GENERAL CHEMISTRY: CHAPTER EIGHTGENERAL CHEMISTRY CHAPTER 8 3/11/2010 1GENERAL CHEMISTRY: CHAPTER EIGHT SOLUTIONS AND MIXTURES 1.1 INTRODUCTION. A mixture may either be homogeneous (single phase) or heterogeneous (multiple phase). There are occasions when we work with a homogeneous mixture where the amounts of the components are very different. In that case, the dominant component qualifies for the title of solvent. Any species that are present in relatively modest amounts are called solutes. We shall limit the use of the term solutions, to mixtures where there is clearly a solvent-solute relationship. We consider it to be absurd to talk of gaseous solutions. Smokes (solid suspensions) and aerosols (liquid suspensions) should be considered to be heterogeneous mixtures. The term solid solutions is frequently applied to alloys, amalgams and “doped” crystals, where solute species can produce dramatic alterations to the physical properties of the solid host solvent. We shall not have occasion to pay much attention to solid solutions. Our prime concern will be with liquid solutions and liquid mixtures. In so far as solutions are concerned, we are thus limiting our interest to liquid solvents. 1GENERAL CHEMISTRY CHAPTER 8 3/11/2010 2 If one scours the literature, one can find references to a vast assortment of liquid (or fluid) solvents. We shall confine our interest to no more than a few types. By far the most important solvent, from a general chemist’s viewpoint, is water. Water is highly polar and can use its hydrogen bonding ability to accomplish some interesting solvating activities. The liquid hydrocarbons and halocarbons are grouped together under the general heading of non-polar solvents. We make a distinction between those non-polar solvents, notably the alkanes, which are not only non-polar but only very weakly polarizible and those which, like benzene and tetrachloromethane have quite large polarizibilities. High polarizibility means an added opportunity to interact favorably with potential solutes by means of dispersion forces. There are a number of organic liquids that are classified as being polar solvents. Acetone { (CH3)2 CO } and ethanol (C2H5OH) are good examples. We should take note of the fact that mixing pairs of solvents together produces mixed solvents that can bring into solution pairs of reactant species that neither individual solvent could accommodate. 2GENERAL CHEMISTRY CHAPTER 8 3/11/2010 31.1 Concentration Units. If we are going to deal with solutions in any type of quantitative context, we will need to have some means of expressing the composition. One of the more practical expressions of composition is that which is widely used for solubilities. This is the number of grams of solute that can be dissolved in 100 mL. of solvent. We are often faced with the alternative of expressing the amount of a substance in terms of its mass or of its number of moles. Any time that we are working with chemical reactions, the number of moles is the more appropriate measure to use. Since we find it to be most convenient to dispense liquids by volume, one very useful concentration unit is the molarity, which is defined to be the number of moles of the solute species that are present in each liter of solution. The major drawback of using molarities as concentration units is that the molarity of a solution changes if one changes the temperature. Typically, an increase of temperature means an increase in volume and that, in turn, means a decrease in molarity. The change may be so trivial as to be unproblematic. On the other hand, if one wants to be as precise as possible, one should adopt a different way of expressing composition. 3GENERAL CHEMISTRY CHAPTER 8 3/11/2010 4While volumes change with temperature, masses and moles do not. The number of moles of solute that are dissolved in 1 kilogram of solvent is called the molality of the solution. 1.2 Gaseous Solutes. Although we shall limit our attention to liquid solvents we shall need to concern ourselves with gaseous, liquid and solid solutes. 1.3 All of the substances that exist as gases at ambient temperatures are molecular in nature and of relatively low molar mass. Most of them are non-polar. Gas solubilities, in water, tend to be fairly low. Carbon dioxide is relatively soluble in water because of the interaction with water to produce carbonic acid. The oxygen that dissolves in water is necessary to maintain many types of aquatic life. Atmospheric oxygen and nitrogen dissolve to a modest but non - trivial extent in our blood streams. The solubilities of gases decrease with increasing temperature. Carbonated beverages tend to go “flat” at higher than ice - box temperatures. If the temperatures of lakes and rivers become too high, the solubility of oxygen falls to a level where fish suffocate. That is what is implied by the term thermal pollution. The solubilities of gases vary in direct proportion to their partial pressures above the solution. This relationship is called Henry’s law. Artificial carbonation of beverages is accomplished by applying a CO2 4GENERAL CHEMISTRY CHAPTER 8 3/11/2010 5pressure of the order of 5.0 atmospheres. Of course when the bottle or can is opened the CO2 pressure drops and the solubility drops with it. The release of gas bubbles is not however instantaneous which means that recapping the bottle, after a few minutes, provides a means of retaining a decent level of carbonation. Deep sea and scuba divers are inhaling air at substantially higher pressures than atmospheric, in order that their lungs should not collapse. That means that their blood will dissolve greater amounts of oxygen and nitrogen. The body can use the oxygen but not the nitrogen. If the diver returns to the surface too quickly nitrogen bubbles form in the blood stream causing a condition called “bends” which can be extremely painful. Gaseous hydrocarbons like methane and ethane are quite soluble in liquid hydrocarbon solvents. This is a good example of the miscibility of chemically similar substances: like dissolves like. 1.3 Solid Solutes. 5GENERAL CHEMISTRY CHAPTER 8 3/11/2010 6We draw an important distinction between non-electrolyte and electrolyte solutes. With non-electrolytes, we again find it to be useful to keep in mind the old cliché “Like dissolves like”. That means that a solid nonpolar substance, like a high molar mass