Question

When 4 g of iron is burnt to ferric oxide at constant pressure, 29.28 kJ of heat is evolved. What is the enthalpy of formation of ferric oxide?

(Atomic mass of Fe = 56)

Options

• −81.98 kJ

• −819.8 kJ

• −40.99 kJ

• +819.8 kJ

Answer 1

−819.8 kJ

Explanation:

Required equation is \[\ce{2Fe + 3O2 -> Fe2O3}\]

∴ ∆H = `(-29.28  "kJ" xx 2  "mol" xx 56  "g mol"^-1)/(4  "g")`

= −819.8 kJ