Principles of Chemistry II © Vanden BoutHave you been reading my “notes”Two topicsRelating ΔH and ΔS fora phase transitionWhat is special about waterPrinciples of Chemistry II © Vanden BoutFree Energy ChangeWhat is the sign of the change in free energyfor me dropping an eraser?! A.! ! negative (free energy decreases)! B.! ! positive (free energy increases)! C.! ! zero (free energy is constant)! D.! ! it depends on the temperature!Things that happen decrease lower free energy (equilibrium is zero change)Principles of Chemistry II © Vanden BoutMixturesWhat is different than pure substances?What is the free energy change for gases mixing?! A.! ! negative (free energy decreases)! B.! ! positive (free energy increases)! C.! ! zero (free energy is constant)! D.! ! it depends on the temperature!it happens. therefore free energy decreasesPrinciples of Chemistry II © Vanden BoutMixturesWhat is different than pure substances?What is the free energy change for gases mixing?! A.! ! negative (free energy decreases)! B.! ! positive (free energy increases)! C.! ! zero (free energy is constant)! D.! ! it depends on the temperature!it happens. therefore free energy decreasesPrinciples of Chemistry II © Vanden BoutMixturesWhat is different than pure substances?Why does the free energy decrease? (ΔG=ΔH-TΔS)! A.! ! ΔH is positive, ΔS is zero! B.! ! ΔH is zero, ΔS is positive!C.!!ΔH is negative, ΔS is positive!D.!!ΔH is negative, ΔS is zero!Gases ~ no IMF. ThereforeΔH = 0The volume of each gas increasestherefore ΔS > 0Take home lesson generallyentropy increases with mixingPrinciples of Chemistry II © Vanden BoutWhen we think of mixtures we typically think about solutionsSolvent: the majority of the moleculesIMF only slightly changed(most solvent molecules interacting with solvent molecules)Solute: the minority substancethe “stuff that is dissolved”could be a solid, liquid, or a gasIMF total differentin solution, solute molecules only interacting with solvent moleculesPrinciples of Chemistry II © Vanden BoutSolutions typically have a higher entropy than the unmixed compoundsTherefore ΔSsolution > 0For most casesEntropy of Solution ΔSsolutionusually easy to predictSince entropy almost always favors mixing,the differences between different substances are the result of enthalpy (intermolecular forces)Principles of Chemistry II © Vanden BoutWhat is enthalpy change for making a solution?Lose solute-solute interactions (IMF)Lose solvent-solvent interactions (IMF) (this is small)Gain solute-solvent interactionsFigure Copyright Houghton Mifflin Company. All rights reservedPrinciples of Chemistry II © Vanden BoutHow we generally think of thisWhat is ΔHsolution?this is the enthalpy change for making the solution?Two stepsFirst break up the solute ΔHLattice Energy(loss of solute-solute interactions) “costs energy” positiveNext put solute into solvet ΔHsolvation(add of solute-solvent interactions) “releases energy” negativeΔHsolution = ΔHLattice Energy + ΔHsolvationPrinciples of Chemistry II © Vanden BoutEnthalpy of Solution ΔHsolutionhard to predictΔHsolution = 0Ideal solutionSolute-solvent interactions are identical to solute-solute (and solvent-solvent)ΔHsolution > 0TypicalSolute-solvent interactions are weaker thansolute-solute (and solvent-solvent)ΔHsolution < 0Unusual but possibleSolute-solvent interactions are stronger thansolute-solute (and solvent-solvent)Principles of Chemistry II © Vanden Bout! A.! ! a chloride ion ! B.! ! water! C.! ! they are the same!Which do you think has a stronger interactions with a sodium ion?Ion-Ion interactions will be stronger thanion-dipole interactions(but ion dipole interactions are still strong)Principles of Chemistry II © Vanden Bout! A.! ! positive ! B.! ! negative! C.! ! zero!What do you predict for the sign of the enthalpy of solution of NaCl in water?Because the solute/solvent interactions (ion-dipole) are weaker than the solute/solute (ion-ion)it will “cost” energy to get the salt into the waterPrinciples of Chemistry II © Vanden Bout! A.! ! ΔH > TΔS ! B.! ! ΔH = TΔS!C.!!ΔH < TΔS!For dissolving salt in water at room temperatureΔHsolution > 0ΔSsolution > 0which is larger?This actually happens. ΔG < 0. This means that ΔH < TΔSPrinciples of Chemistry II © Vanden Bout! A.! ! ΔH > TΔS ! B.! ! ΔH = TΔS!C.!!ΔH < TΔS!For dissolving water in olive oil at room temperatureΔHsolution > 0ΔSsolution > 0which is larger?This does not happen. ΔG > 0. This means that ΔH > TΔSPrinciples of Chemistry II © Vanden BoutWhen things will not dissovleΔHsolution is too large (bigger than TΔS)IMF between the solvent/solute are much lessfavorable than solute/solute (solvent/solvent)When will this happen?Very different IMF Oil (dispersion/nonpolar) and water(H-bonding, polar)Very strong ion/ion (MgO)Principles of Chemistry II © Vanden BoutOther problems for small high charge density ionsPrinciples of Chemistry II © Vanden BoutIf ΔGsoln < 0 solution strongly favoredIf ΔGsoln > 0 undissolved state is strongly favoredΔGsoln = ΔHsoln - T ΔSsoln Typically ΔSsoln >0, ΔHsoln > 0 need |TΔS| > |ΔH|Gibb's Free Energy of Solvation ΔGsolnPrinciples of Chemistry II © Vanden BoutWhat makes an ideal solution?Same IMF for solute-solvent and solute-solute and solvent-solvent"like dissolves like"Polar compounds dissolve polar compounds (ionic)Nonpolar compound dissolve
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