Solution boiling-point elevation

For the same vacuum level, a crystallizing slurry will have a higher temperature than predicted for the pure solvent because the vapor pressure of the solvent is reduced by the presence of the solute (boiling point elevation). For adiabatic crystallization with the contents temperature as the input to the master control loop, the same temperature profile appropriate for crystallization by jacket cooling would apply here. However, the capability of the vacuum source and the line pressure drop should be considered in conjunction with the boiling point elevation to ensure that the desired final temperature can be met. If this is not satisfied, the desired yield may be achieved by removing some of the distillate, provided the saturation of an impurity is not reached. For most... 

Boiling Point Elevation LiquidA/apor Equilibrium Boiling Point Elevation Addition of a Solute Boiling Point Elevation Solution/Vapor Equilibrium... 

Calculate the following colligative properties of a dilute solution boiling-point elevation, freezing-point depression, and osmotic pressure. 

Boiling point elevation. A solute which does not enter the vapor phase to any significant extent raises the boiling point of the solvent. As above, the solute lowers the activity of the solvent, which, in turn, lowers the vapor pressure. Therefore the solution must be raised to a higher temperature before its vapor pressure reaches 1.0 atm. At equilibrium... 

The properties of a solution differ considerably from those of the pure solvent Those solution properties that depend primarily on the concentration of solute particles rather than their nature are called colligative properties. Such properties include vapor pressure lowering, osmotic pressure, boiling point elevation, and freezing point depression. This section considers the relations between colligative properties and solute concentration, with nonelectrolytes that exist in solution as molecules. 

When a solution of a nonvolatile solute is heated, it does not begin to boil until the temperature exceeds the boiling point of the solvent. The difference in temperature is called the boiling point elevation, ATb. 

Boiling point elevation is a direct result of vapor pressure lowering. At any given temperature, a solution of a nonvolatile solute has a vapor pressure lower than that of the pure solvent. Hence a higher temperature must be reached before the solution boils, that is, before its vapor pressure becomes equal to the external pressure. Figure 10.8 (p. 270) illustrates this reasoning graphically. 

The freezing point lowering, like the boiling point elevation, is a direct result of the lowering of the solvent vapor pressure by the solute. Notice from Figure 10.8 that the freezing point of the solution is the temperature at which the solvent in solution has the same vapor pressure as the pure solid solvent. This implies that it is pure solvent (e.g., ice) that separates when the solution freezes. 

Boiling point elevation and freezing point lowering, like vapor pressure lowerings are colligative properties. They are directly proportional to solute concentration, generally expressed as molality, m. The relevant equations are... 

Boiling point elevation (ATb) Increase in the boiling point caused by addition of a nonvolatile solute, 269-271 Bomb calorimeter Device used to measure heat flow, in which a reaction is carried out within a sealed metal container, 202-203... 

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. 

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. 

In physical chemistry, we apply the term colligative to those properties that depend upon number of molecules present. The principal colligative properties are boiling point elevation, freezing point depression, vapour pressure lowering, and osmotic pressure. All such methods require extrapolation of experimental data back to infinite dilution. This arises due to the fact that the physical properties of any solute at a reasonable concentration in a solvent are... 

A = Kj- Cflj A 7b = Ki) Cflj We use molality in these equations because they describe temperature changes. The constant Zf is called the freezing point depression constant, and is called the boiling point elevation constant. These constants are different for different solvents but do not depend on the identity of the solutes. For water, Zf is 1.858 °C kg/mol and is 0.512 °C kg/mol. 

Like freezing point depression and boiling point elevation, osmotic pressure is proportional to the concentration of solute molecules. Experiments show that osmotic pressure is proportional to both concentration (expressed as... 

In 1910 Stobbe and Posnjak drew the conclusion that polystyrene is a colloidal body after noting that boiling point elevations of its solutions are negligible. They proposed cyclic formulas composed of four, five, or possibly more structural units, e.g. 

The solvent s activity can be determined by measuring the saturation vapor pressure above the solution. Such measurements are rather tedious and their accuracy at concentrations below 0.1 to 0.5M is not high enough to produce reliable data therefore, this method is used only for concentrated solutions. The activity can also be determined from the freezing-point depression or boiling-point elevation of the solution. These temperature changes must be ascertained with an accuracy of about 0.0001 K, which is quite feasible. This method is used primarily for solutions with concentrations not higher than 1M. 

The table above shows the effects of various solutes in a given volume of water. Without knowing the acmal values, which of these is the most likely reason that the Na2C03 will cause the greatest boiling point elevation ... 

Colligative1 properties of dilute polymer solutions depend only on the number of dissolved molecules and not on properties of the molecules themselves, such as mass or size. Osmotic pressure, freezing point depression, boiling point elevation, and vapour pressure lowering are the most prominent examples. These methods essentially allow one to count the number n of solute molecules. From n and the known total mass m of the solute the molar mass M is readily obtained as... 

What is the evaporation rate and yield of the sodium acetate hydrate CH3C00Na.3H20 from a continuous evaporative crystalliser operating at 1 kN/m2 when it is fed with 1 kg/s of a 50 per cent by mass aqueous solution of sodium acetate hydrate at 350 K The boiling point elevation of the solution is 10 degK and the heat of crystallisation is 150 kJ/kg. The mean heat capacity of the solution is 3.5 kJ/kg K and, at 1 kN/m2, water boils at 280 K at which temperature the latent heat of vaporisation is 2.482 MJ/kg. Over the range 270-305 K, the solubility of sodium acetate hydrate in water s at T(K) is given approximately by ... 

Allowing for the boiling point elevation, the temperature of the solution at equilibrium is ... 

B. In the equation At = i m - Kb, where At is the boiling-point elevation, m is the molality of the solution, and Kb is the boiling-point-elevation constant for water, i (the van t Hoff factor) would be expected to be 4 if H3B03 were completely ionized. According to data provided, i is about 1.5. Therefore, H3B03 must have a relatively low Ka. 

C. The boiling-point-elevation constant of 0.512°C kg/mole would be expected to raise the B.P. 0.0256°C for a 0.05 m solution when i = 1. The data show that the boiling-point elevation is 0.0255°C. This agrees with the theory. Therefore, C6Hi206 does not dissociate. With few or no ions in solution, poor electrical conductivity is expected. This is supported by the evidence in the table. 

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