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).



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