The enthalpy of combustion at 25^ C of H_2 , cyclohexane (C_6 H_ 12 ) and cyclohexene (C_6 H_ 10 ) are -241,-3920 and -3800 ~kJ / mol respectively. The heat of hydrogenation of cyclohexene is :

Correct answer: A. -121 ~kJ / mol aligned & H_2+(1)/(2) O_2 H_2 O & Δ H=-241 kJ . . aligned aligned C_6 H_ 10 +(17)/(2) O_2 6 CO_2+5 H_2 O Δ H=-3800 kJ (ii) aligned aligned C_6 H_ 12 +9 O_2 & 6 CO_2+6 H_2 O & Δ H=-3920 kJ (iii) aligned hydrogenation of cyclohexene- aligned & C_6 H_ 10 +H_2 C_6 H_ 12 & Eq (i) aligned Eq & (ii) -Eq (iii) Δ H & =-241-3800-(-3920) & =-4041+3920 & =-121 kJ aligned aligned

JIPMER 2006

Exam: JIPMER
Year: 2006
Subject: Chemistry
Chapter: Thermodynamics (C)

JIPMER 2006 · Chemistry · Thermodynamics (C)

The enthalpy of combustion at 25^ C of H_2 , cyclohexane (C_6 H_ 12...

The enthalpy of combustion at

25^{\circ} \mathrm{C}

\mathrm{H}_2

, cyclohexane \left(\mathrm{C}_6 \mathrm{H}_{12}\right)

and cyclohexene \left(\mathrm{C}_6 \mathrm{H}_{10}\right)

are -241,-3920 and

-3800 \mathrm{~kJ} / \mathrm{mol}

respectively. The heat of hydrogenation of cyclohexene is :

Options

A $-121 \mathrm{~kJ} / \mathrm{mol}$
B $+121 \mathrm{~kJ} / \mathrm{mol}$
C $-242 \mathrm{~kJ} / \mathrm{mol}$
D $+242 \mathrm{~kJ} / \mathrm{mol}$

Correct answer

A. $-121 \mathrm{~kJ} / \mathrm{mol}$

Step-by-step solution

\begin{aligned} & \mathrm{H}_2+\frac{1}{2} \mathrm{O}_2 \longrightarrow \mathrm{H}_2 \mathrm{O} \\ & \Delta H=-241 \mathrm{ kJ} . .\end{aligned}

\begin{aligned} \mathrm{C}_6 \mathrm{H}_{10}+\frac{17}{2} \mathrm{O}_2 \longrightarrow 6 \mathrm{CO}_2+5 \mathrm{H}_2 \mathrm{O} \\ \Delta H=-3800 \mathrm{ kJ} \ldots \text{ (ii) }\end{aligned}

\begin{aligned} \mathrm{C}_6 \mathrm{H}_{12}+9 \mathrm{O}_2 \longrightarrow & 6 \mathrm{CO}_2+6 \mathrm{H}_2 \mathrm{O} \\ & \Delta H=-3920 \mathrm{ kJ} \ldots \text{ (iii) }\end{aligned}

hydrogenation of cyclohexene-

\begin{aligned} & \mathrm{C}_6 \mathrm{H}_{10}+\mathrm{H}_2 \longrightarrow \mathrm{C}_6 \mathrm{H}_{12} \\ & \mathrm{Eq} \text{ (i) } \begin{aligned} \mathrm{Eq} & \text{ (ii) }-\mathrm{Eq} \text{ (iii) } \\ \Delta H & =-241-3800-(-3920) \\ & =-4041+3920 \\ & =-121 \mathrm{ kJ}\end{aligned}

\end{aligned}

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