Question

Given that, the molar combustion enthalpy of benzene, cyclohexane and hydrogen are $x, y,$ and $z,$ respectively, the molar enthalpy of hydrogenation of benzene to cyclohexane is

• A. $x - y + z$
• B. $x - y + 3z$
• C. $y - x + z$
• D. $y - x + 3z$

Answer 1

Answer: (B)

Hydrogenation of benzene is given by
$C_{6}H_{6} +3H_{2}\rightarrow C_{6}H_{12}\ldots\left(a\right)$
Enthalpies are given for,
$C_{6}H_{6}+\frac{15}{2}O_{2}\rightarrow 6CO_{2}+3H_{2}O;\Delta H=x\ldots\left(b\right)$
$C_{6}H_{12}+9O_{2}\rightarrow 6CO_{2}+6H_{2}O;\Delta H=y\ldots\left(c\right)$
$H_{2}+\frac{1}{2}O_{2}\rightarrow H_{2};\Delta H=z \ldots\left(d\right)$
Reaction$\left(a\right)=\left(b\right)-\left(c\right)+3\left(d\right),$
$\therefore \Delta H$ of $a=x-y+3z$