al2O3(s) ΔH°rxn = − 850 kJ
when 6.00 mol of Al undergoes the reaction with a stoichiometrically equivalent amount of Fe2O3.
Thermite is a composition of metal powder and metal oxide. In this reaction, iron oxide, Fe2O3(s), and aluminum powder, Al(s), react exothermically to release a large amount of heat. Aluminum reduces the iron oxide to form metallic iron.
2Al(s) + Fe2O3(s) → 2Fe(s) + Al2O3(s)
An extensive property depends upon the quantity of matter present in the substance. Suppose you consider the volume of water in a beaker; if you add more water to the beaker, the volume of the water increases. Thus, volume depends on the amount of substance present, so it is an extensive property.
Like mass and volume, heat content or enthalpy is also an extensive property. Therefore, the magnitude of ΔH rxn is directly proportional to the amount of reactants consumed in the reaction.
For example, consider the decomposition of hydrogen peroxide (H2O2) to form water (H2O) and oxygen (O2):
2H2O2(l) → 2H2O(l) + O2(g), ΔH°rxn = − 196 kJ
The value of ΔH°rxn is − 196 kJ when 2 mol of H2O2 decomposes. However, when 4 mol of H2O2 decomposes, ΔH rxn will be 4/2 × ( − 196 kJ) = − 392 kJ.
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