Colligative properties of water

Water is the ideal biological solvent. It easily dissolves a wide variety of the constituents of living organisms. Examples include ions (e.g., Na+, K+, and CF), sugars, and many of the amino acids. Its inability to dissolve other substances, such as lipids and certain other amino acids, makes supramolecular structures (e.g., membranes) and numerous biochemical processes (e.g., protein folding) possible. In this section the behavior of hydrophilic and hydrophobic substances in water is described. This discussion is followed by a brief review of osmotic pressure, one of the colligative properties of water. Colligative properties are physical properties that are affected not by the specific structure of dissolved solutes, but rather by their numbers. 

This is a situation where you need to employ the colligative properties of water solution, specifically, to lower the freezing point of the water covering the streets. You check a physical chemistry textbook (or an Internet source) and find the following formula for the freezing point depression, ATp... 

As noted earlier, colligative properties of solutions are directly proportional to the concentration of solute particles. On this basis, it is reasonable to suppose that, at a given concentration, an electrolyte should have a greater effect on these properties than does a nonelectrolyte. When one mole of a nonelectrolyte such as glucose dissolves in water, one mole of solute molecules is obtained. On the other hand, one mole of the electrolyte NaCl yields two moles of ions (1 mol of Na+, 1 mol of Cl-). With CaCl three moles of ions are produced per mole of solute (1 mol of Ca2+, 2 mol of Cl-). 

Salt is a strong electrolyte that produces two ions, Na+ and Cl, when it dissociates in water. Why is this important to consider when calculating the colligative property of freezing point depression ... 

As solutes are added to the liquid phase and the mole fraction of water is thereby lowered, water molecules have less tendency to leave the solution. Hence, the water vapor partial pressure in the gas phase at equilibrium becomes less —this is one of the colligative properties of solutions that we mentioned earlier. In fact, adjacent to dilute solutions P at equilibrium depends linearly on the mole fraction of water (Nw) in the liquid phase. This is Raoult s law (also mentioned in Appendix IV). For pure water, Nw equals 1 and Pwv has its maximum value, namely P w> the saturation vapor pressure. 

As osmosis proceeds, pressure builds up on the side of the membrane where volume has increased. Ultimately, the pressure prevents more water from entering, so osmosis stops. The osmotic pressure of a solution is the pressure needed to prevent osmosis into the solution. It is measured in comparison with pure solvent. The osmotic pressure is directly related to the different heights of the liquid on either side of the membrane when no more change in volume occurs. Osmotic pressure depends on the temperature and the original concentration of solute. Interestingly, it does not depend on what is dissolved. Two solutions of different solutes, for example alcohol and sugar, will each have the same osmotic pressure, provided they have the same concentration. Osmotic pressure is therefore a colligative property of solutions, one which depends only on the concentration of dissolved particles, not on their chemical identity. 

The additional water molecules on the solution side of the membrane create pressure and push some water molecules back across the membrane. The amount of additional pressure caused by the water molecules that moved into the solution is called the osmotic pressure. Osmotic pressure depends upon the number of solute particles in a given volume of solution. Therefore, osmotic pressure is another colligative property of solutions. 

Evidence for dimerization of the hydroxoaqua ion is based on the decrease of the number of chromium particles in reaction 10 and its increase in reaction 11. The number, v, of discrete chromium particles per diaqua ion should decrease from v = 1 to v = as the diaqua ion is titrated with 1 mol of OH" and then rise again to v = 1 as a second mole of OH" is added, v may be determined by measuring a colligative property of the solution. A most suitable method for ionic solutes is Three-Phase Vapor Tensiometry, TPVT i The three-phase solvent system consists of a saturated solution of an electrolyte in water, in equilibrium with the crystalline phase of that electrolyte and with water vapor. An isobaric temperature difference (AT)p is established when the pure solvent is equilibrated with a solution of a foreign solute in the same solvent, at constant pressure. The apparent number, Vm of free particles per formula of solute depends on the molality of the solute (m), the three-phase ebulioscopic constant Kg, and (AT)p... 

The colligative 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 applications. 

FIGURE 2-11 Solutes alter the colligative properties of aqueous solutions. (a) At 101 kPa (1 atm) pressure, pure water boils at 100°C... 

The interactions of ions with water molecules and other ions affect the concentration-dependent (colligative) properties of solutions. Colligative properties include osmotic pressure, boiling point elevation, freezing point depression, and the chemical potential, or activity, of the water and the ions. The activity is the driving force of reactions. Colligative properties and activities of solutions vary nonlinearly with concentration in the real world of nonideal solutions. 

Another colligative property of solutions is the lowering of the freezing point. (As mentioned before, a 1 M NaCl solution freezes at approximately -1 °C.) This property is particularly important in cold areas of the world where salt is applied to icy roads. When the applied salt dissolves in the thin layer of water on the surface of ice, forming a very concentrated solution, it causes the ice to melt (it lowers the freezing point of water). The same property is also used to protect automobile engines. The coolant used... 

Ionic solutions are formed when the solute ionizes in water. The ions of the molecule separate in water and are surrounded or hydrated by the water molecules. Ionic molecules greatly influence the mobility of water molecules surrounding them and affect the colligative properties of solvent water. The degree to which the structure of bulk water is disrupted depends on the valence, size, and concentration of the ion in solution. In ice, the presence of ions interferes with intermolecular forces between water molecules and disrupts the crystal lattice structure. Hence the presence of salt decreases the melting point of water. 

A U. luO-L solution is made by dissolving u.44i gof CaCl2(s) in water, (a) Calculate the osmotic pressure of this solution at 27 "C, assuming that it is completely dissociated into its component ions, (b) The measured osmotic pressure of this solution is 2.56 atm at 27 C. Explain why it is less than the value calculated in (a), and calculate the van t Hoff factor, i, for the solute in this solution. (See the A Closer Look box on Colligative Properties of Electrolyte Solutions in Section 13.5.) (c) The enthalpy of solution for CaCl2 is AH = —81.3 kj/mol. If the final temperature of the solution is 27 °C, what was its initial temperature (Assume that the density of the solution is 1.00 g/mL, that its specific heat is 4.18 J/g-K, and that the solution loses no heat to its surroundings.)... 

The next major step was the enunciation of Raoult s law (Raoult 1887, 1888). In 1887, Francois Raoult published his investigations on the vapor pressure of the solvent in dilute solutions. He studied five solutes in water and 14 solutes in each of 11 organic solvents and found that the diminution of the vapor pressure of the solvent upon addition of a given (small) amount of solute was proportionally the same for all cases. The proportionality factor is the mole fraction of the solute. This may be expressed in the currently accepted notation as p° -pi p°X2 this is known as Raoult s law. Raoult had previously discovered the laws of freezing point depression and boiling point elevation (Raoult 1878, 1882), three of the so-called colligative properties of dilute solutions. 

Explain each of the following statements (a) The boiling point of seawater is higher than that of pure water, (b) Carbon dioxide escapes from the solution when the cap is removed from a carbonated soft-drink bottle, (c) Molal and molar concentrations of dilute aqueous solutions are approximately equal, (d) In discussing the colligative properties of a solution (other than osmotic pressure), it is preferable to express the concentration in units of molality rather than in molarity, (e) Methanol (b.p. 65°C) is useful as an antifreeze, but it should be removed from the car radiator during the summa- season. 

When you dissolve something in water, for example, salt, the solution has slightly different properties than pure water. For example, a salty solution boils at a higher temperature and freezes at a lower temperature than pure water. Physical chemists have established that these changes, from pure solvent to solution, depend on the number of particles of the solute and not so much on the type of solute. The common name for these effects is colligative properties of mixtures and solutions. Outside of a physical chemical laboratory those of us who live in colder climates are well familiar with the phenomenon of freezing point depression or cryoscopy. This is also a physical chemical experiment actively practiced by the fish swimming in arctic seas. 

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