Chapter 13 Physical Properties of Solutions Types of Solutions According to Components I A B 1 2 3 4 A 1 2 According to Capacity to Dissolve a Solute Saturated solutions contain the maximum amount of solute that will dissolve in a given amount of solvent at a specified temperature Unsaturated solutions contain less solute than the maximum amount Supersaturated solutions contain more solute than is present in a saturated solution These solutions are not stable The excess solute easily crystallizes and settles out until the solution becomes saturated Solubility refers to the amount of solute that can be dissolved in at a given temperature in order to form a saturated solution II A Molecular View of the Solution Process The Importance of Intermolecular Forces When a solute dissolves in a solvent molecules particles of the solute disperse throughout the solvent The solute particles are separated from one another and are surrounded by solvent particles a process known as solvation 1 3 B 1 2 3 4 1 2 3 4 The ease with which solute molecules are separated from one another and solvated by solvent molecules depends on the relative strength of the interactions listed below a b c Solute solute interactions Solvent solvent interactions Solute solvent interactions Solution Formation Includes the Following Steps Separation of solvent into its component particles energy required H1 Separation of solute into its component particles energy required H2 Association of solute and solvent particles energy released H3 Heat of solution Hsoln H1 H2 H3 If Hsoln 0 solution formation is exothermic If Hsoln 0 solution formation is endothermic C Energy and Entropy in Solution Formation Processes that occur spontaneously tend to be exothermic because the potential energy of the system decreases from the initial to the final state Because the dissolution of some substances is endothermic the potential energy of the system increases and that should make the process nonspontaneous that is it should not happen But it does Therefore there must be some other factor that drives the endothermic solution process That other factor is called entropy Entropy is the measure of the degree of dispersion of the energy of the system In this context the system is the solution 2 5 6 7 D 1 2 There is a natural tendency for entropy to increase and any process in which entropy increases is likely to occur The dissolution of NH4NO3 is an endothermic process While the enthalpy of solution is greater than zero the reason ammonium nitrate dissolves is because the dissolution process is accompanied by a large increase in entropy when the ammonium and nitrate ions are separated and are hydrated H2O NH4NO3 s NH4 aq NO3 aq In some cases the process is so endothermic that entropy cannot overcome the energy barrier Predicting Soubility Rule of solubility like dissolves like Polar molecules dissolve in polar solvents while non polar molecules dissolve in non polar solvents If the molecular properties of the solute and the solvent are known solubility can be predicted Water Octane Polar Nonpolar Immiscible Insoluble Water Polar Ethanol Polar Miscible Soluble 3 Circle the solvent in which the highlighted species is soluble Br2 C6H6 or water b KCl CCl4 or NH3 CH2O CS2 or water d I2 CS2 or water a c 3 III Concentration Units A 1 2 B 1 Previously Covered Concentration Units Molarity M moles of solute mol M liters of solution L Mole fraction XA moles of A sum of moles of all components New Concentration Units Molality m moles of solute mol m kg of solvent kg Molality is independent of temperature What is the molality of a solution that contains 7 78 g of urea NH2 2CO in 203 g of water The molar mass of urea is 60 1 g mol 2 Percent by mass mass of solute x 100 mass of solute mass of solvent If 16 5 grams of CaCl2 are dissolved in 456 grams of water what is the concentration of the solution in percent by mass 3 Sample calculations 4 56 grams of urea NH2 2CO molar mass 60 1 g mol are dissolved in 215 grams of water Express the concentration of urea in a percent by mass b molality 4 Comparison of Concentration Units Which concentration unit is used depends on the experiment Molarity is used as the concentration term for reactions in aqueous solutions such as titrations and gravimetric analyses Mole fraction is used to express the concentration of gases in a mixture of gases Molality is used to calculate the magnitude of colligative properties Molality is a concentration unit that is independent of the temperature Percent by mass is similar to molality in that it does not depend on temperature because it is a ratio of masses At times it is necessary to convert one concentration to another A 16 0 by mass H2SO4 has a density of 1 11 g mL at 25 C Determine the C 1 2 3 IV A 1 Sample calculations a molarity b molality Factors that Affect Solubility Temperature Solids Generally the solubility of solids increases with increasing temperature however there is no clear correlation between the sign of Hsoln and variations in solubility with temperature Hsoln for CaCl2 is negative for NH4NO3 it is positive but the solubility of both salts increases with increasing temperature 5 2 Solubility of solids varies considerably with temperature 3 Note that the solubility of Ce2 SO4 3 decreases with increasing temperature while the solubility of NaCl remains almost the same between 0 C to 100 C Gases Pressure The solubility of most gases decreases with increasing temperature A soft drink loses carbonation as the temperature increases Pressure is significant only in the solubility to gases As the pressure of a gas above a liquid is increased the solubility of the gas increases The quantitative relationship between gas solubility and pressure is given by Henry s law c kP where c is concentration in mol L k is Henry s constant mol L atm P is the pressure of the gas above the liquid in atm Henry s law applies to gases that do not react with the solvent Sample calculation To make a solution that is 0 034 M in oxygen at 25 C what pressure must be exerted by the gas above the liquid Henry s constant for oxygen at 25 C is 1 3 x 10 3 mol L atm B 1 2 3 4 6 V A 1 B 1 2 3 4 5 Colligative Properties General Information Colligative properties depend only on the number of particles in a solution and not on what those particles are Two molecules affect the boiling point as much as two ions 2 Colligative properties are a b c d Vapor pressure lowering Boiling point elevation Freezing point