Ideal solution Raoult’s law

At equilibrium, a component of a gas in contact with a liquid has identical fugacities in both the gas and liquid phase. For ideal solutions Raoult s law applies ... 

A hypothetical solution that obeys Raoult s law exactly at all concentrations is called an ideal solution. In an ideal solution, the interactions between solute and solvent molecules are the same as the interactions between solvent molecules in the pure state and between solute molecules in the pure state. Consequently, the solute molecules mingle freely with the solvent molecules. That is, in an ideal solution, the enthalpy of solution is zero. Solutes that form nearly ideal solutions are often similar in composition and structure to the solvent molecules. For instance, methylbenzene (toluene), C6H5CH, forms nearly ideal solutions with benzene, C6H6. Real solutions do not obey Raoult s law at all concentrations but the lower the solute concentration, the more closely they resemble ideal solutions. Raoult s law is another example of a limiting law (Section 4.4), which in this case becomes increasingly valid as the concentration of the solute approaches zero. A solution that does not obey Raoult s law at a particular solute concentration is called a nonideal solution. Real solutions are approximately ideal at solute concentrations below about 0.1 M for nonelectrolyte solutions and 0.01 M for electrolyte solutions. The greater departure from ideality in electrolyte solutions arises from the interactions between ions, which occur over a long distance and hence have a pronounced effect. Unless stated otherwise, we shall assume that all the solutions that we meet are ideal. 

Just as the ideal gas law discussed in Section 9.3 applies only to "ideal" gases, Raoult s law applies only to ideal solutions. Raoult s law approximates the behavior of most real solutions, but significant deviations from ideality occur as the solute concentration increases. The law works best when solute concentrations are low and when solute and solvent particles have similar intermolecular forces. 

Further, Raoult s law can be applied when two volatile components are mixed. In systems of liquids that mix in all proportions to form ideal solutions, Raoult s law in the form of the second equation applies to the partial pressure of each volatile component separately. 

Eq. (4) is known as Henry s law, and solute is the Henry s law constant, which is less than Psoiute- Therefore, Henry s law applies to the solute in dilute solutions, and Raoult s law applies to solvent in dilute non-ideal solutions. Note the similarities between Eqs. (1) and (2) and between Eqs. (3) and (4) for the non-ideal dilute solution case. When the solution is ideal, Henry s law becomes identical to Raoult s law, and fsoiute becomes identical to f oiute- When the partial pressures of the solute and the solvent are directly proportional to their molefractions over the entire range, the solution is ideal. In a non-ideal solution, Raoult s law will apply to the solvent over the entire concentration range, whereas Henry s law will apply to the solute in a limited concentration range in which it is in a sufficiently diluted form. 

Relation (3.11) was obtained empirically by Raoult and is called Raoult s law. Hence in ideal solutions Raoult s law holds over the entire range of compositions, this being represented by a straight line in the v or pressure composition curve (Fig. 3.1). In reality, most solutions do not obey Eqs. (3.9) and (3.11). Such solutions are called nonideal, or real. Polymer solutions characteristically display sharp negative deviations from ideality, as can be seen in Fig. 3.1. 

A liquid-liquid solution that obeys Raoult s law is called an ideal solution. Raoult s law is to solutions what the ideal gas law is to gases. As with gases, ideal behavior for solutions is never perfectly achieved but is sometimes closely approached. Nearly ideal behavior is often observed when the solute-solute, solvent-solvent, and solute-solvent interactions are very similar. That is, in solutions where the solute and solvent are very much alike, the solute simply acts to dilute the solvent. However, if the solvent has a special affinity for the solute, such as if hydrogen bonding occurs, the tendency of the solvent molecules to escape will be lowered more than expected. The observed vapor pressure will be lower than the value predicted by Raoult s law there will be a negative deviation from Raoult s law. 

Consider a binary mixture of two types of molecules that are roughly identical in size, shape, and external force field. Such a mixture constitutes an ideal solution. Thus, one of the components of an ideal solution may replace another without seriously disturbing the circumstances of immediate neighbors in the solution. Raoult s law provides an appropriate basis for the treatment of an ideal solution. Raoult s law states that the activity, a, of a solvent in the solution is equal to its mole fraction n, ... 

Distillation is a process of differential vaporization of components of a liquid mixture. The tendency of a given component to vaporize is related to its vapor pressure. In an ideal solution, Raoult s law states that the partial pressure of a component in the vapor over a liquid is equal to the product of its vapor pressure and liquid-phase mole fraction ... 

A solution that obeys Raoult s law exactly is called an ideal solution. Solutions that are associated with either exothermic or endothermic reactions are not ideal solutions. Raoult s law is most accurate when used to describe components of a solution that are present in high concentrations. At low concentrations there are often significant departures from Raoult s law. At very low concentrations the vapor pressure of a solute is given by Henry s law. 

In a ideal solution, Raoult s law is valid for any concentration of the components therefore each component can be considered a solvent. Hence the partial vapour pressure may be expressed by... 

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