The enthalpy of combustion at 25° C of H2, cyclohexane (C6H12) and cyclohexene (C6H10) (C6H10) are - 241, - 3920 and - 3800 kJ/mol respectively. The heat of hydrogenation of cyclohexene is:

Question

The enthalpy of combustion at 25° C of H2, cyclohexane (C6H12) and cyclohexene (C6H10) (C6H10) are - 241, - 3920 and - 3800 kJ/mol respectively. The heat of hydrogenation of cyclohexene is: (A) -121 kJ/mol (B) +121 kJ/mol (C) -242 kJ/mol (D) +242 kJ/mol

Tardigrade Admin · Accepted Answer

Use Hess's law to solve the problem. It states that total heat changes during a reaction are independent of path.

H_{2} + \frac{1}{2} O_{2} H_{2} O

Δ H = - 241 k J

.. (i)

C_{6} H_{10} + \frac{17}{2} O_{2} 6 C O_{2} + 5 H_{2} O

Δ H = - 3800 k J

...(ii)

C_{6} H_{12} + 9 O_{2} 6 C O_{2} + 6 H_{2} O

Δ H = - 3920 k J

...(iii)

C yc l o h e x e n e C_{6} H_{10} + H_{2} C_{6} H_{12}

Δ H = ?

Adding Eqs. (i) and (ii) and subtracting Eq (iii) we get,

C_{6} H_{10} + H_{2} C_{6} H_{12}

H = - 241 - 3800 - (- 3920)

= - 4041 + 3920

= - 121 k J

Q. The enthalpy of combustion at 25∘C of H2​, cyclohexane (C6​H12​) and cyclohexene (C6​H10​) (C6​H10​) are - 241, - 3920 and - 3800 kJ/mol respectively. The heat of hydrogenation of cyclohexene is:

-121 kJ/mol

+121 kJ/mol

-242 kJ/mol

+242 kJ/mol

Q. The enthalpy of combustion at $25^{\circ} C$ of $H_{2},$ cyclohexane $(C_{6} H_{12})$ and cyclohexene $(C_{6} H_{10})$ $(C_{6} H_{10})$ are - 241, - 3920 and - 3800 kJ/mol respectively. The heat of hydrogenation of cyclohexene is: