3.1 RAOULT\u2019S LAW FOR A SOLUTION CONTAINING A VOLATILE SOLUTE<\/strong><\/h4>\nRaoult\u2019s law states that the partial vapour pressure of a component of a solution of two miscible liquids A and B at a given temperature is equal to the product of the vapour pressure of the pure component at that temperature and its mole fraction in the solution.<\/p>\n
Mathematical Expression<\/strong>: Let us assume that a solution has nA<\/sub> moles of liquid A and nB<\/sub> moles of liquid B. Let ![\"NEET](\"https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/08\/Raoult-1.png\")
be the vapour pressure of the pure liquid A and ![\"NEET](\"https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/08\/Raoultb.png\")
is the vapour pressure of the pure liquid B.<\/p>\n![\"mathematical](\"https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/08\/mathematical-expression.png\")
<\/p>\n
where PB<\/sub>\u00a0is the partial vapour pressure of liquid B in the solution.<\/p>\nThe total vapour pressure of an ideal solution containing components A and B is the sum of partial vapour pressures of all the components (Dalton\u2019s law of partial pressures)<\/p>\n
![\"dalton\u2019s](\"https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/08\/daltons-law.png\")
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3.2 RAOULT\u2019S LAW & DALTON\u2019S LAW<\/strong><\/h4>\nFurther, by assuming ideal behavior of vapour phase of both components we also can determine vapour phase composition above the surface of solution of equilibrium.<\/p>\n
If x’<\/em>A<\/sub>\u00a0and x’<\/em>B<\/sub>\u00a0are the mole-fractions of gas-phase components A and B, then by the applying Dalton\u2019s Law of partial pressure<\/p>\n![\"dalton\u2019s](\"https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/08\/daltons-law1.png\")
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Now, if the vapour phase is condensed, we get the distillate. Therefore, composition of distillate will remain same as vapour phase composition.<\/p>\n
In terms of number of moles, it also can be represented as<\/p>\n
![\"moles\"](\"https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/08\/moles.png\")
<\/p>\n
where n‘<\/em>A<\/sub> and n‘<\/em>B<\/sub> are number of moles of component A and B in distilled water (Vapour-phase composition).<\/p>\nIn terms of mass-ratio we can transform the above relation as:<\/p>\n
![\"mass](\"https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/08\/mass-ratio.png\")
<\/p>\n
where w and M are representing mass in grams and molar-masses for respective components.<\/p>\n
These formulae are applicable when vapour phase is in equilibrium with the solution and distilled mass here is of the same vapours collected over a solution.<\/p>\n
<\/span>SECTION 4 : IDEAL AND NON-IDEAL SOLUTIONS<\/span><\/h3>\n4.1 IDEAL SOLUTIONS<\/h4>\n
A solution can be termed as ideal, if it falls into any one of the given below categories:<\/p>\n
\n- Raoult\u2019s Law<\/strong> : An ideal solution is that solution in which each component obeys Raoult\u2019s law under all conditions of temperature and This law is described in the earlier section.<\/li>\n
- Thermodynamics<\/strong> : An ideal solution is defined as the one in which no volume change and enthalpy change takes place on mixing the solute and the solvent in any proportion, i.e.,
\n\u2206Vmixing<\/sub> = 0 and \u2206Hmixing<\/sub>\u00a0= 0.<\/li>\n- Molecular Interactions<\/strong>: An ideal solution is defined as the solution in which the intermolecular interactions between the components are of the same magnitude as the intermolecular attractions found in the pure components.<\/li>\n<\/ol>\n
In actual practice, no solution is ideal. However, when concentration of solute is very low the solution behaves ideally. Therefore, very dilute solutions can be considered to be almost ideal in behavior.<\/p>\n
In general, substances possessing similar structure and polarities form nearly ideal solution. For example,<\/p>\n
\n- Benzene + Toluene<\/li>\n
- Hexane + Heptane<\/li>\n
- Ethyl bromide + Ethyl Iodide<\/li>\n
- Chlorobenzene + Bromobenzene<\/li>\n<\/ol>\n
The graphical behavior of ideal solutions has been shown here.<\/p>\n
![\"graphical\"](\"https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/08\/graphical.png\")
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4.2 NON-IDEAL SOLUTIONS<\/h4>\n
A non-ideal solution is defined as the one which does not obey Raoult\u2019s law. These are divided into two types as explained below :<\/p>\n
Non-Ideal Solutions showing Positive Deviations<\/h4>\n
These can be defined on the basis of :<\/p>\n
\n- Molecular interactions :<\/strong> A non-ideal solution showing positive deviation is defined as the one in which the intermolecular forces of interactions between the molecules are weaker as the interactions found in any one of the pure components or both.<\/li>\n
- Thermodynamics :<\/strong> A non-ideal solution is defined as the one in which there is a slight increase in volume on mixing and absorption of heat takes place on mixing DVmixing<\/sub> > 0, DHmixing<\/sub> > 0. Some examples are:\n
\n- Acetone + carbon disulphide<\/li>\n
- Acetone + ethyl alcohol<\/li>\n
- Acetone + benzene<\/li>\n
- Carbon tetrachloride + Chloroform<\/li>\n
- Methyl alcohol + water<\/li>\n
- Ethyl alcohol + water<\/li>\n<\/ol>\n<\/li>\n<\/ol>\n
In the given examples from (i) to (vi), the molecules possessing weaker interactions get in between the molecules possessing stronger interactions thereby reducing them. In examples (v) and (vi), though both the components have hydrogen bonding but the extent of hydrogen bonding in the case of water is more than that in the case of alcohols.<\/p>\n
<\/span>Graphical Representation<\/span><\/h3>\nFor one intermediate composition, the total vapour pressure of such a solution will be the highest and the boiling point will be the lowest. This solution acquires the property of boiling at a constant temperature and its composition remains unchanged. Liquid mixtures, which boil out without any change in the composition are called azeotropes or azeotropic mixtures<\/strong>. In case of positive deviations, we get minimum boiling azeotropes. The topic on azeotropes is discussed in the forthcoming section.<\/p>\n![\"positive](\"https:\/\/www.meniit.com\/study-material\/wp-content\/uploads\/2024\/08\/positive-deviations.png\")
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