Question

When $4g$ of iron is burnt to ferric oxide at constant pressure, $29.28kJ$ of heat is evolved. What is the enthalpy of formation of ferric oxide? (At. wt. of $Fe=56$ )

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

Answer 1

Answer: (D)

Weight of iron burnt $=4g$

Heat liberated $=29.28kJ$

and atomic weight of iron $=56$

In ferric oxide $\left(F{e}_2{O}_3\right),2$ moles of iron or $2\times 56=112g$ of iron are burnt.

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

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

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

$=-819.8kJ$

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