The vapour pressure of pure cyclohexane is 12.2 kPa at 25°C and 25.1 kPa at 42°C. Determine the enthalpy of vaporization of cyclohexane.

Question

Quizplus · Accepted Answer

Using the Clausius-Clapeyron equation, we can solve for the enthalpy of vaporization ($\Delta H_{vap}$):

$\ln\frac{P_2}{P_1}=\frac{\Delta H_{vap}}{R}\left(\frac{1}{T_1}-\frac{1}{T_2}ight)$

$P_1 = 12.2 	ext{ kPa}$, $P_2 = 25.1 	ext{ kPa}$, $T_1 = 298	ext{ K}$, $T_2 = 315	ext{ K}$, and $R = 8.314	ext{ J/mol K}$.

Plugging in these values, we get:

$\ln\frac{25.1}{12.2}=\frac{\Delta H_{vap}}{8.314}\left(\frac{1}{298}-\frac{1}{315}ight)$

Solving for $\Delta H_{vap}$, we get:

$\Delta H_{vap} = 33.1 	ext{ kJ/mol}$

Therefore, the best choice is B, which matches this value.

The Vapour Pressure of Pure Cyclohexane Is 12