How to judge whether the gas-liquid phase has reached equilibrium in the experiment
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Is it enough to ensure that the temperatures of the gas phase and liquid phase remain unchanged? Thank you.
When temperature, pressure and volume remain constant, it must be in equilibrium.
1. The gas phase and liquid phase temperatures remain unchanged 2. The gas phase pressure does not change
I think the key is the gas phase pressure. If the gas phase pressure is constant, it will be balanced.
From a macroscopic point of view, if the liquid volume remains unchanged and the gas phase pressure is constant, it means that the gas and liquid phases are in balance. From a microscopic point of view, the rate of vaporization is equal to the rate of condensation. In the laboratory, generally observe whether the thermometer changes.
As long as the pressure does not change, generally the temperature and volume will not change if the pressure does not change.
What everyone mentioned above is about the situation of single-component gas-liquid phase. In fact, there are three strict conditions for multi-component gas-liquid phase equilibrium. All of them are indispensable.: 1. The temperature of the two phases is the same 2. The pressure of the two phases is the same 3. The chemical potential of each component in the two phases must be equal (or the fugacity of each component in the two phases must be equal)
The simplest way is to just see that the pressure remains unchanged.
In the laboratory, the pressure is generally constant. For a binary system, the degree of freedom is two. Therefore, as long as the temperature is constant, it is considered to be in equilibrium.
It mainly depends on whether the temperature of the gas and liquid phases has changed (confined space)
Under different temperature, pressure and composition conditions, the vapor-liquid system composed of a volatile liquid mixture (or single-component substance) and its vapor reaches the ultimate state. At this time, the mass transfer rates of each component between the vapor and liquid phases from the vapor phase to the liquid phase and from the liquid phase to the vapor phase are equal, and the net mass transfer rate is zero. ； Macroscopically, it means that the concentration of each component of a mixture (or single-component substance) in the liquid phase or vapor phase is constant, reaching vapor-liquid equilibrium. When gas and liquid are in contact, the gas dissolves in the liquid, resulting in a certain solubility ； Gas dissolved in a liquid, as a solute, must produce a certain partial pressure. Gas-liquid equilibrium is reached when the partial pressure produced by the solute is equal to the partial pressure of the gas in the gas phase. The establishment of phase equilibrium indicates that the mass transfer reaches the limit and the absorption process stops. It is an important factor in controlling the operation of the absorption system. For most dilute solutions of gases, the equilibrium relationship between gas and liquid can be expressed by Henry's law
The gas and liquid phases have the same temperature； The pressures of the gas and liquid phases are the same
I think the key is that the gas phase pressure remains unchanged
If the temperature remains constant and the gas phase pressure does not change, it is considered equilibrium.