Elevation of boiling point

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. 

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. 

P RTo2 lip = 1 atm., ST is the elevation of boiling-point, To is the boiling-point of the pure solvent. 

Corollary.—The molecular elevation of boiling-point is independent of the nature of the solute. 

Hence the water in the solution freezes at —0.2°C. A similar expression (boiling-point elevation = kb X molality) is used to relate the elevation of boiling point to the molality of the solute. 

Elevation of boiling point by dissolved solids results in differences of 3-10°F between solution and saturated vapor. 

Elevation of boiling point and depression of freezing point of the solution. Within certain limits, the change in temperature of these phase transitions obeys the eqnation... 

A liquid with no appreciable elevation of boiling-point is concentrated in a triple-effect evaporator. If the temperature of the steam to the first effect is 395 K and vacuum is... 

The depression of a melting point is one of the simplest manifestations of a colligative property. Other everyday examples include pressure, osmotic pressure, vapour pressure and elevation of boiling point. 

But salted water boils at a higher temperature than does pure water, so the food cooks more quickly. (We saw on p. 203 how a hotter temperature promotes faster cooking.) The salt causes an elevation of boiling point, which is another colligative property. We call the determination of such an elevation ebullioscopy. 

An almost identical equation relates the elevation of boiling point to the molality ... 

A triple-effect evaporator concentrates a liquid with no appreciable elevation of boiling point. If the temperature of the steam to the first effect is 395 K, and vacuum is applied to the third effect so that the boiling point is 325 K, what are the approximate boiling points in the three effects The overall heat transfer coefficients may be taken as 3.1, 2.3, 1.3 kW/m2 K in the three effects, respectively. 

The elevation of boiling point in this case is much greater than for any of the other systems. The assumptions that the heat term in the Gibbs-Duhem equation can be neglected and that the effect of the salt can be expressed in terms of its effect on the vapor pressure of each solvent independently become less viable as the boiling point elevation increases. 

Because of the elevation of boiling point by dissolved solids, the difference in temperatures of saturated vapor and boiling solution may be 3-10T which reduces the driving force available for heat transfer. In backward feed [Fig. 8.17(b)] the more concentrated solution is heated with steam at higher pressure which makes for lesser heating surface requirements. Forward feed under the influence of pressure differences in the several vessels requires more surface but avoids the complications of operating pumps under severe conditions. 

Even with this unequal distribution there may be little effect on yield of distillate from a substantially fresh water feed hence the high output of the still from distilled water feed. With sea water, 3 to 4% NaCl equivalent, the average or effective boiling point elevation becomes unequal on the two rotors. Thus if a 50% cut is secured and the lower rotor receives twice the feed of the upper, the average residue concentrate of 7% brine from 3.5% feed could be an actual 10% from the upper periphery and 5% from the lower, supposing equal rates of distillation. Actually because of -the different elevations of boiling point (1.1° and 1.8° F.) the rate of evaporation from the upper rotor decreases while that from the lower rotor increases but less than proportionally because of the added thickness of the feed layer. Later experiments at Columbus on the No. 4 machine suggest that this situation existed in the No. 5 still. 

The activity a2 of an electrolyte can be derived from the difference in behavior of real solutions and ideal solutions. For this purpose measurements are made of electromotive forces of cells, depression of freezing points, elevation of boiling points, solubility of electrolytes in mixed solutions and other characteristic properties of solutions. From the value of a2 thus determined the mean activity a+ is calculated using the equation (V-38) whereupon by application of the analytical concentration the activity coefficient is finally determined. The activity coefficients for sufficiently diluted solutions can also be calculated directly on the basis of the Debye-Hiickel theory, which will bo explained later on. 

When electrolyte solutions are subjected to such measurements, abnormal results are obtained. When substances like sodium chloride or magnesium sulphate are examined, the depression of freezing point or the elevation of boiling point is about twice that calculated from the relative molecular mass, with calcium chloride or sodium sulphate these quantities are three times those expected. Keeping in mind what has been said above, we can say that the number of particles in the solution of sodium chloride or magnesium sulphate is twice the number of molecules present, while in the case of calcium chloride or sodium sulphate there are three particles present for each molecule. 

In the 1920s, it was not feasible to accurately measure the molecu-f lar weight of natural or synthetic polymers. Classical methods 1 of molecular weight determina-V tion, those based upon colligative x properties, elevation of boiling point, depression of freezing point and lowering of vapor pressure, worked very well for low-molar-mass compounds, but were essentially useless for macromolecules. Modern instrumental methods that... 

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