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? (At. wt. of $Fe = 56$ )

• A. $-40.99 \,kJ$
• B. $-81.98 \,kJ$
• C. $+818.8 \,kJ$
• D. $-819.8 \,kJ$

Answer 1

Answer: (D)

Weight of iron burnt $=4 \,g$

Heat liberated $=29.28 \,kJ$

and atomic weight of iron $=56$

In ferric oxide $\left( Fe _{2} O _{3}\right), 2$ moles of iron or $2 \times 56=112 \,g$ of iron are burnt.

Also, when $4 \,g$ of iron are burnt, then heat liberated $=29.28 \,kJ$.

when $112 \,g$ of iron are burnt, then heat liberated

$=-\frac{29.28 \times 112}{4}$

$=-819.8 \,kJ$

Therefore, enthalpy of formation of ferric oxide is $-819.8 \,kJ$.