Question

AB, A2 and B2 are diatomic molecules. If the bond enthalpies of A2, AB & B2 are in the ratio

1 : 1 : 0.5 and enthalpy of formation of AB from A2 and B2 is – 100 kJ/mol–1. What is the bond enthalpy of A2.

• A. 400 kJ/mol
• B. 200 kJ/mol
• C. 100 kJ/mol
• D. 300 kJ/mol

Answer 1

Answer: (A)

400 kJ/mol

Answer 2

$\frac{1}{2}$A – A + $\frac{1}{2}$ B – B$\rightarrow$ AB $\Delta$H = – 100 KJ/mole

$\frac{1}{2}$x + $\frac{1}{2}$ (0.5x) – x = – 100 $\Rightarrow$ $\frac{x}{2}$+ 0.25x – x = – 100

$\Rightarrow$ – 0.25 x = – 100 $\Rightarrow$ x = 400 KJ/mole

Bond enthalpy = 400 kJ/mol.