Exercise 9 | Thermochemistry

The oxidation of iron has a change of enthalpy ΔH0rxn = -1648 kJ.mol-1 and may be represented as:

4 Fe (s) + 3 O(g) → 2 FeO(s)


1) Is this reaction endothermic or exothermic?

2) Calculate the energy released as heat when 25.0 g of iron undergo oxidation at constant pressure.

1) ΔH0rxn < 0 ⇒ the reaction is exothermic


2) ΔH0rxn = - 1648 kJ.mol-1 ⇒ energy released per mole of the stated reaction equation

4 molecules of iron react during the reaction

⇒ -1648 kJ per 4 moles of Fe(s) is released as heat


nFe = mFe / MFe = 25.0 / 55.8 = 4.48 x 10-1 mol

Energy released = ΔH = H0rxn4 nFe

⇒ ΔH = - 1648 × 1034 × 4.48 × 10-1

⇒ ΔH = - 1.85 x 105 J = - 185 kJ