Calculate the delta H for 3Fe2O3 = 2Fe3O4 + 1/2 O2

Question

Given the following data:

2Fe + 3/2 O2 = Fe2O3 delta H= -823 kJ

3Fe + 2O2 = Fe3O4 delta H= -1120 kJ

Answer 1

This is an application of Hess's law.

Write the first reaction in reverse and multiply all coefficients and ΔH by 3 after reversing the sign of the original ΔH .

3 Fe2O3 ======> 6 Fe + 9/2 O2 ΔH = +2469

Write the second reaction as is but multiply all coefficients and ΔH by 2

6 Fe + 4 O2 =======> 2 Fe3O4 ΔH = - 2240

Now add the two equations and cancel similar quantities on each side. Add the two new ΔH s.

3 Fe2O3 =======> 2 Fe3O4 + 1/2 O2 ΔH = 229 kj