On the relationship between vapor pressure and pressure
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In Physical Chemistry, 5th Edition, by Fu Xiancai, on page 280, regarding the relationship between external pressure and vapor pressure: adding an inert gas to the liquid surface increases the external pressure, which in turn raises the vapor pressure. The vapor pressure of a liquid increases as the external pressure rises, as increased external pressure enhances the tendency of molecules in the liquid to escape. However, according to common understanding, in the pxy diagram on page 287 of this book, at a certain temperature, as pressure increases, the gas phase transforms into the liquid phase. This indicates that an increase in pressure reduces the escape of liquid-phase molecules. There are two questions now. The first is: what does this external pressure include? Is it the pressure generated by the vaporization of materials within the system, the pressure of inert gases from the outside, or the pressure exerted mechanically from the outside (such as by a piston)? The second question is that there seems to be a contradiction regarding the increase in pressure mentioned above. I’m not sure what the cause is – whether it’s a misunderstanding regarding external pressure or an understanding issue related to closed systems and open systems. In the first case, it’s because the inert gas comes into contact with the materials inside the system; in the second case, pressure increases as the inert gas acts on the piston, without coming into contact with the system’s materials ? I would appreciate it if someone could point out where my understanding is incorrect.
Firstly, external pressure includes the pressure generated by the vaporization of materials within the system, the pressure of external inert gases, and the pressure exerted by external mechanical forces (such as a piston). The increase in these pressures all increases the tendency of molecules in the liquid to escape, thereby causing the vapor pressure of the liquid to rise as the external pressure increases. Secondly, the relationship between the vapor pressure of a liquid and pressure is not simple. When, at a certain temperature, the gas phase transforms into the liquid phase as pressure increases, it is actually because the chemical potentials of the liquid and gas phases reach equilibrium; at this point, the chemical potentials of the gas and liquid phases are equal. Before that, as pressure increases, the tendency of liquid-phase molecules to escape also increases, and thus the vapor pressure of the liquid rises as well. But when the pressure reaches a certain level, the gas-phase molecules can no longer escape, and thus a transition from the gas phase to the liquid phase occurs. Regarding the doubt you mentioned, the answer to the first question has already been explained above. Regarding the second question, increasing pressure affects the chemical potentials of both the liquid and gas phases; the specific outcome depends on the combination of temperature and pressure. In some cases, an increase in pressure reduces the chemical potential difference, thereby promoting the transition from the gas phase to the liquid phase. In other cases, an increase in pressure may lead to an increase in the vapor pressure of the liquid, but it will not cause the gas phase to transform into the liquid phase. Therefore, to understand this, it is necessary to consider more specifically the relationship between temperature, pressure, and matter (liquids and gases). .
You’re really thorough; posts like this just seem unnecessary once you see them.