Question

At \(25 ^{\circ}\text{C}\) standard enthalpy of combustion of \(\text{H}_2\), Cyclohexene and cyclohexane are -241, -3800 and -3920 kJ \(\text{mol}^{-1}\) respectively. What is the heat of hydrogenation of cyclohexene?

• A. \(-121\) kJ
• B. \(-242\) kJ
• C. \(121\) kJ
• D. \(363\) kJ

Hint:

Apply Hess's law: \(\Delta H_{hyd} = \Delta H_c(\text{H}_2)+\Delta H_c(\text{cyclohexene}) - \Delta H_c(\text{cyclohexane})\).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept
Hydrogenation of cyclohexene is \(\text{C}_6\text{H}_{10} + \text{H}_2 \to \text{C}_6\text{H}_{12}\). By Hess's law we can build it from the combustion equations.

Step 2: Key Formula or Approach
Add the combustion of cyclohexene and of H\(_2\), then reverse the combustion of cyclohexane:
\[ \Delta H_{hyd} = \Delta H_c(\text{H}_2) + \Delta H_c(\text{C}_6\text{H}_{10}) - \Delta H_c(\text{C}_6\text{H}_{12}) \]

Step 3: Detailed Explanation
\[ \Delta H_{hyd} = (-241) + (-3800) - (-3920) \]
\[ = -4041 + 3920 = -121\ \text{kJ} \]
The negative sign means the reaction is exothermic. The value \(-242\) would double the H\(_2\) term, and 121 or 363 have wrong signs or terms.

Final Answer:
The heat of hydrogenation is \(-121\) kJ, option (A). \[ \boxed{-121\ \text{kJ (A)}} \]