acids & bases acids & bases acids & bases acids & bases

Generated by Foxit PDF Creator © Foxit Softwarehttp://www.foxitsoftware.com For evaluation only.PArrheniusP Arrhenius definition.PACID - a proton donorACIDS & BASESPBASE - a hydroxide ion donorPNEUTRALIZATION - reaction of acid andbase to give salt and WATERPHCl + NaOH --> NaCl + H 2 O orPH + + OH - --> H 2OPBronsted-LowryPACID - a proton donorACIDS & BASESPBASE - a proton acceptorPNEUTRALIZATION - exchange of a protonPHCl + H 2O --> H 3O + + Cl -PHCl + NH 3 --> NH 4 + + Cl - no possible OH -cause no OACIDS & BASESPamphoprotic behaviorPHSO 3- + H 2 O --> SO 42-+ H 3 O + andPHSO 3 - + HCl --> H 2 SO 3 + Cl -PProton in water: H + + H 2 O --> H 3 O + . ActuallyH 9 O 4+PAlso, remember:PH 2O H + + OH -PK w = [H + ][OH - ] = 1 x 10 -14PConjugate acid/baseACIDS & BASESPacid 1 + base 2 acid 2 + base 1HCl + H 2 O H 3 O + + Cl -P HNO 3+ H 2O H 3O + + NO 3-P a 1 b 2 a 2 b 1PCO 32-+ H 2 O HCO 3- + OH -P b 1 a 2 a 1 b 2P NH 3 + H 2 S NH 4 + + HS -P b 2a 1a 2b 1

Generated by Foxit PDF Creator © Foxit Softwarehttp://www.foxitsoftware.com For evaluation only.ACIDS & BASESPThe classic example is BF 3 + NH 3ACIDS & BASESPThe ones that interest us most will be theinteraction of metal ions with electron pairdonors.FFBFH:N HHF HF B N HF HNa + :O H HNa + :OHHA coordinate covalent bondACIDS & BASESPHARD-SOFT LEWIS ACID-BASE THEORYPHgCl 2 + KI.PWhy? Are there any means available topredict which acid (often, a metal or metalion) will react with which base (ligand)?HSAB tries to do so. Simple rule, by RalphPearson: Hard (Lewis) acids tend tocombine with hard (Lewis) bases; soft acidswill combine with soft bases.PNeed to find a way of classifying hard/soft. Aprecursor - the Chatt-Arhland theory -class aand b.ACIDS & BASESP Hard to develop general measure of hardness -- theHSAB behavior depends on reaction medium.Order of stability of complexes of metal ions in gasphase with halide ions is F - >Cl - >Br - >I - , which makesall metal ions appear hard in the gas phase. Samemetal ions in water have the order of stability I - >Br ->Cl - >F - -- so seem soft. Some attempts to providea quantitative measure in water, but haven’t metwith much success.P The book lists hard and soft acids & bases, page184.

Generated by Foxit PDF Creator © Foxit Softwarehttp://www.foxitsoftware.com For evaluation only.ACIDS & BASESPSOFT ACIDS. General characteristics. Insolublechlorides, chemically rather inert, so low on theactivity series. Occur in nature as free elements oras sulfides. Relatively electronegative metals.Relatively large and polarizable.PSOFT BASES.Tend to be large, polarizable anions,relatively low charge/size ratio for the anions.PHARD ACIDS AND BASES - opposite of above.PHard acid/base interactions mainly electrostatic.Soft acid/base interactions mainly covalent.PCd experimentsACIDS & BASESEtOHPh 3 PS: + CdI 2 ------------> NR ????EtOH competed successfully for coordination sitesEtOHPh 3PS: + CdI 2------------> (Ph 3PS) 2CdI 2C 6H 6ACIDS & BASESPRelative and Borderline. Suppose we have areaction where all are soft: CdSe + HgS CdS + HgSe This reaction goes to the right:Se is softer than S and Hg softer than Cd.PHardness or softness of an atom is notentirely an inherent property of that atom - itmay be altered. For instance, changing theoxidation state will alter. Co 2+ borderlinewhile Co 3+ hard.ACIDS & BASESQuiz Time!!!Which of the following will tend to go to the right?R 3 P-BBr 3 + R 3 N-BF 3 R 3 P-BF 3 + R 3 N-BBr 3SO 2 + Ph 3 P-HOCMe 3 Ph 3 P-SO 2 + HOCMe 3CH 3 HgI + HCl CH 3 HgCl + HI[AgCl 2] - (aq) + 2 CN - (aq) [Ag(CN) 2] - (aq) + 2Cl - (aq)

Generated by Foxit PDF Creator © Foxit Softwarehttp://www.foxitsoftware.com For evaluation only.ACIDS & BASESQuiz Time!!!Pa) Phosphines softer bases than amines and BBr 3softer acid than BF 3 so lies to left (K1)Pc) I - softer base than Cl - and CH 3 Hg + soft acid, solies left (K1)ACIDS & BASESP Drago-Wayland Equation, p. 189. ΔH =E A E B +C A C B . E A and E B ionic contributions towardbonding of the acid A and base B, while C A and C Bare covalent contributions. Pretty good in solventsof low dielectric constant. But to fit in water, a thirdset of parameters D A D B must be subtracted. The Dparameters appear to represent the extent of stericstrain produced when a water molecule coordinatedto the Lewis base is replaced by the incoming base.P So far, anyhow, there appears little likelyhood that auniversal measure of hardness and softness willoccur.ACIDS & BASESPacid strength - distinguish between strengthand concentration. HA + H 2 O H 3 O + +A -PK = a(H 3 O + )a(A - ) Usually a(H 2 O) = 1a(H 2 O)a(HA)Pand we often use concentrations:PK = [H 3 O + ][A - ][HA]PFor water: H 2O + H 2O H 3O + + OH -PSo K w = [H 3 O + ][OH - ] =1 x 10 -14Pand pH = -log[H 3O + ] = log(1/[H 3O + ]) andACIDS & BASESPStrong/weak acid/basePCompletely ionizePThe weaker the acid the stronger is itsconjugate base.PpK a = - logK a

Generated by Foxit PDF Creator © Foxit Softwarehttp://www.foxitsoftware.com For evaluation only.ACIDS & BASESPHA + H 2O H 3O + + A -P or, HA H + + A -PRelationship between acid and conjugatebase. The stronger the acid the weaker theconjugate base.PFrom the equation - if HA is a relativelystrong acid, it loses its proton relatively easily- the equilibrium tends to lie further right -thatmeans that the reverse reaction - thereaction between H + and A - is less favored -A - is a relatively weak base.ACIDS & BASESPVery important relationship between K a, K b,and K w. We need this relationship so that wecan relate a Bronsted acid and its conjugatebase.ACIDS & BASESPFor the acid HA, HA H + + A - . If A - isa weak base, HA is a strong acid, and visaversa.PIf A - weak base, H 2 O + A - HA + OH -does not proceed very far to the right.ACIDS & BASESP For H 2 O:H 2 O H + + OH - ; K I = [H + ][OH - ]P For H + + A - HA; K II = [HA][H + ][A - ]P A - + H 2 O HA + OH - ; K III = [HA][OH - ][A - ]P K Ix K II= [H + ][OH - ] x [HA] = [HA][OH - ] = K III[H + ][A - ] [A - ]P But K I = K wand K II = 1 and K III = K BK AP So: K w = K BK Aor K w = K A X K B

Generated by Foxit PDF Creator © Foxit Softwarehttp://www.foxitsoftware.com For evaluation only.ACIDS & BASESLeveling EffectP Compare the strength of various acids bycomparing their tendancy to donate a proton. Try:P HCl + H 2 O H 3 O + + Cl - 100% to the right HF+ H 2 O H 3 O + + F - Incomplete K a . 10 -4P So HCl is stronger than HFP HCN+ H 2O H 3O + +CN - very weak K a.10 -10P So HCl > HF > HCN. Easy. But what aboutHCl/HBr?ACIDS & BASESLeveling EffectPHCl, HBr, HI same strength in waterPHCl + H 2 O --> H 3 O + + Cl -P HBr + H 2 O --> H 3 O + + Br -P HI + H 2 O --> H 3 O + + I -P How to distinguish?PNeed a weaker base than water.ACIDS & BASESPConsider relative strengths of four HX acids.PHF weakest (pK a= 3.2), HCl next (pK a= -7),HBr next (pK a= -9) and HI strongest (pK a=-10)ACIDS & BASESPThe strength of an acid: HX H + + X -PIs the degree to which the reaction occurs.PRecall that ΔG for the reaction is:PΔG = -RTlnK, so ΔG is related to KPLarge negative ΔG values indicates that thereaction proceeds to the right, and so there isa large K.PThus, the size of K indicates the degree towhich the reaction proceeds to the right.

Generated by Foxit PDF Creator © Foxit Softwarehttp://www.foxitsoftware.com For evaluation only.ACIDS & BASESACIDS & BASESPAlso, note that ΔG = ΔH - TΔSPThe factors that affect the ionization of anacid in water summarized by atheromochemical cyclePThe sum around the cycle is:PΔH ion = -ΔH(HX) + ΔH(H + ) + ΔH(X - ) + Bondenergy(HX) + IP(H) - EA(X)PAlways problems with solvation enthalpies forthe ionic parts, but approximations areavailable.PA tabulation:ACIDS & BASESP HF HCl HBr HIPΔH(HX) -11.5 -4.2 -5.0 5.5PΣΔH(H + ) + ΔH(X - ) -282 -349 -341 -340PBond energy (HX) 134 103 87.5 71PIP(H) 315 315 315 315PEA(X) 82 87 82 76PΔH ion(Calc) - 3.0 -13.7 -15.2 -14PTΔS(calc) -6.0 - 4.0 -3.0 -1.0PΔG ion (calc) 3 -10 -12 -13PpK(calc) 2 - 7 - 9 -10ACIDS & BASESPThe pK values indicate that only HF is weaklyionizedPIn that system, the molecular system isdetected.PIn the others, only H + and X - are foundPThe relatively large bond energy for HF is theprime source of this effect.

Generated by Foxit PDF Creator © Foxit Softwarehttp://www.foxitsoftware.com For evaluation only.ACIDS & BASESpolyproticsPH 2 SO 4 2 H + 2-+ SO 4PActually H 2 SO 4 H + 1-+ HSO 4PAnd HSO 1- 4 H + 2-+ SO 4PK a1always greater than K a2.PWhy???ACIDS & BASESQuiz Time!!!P Arrange in order of increasing acidity:P [Na(OH 2 ) n ] + ; [Sc(OH 2 ) 6 ] 3+ ; [Mn(OH 2 ) 6 ] 2+ ; [Ni(OH 2 ) 6 ] 2+P Increasing positive charge on the center atom anddecreasing ionic radius increases acid strength.Strongest acid -highest charge, for acids where ionicradius (r) + diameter (d) of the water molecule varieslittle. Here d constant. So [Na(H 2 O) 6 ] + weakest and[Sc(H 2 O) 6 ] 3+ strongest. The two remaining oneshave same charge, so one with smaller r will have thesmaller value of r+d and so the greater acidity. SinceNi 2+ has a greater Z eff than Mn 2+ it has a smallerradius, and so [Ni(H 2 O) 6 ] 2+ will be more acidic.ACIDS & BASESP Pauling's Rules P. 196. For the oxoaxid H yMO x,the strength is given by pK a= 14 -pK b. At a level ofapproximation, the strength of the oxo aciddepends only on the number of oxo groups present:pK a = 8.5 -5.7(x-y), where the oxoacid is MO x-y(OH) y to show the no. of oxo groups directly.P With each proton transferred, pK agoes up by fiveunits.P Since existing oxo acids of this type have from zeroto three oxo groups, we can categorize the oxoacids:ACIDS & BASESP 1. Three oxo groups (in practice, HMO 4 ) - pk a 's ofabout -8.6 - very strong acids.P 2. Two oxo groups (HMO 3 , H 2 MO 4 , and H 4 MO 6 ) -pK a 's of about -2.9 - strong acids. HNO 3 & H 2 SO 4 .P 3. One oxo group (HMO 2, H 2MO 3, H 3MO 4, andH 5MO 6) - pk a's of 2.8 -moderately acidic.P 4. No oxo groups (hydroxides of nonmetals) -pK a 'sof about 8.5 - weak acids.P 5. The second acid dissociation constants of theseoxo acids can be estimated by adding 4.5 to thefirst ionization constant, and so forth for the thirdconstant, etc.

Generated by Foxit PDF Creator © Foxit Softwarehttp://www.foxitsoftware.com For evaluation only.ACIDS & BASESPNote the exceptions for H 3PO 4(HO) 3PO,H 3PO 3(HO) 2HPO, and H 3PO 2(HO)H 2POPConsider H 3 PO 3 . Here, x-y = 0, but pK a is1.8. Actually, noting structure, x = 3 and y =2, so 8.5 -5.7 = 2.8 -close, cause has twooxo groups.ACIDS & BASESQuiz Time!!!-PFind pK a of H 2 SO 3 and HSO 3-P(HO) 2SO and (HO)SO 2PFor (HO) 2 SO pK a = 8.5 - 5.7(1) = 2.8PAnd for (HO)SO - 2 = 7.8PActual 1.9 and 7.2HOCl7.2Si(OH) 410Te(OH) 67.8, 11.2B(OH) 39.1ρ = 0ACIDS & BASESStructure & pK a Values of Some OxoacidsOC(OH) 23.6OP(OH) 32.1, 7.4, 12.7OI(OH) 51.6, 7.0OAs(OH) 32.3, 6.9, 11.5OClOH2.0O(H)P(OH) 21.8, 6.6ρ = 1OSe(OH) 22.6, 8.0O 2 N(OH)-1.4O 2 S(OH) 2-2.0, 1.9O 2 ClOH-1.0ρ = 2O 3 ClOHρ = 3ACIDS & BASESPAcidic and basic oxides. Metal vs nonmetaloxides.PNa 2 O + H 2 O --> 2 NaOHP2 P 2O 5+ 6 H 2O --> 4 H 3PO 4PAlso called acidic or basic anhydridesPAl 2O 3an example of an amphoteric oxide.PAl 2O 3+ 6 H 3O + + 3 H 2O --> 2[Al(OH 2)] 3+PAl 2 O 3 + 2 OH - + 3 H 2 O --> 2[Al(OH) 4 ] -

Generated by Foxit PDF Creator © Foxit Softwarehttp://www.foxitsoftware.com For evaluation only.ACIDS & BASESPPolymerization of aqua ions to polycations.POne strong characteristic of Cr(III) is itstendency to hydrolyze and form polynuclearcomplexes containing OH - bridges. Thoughtto occur by the loss of a proton fromcoordinated water, followed by coordinationof the OH - so formed to a second cation:ACIDS & BASESP[Cr(H 2 O) 6 ] 3+ [Cr(H 2 O) 5 OH] 2+ OH -[(H 2 O) 4 Cr Cr(H 2 O) 4 ] 4+ + 2 H 2 OOH -Or [Cr(H 2 O) 6 ] 3+ + [Cr(H 2 O) 5 OH] 2+ [H 2 O) 5 Cr Cr(H 2 O) 5 ] 5+ + H 2 OOH -