Boiling point-composition curves

Boiling-point composition curves, 7-12 Bond refractions, 1036 Boring of corks, 55, 56 Boron trifluoride, 183, 862, 864, 865 Bouveault-Blanc reduction, 247, 249, 250, 812, 832... 

Vapour Pressure (or Boiling Point) Composition Curves of Completely Miscible Binary Solutions... 

FIG. 2 Boiling point-composition curves for cyclohexane-toluene mixtures. 

Not all liquids form ideal solutions and conform to Raoult s law. Ethanol and water are such liquids. Because of molecular interaction, a mixture of 95.5% (by weight) of ethanol and 4.5% of water boils below (78.15°C) the boiling point of pure ethanol (78.3°C). Thus, no matter how efficient the distilling apparatus, 100% ethanol cannot be obtained by distillation of a mixture of, say, 75% water and 25% ethanol. A mixture of liquids of a certain definite composition that distills at a constant temperature without change in composition is called an azeotrope 95% ethanol is such an azeotrope. The boiling point-composition curve for the ethanol-water mixture is seen in Fig. 4. To prepare 100% ethanol the water can be removed chemically (reaction with calcium oxide) or by removal of the water as an azeotrope (with still another liquid). An azeotropic mixture of 32.4% ethanol and 67.6% benzene (bp 80.1 °C) boils at 68.2°C. A ternary azeotrope (bp 64.9°C) contains 74.1% benzene, 18.5% ethanol, and 7.4% water. Absolute alcohol (100% ethanol) is made by addition of benzene to 95% alcohol and removal of the water in the volatile benzene-water-alcohol azeotrope. 

Figure 2.15 Boiling point-Composition curves of mixtures of benzene and toluene at 760 mm...

Mixtures that display a maximum in the boiling point-composition curve can lead to initial separation of pure A on fractionation but as the composition of the liquid moves towards B and reaches the maximum, a constant boiling mixture L3 is reached that will distill over unchanged. An example of an azeotropic mixture of maximum boiling point is water (b.p. 100°C) and hydrogen chloride (b.p. -80°C), the azeotrope being 80% water and boiling at 108.6°C. 

Table 14.42 Boiling Point Composition Curves for Aqueous Ethylenediamine Solutions (2)...

The temperature-composition or boiling point-composition curves can be drawn. The shapes of toe curves depend on toe behaviour of toe system as a function of temperature at constant pressure. 

What is the principle of fractional distillation With the help of boiling point -composition curve, explain the separation of an ideal binary liquid mixture. 

A distillation is carried out with a still-head kept at a constant temperature (G). The composition of the distillate is determined in the usual manner that of the mixture distilled is ascertained from the temperature of the still-head, the boiling-point-composition curve having been previously constructed. 

It will be seen that the agreement is very satisfactory, and it is evident that this method may be used to determine the relation between the composition of liquid and of vapour if the boiling point-composition curve has previously been constructed. Further experiments are desirable to test its applicability and the accuracy attainable. 

Form of Boiling Point-Composition Curve.—It is, however, found practically to be impossible in either case to separate the mixture of minimum boiling point even from the less volatile component, although the difference between their boiling points is considerable. In both cases the boiling point-composition curve is very flat where the more volatile component is in large excess, and it is in such cases —when the curve is very flat at either one end or the other —that one at least of the components is exceedingly difficult to separate, and that the distillation method cannot be relied on for the determination of composition. 

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