Chemical Thermodynamics Question 82
Question: If the bond dissociation energies of $ XY,,X_2 $ and $ Y_2 $ (all diatomic molecules) are in the ratio of 1 : 1 : 0.5 and $ {\Delta_{f}}H $ for the formation of $ XY $ is $ -200,kJ,mol{{e}^{-1}} $ . The bond dissociation energy of $ X_2 $ will be [AIEEE 2005]
Options:
A) $ 100,kJ,mo{{l}^{-1}} $
B) $ 800,kJ,mo{{l}^{-1}} $
C) $ 300,kJ,mo{{l}^{-1}} $
D) $ 400,kJ,mo{{l}^{-1}} $
Show Answer
Answer:
Correct Answer: B
Solution:
$ XY\xrightarrow{{}}{X_{(g)}}+{Y_{(g)}} $ ; $ \Delta H=+akJ/mole $ —.(i) $ X_2\xrightarrow{{}}2X $ ; $ \Delta H=+akJ/mole $ —. (ii) $ Y_2\xrightarrow{{}}2Y $ ; $ \Delta H=+0.5,akJ/mole $ —.(iii) $ \frac{1}{2}\times \text{(ii) }+\frac{1}{2}\times \text{(iii)-(i),} $ gives $ \frac{1}{2}X_2+\frac{1}{2}Y_2\xrightarrow{{}}XY $ ; $ \Delta H=( +\frac{a}{2}+\frac{0.5}{2}a-a )kJ/mole $
$ +\frac{a}{2}+\frac{0.5a}{2}-a=-200 $
$ a=800 $
