What Is Raoult's Law?
Raoul's law is one of the basic laws of physical chemistry. As the basis of the study of solution thermodynamics, it has played a guiding role in the study of phase equilibrium and the thermodynamic function of solutions.
- Raoult's law: One of the basic laws of physical chemistry was discovered by French physicist F.-M. Raoul in 1887 when he studied the behavior of dilute solutions containing non-volatile solutes. For: "At a certain temperature, the vapor pressure of the solvent in the dilute solution is equal to the vapor pressure of the pure solvent times the mole fraction of the solvent in the solution."
- This law was proposed on the basis of experiments in 1887, and it is one of the basic laws of thin solutions [1]
- Its mathematical expression is:
- p = p * nA / (nA + nB)
- among them:
- p: vapor pressure of the solution
- p *: vapor pressure of pure solvent
- nA: the amount of the substance of the solvent
- nB: the amount of solute substance [2]
- An important application of Raoul's law is often defined as the ideal solution (although it is described differently in different physical chemistry textbooks). The ideal solution requires that all components in the system meet the Raoul's law in the entire concentration range (or within the concentration range treated as the ideal solution), and then the thermodynamic expression of Raoul's law can be obtained, and the Some properties.
- After the introduction of Raoul's law, it is inevitable to make a qualitative explanation of Raoul's law, but the explanation often involves only intermolecular interactions. For example, there is an explanation: if the difference in the interaction between the solute molecules and the solvent molecules can be ignored, then the number of solvent molecules per unit volume is reduced after the solute is added to the pure solvent, which also reduces the potential for leaving in a unit time. The number of molecules of the solvent entering the gas phase on the surface of the liquid phase, so that the solvent and its vapor pressure reach equilibrium at a lower solvent vapor pressure, that is, the vapor pressure of the solvent in the solution is lower than that of the pure solvent. There are similar expressions in different textbooks, and they are based on the fact that there is no dominant interaction between the molecules of the components. When we examine the deviation of the actual solution from Raoul's law, we naturally attribute the deviation to Different molecular interactions. For example, in a system consisting of acetone and carbon disulfide, both have a positive deviation from Raoul's law. This is because the interaction (attraction) between solvent molecules and solute molecules is smaller than the interaction between the same molecules. Action, so that the molecules of the two become easier to escape in solution than in their pure state, so both produce positive deviations. In systems that have a negative deviation from Raoul's law, for example, a solution composed of acetone and chloroform, because the attractive force between heterogeneous molecules is greater than the interaction between the same molecules, the escape ability of both It is lower than in the pure state formed by itself. But just explaining the magnitude of the intermolecular interaction is not enough in some cases.
- When the molecules of the solute are larger than the molecules of the solvent, a deviation from Raoul's law can also be seen. This phenomenon is particularly pronounced in polymer solutions. Generally, when the molar volume ratio of solute to solvent exceeds 1:10, deviation from Raoul's law can be observed.
- The effect of the increase in the solute molecular volume on the solvent vapor pressure can also be qualitatively explained using the simple physical image described above. As the volume of solute molecules increases, the number of solvent molecules per unit volume is reduced, and the vapor pressure of the solvent is also reduced. From this we also see that the deviation of polymer solutions from Raoul's law is based on the same physical principles as small molecules and is an extension of small molecule behavior. It is pointed out that the significance of the effect of molecular size on the solvent vapor pressure is that Raoul's law is derived from the thermodynamic relationship when the difference in molecular size and the difference in interaction are not considered.
- Therefore, when making a qualitative interpretation of Raoul's law, in addition to requiring that the interactions between the component molecules are the same, the size of the component molecules should also be the same. [3]
- For different solutions, although the law applies to different concentration ranges, any solution can strictly follow the above formula under the condition of xA 1. Raoul's law was first summarized when studying dilute solutions of non-volatile non-electrolyte, but later found that it is also true for solvents in other dilute solutions. In any solution that satisfies xA 1, the acting force on the solvent molecules is almost the same as that in the pure solvent. So, in
- Atomic mole fraction