Chemical Thermodynamics Question 263
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) [AIIMS 1999]
Options:
A) - 81.98 kJ
B) - 819.8 kJ
C) - 40.99 kJ
D) + 819.8 kJ
Show Answer
Answer:
Correct Answer: B
Solution:
Given: Weight of iron burnt $ =4g; $ Heat liberated $ =29.28,KJ $ and atomic weight of iron $ (Fe)=56 $ .
We know that in ferric oxide $ (Fe_2O_3),,2moles $ of iron or $ 2\times 56=112,gram $ of iron are burnt.
We also know that when $ 4grams $ of iron are burnt, then heat liberated = 29.28 kJ, therefore when $ 112,grams $ of the iron are burnt, then heat liberated $ =\frac{29.28\times 112}{4}=-819.8,kJ $ (Minus sign due to liberation of heat).
