Chapter 12 Solutions

Chemistry: A Molecular Approach, 1st Ed.Nivaldo Tro Chapter 12Solutions Roy Kennedy Massachusetts Bay Community College Wellesley Hills, MA 2008, Prentice Hall

Solution • homogeneous mixtures • composition may vary from one sample to another • appears to be one substance, though really contains multiple materials • most homogeneous materials we encounter are actually solutions • e.g., air and sea water • nature has a tendency toward spontaneous mixing • generally, uniform mixing is more energetically favorable Tro, Chemistry: A Molecular Approach

Solutions • soluteis the dissolved substance • seems to “disappear” • “takes on the state” of the solvent • solvent is the substance solute dissolves in • does not appear to change state • when both solute and solvent have the same state, the solvent is the component present in the highest percentage • solutions in which the solvent is water are called aqueous solutions Tro, Chemistry: A Molecular Approach

Seawater • drinking seawater will dehydrate you and give you diarrhea • the cell wall acts as a barrier to solute moving • the only way for the seawater and the cell solution to have uniform mixing is for water to flow out of the cells of your intestine and into your digestive tract Tro, Chemistry: A Molecular Approach

Common Types of Solution • solutions that contain Hg and some other metal are called amalgams • solutions that contain metal solutes and a metal solvent are called alloys Tro, Chemistry: A Molecular Approach

Brass

Solubility • when one substance (solute) dissolves in another (solvent) it is said to be soluble • salt is soluble in water • bromine is soluble in methylene chloride • when one substance does not dissolve in another it is said to be insoluble • oil is insoluble in water • the solubility of one substance in another depends on two factors – nature’s tendency towards mixing, and the types of intermolecular attractive forces Tro, Chemistry: A Molecular Approach

Spontaneous Mixing Tro, Chemistry: A Molecular Approach

Solubility • there is usually a limit to the solubility of one substance in another • gases are alwayssoluble in each other • two liquids that are mutually soluble are said to be miscible • alcohol and water are miscible • oil and water are immiscible • the maximum amount of solute that can be dissolved in a given amount of solvent is called the solubility • the solubility of one substance in another varies with temperature and pressure Tro, Chemistry: A Molecular Approach

Mixing and the Solution ProcessEntropy • formation of a solution does not necessarily lower the potential energy of the system • the difference in attractive forces between atoms of two separate ideal gases vs. two mixed ideal gases is negligible • yet the gases mix spontaneously • the gases mix because the energy of the system is lowered through the release of entropy • entropy is the measure of energy dispersal throughout the system • energy has a spontaneous drive to spread out over as large a volume as it is allowed Tro, Chemistry: A Molecular Approach

Intermolecular Forces and the Solution ProcessEnthalpy of Solution • energy changes in the formation of most solutions also involve differences in attractive forces between particles • must overcome solute-solute attractive forces • endothermic • must overcome some of the solvent-solvent attractive forces • endothermic • at least some of the energy to do this comes from making new solute-solvent attractions • exothermic Tro, Chemistry: A Molecular Approach

Intermolecular Attractions Tro, Chemistry: A Molecular Approach

Relative Interactions and Solution Formation • when the solute-to-solvent attractions are weaker than the sum of the solute-to-solute and solvent-to-solvent attractions, the solution will only form if the energy difference is small enough to be overcome by the entropy Tro, Chemistry: A Molecular Approach

Solution Interactions Tro, Chemistry: A Molecular Approach

Will It Dissolve? • Chemist’s Rule of Thumb – Like Dissolves Like • a chemical will dissolve in a solvent if it has a similar structure to the solvent • when the solvent and solute structures are similar, the solvent molecules will attract the solute particles at least as well as the solute particles to each other Tro, Chemistry: A Molecular Approach

Classifying Solvents Tro, Chemistry: A Molecular Approach

Example 12.1a  predict whether the following vitamin is soluble in fat or water The 4 OH groups make the molecule highly polar and it will also H-bond to water. Vitamin C is water soluble Vitamin C Tro, Chemistry: A Molecular Approach

Example 12.1b  predict whether the following vitamin is soluble in fat or water The 2 C=O groups are polar, but their geometric symmetry suggests their pulls will cancel and the molecule will be nonpolar. Vitamin K3 is fat soluble Vitamin K3 Tro, Chemistry: A Molecular Approach

Energetics of Solution Formation • overcome attractions between the solute particles – endothermic • overcome some attractions between solvent molecules – endothermic • for new attractions between solute particles and solvent molecules – exothermic • the overall DH depends on the relative sizes of the DH for these 3 processes DHsol’n = DHsoluteDHsolvent + DHmix Tro, Chemistry: A Molecular Approach

1. add energy in to overcome solute-solute attractions 2. add energy in to overcome some solvent-solvent attractions Solution Process 3. form new solute-solvent attractions, releasing energy Tro, Chemistry: A Molecular Approach

if the total energy cost for breaking attractions between particles in the pure solute and pure solvent is less than the energy released in making the new attractions between the solute and solvent, the overall process will be exothermic if the total energy cost for breaking attractions between particles in the pure solute and pure solvent is greater than the energy released in making the new attractions between the solute and solvent, the overall process will be endothermic Energetics of Solution Formation Tro, Chemistry: A Molecular Approach

Heats of Hydration • for aqueous ionic solutions, the energy added to overcome the attractions between water molecules and the energy released in forming attractions between the water molecules and ions is combined into a term called the heat of hydration • attractive forces in water = H-bonds • attractive forces between ion and water = ion-dipole • DHhydration = heat released when 1 mole of gaseous ions dissolves in water Tro, Chemistry: A Molecular Approach

Heat of Hydration Tro, Chemistry: A Molecular Approach

Ion-Dipole Interactions • when ions dissolve in water they become hydrated • each ion is surrounded by water molecules Tro, Chemistry: A Molecular Approach

Solution Equilibrium • the dissolution of a solute in a solvent is an equilibrium process • initially, when there is no dissolved solute, the only process possible is dissolution • shortly, solute particles can start to recombine to reform solute molecules – but the rate of dissolution >> rate of deposition and the solute continues to dissolve • eventually, the rate of dissolution = the rate of deposition – the solution is saturated with solute and no more solute will dissolve Tro, Chemistry: A Molecular Approach

Solution Equilibrium Tro, Chemistry: A Molecular Approach

Solubility Limit • a solution that has the maximum amount of solute dissolved in it is said to be saturated • depends on the amount of solvent • depends on the temperature • and pressure of gases • a solution that has less solute than saturation is said to be unsaturated • a solution that has more solute than saturation is said to be supersaturated Tro, Chemistry: A Molecular Approach

How Can You Make a Solvent Hold More Solute Than It Is Able To? • solutions can be made saturated at non-room conditions – then allowed to come to room conditions slowly • for some solutes, instead of coming out of solution when the conditions change, they get stuck in-between the solvent molecules and the solution becomes supersaturated • supersaturated solutions are unstable and lose all the solute above saturation when disturbed • e.g., shaking a carbonated beverage Tro, Chemistry: A Molecular Approach

Adding Solute to a Supersaturated Solution of NaC2H3O2 Tro, Chemistry: A Molecular Approach

Temperature Dependence of Solubility of Solids in Water • solubility is generally given in grams of solute that will dissolve in 100 g of water • for most solids, the solubility of the solid increases as the temperature increases • when DHsolution is endothermic • solubility curves can be used to predict whether a solution with a particular amount of solute dissolved in water is saturated (on the line), unsaturated (below the line), or supersaturated (above the line) Tro, Chemistry: A Molecular Approach

Solubility Curves Tro, Chemistry: A Molecular Approach

Tro, Chemistry: A Molecular Approach

Temperature Dependence of Solubility of Gases in Water • solubility is generally given in moles of solute that will dissolve in 1 Liter of solution • generally lower solubility than ionic or polar covalent solids because most are nonpolar molecules • for all gases, the solubility of the gas decreases as the temperature increases • the DHsolution is exothermic because you do not need to overcome solute-solute attractions Tro, Chemistry: A Molecular Approach

Pressure Dependence of Solubility of Gases in Water • the larger the partial pressure of a gas in contact with a liquid, the more soluble the gas is in the liquid

Henry’s Law • the solubility of a gas (Sgas) is directly proportional to its partial pressure, (Pgas) Sgas = kHPgas • kH is called Henry’s Law Constant Tro, Chemistry: A Molecular Approach

Relationship between Partial Pressure and Solubility of a Gas Tro, Chemistry: A Molecular Approach

persrst Tro, Chemistry: A Molecular Approach

[CO2] P Ex 12.2 – What pressure of CO2 is required to keep the [CO2] = 0.12 M at 25°C? Given: Find: S = [CO2] = 0.12 M, P of CO2, atm Concept Plan: Relationships: S = kHP, kH = 3.4 x 10-2 M/atm Solve: Check: the unit is correct, the pressure higher than 1 atm meets our expectation from general experience Tro, Chemistry: A Molecular Approach

Concentrations • solutions have variable composition • to describe a solution, need to describe components and relative amounts • the terms dilute and concentrated can be used as qualitative descriptions of the amount of solute in solution • concentration = amount of solute in a given amount of solution • occasionally amount of solvent Tro, Chemistry: A Molecular Approach

moles of solute liters of solution molarity = Solution ConcentrationMolarity • moles of solute per 1 liter of solution • used because it describes how many molecules of solute in each liter of solution • if a sugar solution concentration is 2.0 M, 1 liter of solution contains 2.0 moles of sugar, 2 liters = 4.0 moles sugar, 0.5 liters = 1.0 mole sugar Tro, Chemistry: A Molecular Approach

Molarity and Dissociation • the molarity of the ionic compound allows you to determine the molarity of the dissolved ions • CaCl2(aq) = Ca+2(aq) + 2 Cl-1(aq) • A 1.0 M CaCl2(aq) solution contains 1.0 moles of CaCl2 in each liter of solution • 1 L = 1.0 moles CaCl2, 2 L = 2.0 moles CaCl2 • Because each CaCl2 dissociates to give one Ca+2 = 1.0 M Ca+2 • 1 L = 1.0 moles Ca+2, 2 L = 2.0 moles Ca+2 • Because each CaCl2 dissociates to give 2 Cl-1 = 2.0 M Cl-1 • 1 L = 2.0 moles Cl-1, 2 L = 4.0 moles Cl-1 Tro, Chemistry: A Molecular Approach

Solution ConcentrationMolality, m • moles of solute per 1 kilogram of solvent • defined in terms of amount of solvent, not solution • like the others • does not vary with temperature • because based on masses, not volumes Tro, Chemistry: A Molecular Approach

Percent • parts of solute in every 100 parts solution • mass percent = mass of solute in 100 parts solution by mass • if a solution is 0.9% by mass, then there are 0.9 grams of solute in every 100 grams of solution • or 0.9 kg solute in every 100 kg solution Tro, Chemistry: A Molecular Approach

Percent Concentration Tro, Chemistry: A Molecular Approach

Using Concentrations asConversion Factors • concentrations show the relationship between the amount of solute and the amount of solvent • 12%(m/m) sugar(aq) means 12 g sugar  100 g solution • or 12 kg sugar  100 kg solution; or 12 lbs.  100 lbs. solution • 5.5%(m/v) Ag in Hg means 5.5 g Ag  100 mL solution • 22%(v/v) alcohol(aq) means 22 mL EtOH  100 mL solution • The concentration can then be used to convert the amount of solute into the amount of solution, or vice versa Tro, Chemistry: A Molecular Approach

Chapter 12 Solutions