Binary Mixtures

Binary mixtures of miscible liquids such as Water and ethanol are of interest because of the differences in vapor pressure that they exhibit and the influence that vapor pressure has upon their separation by distillation. All mixtures fall into one of three classes, and their properties are considerably different.

Class I

Class 1 includes all mixtures with total vapor pressure, regardless of the composition of the mixture, that is always less than that of the most volatile component and always more than that of the least volatile component; consequently, the boiling point of Class I mixtures is always between those of the two components. The com-

Figure 3.1

Composition of liquid and vapor phases during distillation of Class I binary mixtures.

Figure 3.1

Composition of liquid and vapor phases during distillation of Class I binary mixtures.

position of the vapor is always richer in the more volatile component than the liquid from which it distills. Such mixtures are amenable to essentially complete separation by means of fractional distillation.

A diagram showing the composition of the liquid and vapor phases, and how separation of the two components of Class I mixtures can be accomplished by fractional distillation, is given in Fig. 3.1. If a mixture of A and B having the composition represented by x is heated to its boiling point, the liquid will have a temperature corresponding to I, and the vapor produced will have a composition corresponding to v on the vapor curve. If the vapor at v is condensed, a liquid corresponding to x', much richer in B, is obtained. Redistillation of the mixture x' results in a vapor with a composition x". Through successive condensations and evaporations, normally accomplished by a fractionating column, a distillate of essentially pure B can be obtained, and A will remain as a relatively pure residue in the still. A wide variety of compounds form Class I binary mixtures.

Class II

Class n binary mixtures include those that at certain mole ratios have vapor pressures less than either of the components and, consequently, at these ratios have boiling points that are greater than that of either of the components. Upon distillation of such mixtures, one or the other of the components may be fractionated into relatively pure form until the liquid mixture reaches a composition of minimum vapor pressure or maximum boiling point. From that point on, a constant boiling mixture is obtained, the compositions of vapor and liquid are identical, and further separation is impossible by this means.

A diagram showing the composition of the liquid and vapor phases of a Class II binary mixture is given in Fig. 3.2. If a mixture corresponding to x' is distilled, the

Mole fraction B.

Figure 3,2

Composition of liquid and vapor phases during distillation of a Class II binary mixture.

Mole fraction B.

Figure 3,2

Composition of liquid and vapor phases during distillation of a Class II binary mixture.

vapor formed is rich in component A and may be recovered in part in relatively pure form by fractionation. However, as A is removed from the liquid phase, the liquid phase grows richer in B until it equals the composition shown by x. At this point the compositions of the vapor and liquid phases are identical, and a constant boiling mixture that cannot be fractionated results. Likewise, if a mixture with a composition represented by xf is distilled, a distillate of B can be obtained, the liquid remaining will approach a composition equal to x, and a constant boiling mixture will result. Hydrochloric, hydrobromic, hydroiodic, hydrofluoric, nitric, and formic acid in aqueous solution are all binary mixtures of Class II The constant boiling mixture of hydrochloric acid at 1 atm pressure contains 20.2 percent HC1 and is often used as a primary standard in quantitative analysis.

Class m

Class III binary mixtures include those that at certain mole ratios have vapor pressure greater than that of either of the components, and therefore the boiling points at such mole ratios are lower than that of either component. Upon distillation of Class III mixtures, the results are opposite to those obtained with Class II mixtures. A distillate is obtained that contains both components in a constant ratio, and the residue remaining in the flask consists of one or the other component in pure form.

A diagram showing the composition of the liquid and vapor phases of a Class III binary mixture is given in Fig. 3.3. A mixture corresponding to x' will produce a vapor with composition v. Fractionation of this vapor will produce a distillate with composition x, and the liquid phase will grow richer in component A. Distillation of a mixture corresponding to x" will produce a vapor with a composition v'. Fractionation will yield a distillate with composition x, and the liquid will become richer in component B. Eventually either pure A or pure B will remain in the liquid phase.

Mole fraction B

Figure 3.3

Composition of liquid and vapor phases during distillation of a Class III binary mixture.

Mole fraction B

Figure 3.3

Composition of liquid and vapor phases during distillation of a Class III binary mixture.

Ethanol (ethyl alcohol) and water form a binary mixture of this class. The distillate, regardless of the composition of the original mixture, will always contain 95.6 percent of alcohol at 1 atm pressure as long as both components are present in the liquid phase,

Healthy Chemistry For Optimal Health

Healthy Chemistry For Optimal Health

Thousands Have Used Chemicals To Improve Their Medical Condition. This Book Is one Of The Most Valuable Resources In The World When It Comes To Chemicals. Not All Chemicals Are Harmful For Your Body – Find Out Those That Helps To Maintain Your Health.

Get My Free Ebook


Post a comment