Colligative Properties Freezing Point

The fact that the chemical potential of a solvent in a solution (composed of solute + solvent) is less than that of the chemical potential of the pure solvent gives rise to a number of properties  

Such properties are termed colligativeproperties (colligative = binding together) and they depend not so much on the nature of the solute particles present but rather on their number, which is expressed by the molality, /Wsoiute, in the final form of the equations involved. 

Consider the effect on the freezing (melting) point, Tm of a liquid of adding some solid (solute) to it. As a practical application we can think of the road gritting protocol in winter. The addition of solids (usually sand/salt mixtures) to road surface water depresses the freezing point to below the lowest temperature likely to be reached and hence inhibits the creation of ice surfaces on the road. 

A pure liquid (say water), once solute is added acts as the solvent and provided only relatively small amounts of solute are added we create a so called ideal solution (Frames 32 and 33). The entropy is increased (i.e. 5ideai solution Spure solvent) by the addition of solute (more randomness in molecular arrangement is created compared with the pure solvent). The chemical potential is lowered  

The process offusion is defined as the phase change taking place when a liquid is converted into a solid. The reverse process (liquid —f solid) is called melting. 

IX for pure solid solvent in solution =n for pure liquid solvent (= m. pt. of solvent) 

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This procedure and the / V I Hall-Heroult process discussed below are examples of commercial uses of a colligative property (freezing point depression) to enable electrolytic reactions to be carried out more economically. 

Colligative properties freezing point depression and osmotic pressure... 

Another application of a power series in physical chemistry is in the discussion of colligative properties (freezing-point depression, boiling-point elevation, and osmotic pressure). If is the mole fraction of solvent, AyapHm is the molar heat of vaporization of the solvent, Tq is the pure solvent s boiling temperature, and T is the solution s boiling temperature, it is shown in physical chemistry textbooks that... 

Methods that depend on end-group analysis or colligative properties (freezing-point depressirai, boiling-point elevation, osmotic pressure) can be employed to determine M . 

BoUmg-point elevation Colligative properties Freezing-point depression MoM boiling-point elevation constant... 

Co to http //now.brookscole.com/ cracoliceSe and click Coached Problems for an exercise on Colligative Properties Boiling Point and Freezing Point. 

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. 

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. 

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... 

Colligative properties, particularly freezing point depression, can be used to determine molar masses of a wide variety of nonelectrolytes. The approach used is illustrated in Example 10.9. 

Freezing point lowering (or other colligative properties) can be used to determine the extent of dissociation of a weak electrolyte in water. The procedure followed is illustrated in Example 10.11. 

The lowering of freezing point and the generation of osmotic pressure both depend on the total concentration of solute particles. Therefore, by using the colligative property to determine the amount of solute present, and knowing its mass, we can infer its molar mass. 

Colligative properties can be sources of insight into not only the properties of solutions, but also the properties of the solute. For example, acetic acid, CH.COOH, behaves differently in two different solvents, (a) The freezing point of a 5.00% by mass aqueous acetic acid solution is — l.72°C. What is the molar mass of the solute Explain any discrepancy between the experimental and the expected molar mass, (b) The freezing-point depression associated with a 5.00% by mass solution of acetic acid in benzene is 2.32°C. Whar is the experimental molar mass of the solute in benzene What can you conclude about the nature of acetic acid in benzene ... 

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... 

The depression of the freezing point of a solvent due to the presence of a dissolved solute is an example of a colligative property, that is, a property of a dilute solution that depends on the number of dissolved particles and not on the identity of the particles. Water has a freezing point depression constant, Kf, of 1.86 K kg mol-1. In other words, for every mole of nonvolatile solute dissolved in a kilogram of water, the freezing point of water is lowered by 1.86°C. The change in freezing point, A T, can be calculated from the equation... 

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