# What is the standard heat of formation of Fe2O3 (s) ?Fe2O3 (s) + 2Al (s) -> Al2O3 (s) + 2Fe (l) ΔH= -800.3 kJ/mol If the standard heat of formation of Al2O3 (s) is -1675.7 kJ/mol and the heat...

What is the standard heat of formation of Fe2O3 (s) ?

Fe2O3 (s) + 2Al (s) -> Al2O3 (s) + 2Fe (l) ΔH= -800.3 kJ/mol

If the standard heat of formation of Al2O3 (s) is -1675.7 kJ/mol and the heat of fusion for iron (Fe (s) -> Fe(l)) is 5.2 kj/mol, determine the standard heat of formation of Fe2O3 (s).

llltkl | Student

The problem can be solved by applying Hess' law of constant heat summation. Consider the equations:

2Al (s) + 3/2 O2 -> Al2O3 (s)  ΔH= -1675.7 kJ/mol.......(i)

Fe (s) -> Fe (l) ΔH= 5.2 kJ/mol.........(ii)

2Fe (s) + 3/2 O2 -> Fe2O3 (s)  ΔH= x kJ/mol.......(iii)

Performing the operation (i) + 2* (ii) - (iii) we get

2Al (s) + 3/2 O2 + 2Fe (s) + Fe2O3 (s) -> Al2O3 (s) +  2Fe (l) + 2Fe (s) + 3/2 O2 ,

canceling similar terms on both sides,

2Al (s) + Fe2O3 (s) -> Al2O3 (s) +  2Fe (l) , which is nothing but the reaction given above. Hence applying the heat summation one can write,

-1675.7+ 2*5.2 + -x = -800.3 kJ/mol

or, x = -1675.7+ 2*5.2 + 800.3 = - 865.0 kJ/mol. This is the  standard heat of formation of Fe2O3 (s). (answer)