The boiling point elevation

The coUigative properties of antifreeze chemicals may also result in boiling point elevation. As the chemical is added to water, the boiling point of the mixture increases. Unlike the freeze depression, the boiling elevation does not experience a maximum the boiling point versus concentration curve is a smooth curve that achieves its maximum at the 100% antifreeze level. The boiling point elevation can be another important characteristic for antifreeze fluids in certain heat-transfer appHcations. 

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. 

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. 

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. 

The depression of the activity may be measured in various ways. The most obvious would involve a measurement of the vapor pressure lowering, but this method is superseded by others both in accuracy and in simplicity of execution. The boiling point elevation and freezing point depression methods relegated vapor pressure measurement... 

We compute the value of the van t Hoff factor, then use this value to compute the boiling point elevation. 

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. 

In this equation, ATb is the number of degrees that the boiling point has been elevated (the difference between the boiling point of the pure solvent and the solution), Kb is the boiling-point elevation constant, m is the molality of the solute, and i is again the van t Hoff factor. 

The freezing point depression and boiling point elevation techniques are useful in calculating the molar mass of a solute or its van t Hoff factor. In these cases, you will begin with the answer (the freezing point depression or the boiling point elevation), and follow the same steps as above in reverse order. 

At 13.5 kN/m2, water boils at 325 K and, in the absence of data on the boiling point elevation, this will be taken as the temperature of evaporation, assuming an aqueous solution. The total enthalpy of steam at 325 K is 2594 kJ/kg. 

What is the yield of sodium acetate crystals (CH3C00Na.3H20) obtainable from a vacuum crystalliser operating at 1.33 kN/m2 when it is supplied with 0.56 kg/s of a 40 per cent aqueous solution of the salt at 353 K The boiling point elevation of the solution is 11.5 deg K. 

By a similar set of arguments, it can be demonstrated that the boiling point elevation for dilute solutions containing a nonvolatile solute is given by the expression... 

It can be observed that g is the ratio between the observed osmotic pressure and the osmotic pressure that would be observed for a completely dissociated electrolyte that follows Henry s law [see Equation (15.47)], hence the name, osmotic coefficient. A similar result can be obtained for the boiling point elevation, the freezing point depression, and the vapor pressure lowering. 

Boiling point elevations are directly proportional to the molality of a solution, but chemists have found that some solvents are more susceptible to this change than others. The formula for the change in the boiling point of a solution, therefore, contains a proportionality constant, abbreviated K, which is a property determined experimentally and must be read from a table such as Table 13-2. The formula for the boiling point elevation is... 

A. 120.6°C. The problem asks for the boiling point of the solution, so you know that first you have to calculate the boiling point elevation. This means you need to know the molality of the solution and the value of the solvent (acetic acid). Table 13-2 tells you that the of acetic acid is 3.07. To calculate the molality, you must convert 45.2 g of menthol to moles ... 

What is the boiling point of a solution containing 158 g of sodium chloride (NaCl) and 1.2 kg of water What if the same number of moles of calcium chloride (CaCl2) is added to the solvent instead Explain why there s such a great difference in the boiling point elevation. 

Find the boiling point elevation or freezing point depression. 

In the present study, systems composed of two solvents and a salt are treated as ternary systems. Data on the vapor pressure depression of the solvent by the salt for isothermal systems and on the boiling point elevation of the solvent in the presence of salt for isobaric systems are used to develop the parameters for the solvent-salt binaries. For such binaries only the activity coefficients for the solvent are considered. The parameters for all three binary sets are generated from the binary data by a regression subroutine. 

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. 

For certain liquids, tile temperature of a boiling solution of the unknown may be compared with that of boiling water at the same pressure, For a given solution, the boiling-point elevation may be calibrated in terms of specific gravity at standard temperature. Usually two resistance thermometers are used. The system finds use in the control of evaporators to determine the endpoint of evaporation, Good accuracy is achieved in the determination of one dissolved component, or of mixtures of fixed composition. 

Estimate the boiling-point elevation and the normal boiling points of (a) 0.10 m C12H22On(aq) ... 

Before proceeding to use this equation some consideration must be given to the boiling point elevation of sodium chloride solutions. This varies with concentration and with temperature level. Because somewhat conflicting values have appeared in some of the recent literature on saline-water conversion, it seems desirable to digress for a moment to give the basis for the present values. The International... 

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