Depression, freezing point

0 Freezing Point Depression and Boiling Point Elevation Making Water Freeze Colder and Boil Hotter 

A Sprinkling salt on icy roads lowers the freezing point of water, so the ice melts even if the temperature is below 0 °C. 

Note that molality Is abbreviated with a low-eroase m, while molarity Is abbreviated with a oapital M. 

Have you ever wondered why salt is added to ice in an ice-cream maker Or why salt is scattered on icy roads in cold climates Salt actually lowers the melting point of ice. A salt-and-water solution will remain a liquid even below 0 °C. By adding salt to ice in the ice-cream maker, you form a mbcfure of ice, salf, and wafer fhaf can reach a temperature of about —10 °C, cold enough to freeze the cream. On the road, the salt allows the ice to melt, even if the ambient temperature is below freezing. 

Notice that molality is defined wifh respect to kilograms of solvent, not kilograms of solution. 

The freezing point depression follows from the lowering of vapor pressure. From the Clausius-Clapeyron equation and Raoulfs law, it follows enthalpy of 

Taking the molality m (defined as the ratio of the amount of dissolved matter and the mass of water, expressed in mol kg and in contrast to the molarity not depending on T in dissolved solution molarity is proportional to molahty) we obtain  

Although recognized for hundreds of years, artificial weather modification was not applied before the turn of the 20 century. It started by simply introducing artificial CCN to the cloud layers by guns, later rockets and aircrafts. By end of the 1940s the introduction of crystallization nuclei (e. g. Agl, dry ice and many others) affected the ice phase processes in supercooled clouds and larger hygroscopic particles (artificial precipitation embryos about 30 pm in diameter) to stimulate collision-coalescence processes in warm clouds. About 10 countries applied different techniques with variable success. 

The equilibrium freezing point of pure water at atmospheric pressure is 0 °C. When a solute, (e.g. sugar or salt) is present, the solute molecules do not fit comfortably into the ice crystal lattice. They effectively get in the way of the water molecules trying to join onto the crystal, so that it 

The amount by which the freezing point changes is known as the freezing point depression , and depends on the number of solute molecules present, but not their type. It can be shown that (for low solute concentrations) the freezing point depression, AT, is given by 

What is the freezing point depression constant of naphthalene  

K( is the freezing point depression constant in C-kg/mol, and m is the molal concentration in mol/kg. 

Read over the entire laboratory activity. Use the periodic table in your textbook to answer the following questions. 

The solution in question 3 freezes at -0.192°C. Because water normally freezes at 0°C, this means that the freezing point has decreased by 0.192°C. Thus, ATf = -0.192°C. What is the freezing point depression constant of water, Kfl 

Add about 400 mL of water to a 600-mL beaker. Using a hot plate, heat the water until it boils. CAUTION The hot plate and boiling water can cause burns. 

Solvent Normal Boiling Point (X) Kfc(X/m) Normal Freezing Point (X) KfCC/m) 

Like the boiling-point elevation, the change in freezing point AT is directly proportional to solute molality, taking into account the van t Hotf factor i  

The proportionality constant K( is the molal freezing-point-depression constant, analogous to for boiling point elevation. Note that because the solution freezes at a lower temperature than does the pure solvent, the value of AT is negative. 

Some typical values of and K for several common solvents are given in A Table 13.3. For water, the table shows K, = 0.51 °Cfm, which means that the boiling point of any aqueous solution that is 1 m in nonvolatile solute particles is 0.51 °C higher than the boiling point of pure water. Because solutions generally do not behave ideally, the constants listed in Table 13.3 serve well only for solutions that are rather dilute. 

For water, Kf is 1.86 °Cfm. Therefore, any aqueous solution that is 1 min nonvolatile solute particles (such as 1 m C5H12O5 or 0.5 m NaCl) freezes at the temperature that is 1.86 °C lower than the freezing point of pure water. 

If a solute is dissolved in a liquid the freezing point of the liquid is usually lowered. The thermodynamics of this process are very simple if the solute dissolves only in the liquid phase of the solvent and does not form solid solutions with it. Then the phase that separates out on cooling is the pure solvent in its solid state. 

Equating the chemical potentials of the solvent A in both phases, 

If the subscript 1 (used to denote that it is the solvent that is in equilibrium) is dropped from all but the mole fraction term xx, then 

We notice that this equation is similar to that obtained for the ideal solubility of solids (Section 6.4). Indeed the two situations are thermodynamically identical, but the enthalpy of fusion is that of the solute in the ideal solubility equation. In the case now being examined the solid state is the pure solvent (Fig. 6.8). We may integrate as before from Xj = 1, T — Tfus to xx = xt T —T and obtain 

Frequently the equation is used in a simplified form. The approximations commonly made are as follows  

Sweetener Sucrose equivalence value (SE)a Freezing point equivalence factor  

You can observe the effect of freezing point depression by doing the miniLAB on this page. 

A solution s freezing point depression, AT, is the difference in temperature between its freezing point and the freezing point of its pure solvent. For nonelectrolytes, the value of the freezing point depression, which is symbolized as AT, is directly proportional to the solution s molality. 

This phase diagram shows how temperature and pressure affect the solid, liquid, and gas phases of a pure solvent (solid lines) and a solution (dashed lines). The difference between the solid and dashed lines corresponds to vapor pressure lowering (AP), boiling point elevation (AT[,), and freezing point depression (ATf). 

Measuring The colligative property of freezing point depression can be observed in a simple laboratory investigation. You will measure the temperatures of two beakers and their contents. 

Materials 400-mL beakers (2), crushed ice, rock salt (NaCI), water, stirring rods (2), graduated cylinder, thermometers (2), balance 

TABLE 13.3 Molal Boiling-Point-Elevation and Freezing-Point-Depression Constants 

A solute dissolved in water causes the boiling point to increase by 0.51 °C. Does 

The vapor-pressure curves for the liquid and solid phases meet at the triple point, a. (Section 11.6) In FIGURE 13.24 we see that the triple-point temperature of the solution is lower than the triple-point temperature of pure liquid because the. solution has a lower vapor pressure than the pure liquid. 

A FIGURE 13.24 Phase diagram illustrating freezing-point depression. 

The boiling point of 0.2 m solution of glucose is 100 + 0.1 = 100.1°C Freezing-point Depression 

The solute concentration affects the fi-eezing point of a solvent. The freezing point of a pure solvent is decreased when solutes are added to it. Just like the boiling-point elevation, freezing-point depression is proportional to the molal concentration. The relationship is shown  

Calculate the freezing point of 2 m aqueous solution of glucose. K, of water is 

In this problem, we have.all the necessary values to find the freezing-point depression. The ionization factor is 1. 

As in Sec. 12.2.1, we assume the solid that forms when a dilute solution is cooled to its freezing point is pure component A. 

Equation 12.3.6 on page 375 gives the general dependence of temperature on the composition of a binary liquid mixture of A and B that is in equilibrium with pure solid A. We treat the mixture as a solution. The solvent is component A, the solute is B, and the temperature is the freezing point Tf. 

Consider the expression on the right side of this equation in the limit of infinite dilution. In this limit, Tf becomes T, the freezing point of the pure solvent, and Aso, aH becomes Afus,A the molar enthalpy of fusion of the pure solvent. 

To deal with the partial derivative on the right side of Eq. 12.4.1 in the limit of infinite dilution, we use the fact that the solvent activity coefficient ya approaches 1 in this limit. Then the solvent chemical potential is given by the Raoult s law relation 

If the solute is an electrolyte, Eq. 12.4.2 can be derived by the same procedure as described in Sec. 9.4.6 for an ideal-dilute binary solution of a nonelectrolyte. We must calculate xa from the amounts of all species present at infinite dilution. In the limit of infinite dilution, any electrolyte solute is completely dissociated to its constituent ions ion pairs and weak electrolytes are completely dissociated in this limit. Thus, for a binary solution of electrolyte B with v ions per formula unit, we should calculate xa from 

As shown before, small amounts of a solute (2) dissolved in a solvent (1) will lower the freezing point. An expression for the freezing point depression can directly be derived starting from Eq. (8.10) for simple eutectic systems, where in the case of diluted solutes ideal behavior can be assumed. 

Following Eq. (8.12) the freezing point depression is proportional to the cryoscopic constant and the concentration of the solute (m2 (mol/kg)) in the molality scale. Since the cryoscopic constant K ry of the solvent depends on the molar mass, the melting temperature T and the enthalpy of fusion, the various liquids show different values. From Eq. (8.12) it can be seen that the measurement of the freezing point depression allows the determination of the molar mass of unknown compounds, if the melting temperature and the enthalpy of fusion is known. 

Calculate the cryoscopic constant of water and naphthalene using the following pure component data. 

If you have ever lived in a cold climate, you may have seen roads and sidewalks that were salted in the winter. The application of a salt such as NaCl or CaCU thaws ice (or prevents its formation) by lowering the freezing point of water. 

Because Tf Tf, ATf is a positive quantity. Again, the change in temperature is proportional to the molal concentration of the solution  

We get the freeziig point of the solution by sJjtTicting ATf from the freezing point of the puresoHiem Tf = Tf ATt 

Sample Problem 13.7 demonstrates a practical application of freezing-point depression and boiling-point elevation. 

When a solution freezes the solid that separates out Is actually pure solvent The solute remains in the liquid solution. 

Like boiling-point elevation, freezing-point depression can be explained in terms of differences in entropy. Freezing involves a transition from the more disordered liquid state to the more ordered solid state. For this to happen, energy must be removed from the system. Because a 

TABLE 13.2 Molal Boiling-Point Hlc alion and Freezing-Point Depression Conslanls of Sc cral Common Sohcnls 

Each individual liquid has a specific temperature at which it freezes. If you use a particular liquid as a solvent in a solution, though, you find that the freezing point of the solution is always lower than the pure liquid. This is called the freezing point depression, and it s a colligative property of a solution. 

In case you re interested, you can actualiy caicuiate the eunount the freezing point wiii be depressed  

Keeping blood celts atii/e and iPett Osmotic pressure 

Suppose that you take a container and divide it into two compartments with a thin membrane containing microscopic pores large enough to allow water molecules but not solute particles to pass through. This membrane type is called a semipermeable membrane it lets some small particles pass through but not other, larger particles. 

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Beckmann thermometer A very sensitive mercury thermometer with a small temperature range which can be changed by transferring mercury between the capillary and a bulb reservoir. Used for accurate temperature measurements in the determination of molecular weights by freezing point depression or boiling point elevation. 

This is an expression of Raoult s law which we have used previously. Freezing point depression. A solute which does not form solid solutions with the solvent and is therefore excluded from the solid phase lowers the freezing point of the solvent. It is the chemical potential of the solvent which is lowered by the solute, so the pure solvent reaches the same (lower) value at a lower temperature. At equilibrium... 

Important physical and functional properties of maltose and maltose symps include sweetness, viscosity, color stabiUty, humectancy, freezing point depression, and promotion of beneficial human intestinal microflora growth. Maltose possesses ca 30—40% of the sweetness of sucrose in the pure state (32). 

Commercially, sulfolane is available as a crystalline anhydrous material, and containing 3 wt % deionized water as a freezing point depressant, as Sulfolane-W. 

Sweetness is primarily a function of the levels of dextrose and maltose present and therefore is related to DE. Other properties that increase with increasing DE value are flavor enhancement, flavor transfer, freezing-point depression, and osmotic pressure. Properties that increase with decreasing DE value are bodying contribution, cohesiveness, foam stabilization, and prevention of sugar crystallization. Com symp functional properties have been described in detail (52). 

Many chemicals when added to water cause a freezing point depression, as shown in Table 1, and thus are termed antifreezes. The antifreeze properties of these chemicals vary widely as a function of their coUigative, or concentrative, properties. The reduction in freeze point depends both on the chemical itself and the concentration of the chemical in water. The freeze point depression increases as the antifreeze chemical is added to the water, until a characteristic concentration is achieved. Further addition of the antifreeze chemical to water will either result in insolubility or serve to increase the freezing point of the mixture, as illustrated in Figure 1. 

Table 1. Freeze Point Depression of Antifreeze Chemicals ...

Component CAS Registry Number Molecular formula Concentration in water, wt % Freeze point depression, °C... 

Fig. 1. Freeze point depression as a function of solute concentration (1,2). Calcium chloride sucrose (-------), and urea (------) become...

Like brines, alcohols were readily available and widely used as antifreeze Hquids in the early 1900s. Both methanol and ethanol offer exceUent heat transfer and efficient freeze point depression. However, the alcohols have the distinct disadvantage of their low boiling points. During the summer months when the engines operate hot, significant amounts of the alcohols are lost because of evaporation. These evaporative losses result in cosdy make-up requirements. Additionally, the alcohols have very low flash points and potentially flammable vapors. These safety concerns have, particularly in recent years, caused the use of alcohols to be completely discontinued for most heat-transfer systems. 

Freeze Point Depression. The slight heat-transfer penalty incurred when an antifreeze is added to the aqueous heat-transfer fluid is necessitated by the need for increased operating temperature range in most internal combustion engines. Because most parts of the world achieve temperatures below freezing during some time of the year, an antifreeze fluid is required to keep equipment operational in these subfreezing temperatures. 

The toxicity of antifreeze and deicing fluids is predorninantly a function of the main component, the freezing point depressant. Eor ethylene glycol-based fluids, the toxicity is well-defined, as the toxicity of ethylene glycol has been studied extensively because of its wide usage in varied appHcations (16). 

M depends not on the molecular sizes of the particles but on the number of particles. Measuring colligative properties such as boiling point elevation, freezing point depression, and vapor pressure lowering can determine the number of particles in a sample. 

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