Elevation of the boiling-point

Consider a solution that is in equilibrium with the vapor of the pure solvent. The equilibrium condition is that 

The boiling point T of the solution is expressed in terms of the heat of vaporization and the boiling point of the pure solvent, and Tq, and the mole fraction x of solvent in the 

Methyl alcohol Ethyl alcohol Acetone Acetic acid Benzene Cyclohexane Ethyl bromide 

The elevation of the boiling point is used to determine the molecular weight of a solute in the same way as is the freezing-point depression. It is desirable to use a solvent that has a large value of K. In Eq. (13.30) if is replaced by Tq then 

But many liquids follow Trouton s rule AiS ap 90 J/K mol. Since = 8.3 J/K mol, then, 

The last two sections were concerned with the equilibrium between an ideal solution and one of its components as a pure solid phase. An exactly similar theory will apply to the equilibrium between an ideal solution and one of its components as a pure vapour. When one or more involatile solutes are dissolved in a volatile solvent, and when the solution is sufficiently dilute to be ideal, it is readily confirmed that the elevation of the boiling-point of the solvent is given by equations closely similar to (8-43), (8 47) and (8 60). 

Let T be the equilibrium temperature when the mole fraction of the solvit is X, Then the equations in question are 

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Raoult s law When a solute is dissolved in a solvent, the vapour pressure of the latter is lowered proportionally to the mole fraction of solute present. Since the lowering of vapour pressure causes an elevation of the boiling point and a depression of the freezing point, Raoult s law also applies and leads to the conclusion that the elevation of boiling point or depression of freezing point is proportional to the weight of the solute and inversely proportional to its molecular weight. Raoult s law is strictly only applicable to ideal solutions since it assumes that there is no chemical interaction between the solute and solvent molecules. 

If the thermometer is to be used to determine the elevation of the boiling-point of a liquid on the addition of a solute, it must be remembered that at the boiling-point of the pure solvent the mercury must now be about 1-2 above the bottom of the scale S, and hence for adjustment purposes the temperature of the beaker of water should be 6—7 above the boiling-point of the liquid itself, instead of 1-2 as before. 

Molecular weight determinations by depression of the freezrng-point are more accurate and far less troublesome than those by the elevation of the boiling-point (p. 440), and the former method should always be preferentially employed if the solubility of the organic compound in the soh ent is sufficiently high. 

The use of water as a solvent should be avoided whenever possible, as its Molecular Elevation Constant is so low that only a small elevation of the boiling-point is obtained. 

Elevation of the boiling point by dissolved solids. Organic substances dissolved in organic solvents cause a rise in boiling point which is proportional to the concentration of the substance, and the extent of rise in temperature is characteristic of the solvent. The following equation applies for dilute solutions and non-associating substances ... 

Where M is the molecular weight of the solute, Dt is the elevation of boiling point in °C, c is the concentration of solute in grams for lOOOgm of solvent, and K is the Ebullioscopic Constant (molecular elevation of the boiling point) for the solvent. K is a fixed property (constant) for the particular solvent. This has been very useful for the determination of the molecular weights of organic substances in solution. 

Because the presence of a nonvolatile solute lowers the vapor pressure of the solvent, the boiling point of the solvent rises. This increase is called boiling-point elevation. The elevation of the boiling point has the same origin as vapor-pressure lowering and is also due to the effect of the solute on the entropy of the solvent. 

TABLE 11.9 Molecular Elevation of the Boiling Point (Continued)... 

Derive Equation (15.71), the van t Hoff expression for the elevation of the boiling point. 

Solid bodies dissolved in water or other liquida produce, to a certain extent, the same effect as high pressure generally, the elevation of the boiling point varies according to the nature of the substance in solution, but it uniformly increases with the same body till the liquid is saturated. Tables founded upon this law have been drawn up, for the purpose of estimating the amount of saline matter in a liquid from its boiling point. 

The constant, K, for the molecular elevation of the boiling-point of carbon disulphide is 23-7.1 The specific heat, C, of liquid carbon disulphide is given by2... 

The colligative properties of solutions are those properties that depend upon the number of dissolved molecules or ions, irrespective of their kind. They are the lowering of the vapor pressure, the depression of the freezing point, the elevation of the boiling point, and the osmotic pressure. These properties may be used in determining molecular weights of dissolved substances. 

BECKMANN METHOD. A method of measuring elevation of the boiling point or depression of the freezing point of a solution, It may be used to measure concentration if the nature of the solute is known, or the molecular weight of the solute if the volume concentration is known. 

Since the osmotic pressure is related to the concentration of dissolved solute particles, it is related to the lowering of the freezing point and elevation of the boiling point. 

The relation between osmotic pressure and lowering of the freezing point and the elevation of the boiling point may be expressed by the relation ... 

The vapor pressure of a solvent is lowered on dissolving the solute in it. This lowering for dilute solutions is proportional to die mole fraction of the solute (Raoult s Law). The lowering of the vapor pressure of the solution can be related to the lowering of the freezing point and the elevation of the boiling point. These phenomena serve as a basis for molecular weight determinations. If both components of the solution are volatile, each lowers the vapor pressure of the other and the ratios of the two substances in the liquid and vapor phase are not necessarily the same. Use is made of this fact to separate the two substances by distillation. 

The study of the elevation of the boiling point of a solvent by the addition of a nonvolatile solute is a special case of isopiestic vapor-liquid equilibria. The solvent is the only volatile component, and the standard state of the solvent is chosen as the pure solvent. Under these conditions Equation (10.81) may be written as... 

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