Question

The standard molar enthalpies of formation of cyclohexane (l) and benzene (l) at ${25}^{\circ }C$ are -156 and +49 kJ/mol. The standard enthalpy of hydrogenation of cyclohexene (l) at ${25}^{\circ }C$ is -119 kJ/mol. Estimate the magnitude of resonance energy.

• A. 357 kJ/mol
• B. 205 kJ/mol
• C. 152 kJ/mol
• D. None of the above

Answer 1

The correct option is C

152 kJ/mol

$C_6{H}_{10}\left(l\right)+H_2\left(g\right)\to C_6{H}_{12}\left(l\right)\Delta H=-119kJ/mol$
$6C\left(s\right)+6H_2\left(g\right)\to C_6{H}_{12}\left(l\right){\Delta }_{f}H=-156kJ/mol$
$6C\left(s\right)+3H_2\left(g\right)\to C_6{H}_6\left(l\right){\Delta }_{f}H=+49kJ/mol$
Two things are needed here,
$C_6{H}_6\left(l\right)+3H_2\left(g\right)\to C_6{H}_{12}\left(l\right)$
and benzene losing all its double donds and forming cyclohexane.
Subtracting the first from the second will give us the resonance energy.
We flip the formation of benzene equation, add it to the formation of cyclohexane equation:
$C_6{H}_6\left(l\right)\to 6C\left(s\right)+3H_2\left(g\right)\Delta H=-49kJ/mol$
$\underset{–}{6C\left(s\right)+6H_2\left(g\right)\to C_6{H}_{12}\left(l\right){\Delta }_{f}H=-156kJ/mol}$
$C_6{H}_6\left(l\right)+3H_2\left(g\right)\to C_6{H}_{12}\left(l\right)\Delta H=-205kJ/mol$
Hypothetical heat of hydrogenation of benzene

Resonance energy = |–357 – (–205)|
= |–152| kJ/mol
= 152 kJ/mol