Chapter 16 Acids and Bases Chemistry The Molecular Science[第16章酸和堿的化學(xué)分子科學(xué)](-77)

1、Chapter 16:Acids and BasesChemistry: The Molecular ScienceMoore, Stanitski and JursArrhenius DefinitionArrhenius: any substance which ionizes in water to produceBetter version of the Arrhenius definition:Acid: hydronium ions (H3O+) in water are acidic.Base: hydroxide ions (OH-) in water are basic.pr

2、otons (H+) is an acid. hydroxide ions (OH-) is a base. Acid-Base ReactionsA strong acid is an acid that ionizes completely in water (product favored). There are six strong acids:HCl - hydrochloric acid HBr hydrobromic acidHI hydroiodic acidH2SO4 - sulfuric acidHNO3 nitric acidHClO4 perchloric acidAc

3、id-Base ReactionsStrong Acids and BasesA strong base is a base that is present entirely as ions (product favored). The hydroxides of Group IA, IIA (except Be and Mg hydroxides) are strong bases. So, why is NH3 (aq) basic?Brnsted-Lowry ConceptAn alternative definition:NH3(g) + H2O(l) NH4+(aq) + OH-(a

4、q)indirectly producedBase:H+ acceptorAcid:H+ donoracid= proton (H+) donorbase= proton (H+) acceptorWorks for non-aqueous solutions and explains why NH3 is basic:Weak acids and bases do not fully ionize (reactant favored).Brnsted-Lowry ConceptStrong acids and bases almost completely ionize (product f

5、avored).HNO3(aq) + H2O(l)H3O+(aq) + NO3-(aq)Note: the products are a new acid and base pair.LeChatelier a more dilute solution (more water), will ionize more.HF(aq) + H2O(l)H3O+(aq) + F-(aq)Base:H+ acceptorAcid:H+ donorBase:H+ acceptorAcid:H+ donor100%3%Waters Role as Acid or BaseWater acts as a bas

6、e when an acid dissolves in water: HBr(aq) + H2O(l) H3O+(aq) + Br-(aq)acid base acid baseWater is amphiprotic - it can donate or accept a proton (act as acid or base).But water acts as an acid for some bases:H2O(l) + NH3(aq) NH4+(aq) + OH-(aq) acid base acid basePracticeComplete the equations: (acid

7、 or base?, mass/charge balance?, single or double arrow?)HClO4 + H2OCN- + H2OH2S + H2ONO2- + H2OCa(OH)2 + H2O ClO4- + H3O+HCN + HO-HS- + H3O+HNO2 + HO- Ca+2 + 2OH-Conjugate Acid-Base PairsMolecules or ions related by the loss/gain of one H+.Conjugate AcidConjugate BaseH3O+H2OCH3COOH CH3COO-NH4+NH3H2

8、SO4HSO4-HSO4-SO42-HClCl-donate H+accept H+NH4+ and NH2- are not conjugate(conversion requires 2 H+)0Identify the base conjugate to HC2H3O2(aq) and the acid conjugate to HCO3-(aq)160 of 200HCO3- + H2O OH- +HC2H3O2 + H2O H3O+ +Conjugate AcidBase:H+ acceptorConjugateBaseAcid:H+ donorConjugate Acid-Base

9、 PairsShow that HCO3- is amphoteric. C2H3O2 - H2CO3Practice: Conjugate Acid-Base PairsWrite the equation for the acid or base in water and identify the acid-base pairs.HBrHSO3- (as an acid)HSO3- (as a base)PH4+Relative Strength of Acids & BasesStrong acids are better H+ donors than weak acids.Strong

10、 bases are better H+ acceptors than weak bases.Strong acids have weak conjugate bases.Weak acids have strong conjugate bases. Strong acid+ H2OH3O+ + weak conjugate baseFully ionized, reverse reaction essentially does not occur.The conjugate base is weak.Weakly ionized, reverse reaction readily occur

11、s.The conjugate base is strong. Weak acid + H2OH3O+ + strong conjugate baseConjugate acid Conjugate baseH2SO4HSO4-HBrBr-HClCl-HNO3NO3-H3O+H2OH2SO3HSO3-HSO4-SO42-H3PO4H2PO4-HFF-CH3COOHCH3COO-H2SHS-H2PO4-HPO42-NH4+NH3HCO3-CO32-H2OOH-OH-O2-H2H-CH4CH3-Acid strength increasingBase strength increasingston

12、g acidsstrong baseextremelyweak acidsextremelyweak basesRelative Strength of Acids & BasesHF is a stronger acid than NH4+. (NH3 is a stronger base than F-)HF has greater tendency to ionize than NH4+. (NH3 more readily accepts H+ than F-)Reactant FavoredNH4+ + F- NH3 + HFProblemIs the following aqueo

13、us reaction product or reactant favored?Acid strength increasingBase strength increasingConj acid. Conj. baseH2SO4HSO4-HBrBr-HClCl-HFF-NH4+NH3OH-O2-H2H-CH4CH3-Relative Strength of Acids & BasesCarboxylic AcidsC3H7COOHCH3COOHAmines (bases)CH3NH2 (CH3)2NH(CH3)3Nproton acceptorsAutoionization of WaterP

14、ure water conducts a very small electrical current.Autoionization occurs:H2O(l) + H2O(l) H3O+(aq) + OH-(aq)Kw = ionization constant for waterHeavily reactant favored.Kw = H3O+ OH- Kw = 1.0 x 10-14 (at 25C)BaseAcidAcidBaseIonization Constant for WaterKw is a special equilibrium constant for the autoi

15、onization of water. It is T-dependent.T = 25C (77F) is usually used as the standard T.T (C) Kw100.29 x 10-14150.45 x 10-14200.68 x 10-14251.01 x 10-14301.47 x 10-14505.48 x 10-14Kw increases with temperature. Is it endo or exothermic?Autoionization of WaterHydronium and hydroxide ions are produced i

16、n equal numbers in pure water,Thus, the concentrations of H3O+ and OH- in pure water are both 1.0 x 10-7 M.Kw = H3O+ OH- 2H2O(l) H3O+(aq) + OH-(aq)Autoionization of WaterH3O+ and OH- are present in all aqueous solutions.Neutral solution.Pure water ( 25C): H3O+ = 10-7 M = OH-Acidic solutionIf acid is

17、 added to water: H3O+ is increased, disturbing the equilibrium: 2 H2O H3O+ + OH-LeChatelier: equilibrium shifts to the left. OH- Equilibrium is reestablished: H3O+ 10-7 M OH-Product of ion concentrations is the same H3O+ OH- = Kw = 1.0 x 10-14 Autoionization of WaterBasic solution.If base is added t

18、o water.LeChatelier: equilibrium shifts to the left, H3O+New equilibrium: H3O+ 10-7 M 1.0 x 10-7 M.In a neutral solution, H3O+ = OH- = 1.0 x 10-7 M.In a basic solution, H3O+ 1.0 x 10-7, pH 7.00 basic solutions: H3O+ 7.00 The pH of a SolutionFor a solution in which the hydronium-ion concentration is

19、1.0 x 10-3, the pH is:Note that the number of decimal places in the pH equals the number of significant figures in the hydronium-ion concentration.Example pH given H3O+ ?A sample of orange juice has a hydronium-ion concentration of 2.9 x 10-4 M. What is the pH?Example - H3O+ given pH?The pH of human

20、 arterial blood is 7.40 (acidic or basic?). What is the hydronium-ion concentration? -7M?)10logx = xThe pOH scalepOH = log10OH-A neutral solution (25C) has:pOH = log101.0 x 10-7Since Kw = H3O+ OH- = 1.0 x 10-14log(KW)= logH3O+ + (logOH-) = log(1.0 x 10-14)pKwExampleAn ammonia solution has a hydroxid

21、e-ion concentration of 1.9 x 10-3 M. What is the pH of the solution?we first calculate the pOH:Then the pH is:pH CalculationsSolution A:pOH = log OH-pH + pOH = pKwSolution B:pH = log H3O+A has higher pH, B is more acidicGiven two aqueous solutions (25C).Solution A: OH- = 4.3 x 10-4 M,Solution B: H3O

22、+ = 7.5 x 10-9 M.Which has the higher pH? Which is more acidic?Practice - WSsH+ Kw =1x10-14 = H+ OH- OH-pH 14 = pH + pOH pOHpH= -log H+H+ = 10-pH pOH =-log OH-OH- = 10-pOH pH of Aqueous SolutionsStomach FluidsLemon JuiceVinegarWineTomatoesBlack CoffeeMilkPure waterBlood, seawaterSodium bicarbonateBo

23、rax solutionMilk of MagnesiaDetergentsAqueous ammonia1 M NaOHH3O+ OH- 10010-14 10-110-13 10-210-12 10-310-11 10-410-10 10-510-9 10-610-8 10-710-7 10-810-6 10-910-5 10-1010-4 10-1110-3 10-1210-2 10-1310-1 10-141ExampleBattery AcidpH 0 1 2 3 4 5 6 7 8 91011121314BleachMeasuring pHH3O+ concentrations c

24、an be measured with an:Electronic pH meter:fast and accuratepreferred method.Acid-base Indicator:substance that changes color within a narrow pH rangemay have multiple color change (e.g. bromthymol blue)one “color” may be colorless (e.g. phenolphthalein)cheap and convenient.HA(aq) + H2O(l) H3O+(aq)

25、+ A-(aq)The acid ionization constant is used to report the degree of ionization:A-H3O+HA Ka =(Water omitted, as usual)Strong acids have large Ka valuesWeak acid have small Ka valuesIonization Constants of Acids and BasesWhen an acid ionizes in water:Base Ionization ConstantsFor a base in water:The b

26、ase ionization constant, Kb, is: Kb =BH+OH-BA-(aq) + H2O(l) HA(aq) + OH-(aq) Kb =HAOH-A-B(aq) + H2O(l) BH+(aq) + OH-(aq)If the base is an anion:Larger Ka = stronger acidLarger Kb = stronger baseIonization ConstantsLarger Ka = stronger acidLarger Kb = stronger baseIonization ConstantsAcid and Base St

27、rength Small value of Ka indicates that the numerator is smaller than the denominator. Equilibrium favors reactants or only a small amount dissociates a weak acid. Small value of Kb indicates that the numerator is smaller than the denominator. Equilibrium favors reactants or only a small amount diss

28、ociates a weak base. Write the ionization equation and ionization constant expression for the following acids and bases: a.) HFb.) HBrOc.)NH3d.) CH3NH2ExampleKa Values for Polyprotic AcidsSome acids can donate more than one H+FormulaNameAcidic HsH2SHydrosulfuric Acid2H3PO4Phosphoric Acid 3H2CO3Carbo

29、nic Acid2HOOC-COOHOxalic acid2C3H5(COOH)3Citric acid3Each H+ ionization has a different Ka.The 1st proton is easiest to removeThe 2nd is harder, etc.Ka Values for Polyprotic AcidsPhosphoric acid (H3PO4) is weak. It has three acidic protons:H3PO4(aq) + H2O(l) H3O+(aq) + H2PO4- (aq)Ka = 7.5 x 10-3H2PO

30、4-(aq) + H2O(l) H3O+(aq) + HPO42- (aq)Ka = 6.2 x 10-8HPO42-(aq) + H2O(l) H3O+(aq) + PO43- (aq)Ka = 3.6 x 10-13Ka decreasingH+ is harder to removePractice Relative Strength (Ka and Kb)The following reactions all have Keq1.NO2- + HFHNO2 + F-CH3COO- + HFCH3COOH + F-HNO2 + CH3COO HNO2- + CH3COOHArrange

31、the substances based on their relative base strength. F-CH3COOHHFHNO2-CH3COO-HNO21 strongest base2 intermediate base3 weakest base4 not a B-L base344214Molecular Structure and Acid StrengthWhat makes a strong acid?A weak HA bond, so H+ can be easily removed! smaller bond energylarger acid strengthHX

32、 Bond Energy (kJ) KaHFweak acid5667 x 10-4HCl4311 x 107HBr3661 x 108HI2991 x 1010strong acidsConsider the binary acids HF, HCl, HBr, HI.H-A, bond energy decreases down a group.Acid strength increases.Molecular Structure and Acid StrengthWhat makes a strong acid?A weak HA bond, so H+ can be easily re

33、moved!Consider the binary acids SiH4, PH3, H2S, HCl.The higher electronegativity, across a period, makes the bond more polar and hence more ionic a stronger acid.Oxoacidshydrogen, oxygen and one other elementH O-Ehigher the electronegativity on E, stronger the acid as this weakens the bond between t

34、he O and HMolecular Structure and Acid StrengthHOCl HOBr HOIKa: 3.5 10-82.5 10-9 2.3 10-11The acid strength increases as the number of oxygen atoms bonded to E increases. Strong oxoacid has at least two oxygen atoms per acidic H atoms.Molecular Structure and Acid StrengthHClO HClO2 HClO3 *HClO4Ka: 3

35、.5 10-8 1.1 10-2 103 108Problem Solving Using Ka and KbExample: Ka from pHLactic acid is monoprotic. The pH of a 0.100 M solution was 2.43 at 25C. Determine Ka for this acid.HA(aq) + H2O(l) H3O+(aq) + A-(aq)Using the pH information:- log H3O+ = 2.43H3O+ = 10 = 0.0037 MThese hydronium ions are produc

36、ed by:H2O(l) + H2O(l) H3O+(aq) + OH-(aq)and from the autoionization of water: Ka = = 1.4 x 10-4 (0.0037)(0.0037)(0.100 0.0037)HA(aq) + H2O(l) H3O+(aq) + A-(aq) initial 0.100 1.0 x 10-7 0 0 (water autoionization) equilKa =H3O+A-HAProblem Solving Using Ka and KbProblem Solving Using Ka and KbExample:

37、pH from KaDetermine the pH of a 0.100 M propanoic acid solution at 25C. Ka = 1.4 x 10-5. What % of acid is ionized?C2H5COOH + H2O(l) H3O+(aq) + C2H5COO-(aq) Ka = = 1.4 x 10-5H3O+C2H5COO- C2H5COOHHA(aq) + H2O(l) H3O+(aq) + A-(aq) initial 0.100 0* 0change -x +x +x equil 0.100 x x* x*Ignore the water c

38、ontribution 1.4 x 10-5=x2(0.100 x)H3O+A-HAKa = 1.4 x 10-5 =Problem Solving Using Ka and KbDetermine the pH of 0.001 M propanoic acid soln. at 25C. Ka = 1.4 x10-5. What % of acid is ionized?Problem Solving Using Ka and KbBecause Ka is small, assume x HAKa = 1.4 x 10-5 = x2(0.100 x)x20.100 x = (1.4 x

39、10-5 )(0.100) = 0.00118 M(0.00118 0.100 M; the approximation is good)%-ionized = x100 = 1.18 %0.001180.100 7.0). CH3COO-(aq) + H2O(l) CH3COOH(aq) + OH-(aq)Calculate pH of solution from Kb of CH3COO-.A hydrolysis reaction water is broken apart.CH3COOH(aq) + OH-(aq) CH3COO-(aq) + H2O(l)baseacidbaseaci

40、dSalts of Strong Bases and Weak Acids100%pH of a Salt SolutionFor a weak acid + strong base, pH depends on KbLarger Kb = stronger baseExampleWhat is the pH of a 1.50 M aqueous solution of Na2CO3? Kb (CO32-) = 2.1 x 10-4.(or equal volumes of 1.5M H2CO3 and 3.0 M NaOH are mixed, what is the pH of the

41、mixture?)Na+ is the conjugate acid of NaOH (strong base).It remains 100% ionizedNa+ has no effect on pH.CO32- is the conjugate base of HCO3- (weak acid) Most CO32- ions undergo hydrolysisGenerates HCO3- and OH-Changes the pHCO32-(aq) + H2O(l) HCO3-(aq) + OH-(aq)What is the pH of a 1.50 M aqueous sol

42、ution of Na2CO3? Kb = 2.1 x 10-4 pH of a Salt SolutionCO32-(aq) + H2O(l) HCO3-(aq) + OH-(aq) initial 1.50 0 0change - x +x +x equil 1.50 - x xxKb = 2.1 x 10-4 = = HCO3-OH-CO32-x2(1.50 x)x21.50 x = 1.77 x 10-2pOH = log(1.77 x 10-2What is the pH of a 1.50 M aqueous solution of Na2CO3? Kb = 2.1 x 10-4

43、pH of a Salt SolutionSalts of Weak Bases and Strong AcidsNH3(aq) + HCl(aq) NH4+(aq) + Cl-(aq)The products of the neutralization reaction form an acidic solution: NH4+(aq) + H2O(l) NH3(aq) + H3O+(aq)NH3(aq) + H3O+(aq) + Cl-(aq) NH4+(aq) + Cl-(aq) + H2O(l)Net ionic: NH3(aq) + H3O+(aq)NH4+(aq) + H2O(l)

44、The final pH depends on the Ka , larger Ka = more acidicpH of a Salt SolutionFind the pH of a 0.132 M NH4Br. Is this solution acidic, basic or neutral? Ka(NH4+) = 5.6 x 10-10.The solution contains NH4+ ions and Br- ions.NH4+ is the conjugate acid of NH3 (a weak base).It will partially ionize in solu

45、tion giving NH3 and H3O+Forms an acidic solution.Br- is the conjugate base of HBr (a strong acid) Br- ions stay fully ionizedDoes not change the pH.NH4+(aq) + H2O(l) NH3(aq) + H3O+(aq) initial 0.132 0 0change - x +x +x equil 0.132 - x xxx = 8.57 x 10-6 pH = log(8.57 x 10-6Ka = 5.6 x 10-10 = = NH3H3O

46、+NH4+x2(0.132 x)x20.132pH of a Salt SolutionpH of a 0.132 M aq. solution of NH4Br? Is this acidic, basic or neutral? Ka(NH4+) = 5.6 x 10-10.Acidic!Salts of Weak Bases and Weak AcidsThe most difficult. Consider qualitative results. e.g.F-(aq) + H2O(l) HF(aq) + OH-(aq)NH3 (aq) + HF(aq)NH4+ (aq) + F- (aq) NH4+(aq) + H2O(l)NH3(aq) + H3O+(aq)Ka(NH4+) = 5.6 x 10-10 ; Kb(F-) = 1.4 x 10-11Ka (weak acid) Kb (weak base)The ammonium reaction is more favorable.The acid wins! The solution will be acidic!Acids, Bases and SaltsStrong acid + strong base salt solution, p