Chapter 7 Solutions and Colloids 7.1 Physical States of Solutions Solutions are homogeneous mixtures of two or more substances in which the components are present as atoms, molecules, or ions. Properties of solutions. can be in any phase. For example, brass is a solution made from copper and zinc. are clear (gases and liquids), but not necessarily colorless. will not settle over time. Solvent and Solute Solutions are made of a solute and a solvent. Solvent the substance present in the largest amount in the solution. Solute any substance present in an amount less than that of the solvent. A solution may contain more than one solute. 1
Physical States of Solutions The physical state of a solution, solid, liquid, or gas, is usually the same as the physical state of the solvent. 7.2 Solubility Solubility the maximum amount of solute that can be dissolved in a specific amount of solvent at a given temperature and pressure. Also, known as the solubility limit. Soluble vs. Insoluble A solute is soluble if it dissolves to a significant extent in a solvent. A solute is insoluble if it does not dissolve to a significant extent in a solvent. Sugar has limited solubility in water 2
Miscible vs. Immiscible Miscible another way to describe a soluble substance. Immiscible another way to describe an insoluble substance. Commonly used for liquids that do not mix. Degrees of Saturation A saturated solution contains the maximum amount of solute at a given temperature and pressure (at the solubility limit). Saturated solutions are stable solutions. A supersaturated solution contains an amount of solute greater than the solubility limit at a given temperature and pressure. Supersaturated solutions are unstable solutions. An unsaturated solution contains less than the maximum amount of solute (less than the solubility limit) at a given temperature and pressure. Unsaturated solutions are stable solutions. Examples of Solute Solubilities in Water at 0 o C 3
Effect of Temperature on Solubility 7.3 The Solution Process The dissolving of a solute involves interactions between solvent molecules (often water) and the particles of solute. An example of the dissolving (solution) process for an ionic solute in water: Dissolving of a Polar Solute An example of the dissolving process for a polar solute in water: 4
To Dissolve or Not To Dissolve A solute will not dissolve in a solvent if: the forces between solute particles are too strong to be overcome by interactions with solvent particles. the solvent particles are more strongly attracted to each other than to solute particles. A good rule of thumb for solubility is like dissolves like. Polar solvents dissolve polar or ionic solutes. Nonpolar solvents dissolve nonpolar or nonionic solutes. For ionic solutes dissolving in water, the solubility rules are used. Increasing the Rate of Dissolving Crush or grind the solute. Small particles provide more surface area for solvent attack and dissolve more rapidly than larger particles. Heat the solvent. Solvent molecules move faster and have more frequent collisions with solute at higher temperatures. Stir or agitate the solution. Stirring removes locally saturated solution from the vicinity of the solute and allows unsaturated solvent to take its place. 7.4 Solution Concentrations Solution concentrations describe the amount of solute contained in a specific amount of solution. Concentration units to be discussed include molarity and percentage. 5
Molarity Molarity (M) is the number of moles of solute contained in one liter of solution. moles of solute M = liters of solution Example 1: A 250-mL sample of solution contains 0.134 moles of solute. Calculate the molarity of the solution. Example 2: 9.45 g of methyl alcohol, CH 3 OH, was dissolved in enough pure water to give 500 ml of solution. What was the molarity of the solution? Weight/Weight Percent Weight/weight percent, %(w/w), is a concentration that expresses the mass of solute per mass of solution. Any mass units may be used, but the mass of solute and solution must be in the same units. solute mass %( w / w) = 100 solution mass Example: Calculate the %(w/w) of a solution prepared by dissolving 15.0 grams of table sugar in 100 ml of water. The density of the water is 1.00 g/ml. Weight/Volume Percent Weight/volume percent, %(w/v), is a concentration that expresses the grams of solute per ml of solution. This percent concentration is normally used when the solute is a solid and the solvent and resulting solutions are liquids. grams of solute %( w / v) = 100 ml of solution Example: Calculate the %(w/v) of a solution prepared by dissolving 8.95 kg of sodium chloride in enough water to give 5.00 L of solution. 6
Volume/Volume Percent Volume/volume percent, %(v/v), is a concentration that expresses the volume of liquid solute per volume of solution. Any volume units may be used, but they must be the same for both the solute and the solution. solute volume %( v / v) = 100 solution volume Example: A solution is made by dissolving 250 ml of glycerin in enough water to give 1.50 L of solution. Calculate the %(v/v) of the resulting solution. Solution Preparation Examples Example 1: Describe how to prepare 500 ml of 0.250 M NaCl solution. 7.5 Solution Preparation Solutions of known concentration are usually prepared in one of two ways. A necessary quantity of pure solute is measured using a balance or volumetric equipment. The solute is put into a container and solvent, usually water, is added until the desired volume of solution is obtained. Or A more concentrated solution is diluted by adding the appropriate amount of solvent to give a solution of lower concentration. (C c )(V c ) = (C d )(V d ) 7
Solution Preparation Examples Example 2: Describe how to prepare 250 ml of 0.500 M HCl solution from a 1.50 M HCl solution. 7.6 Solution Stoichiometry Solution stoichiometry is similar to regular stoichiometry except instead of starting with or determining grams we are using molarity and volume. M & V (of A) moles of A moles of B M or V (of B) grams of A grams of B In this case we are using molarity as a conversion factor. moles solute liters solution or liters solution moles solute Solution Stoichiometry Examples Example 1: Consider the balanced equation HCl(aq) + NaOH(aq) NaCl(aq) + H 2 O(l) How many ml of 0.100 M HCl solution would exactly react with 25.00 ml of 0.125 M NaOH solution? Example 2: How many grams of solid Na 2 CO 3 will exactly react with 250. ml of a 1.25 M HCl solution? Na 2 CO 3 (s) + 2 HCl(aq) 2 NaCl(aq) + CO 2 (g) + H 2 O(l) 8
7.7 Solution Properties Electrolytes solutes that when dissolved in water conduct a current of electricity. Nonelectrolytes solutes that when dissolved in water do not conduct a current of electricity. Electrolytes Strong electrolytes form solutions that conduct electricity because they dissociate completely into charged ions when they dissolve. Examples: NaCl, KNO 3, HCl, KOH Weak electrolytes form weakly conducting solutions because they dissociate into ions only slightly when they dissolve. Examples: HC 2 H 3 O 2, HF Nonelectrolytes form non-conducting solutions because they do not dissociate into ions at all when they dissolve. Examples: CH 3 CH 2 OH, C 6 H 12 O 6 Colligative Properties of Solutions Colligative properties properties that depend only on the concentration of solute particles in the solution. Four colligative properties will be discussed involving vapor pressure, boiling point, freezing point, and osmotic pressure. Experiments demonstrate that the vapor pressure of a solvent above a solution is lower than the vapor pressure of pure solvent. For example, the vapor pressure of water above an aqueous NaCl solution is lower than the vapor pressure of pure water. Why? 9
Boiling Point Elevation If the vapor pressure of a solution is lower than the pure solvent, then how is boiling point of a solution affected? The boiling point of the solution will always be higher than the boiling point of the pure solvent. The difference in boiling point between pure solvent and solution depends on the concentration of solute particles. The more solute particles in solution the greater the difference in the boiling point of the solution compared to the pure solvent. Freezing Point Depression The freezing point of a solution will always be lower than the freezing point of the pure solvent. Similar to the boiling point, the freezing point of the solution is only dependent on how much solute is dissolved in the solution. The more solute in solution the lower the freezing point of the solution compared to the pure solvent. Osmosis 10
Osmosis When solutions having different concentrations of solute are separated by a semipermeable membrane, solvent flows through the membrane in both directions; however, more solvent flows from the less concentrated solution into the more concentrated solution in a process called osmosis. When the more concentrated solution involved in osmosis is put under sufficient pressure, the net osmotic flow of solvent into the solution can be stopped. The pressure necessary to prevent the osmotic flow of solvent into a solution is called the osmotic pressure of the solution. The more solute particles the higher the osmotic pressure will have to be to stop the flow of solvent. 11