Question

The enthalpies of combustion of cyclohexane, cyclohexene and $H_2$ are respectively $-3920,-3800$ and $-241kJ$ ${mol}^{-1}$. The heat of hydrogenation of cyclohexene is:

• A. $-121kJ$ ${mol}^{-1}$
• B. $121kJ$ ${mol}^{-1}$
• C. $-242kJ$ ${mol}^{-1}$
• D. $242kJ$ ${mol}^{-1}$

Answer 1

Answer: (A)

$-121kJ$ ${mol}^{-1}$

Heat of hydrogenation $={\Delta }_c$ (cyclohexene) + ${\Delta }_c\left(H_2\right)-{\Delta }_c$ (cyclohexane)

$=(-3800-241+3920)KJ{mol}^{-1}$

$=-121KJ{mol}^{-1}$