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The vapor pressure of dichloromethane, CH2Cl2, at 0°C is 134 mmHg. The normal boiling point of dichloromethane is 40°C. Calculate its molar heat of vaporization using the Clausius-Clapeyron equation.
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13. Liquids, Solids & Intermolecular Forces
Clausius-Clapeyron Equation
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Which of the following cannot be determined using the Clausius-Clapeyron equation?
A
The vapor pressure of a liquid at a given temperature
B
The enthalpy of vaporization of a liquid
C
The boiling point of a liquid at a different pressure
D
The specific heat capacity of a liquid
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Understand that the Clausius-Clapeyron equation relates the vapor pressure of a liquid to temperature and the enthalpy of vaporization. It is commonly written as:
\[\ln P = -\frac{\Delta H_{vap}}{R} \cdot \frac{1}{T} + C\]
where \(P\) is the vapor pressure, \(\Delta H_{vap}\) is the enthalpy of vaporization, \(R\) is the gas constant, \(T\) is the temperature in Kelvin, and \(C\) is a constant.
Recognize that using this equation, you can calculate the vapor pressure at a given temperature if you know the enthalpy of vaporization and a reference vapor pressure at another temperature.
You can also determine the enthalpy of vaporization by measuring vapor pressures at different temperatures and plotting \(\ln P\) versus \(1/T\), then finding the slope, which equals \(-\Delta H_{vap}/R\).
The boiling point at a different pressure can be found by setting the vapor pressure equal to the external pressure and solving for the temperature \(T\) using the Clausius-Clapeyron equation.
However, the specific heat capacity of a liquid is unrelated to vapor pressure or phase change enthalpies and cannot be determined using the Clausius-Clapeyron equation.
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