Chapter13PropertiesofSolutions：13章对解的性质.doc

Chapter 13: Properties of SolutionsRecall: From Ch. 4, a solution is a homogeneous mixture of two or more substancesWe dealt specifically with aqueous solutions in Ch 4Defined solvent and solute From Ch 11: we dealt with intermolecular forces and physical properties of pure substances (e.g., H2O(l)Now we ask: how do intermolecular forces affect the properties of solutions?The Solution ProcessWhat determines whether or not a given solute will dissolve in a given solvent?E.g., ethanol (C2H5OH) is infinitely soluble in water, but hexane is not soluble in any proportion in waterIn general: solutions form when the attractive forces between solvent and solute are comparable to those between solvent particles alone and solute particles alone What types of intermolecular forces operate between solute and solvent particles?The intermolecular forces are the same as discussed in chapter 11.Dipole-dipole forces operate between neutral, polar moleculesHow do we recognize hydrogen bonding?London dispersion forces operate between neutral, nonpolar molecules (and polar ones also)Ion-dipole forces operate between permanent ions and a polar solvent moleculeE.g., how does NaCl(s) dissolve in H2O? (review)The ion-dipole interactions between Na+, Cl-, & H2O are strong enough pull the ions from their lattice positionsThis process is called solvation (hydration if H2O is the solvent)The important difference now is this: to determine whether a solution will form, we look at the intermolecular forces within the solvent and solute separately and compare these interactions to that between the solvent and solute molecules.In solution formation we classify the interactions as follows:solute - solute interactionsattractive forces between solute particles onlysolvent - solvent interactionsattractive forces between solvent particles onlysolvent - solute interactionsattractive forces between solvent and soluteE.g., what are the interactions involved in dissolving NaCl(s) in water?E.g., what are the interactions involved in dissolving CH3OH in water?Easiest to think about the energetics of solution formation in terms of a series of steps:Step 1: overcome solute-solute interactionAlways endothermicRequires energy DH1Step 2: overcome solvent-solvent interactionAlways endothermicRequires energy DH2Step 3: let separated solvent and solute particles interactThis is the solvent-solute interactionAssociated energy called DH3If the solvent-solute interaction is very strong, DH3 will be very exothermicIf the solvent-solute interaction is weak, DH3 will be only slightly exothermicThe net enthalpy change in the formation of a solution is called DHsoln and is given byDHsoln = DH1 + DH2 + DH3DHsoln = sum of enthalpy changes associated with each of the interactionsWhat is the sign of DH for the first 2 steps in solution formation? A solution will not form if DHsoln is too endothermic spontaneous processes tend to be exothermicSolvent - solute interactions must be strong enough to make DH3 DH1+ DH2E.g., NaCl dissolves in H2O; would NaCl dissolve in CCl4? Why or why not?Would H2O dissolve in CCl4? Why or why not?Is energy the only factor involved in solution formation?e.g., NH4NO3 spontaneously dissolves in H2O even though DHsoln is endothermic (26.4 kJ)e.g., CCl4 and C6H14 are soluble in all proportions; what are the attractive forces at work?Two factors at work in solution formation:- EnergyProcesses in which the energy content of the system decreases tend to occur spontaneouslySpontaneous processes tend to be exothermicHow to explain the NH4NO3 case?Disorder (entropy)Processes which increase the amount of disorder in the system tend to occur spontaneouslySolution formation is favored by the increase in disorder which accompanies mixingIn general: A solution will form unless solute - solute or solvent - solvent interactions are too strong relative to solute-solvent interactionsE.g., when two nonpolar organic liquids, such as heptane and hexane, are mixed, DHsoln is generally quite small. Why is this?Given that DHsoln 0, why