Question

Enthalpy of hydrogenation is the enthalpy change when one mole of unsaturated compound is converted to saturated compound. Heat of hydrogenation helps to calculate resonance energy.
The standard enthalpy of combustion at ${25}^{\circ }C$ of hydrogen, cyclohexene $\left(C_6{H}_{10}\right)$ and cyclohexane $\left(C_6{H}_{12}\right)$ are –300,–3600 and –3770 KJ/mole respectively. Calculate the heat of hydrogenation of cyclohexene.

• A. +130 KJ/mole
• B. –78 KJ/mole
• C. –130 KJ/mole
• D. –137 KJ/mole

Answer 1

The correct option is C –130 KJ/mole

$H_2+\frac{1}{2}{O}_2\to H_{2}O;\Delta H=-300$ ...equ.(A)
$C_6{H}_{10}+\frac{17}{2}{O}_2\to 6C{O}_2+5H_{2}O;\Delta H=-3600$ ...equ. (B)
$C_6{H}_{12}+9O_2\to 6C{O}_2+6H_{2}O;\Delta H=-3770$ ...equ. (C)
$C_6{H}_{10}+H_2\to C_6{H}_{12};\Delta H$=Reversing equation (C);
$6C{O}_2+6H_{2}O\to C_6{H}_{12}+9O_2;\Delta H=3770$ ...(D)
Adding equation (A), (B) and (D); we get
$C_6{H}_{10}+H_2\to C_6{H}_{12}$ (Required equation)
$\therefore \Delta H=-300-3600+3770$
$=-130KJ/mol$.