Q. If the bond dissociation energies of $XY,\ X_{2}$ and $Y_{2}$ (all diatomic molecules) are in the ratio of 1: 1: 05 and $\mathrm{\Delta}H$ for the formation of $XY$is $- 200\ kJ\ mol^{- 1}$. The bond dissociation energy of $X_{2}$ will be
A) $100\ kJ\ mol^{- 1}$
B) $800\ kJ\ mol^{- 1}$
C) $300\ kJ\ mol^{- 1}$
D) $400\ kJ\ mol^{- 1}$
Explanation
(b)
$XY \rightarrow X(g) + Y(g);\ \mathrm{\Delta}H = + a\$kJ/mol …(i)
$X_{2} \rightarrow 2X;\ \mathrm{\Delta}H = + a\$kJ/mol …(ii)
$Y_{2} \rightarrow 2Y;\ \mathrm{\Delta}H = + 0.5a$ kJ/mol …(iii)
$\frac{1}{2} \times (ii) + \frac{1}{2} \times (iii) - (i)$ gives
$\frac{1}{2}X_{2} + \frac{1}{2}Y_{2} \rightarrow XY$
$\mathrm{\Delta}H = \left( + \frac{a}{2} + \frac{0.5}{2}a - a \right)kJ/mol$
$\therefore\ - 200 = + \frac{a}{2} + \frac{0.5a}{2} - a$
or $a = 800$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.