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General Chemistry I Dr. PHAN TẠI HUÂN Faculty of Food Science and Technology Nong Lam University Module 6: Solutions • Definitions (solutions: solvent, solute, solubility, concentration; interactions between solvent phase and solute molecules; properties of polar and non-polar solvents; properties of colloid). • Chemical equilibrium for solutions (self-ionisation of water, solvation, pH; Acids and Bases; solution equilibrium with precipitation). 2 1 Solution • A solution is defined as a homogeneous mixture of substances in which no settling occurs. • A solution consists of a solvent and one or more solutes, whose proportions vary from one solution to another. • The solvent is the medium in which the solutes are dissolved. The fundamental units of solutes are usually ions or molecules. • Water is the most important solvent, and compounds dissolved in water are said to be in aqueous solution. 3 Solution • In reality, any combination of the three states can be considered a solution. • Usually a solution is formed by dissolving a solid (e.g., sugar) in a liquid (e.g., water). • Air is a solution which is a mixture of various gases. • Carbonated water (soda) is a mixture of a gas (CO2) dissolved in a liquid (H2O). • Even alloys such as gold-silver alloys are solutions containing two solids. A true solution is a solution which has only one solvent with one or more solutes. 4 2 The concept of solubility • Solubility of a substance is defined as the amount of the substance that will dissolve in a particular solvent. • Solubilities vary tremendously. • At one extreme, some substances form solutions in all proportions and are said to be miscible. For example, acetone and water can be mixed in any proportion, from pure water to pure acetone. • At the other extreme, a substance may be insoluble in another. One example is common salt, NaCl, whose solubility in gasoline is virtually zero. 5 Determinants of solubility • Many combinations display solubility that is between the two extremes of miscible and insoluble. In other words, the substance dissolves, but there is a limit to the amount of solute that will dissolve in a given amount of solvent. • Concentrations of solutions are expressed in terms of either the amount of solute present in a given mass or volume of solution, or the amount of solute dissolved in a given mass or volume of solvent. 6 3 Percent by mass • Concentrations of solutions may be expressed in terms of percent by mass of solute, which gives the mass of solute per 100 mass units of solution. The gram is the usual mass unit. percent sulute = mass of solute x 100% mass of solution • Thus, a solution that is 10% calcium gluconate, Ca(C6H11O7)2, by mass contains 10 grams of calcium gluconate in 100 grams of solution. This could be described as 10 grams of calcium gluconate in 90 grams of water. • Unless otherwise specified, percent means percent by mass, and water is the solvent. 7 Molarity vs. Molality • The Molarity, M, of a solution is defined as the number of moles of the solute per liter of solution. Molarity = moles of solute mol = liter of solution L • The molality, m, of a solution is defined as the number of moles of the solute per kilogram of solvent. Molality = moles of solute kilograms of solvent 8 4 Normality • The normality, N, of a solution is the number of equivalents of solute per liter of solution. • The equivalent is usually defined in terms of a chemical reaction. For acid-base reactions, an equivalent is the amount of substance that will react or form 1 mole of hydrogen (H+) or hydroxide (OH-) ions. For redox (oxidation-reduction) reactions, an equivalent is the amount of substance that will react or form 1 mole of electrons. Normality = number of equivalents 1 liter of solution N = nM 9 Exercise • How many grams of H2O must be used to dissolve 50 grams of sucrose to prepare a 1.25 m solution of sucrose, C12H22O11? Ans: 10 5 Exercise • Hydrogen peroxide disinfectant typically contains 3.0% by mass. Assuming that the rest of the contents is water, what is the molality of this disinfectant? Ans: 11 Spontaneity of the dissolution process • A process is favored by (1) a decrease in the energy of the system, which corresponds to an exothermic process, and (2) an increase in the disorder, or randomness, of the system. • The energy change that accompanies a dissolution process is called the heat of solution, ∆Hsolution. It depends mainly on how strongly solute and solvent particles interact. • A negative value of ∆Hsolution designates the release of heat. • The main interactions that affect the dissolution of a solute in a solvent follow: – Weak solute–solute attractions favor solubility. – Weak solvent–solvent attractions favor solubility. – Strong solvent–solute attractions favor solubility. 12 6 Spontaneity of the dissolution process 13 Like dissolves Like • Solubility is a complex phenomenon that depends on the balance of several properties. • The general features of solubility are summarized by the expression like dissolves like. • Substances that dissolve in each other usually have similar types of intermolecular interactions. • One substance dissolves in another if the forces of attraction between the solute and the solvent are similar to the solvent–solvent and solute–solute interactions. 14 7 Dissolution of liquids • Water and methanol are alike in that both substances contain O-H groups that form hydrogen bonds readily. When these liquids are mixed, H2O... H2O hydrogen bonds and CH3OH... CH3OH hydrogen bonds break, but H2O...CH3OH hydrogen bonds form. • The net result is that the degree of hydrogen bonding in the solution is about the same as in either of the pure liquids, making these two liquids miscible. 15 Dissolution of liquids • The intermolecular interactions of octane and cyclohexane are alike. • Octane and cyclohexane have low polarities, so these molecules in the pure liquids are held together by the dispersion forces caused by their polarizable electron clouds. • Dispersion forces in solutions of octane and cyclohexane are about the same as in the pure liquids. So these two liquids are miscible. 16 8 Dissolution of liquids • Water and octane are not alike and nearly insoluble in each other. • Octane does not form hydrogen bonds, so the only forces of attraction between water molecules and octane molecules are dispersion forces. • Because hydrogen bonds are stronger than dispersion forces, the cost of disrupting the hydrogen-bonding network in water is far greater than the stability gained from octane–water dispersion forces. 17 Dissolution of liquids • Some liquids can interact with other substances in multiple ways. Acetone, for instance, has a polar CO bond and a three-carbon bonding framework. • The bonding framework is similar to that of a hydrocarbon, so acetone mixes with cyclohexane and octane. • The polar CO group makes acetone miscible with other polar molecules such as acetonitrile . • The polar oxygen atom in acetone has lone pairs of electrons that can form hydrogen bonds with hydrogen atoms of ammonia or water. 18 9 Exercise • Give a molecular explanation for the following trend in alcohol solubilities in water: n-Propanol n-Butanol n-Pentanol n-Hexanol CH3CH2CH2OH Completely miscible CH3CH2CH2CH2OH 1.1 M CH3CH2CH2CH2CH2OH 0.30 M 0.056 M CH3CH2CH2CH2CH2CH2OH Strategy • Solubility limits depend on the stabilization generated by solute–solvent interactions balanced against the destabilization that occurs when solvent–solvent interactions are disrupted by solute. • Intermolecular interactions involving water and alcohol molecules must be examined. 19 Solubility of solids: network solids • Network solids such as diamond, graphite, or silica cannot dissolve without breaking covalent chemical bonds. • Because intermolecular forces of attraction are always much weaker than covalent bonds, solvent–solute interactions are never strong enough to offset the energy cost of breaking bonds. • Covalent solids are insoluble in all solvents, although they may react with specific liquids or vapors. 20 10