Question

The enthalpies of combustion at ${25}^{o}C$ of $H_2$, cyclohexane $\left(C_6{H}_{12}\right)$ and cyclohexene $\left(C_6{H}_{10}\right)$ are $-241,-3920$ and $-3800$ $kJ/mol$ respectively. The heat of hydrogenation of cyclohexene is:

• A. $-121kJ/mol$
• B. $+121kJ/mol$
• C. $-242kJ/mol$
• D. $+242kJ/mol$

Answer 1

Answer: (A)

$-121kJ/mol$

$H_2\left(g\right)+\frac{1}{2}{O}_2\left(g\right)⟶H_{2}O\left(l\right);\Delta H=-241kJ...\left(i\right)$
$C_6{H}_{10}+\frac{17}{2}{O}_2\left(g\right)⟶6C{O}_2\left(g\right)+5H_{2}O\left(l\right);\Delta H=-3800kJ..\left(ii\right)$
$C_6{H}_{12}+9O_2\left(g\right)⟶6C{O}_2\left(g\right)+6H_{2}O\left(l\right);\Delta H=-3920kJ...\left(iii\right)$
$eq.\left(i\right)+eq.\left(ii\right)-eq.\left(iii\right)$
$\Delta H=-242-3800-(-3920)=-121kJ$