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 the formation of ferric oxide? (Atomic Mass of $Fe=56g/mole$)

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

Answer 1

The correct option is B $-819.8kJ$

$2Fe+3/2O_2\to F{e}_2{O}_3$

Number of moles of $Fe=\frac{4g}{56g/mol}=0.0714$ mol.

2 moles of $Fe$ gives one mole of $F{e}_2{O}_3$

0.0714 moles of $Fe$ will give $\frac{0.0714}{2}=0.0357$ moles of $F{e}_2{O}_3$.

When 0.0357 moles of $F{e}_2{O}_3$ are formed, 29.28 kJ of heat is evovled.

When 1 mole of $F{e}_2{O}_3$ are formed, $29.28kJ\times \frac{1mol}{0.0357mol}=819.8kJ/mol$ of heat is evovled.

A negative sign will be introduced as heat is evolved.

Hence, the answer is $-819.8$ kJ/mol.