Question

The enthalpy of combustion of ${{H}_{2}}$, cyclohexane (${{C}_{6}}{{H}_{10}}$) and cyclohexane (${{C}_{6}}{{H}_{12}}$) are -241, -3800 and -3920 kJ/mol respectively. Heat of hydrogenation of cyclohexene is:
A. -121kJ/mol
B. +121 kJ/mol
C. +242 kJ/mol
D. -242 kJ/mol

Answer 1

Hint: As per the process of heat combustion, in a highly-temperature exothermic redox chemical reaction between a reductant and an oxidant, a combustion takes place in which large amounts of heat and light energy are released. This reaction is known as rapid combustion.

Complete answer:
- Enthalpy is a capacity to reflect to do non-mechanical work and also reflect the capacity to release heat. It is a thermodynamic property.
- Enthalpy is the sum of the internal energy of a system added to the product of the pressure and its volume.
- It is denoted as H. The formula used is shown below
\[-H=E+PV\]
Here, H = enthalpy, E = internal energy of the system, P = pressure and V = volume
Now, $dH=TdS+PdV$
The required reaction is shown below
$\begin{align}
  & {{C}_{6}}{{H}_{10}}+{{H}_{2}}\to {{C}_{6}}{{H}_{12}}...(1) \\
 & \Delta {{H}_{1}}=? \\
\end{align}$
Let us mention the given facts
$\begin{align}
  & {{H}_{2}}+\dfrac{1}{2}{{O}_{2}}\to {{H}_{2}}O...(2) \\
 & \Delta {{H}_{2}}=-241kJ/mol \\
\end{align}$
 $\begin{align}
  & {{C}_{6}}{{H}_{10}}+\dfrac{17}{2}{{O}_{2}}\to 6C{{O}_{2}}+5{{H}_{2}}O...(3) \\
 & \Delta {{H}_{4}}=-3800kJ/mol \\
\end{align}$
$\begin{align}
  & {{C}_{6}}{{H}_{12}}+9{{O}_{2}}\to 6C{{O}_{2}}+6{{H}_{2}}O...(4) \\
 & \Delta {{H}_{4}}=-3920kJ/mol \\
\end{align}$
The required reaction (1) can be obtained by adding equation (2) and (3), and subtracting (4) from the sum of (2) and (3).
$\begin{align}
  & {{C}_{6}}{{H}_{10}}+{{H}_{2}}\to {{C}_{6}}{{H}_{12}} \\
 & \Delta {{H}_{1}}=(\Delta {{H}_{2}}+\Delta {{H}_{3}})-\Delta {{H}_{4}} \\
\end{align}$
= [-241+(-3800)] - (-3920)
= (-241-3800) – (-3920)
= -4041 + 3920
= -121 kJ/mol
Therefore, the heat of hydrogenation of cyclohexene is -121 kJ/mol.

Thus, the answer to the question is (A) -121kJ/mol.

Note:
The organic fuel combustion in air is always exothermic, because the double bond in oxygen is much weaker than the other double bond in the form of campfires and bonfires. Combustion reaction was the first controlled chemical reaction discovered by humans.