Question

The enthalpy of combustion of h2 , cyclohexene (c6 h10) and cyclohexane (c6 h12) are – 241, –3800 and –3920 kj per mol respectively. Heat of hydrogenation of cyclohexene is

Answer 1

Answer:

Heat of hydrogenation of cyclohexene is -121 kJ/mol.

Explanation:

$H_2+\frac{1}{2}O_2\rightarrow H_2O,\Delta H_c=-241 kJ/mol$...(1)

$C_6H_{10}+\frac{17}{2}O_2\rightarrow 6CO_2+5H_2O,\Delta H_c=-3800 kJ/mol$...(2)

$C_6H_{12}+9O_2\rightarrow 6CO_2+6H_2O ,\Delta H_c=-3920 kJ/mol$...(3)

$C_6H_{10}+H_2\rightarrow C_6H_{12} \Delta H_{hydrogenation}$...(4)

(1)+(2)-(3)=(4)

$\Delta H_{hydrogenation}=-241 kJ/mol+(-3800 kJ/mol)-(-3920 kJ/mol)=-121 kJ/mol$

Heat of hydrogenation of cyclohexene is -121 kJ/mol.

Answer 2

Answer: The heat of hydrogenation of cyclohexene is -121kJ/mol

Explanation:

$H_2+\frac{1}{2}O_2\rightarrow H_2O$ ,$\Delta H_1 =-241 kJ/mol$..(1)

$C_6H_{10}+\frac{17}{2}O_2\rightarrow 6CO_2+5H_2O$ $\Delta H_2= -3800 kJ/mol$...(2)

$C_6H_{12}+9O_2\rightarrow 6CO_2+6H_2O$ ,$\Delta H_3=-3920 kJ/mol$ ..(3)

According to Hess’s law of constant heat summation, the heat absorbed or evolved in a given chemical equation is the same whether the process occurs in one step or several steps.

According to this law, the chemical equation can be treated as ordinary algebraic expression and can be added or subtracted to yield the required equation. That means the enthalpy change of the overall reaction is the sum of the enthalpy changes of the intermediate reactions.

Adding 1 and 2 and subtracting 3 , the net chemical equation:

$C_6H_{10}+H_2\rightarrow C_6H_{12}$ $\Delta H_{4}$ = ?...(4)

$\Delta H_4=\Delta H_1+\Delta H_2-\Delta H_3=(-241)+(-3800)-(-3920)=-121kJ/mol$

The heat of hydrogenation of cyclohexene is -121 kJ/mol