Chapter 14 Solutions. Solutions. Solutions. Solutions. Concentration Units. Concentration Units

Transcription

1 1 Solutions Chapter 14 Solutions and Their Behavior Jeffrey Mack California State University, Sacramento A solution is a homogeneous mixture of two or more substances in a single phase. By convention, the component present in largest amount is identified as the solvent and the other component(s) as the solute(s) Solutions Solutions Solutions can be classified as saturated or unsaturated. A saturated solution contains the maximum quantity of solute that dissolves at that temperature. An unsaturated solution can still take on more solute at a given temperature. SUPERSATURATED SOLUTIONS contain more than is possible and are unstable. Concentration Units Concentration Units Molarity (M) Weight (mass) % M = moles solute L of solution % = mass solute Total mass of solution 100 % by mass = x 100% mass of solute mass of solute + mass of solvent

2 2 Concentration Units Concentration Units X i = Mole Fraction (X) moles solute (i) Total moles in solution Mole Fraction (X) Where i is the moles of one component of the solute. Total moles are all species: mols solute (i) + mols solvent. The sum of all of the mole fractions for each component are 1 exactly. Xi 1 Concentration Units Molarity (M) moles of solute M = liters of solution Molality (m) moles of solute m = mass of solvent (kg) Concentration Units m = Parts Per Million (ppm) mg solute kg solution Parts per million (ppm): grams of solute/grams of solution (then multiplied by 10 6 or 1 million) Solution Concentration Most concentration units are expressed as: Amount of solute Amount of solvent or solution Calculating Concentrations 62.1 g (1.00 mol) of ethylene glycol is dissolved in 250. g of water. Calculate mol fraction, molality, and weight % of the solution. Molarity: moles of solute/liter of solution Percent by mass: grams of solute/grams of solution (then multiplied by 100%) Percent by volume: milliliters of solute/milliliters of solution (then multiplied by 100%) Mass/volume percent: grams of solute/milliliters of solution (then multiplied by 100%) 11

3 3 Calculating Concentrations 62.1 g (1.00 mol) of ethylene glycol is dissolved in 250. g of water. Mole Fraction: 1 mol C H O 62.1 g C H O 1.00 mol C H O 62.1 g mol H2O 250. g H2O 13.9 mol H2O g Calculating Concentrations 62.1 g (1.00 mol) of ethylene glycol is dissolved in 250. g of water. Mole Fraction: 1 mol C H O 62.1 g C H O 1.00 mol C H O 62.1 g mol H2O 250. g H2O 13.9 mol H2O g 1.00 mol XC2 H6O mol 13.9 mol Calculating Concentrations 62.1 g (1.00 mol) of ethylene glycol is dissolved in 250. g of water. Molality: Calculating Concentrations 62.1 g (1.00 mol) of ethylene glycol is dissolved in 250. g of water. Wt. %: m C2H6O g C H O mol C H O 62.1 g 1 kg 250. g H2O 10 3 g mol/kg 62.1 g C H O % C2H6O % 62.1 g C2H6O g H2O What is the molality of a 5.86 M ethanol (C 2 H 5 OH) solution whose density is g/ml? m = moles of solute mass of solvent (kg) Assume 1 L of solution: 5.86 moles ethanol = 270 g ethanol 927 g of solution (1000 ml x g/ml) mass of solvent = mass of solution mass of solute m = moles of solute mass of solvent (kg) M = = 927 g 270 g = 657 g = kg = 5.86 moles C 2 H 5 OH kg solvent moles of solute liters of solution = 8.92 m 17 The Solution Process Solutes dissolve in solvents by a process called solvation. Polar solvent dissolve polar solutes, nonpolar solvents dissolve non-polar solutes. (aka: like dissolves like. If two liquids mix to an appreciable extent to form a solution, they are said to be miscible. In contrast, immiscible liquids do not mix to form a solution; they exist in contact with each other as separate layers.

4 4 The Solution Process Solvation of Ions + When a cation exists in solution, it is surrounded by the negative dipole ends of water molecules. When as anion exists in solution, it is surrounded by the positive dipole ends of water molecules. Energetics of the Solution Process: solutionh Energy must be supplied to separate the ions from their attractive forces. (an endothermic process) latticeh Energetics of the Solution Process: solutionh Energy must be supplied to separate the ions from their attractive forces. (an endothermic process) latticeh Energy is evolved when the individual ions dissolve in water where each ion is stabilized by solvation. Energetics of the Solution Process: solutionh Energy must be supplied to separate the ions from their attractive forces. (an endothermic process) latticeh Energy is evolved when the individual ions dissolve in water where each ion is stabilized by solvation. Energetics of the Solution Process: solutionh We can therefore represent the process of dissolving KF in terms of these chemical equations: Step 1: KF(s) K + (g) + F (g) = latticeh Step 2: K + (g) + F (g) K + (aq) +F (aq) = hydration H This process, referred to as the Energy of Hydration when water is the solvent, is strongly exothermic.

5 5 Energetics of the Solution Process: solutionh The overall reaction is the sum of these two steps. The enthalpy of the overall reaction, called the enthalpy of solution ( soln H), is the sum of the two enthalpies. Energetics of the Solution Process: solutionh Overall: KF(s) K + (aq) + F (aq) solnh = latticeh + hydrationh Energetics of the Solution Process Supersaturated Sodium Acetate If the enthalpy of formation of the solution is more negative than that of the solvent and solute, the enthalpy of solution is negative. The solution process is exothermic! One application of a supersaturated solution is the sodium acetate heat pack. The enthalpy of solution for sodium acetate is ENDOthermic. Supersaturated Sodium Acetate Temperature and Solubility Solid solubility and temperature Sodium acetate has an ENDOthermic heat of solution. NaCH 3 CO 2 (s) + heat Na + (aq) + CH 3 CO 2- (aq) Therefore, formation of solid sodium acetate from its ions is EXOTHERMIC. Na + (aq) + CH 3 CO 2- (aq) NaCH 3 CO 2 (s) + heat solubility increases with increasing temperature solubility decreases with increasing temperature 30

6 6 Factors Affecting: Solubility Pressure & Temperature Henry s Law Temperature and Solubility O 2 gas solubility and temperature solubility usually decreases with increasing temperature Gas solubility (mol/l) Do you like your coke hot or cold? Why? S g =K H P g 32 Chemistry In Action: The Killer Lake Colligative Properties 8/21/86 CO 2 Cloud Released 1700 Casualties Trigger? earthquake landslide strong Winds Lake Nyos, West Africa 33 Relative to a pure solvent, a solution has: Lower vapor pressure Higher boiling point Lower freezing point A higher osmotic pressure Example: Pure water: b.p.=100 C f.p.= 0 C 1.00 m NaCl (aq) b.p.= 101 C f.p. = -3.7 C Colligative properties Upon adding a solute to a solvent, the properties of the solvent are affected: Vapor pressure decreases Melting point decreases Boiling point increases Osmosis is possible (osmotic pressure) Collectively these changes are called COLLIGATIVE PROPERTIES. They depend only on the NUMBER of solute particles relative to solvent particles, not on the KIND of solute particles. Understanding Colligative Properties To understand colligative properties, one must consider the LIQUID-VAPOR EQUILIBRIUM for a solution.

7 Understanding Colligative Properties Changes in Vapor Pressure: Raoult s Law 7 Vapor pressure: pressure exerted by the vapor when a liquid in a closed container is at equilibrium with the vapor. Vapor pressure of solution is decreased by the presence of a solute. More solute particles, lower vapor pressure of solution. LIQUID-VAPOR EQUILIBRIUM Raoult s Law P solution = X solvent P o P solution = the vapor pressure of a mixture of solute and solvent P o = the vapor pressure of the pure solvent X solvent = the mole fraction of the solvent. The expression can also be written in the form: P X P solvent solute P is the change to the vapor pressure of the pure solvent. o Ideal Solution 0 P A = X A P A 0 P B = X B P B P T = P A + P B 0 0 P T = X A P A + X B P B Special case of 2 Liquids 40 P T is greater than predicted by Raoults s law P T is less than predicted by Raoults s law Vapor Pressure Lowering of Benzene + non volatile solute Force A-B Force A-A < & Force B-B Force A-B Force A-A > & Force B-B 41

8 8 Raoult s Law Pure iodine (105 g) is dissolved in 325 g of CCl 4 at 65 C. Given that the vapor pressure of CCl 4 at this temperature is 531 mm Hg, what is the vapor pressure of the CCl 4 I 2 solution at 65 C? (Assume that I 2 does not contribute to the vapor pressure.) Raoult s Law Pure iodine (105 g) is dissolved in 325 g of CCl 4 at 65 C. Given that the vapor pressure of CCl 4 at this temperature is 531 mm Hg, what is the vapor pressure of the CCl 4 I 2 solution at 65 C? (Assume that I 2 does not contribute to the vapor pressure.) 1 mol I 105g I = mol I g mol CCl 325g CCl = 2.11 mol CCl g X CCl mol 2.11 mol mol CCl CCl CCl Raoult s Law Pure iodine (105 g) is dissolved in 325 g of CCl 4 at 65 C. Given that the vapor pressure of CCl 4 at this temperature is 531 mm Hg, what is the vapor pressure of the CCl 4 I 2 solution at 65 C? (Assume that I 2 does not contribute to the vapor pressure.) P X Pº mm Hg 444 mm Hg Boiling Point Elevation & Freezing Point Depression Molecular Compounds The temperature of the normal boiling point of a solution is increased by: T K m b b solute The temperature of the normal freezing point of a solution is decreased by: Tf Kf msolute Where the K s are the respective boiling and freezing point constants and m solute is the molality of the solution. Boiling Point Elevation & Freezing Point Depression Molecular Compounds Boiling Point Elevation & Freezing Point Depression Tb Kb msolute Tf Kf msolute For boiling point elevation: For freezing point depression: K b > 0 (positive) K f < 0 (negative)

9 9 van t Hoff Factor Boiling and Freezing point effects involving ions: When solutions containing ions are involved, the total concentration of solute particles must be considered. The change in b.p. or f.p. is given by the equation: T K msolute i Where m is the calculated molaity based on formula wt. i = the number of ions (van t Hoff factor) compound Type i CH 3 OH molecular 1 NaCl strong electrolyte 2 Ba(NO 3 ) 2 strong electrolyte 3 HNO 2 weak electrolyte 1-2 The Boiling Point of a Solution is Higher Than That of a Pure Solvent Change in Freezing Point Lowering the Freezing Point Pure water Water solution The freezing point of a solution is LOWER than that of the pure solvent. Water with and without antifreeze When a solution freezes, the solid phase is pure water. The solution becomes more concentrated. Freezing Point Depression If 52.5 g of LiF is dissolved in 306 g of water, what is the expected freezing point of the solution? (Assume the van t Hoff factor, i, for LiF is 2.) Freezing Point Depression If 52.5 g of LiF is dissolved in 306 g of water, what is the expected freezing point of the solution? (Assume the van t Hoff factor, i, for LiF is 2.) T K msolute i m LiF 1 mol LiF 52.5 g LiF g 1kg 306 g H2O 3 10 g 6.61 m

10 Freezing Point Depression If 52.5 g of LiF is dissolved in 306 g of water, what is the expected freezing point of the solution? (Assume the van t Hoff factor, i, for LiF is 2.) T K msolute i Molar Mass By Boiling Point Elevation 10 Benzyl acetate is one of the active components of oil of jasmine. If g of the compound is added to 25.0 g of chloroform (CHCl 3 ), the boiling point of the solution is C. What is the molar mass of benzyl acetate? T T T fp fp fp 1.86 C 6.61 m (2) m 24.6 C 0 C ( 24.6 C) 24.6 C Molar Mass By Boiling Point Elevation Benzyl acetate is one of the active components of oil of jasmine. If g of the compound is added to 25.0 g of chloroform (CHCl 3 ), the boiling point of the solution is C. What is the molar mass of benzyl acetate? 1. Solution: The molality of the solution can be found from the freezing point depression. 2. Knowing molality and mass of solvent, one can find the moles of solute. 3. Knowing mass and moles of solute, the molar mass of the compound can be determined. Molar Mass By Boiling Point Elevation Benzyl acetate is one of the active components of oil of jasmine. If g of the compound is added to 25.0 g of chloroform (CHCl 3 ), the boiling point of the solution is C. What is the molar mass of benzyl acetate? m T K m f f solute solute Tf K f Molar Mass By Boiling Point Elevation Benzyl acetate is one of the active components of oil of jasmine. If g of the compound is added to 25.0 g of chloroform (CHCl 3 ), the boiling point of the solution is C. What is the molar mass of benzyl acetate? m T bp benzyl acetate C C 0.12 C T K bp bp 0.12 C 3.63 C m m Molar Mass By Boiling Point Elevation Benzyl acetate is one of the active components of oil of jasmine. If g of the compound is added to 25.0 g of chloroform (CHCl 3 ), the boiling point of the solution is C. What is the molar mass of benzyl acetate? mol benzyl acetate 1 kg 1 kg CHCl 10 g g benzyl acetate mol benzyl acetate g CHCl mol g/mol

11 11 Osmosis Osmosis Solvent Solution Dissolving the shell in vinegar Egg in pure water Egg in corn syrup Semipermeable membrane A semipermeable membrane allows only the movement of solvent molecules. Solvent molecules move from pure solvent to solution in an attempt to make both have the same concentration of solute. Osmosis Osmosis at the Particulate Level Osmosis of solvent from one solution to another will occur as they try to equalize one another s concentration. At the point where they have equal osmotic pressures, they are said to be Isotonic. Process of Osmosis Osmotic Pressure, Equilibrium is reached when the internal pressure of the apparatus equals the external pressure of the open tube. Since the internal pressure, P int is greater then the atmospheric pressure, a column of liquid rises: P atm +P col = P int Recall that: P col = g d h

12 12 Osmotic Pressure, Osmosis & Living Cells P col is referred to as the Osmotic Pressure,. Osmotic pressure = crt c = concentration of solute (mols/l) T = the absolute temperature (K) L atm R mol K is in units of atm. Reverse Osmosis: Water Desalination Colligative Properties of Nonelectrolyte Solutions Colligative properties are properties that depend only on the number of solute particles in solution and not on the nature of the solute particles. 0 Vapor-Pressure Lowering P 1 = X 1 P 1 Water desalination plant Boiling-Point Elevation Freezing-Point Depression Osmotic Pressure ( ) Electrolyte Solutions Boiling-Point Elevation Freezing-Point Depression Osmotic Pressure ( ) T b = K b m T f = K f m = CRT T b = i K b m T f = i K f m = icrt 70 Colloids A colloid is a dispersion of particles of one substance throughout a dispersing medium of another substance. In a solution, dispersed particles are molecules, atoms, or ions (roughly 0.1 nm in size). Solute particles do not settle out of solution. Colloids In a suspension (e.g., sand in water) the dispersed particles are relatively large, and will settle from suspension. In a colloid, the dispersed particles are on the order of nm in size. Although they are larger than molecules/atoms/ions, colloidal particles are small enough to remain dispersed indefinitely. Colloidal dispersions scatter light, a phenomenon known as the The Tyndall effect

13 13 Hydrophobic Colloids A hydrophobic colloid is stabilized by positive ions absorbed onto each particle and a secondary layer of negative ions. Because the particles bear similar charges, they repel one another, and precipitation is prevented. Soap Soap molecules interact with water through the charged, hydrophilic end of the molecule. The long, hydrophobic end of the molecule binds through dispersion forces with nonpolar hydrocarbons and other non polar substances. Emulsifiers & Surfactants An emulsifier (also known as an emulgent) is a substance which stabilizes an emulsion by increasing its kinetic stability. One class of emulsifiers is known as surface active substances, or surfactants. Examples of food emulsifiers are egg yolk (where the main emulsifying agent is lecithin) and honey. In some cases, particles can stabilize emulsions as well through a mechanism called Pickering stabilization. Both mayonnaise and Hollandaise sauce are oil-inwater emulsions that are stabilized with egg yolk lecithin or other types of food additives such as Sodium stearoyl lactylate.