水化学成分和水化学指标学习教案.pptx

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1、会计学 1水化学成分和水化学指标(zhbio)第一页，共77 页。水分子的结构(jigu)与性质在水分子中，氢、氧原子核呈等腰三角形排列，氧核 位 于(wiy)两腰相交的顶角上，而两个氢核则 位 于(wiy)等腰三角形的两个底角上，两腰夹 角 为10445。在水分子中氢、氧原子的这种排列，使水分子在结构上正负电荷静电引力中心不重合，从而(cng r)形成水分子的偶极性质。第 2 页/共 77 页第二页，共77 页。水分子的结构(jigu)与性质以上图象(t xin)为计算机模拟所得的水分子结构图。第 3 页/共 77 页第三页，共77 页。水的独特(dt)性质 由于水分子的结构很特殊，使相邻水

2、分子之间可以由氢键联结，这就导致水在物理化学性质方面具有(jyu)一系列不同于其他液体的独特性质。v 水具有使盐类离子产生水化作用的能力v 水具有高的介电效应 v 水具有良好(lingho)的溶解性能第 4 页/共 77 页第四页，共77 页。天然水的组成(z chn)v 天然水是组成复杂的溶液v 存在于地壳中的87种稳定(wndng)的化学元素中，在天然水中就发现了70种以上v 天然水的化学成分是指v 离子、络阴离子、复杂络合物v 无机分子(O2、CO2、H2、CH4、H4SiO4)v 有机分子v 微生物（细菌、病毒、真菌寄生虫）（存活时间、吸附、酸性土壤）v 胶体(10-9-10-7m)第

3、 5 页/共 77 页第五页，共77 页。v 离子、络阴离子、复杂络合物v 单一离子形式(xngsh)：Ca2+、Mg2+、Na+、K+、Cl-、F-v 络阴离子形式(xngsh)：SO42-、CO32-、HCO3-、NO3-、CrO42-、PO43-v 复杂络合物：包括有机和无机络合物v 地下水中常见的常量组分络合物有10种：v CaSO40、MgSO40、NaSO4-、KSO4-v CaHCO3+、MgHCO3+、NaHCO30 v CaCO30、MgCO30、NaCO3-天然水的组成(z chn)第 6 页/共 77 页第六页，共77 页。n n Dissolved substances

4、 that can donate a proton are called acids and those that can accept a proton are called bases.The key to understanding acid/base equilibria lies in the phenomenon of hydrolysis.n n Hydrolysis is a reaction that accompanies ion hydration.Hydrogen ions are labile and can transfer from one water molec

5、ule to the next in solution.Imagine the hydrogen ions associated with the first neighbor water molecules around a monovalent cation in solution.Ion Hydrolysis第 7 页/共 77 页第七页，共77 页。Ion Hydrolysis第 8 页/共 77 页第八页，共77 页。n n Would Would the the cation cation tend tend to to repel repel and and perhaps pe

6、rhaps eject a hydrogen ion into the bulk solution?eject a hydrogen ion into the bulk solution?n n Certainly Certainly one one would would think think so so and and the the tendency tendency would would be be greater greater the the higher higher the the charge charge on on the the cations cations.Th

7、e The process process of of hydrogen hydrogen ion ion detachment detachment from from hydration hydration sheaths sheaths and and their their ejection ejection into into the the bulk bulk solution solution is called hydrolysis.is called hydrolysis.Ion Hydrolysis第 9 页/共 77 页第九页，共77 页。n n Cation hydro

8、lysis thus results in a decrease in solution pH.n n Anion hydrolysis operates oppositely.Because of their negative charge,anions not only preserve the hydrogen ions of water molecules in their hydration sheaths but attract hydrogen ions from the bulk solution.n n Anion hydrolysis thus results in an

9、increase in the solution pH.Ion Hydrolysis第 10 页/共 77 页第十页，共77 页。n n Monovalent ions are so weakly charged that they rarely hydrolyze.For example,NaCl is considered a neutral salt,i.e.,producing no effect on pH when it is added to water.n n This is because the field strengths(charge/surface area rat

10、ios)of Na+and Cl are not sufficiently high to attract or eject hydrogen ions to or from their hydration sheaths.Ion Hydrolysis第 1 1 页/共 77 页第十一页，共77 页。n n The addition of AlCl3 or Na2CO3 to water,however,causes a marked change in the pH of water.In the case of AlCl3,it is the Al3+ion that hydrolyzes

11、 and several hydrolysis products form representing Al3+ions that have lost one,two,three and four hydrogen ions from their hydration sheaths.n nAlAl3+3+H+H22O AlOHO AlOH2+2+H+H+n nAlAl3+3+2H+2H22O Al(OH)O Al(OH)22+2H+2H+n nAlAl3+3+3H+3H22O Al(OH)O Al(OH)3300+3H+3H+n nAlAl3+3+4H+4H22O Al(OH)O Al(OH)4

12、4+4H+4H+Ion Hydrolysis第 12 页/共 77 页第十二页，共77 页。n n In In the the case case of of the the pH pH increase increase associated associated with with the the addition addition of of Na Na2 2CO CO33 to to water,water,it it is is the the CO CO332 2 ion ion that that hydrolyzes.hydrolyzes.For For CO CO3 322,

13、only only two two hydrolysis hydrolysis products products form:form:n nCOCO3322+H+H+HCO HCO33n nCOCO3322+2H+2H+H H22COCO3300Ion Hydrolysis第 13 页/共 77 页第十三页，共77 页。n n Thus,when Al3+ions are added to water,amounts of AlOH2+,Al(OH)2+,Al(OH)30 and Al(OH)4 ions are formed.Similarly,whenever CO32 ions are

14、 added to water,an equilibrium concentration of the hydrolysis products,HCO3 and H2CO30 form.n n The relative amounts of an ion and its hydrolysis products will depend on the initial pH of the solution and the equilibrium constants describing the hydrolysis reactions.Ion Hydrolysis第 14 页/共 77 页第十四页，

15、共77 页。n nThe The concept concept of of hydrolysis hydrolysis is is really really the the key key to fully understandingto fully understandingn n why elements are found in the forms they are in water;n n what controls acid/base equilibria and pH buffering in solution;n n why the solubility of some mi

16、nerals is pH dependent;n n why pH changes are frequently associated with oxidation/reduction reactions.Ion Hydrolysis第 15 页/共 77 页第十五页，共77 页。n n Monovalent Monovalent ions ions rarely rarely hydrolyze hydrolyze in in solution solution because because the the single single positive positive or or neg

17、ative negative charge charge is is insufficient insufficient to to dislodge dislodge or or attract attract hydrogen hydrogen ions ions to to or or from from the the bulk solution.bulk solution.n n Thus Thus ions ions like like ClCl,II,Na Na+,and and K K+are are only found in one ionic form in water.

18、only found in one ionic form in water.Hydrolysis-Monovalent ions第 16 页/共 77 页第十六页，共77 页。n n However,However,there there are are some some exceptions.exceptions.F F tends tends to to attract attract H+,H+,especially especially at at low low pH pH where where an an abundance abundance of of H+H+ions i

19、ons are are present present in in the the bulk bulk solution.solution.Why Why does does F F hydrolyze hydrolyze and and not Cl?not Cl?n n F F has has a a smaller smaller crystallographic crystallographic radius radius(晶 晶 体 体 学 学 半 半 径 径(bnjng)(bnjng)，而 而 非 非 水 水 合 合 半 半 径 径(bnjng)(bnjng)：hydrated h

20、ydrated radius)radius)and and a a higher higher charge charge density density at at its its surface surface and and so so is is more more likely likely to to attract attract a a hydrogen hydrogen ion.ion.The The hydrolysis hydrolysis reaction reaction(written(written in reverse)is merely an acid dis

21、sociation reaction:in reverse)is merely an acid dissociation reaction:Hydrolysis-Monovalent ions第 17 页/共 77 页第十七页，共77 页。n n Remembering that pH=log aH+and assuming activities concentrations,at what pH would the hydrolysis product HF0 equal the concentration of F,i.e.,at what pH will aF/aHFo=1.0?n n

22、From the above equilibrium expression,it is seen that this would occur at a hydrogen ion activity equal to the value of the hydrolysis reaction constant,i.e.,aH+=104.0 or at a pH of 4.0.Hydrolysis-Monovalent ions第 18 页/共 77 页第十八页，共77 页。n n Assuming for simplicity that concentrations equal activities

23、,i.e.,s=1.0,the concentrations of HF0 and F-can be solved as a function of pH.n n At a pH of 3.0,aF/aHF0 would equal 0.1 and at a pH of 5.0,aF/aHF0 would equal 10.The concentrations of HFo and F at a variety of pHs can be formally calculated by solving the following two simultaneous equations:Hydrol

24、ysis-Monovalent ions第 19 页/共 77 页第十九页，共77 页。n n These results are plotted in the diagram to the left below at two concentrations of FTotal.Hydrolysis-Monovalent ions第 20 页/共 77 页第二十页，共77 页。n n Two features are noticed from this diagram.Two features are noticed from this diagram.First,the First,the h

25、ydrolysis product F hydrolysis product F(the one(the one that has lost a H that has lost a H+with respect with respect to the parent)increases in concentration with increasing pH to the parent)increases in concentration with increasing pH.Think of this as merely due to the greater likelihood that Th

26、ink of this as merely due to the greater likelihood that hydrogen ions will be ejected from hydrogen ions will be ejected from the hydration sheath of the hydration sheath of an ion when hydrogen ions are depleted in the bulk an ion when hydrogen ions are depleted in the bulk solution(pH solution(pH

27、 increase).increase).n n Secondly,the position of equal concentrations of HF Secondly,the position of equal concentrations of HF0 0 and and F F is independent of the is independent of the total concentration of fluorine in total concentration of fluorine in solution.solution.Such a diagram is called

28、 a pH-distribution diagram Such a diagram is called a pH-distribution diagram as it as it shows the distribution of concentration of an elements shows the distribution of concentration of an elements aqueous species as a function of aqueous species as a function of pH.The diagram to the right pH.The

29、 diagram to the right is an alternate way of expressing this information.It shows the is an alternate way of expressing this information.It shows the percentage of the total fluoride that each species comprises as a percentage of the total fluoride that each species comprises as a function of pH.fun

30、ction of pH.Hydrolysis-Monovalent ions第 21 页/共 77 页第二十一页，共77 页。n nDue to their higher charge,elements of 2+or 2 charge are more likely to hydrolyze than monovalent ions.Examine the accompanying distribution diagrams for Se2 and Fe2+.Which of these ions behaves like an acid when added to a solution a

31、nd which behaves like a base?Hydrolysis-Divalent ions第 22 页/共 77 页第二十二页，共77 页。n n Lets now look at the distribution diagrams for Ca Lets now look at the distribution diagrams for Ca2+2+and and Mg Mg2+2+.Both ions hydrolyze to.Both ions hydrolyze to form a hydroxide species that form a hydroxide spec

32、ies that becomes dominant at high pH.becomes dominant at high pH.Why is Mg Why is Mg2+2+more effective more effective at at hydrolysis than Ca hydrolysis than Ca2+2+?Most natural waters have pHs Most natural waters have pHs between 4.0 and 9.0,is it important to between 4.0 and 9.0,is it important t

33、o consider consider hydrolysis reactions for calcium and magnesium for hydrolysis reactions for calcium and magnesium for most waters?No,not really.most waters?No,not really.Hydrolysis-Divalent ions第 23 页/共 77 页第二十三页，共77 页。n n Things begin to look more interesting for Things begin to look more inter

34、esting for trivalent ions like B trivalent ions like B3+3+and Fe and Fe3+3+.For ferric For ferric iron theiron the hydrolysis reactions can be written hydrolysis reactions can be written like:like:n n Fe Fe3+3+H+H2 2O FeOH O FeOH2+2+H+H+n n Fe Fe3+3+2H+2H2 2O Fe(OH)O Fe(OH)2 2+2H+2H+n n Fe Fe3+3+3H+

35、3H2 2O Fe(OH)O Fe(OH)3 30 0+3H+3H+n n Fe Fe3+3+4H+4H2 2O Fe(OH)O Fe(OH)4 4+4H+4H+Hydrolysis-Trivalent ions第 24 页/共 77 页第二十四页，共77 页。n nAlthough Fe3+is exclusively an acid and Fe(OH)4,exclusively a base,the intermediate hydrolysis products FeOH2+,Fe(OH)2+and Fe(OH)30 can accept or donate protons.Speci

36、es capable of such dual behavior are termed amphoteric substances.n nThe hydrolysis equilibria can also be expressed as sequential reactions:n n Fe Fe3+3+H+H2 2O FeOH O FeOH2+2+H+H+n n FeOH FeOH2+2+H+H2 2O Fe(OH)O Fe(OH)2 2+H+H+n n Fe(OH)Fe(OH)2 2+H+H2 2O Fe(OH)O Fe(OH)3 30 0+H+H+n n Fe(OH)Fe(OH)3 3

37、0 0+H+H2 2O Fe(OH)O Fe(OH)4 4+H+H+Hydrolysis-Trivalent ions第 25 页/共 77 页第二十五页，共77 页。n nDifferent values are obtained for the equilibrium constants depending on which way the hydrolysis reactions are written but the final distribution diagram will look the same.This diagram is shown below.Hydrolysis-

38、Trivalent ions第 26 页/共 77 页第二十六页，共77 页。n n One interesting feature of the ferric iron One interesting feature of the ferric iron hydrolysis diagram is that the species hydrolysis diagram is that the species Fe(OH)Fe(OH)3 30 0 never becomes the dominant species never becomes the dominant species in s

39、olution at any pH in solution at any pH.n n It often happens that for steric It often happens that for steric reasons reasons certain hydrolysis products are more likely to certain hydrolysis products are more likely to form than others.form than others.Hydrolysis-Trivalent ions第 27 页/共 77 页第二十七页，共7

40、7 页。n n The diagram for Al The diagram for Al3+3+shows a similar phenomenon.In shows a similar phenomenon.In this case,Al(OH)this case,Al(OH)2+2+never becomes a dominant never becomes a dominant species with pH.It is quite common for species with pH.It is quite common for hydrolysis hydrolysis produ

41、cts with an even number of hydroxide groups products with an even number of hydroxide groups to have a greater stability than those with an odd to have a greater stability than those with an odd number of hydroxide groups number of hydroxide groups.Hydrolysis-Trivalent ions第 28 页/共 77 页第二十八页，共77 页。n

42、 nUsing Fe3+and Al3+hydrolysis as comparisons,the hydrolysis reactions for B3+might be:n n B B3+3+H+H2 2O BOH O BOH2+2+H+H+n n BOH BOH2+2+H+H2 2O B(OH)O B(OH)2 2+H+H+n n B(OH)B(OH)2 2+H+H2 2O B(OH)O B(OH)3 30 0+H+H+n n B(OH)B(OH)3 30 0+H+H2 2O B(OH)O B(OH)4 4+H+H+n nIf the literature is examined,how

43、ever,only two aqueous species are listed for boron B(OH)30and H2BO3-.Hydrolysis-Trivalent ions第 29 页/共 77 页第二十九页，共77 页。n nThe distribution diagram is shown in the accompanying figure.Hydrolysis-Trivalent ions第 30 页/共 77 页第三十页，共77 页。n nTo explain this difference between the anticipated and the actual

44、 hydrolysis pattern for borate species,a digression is necessary.n nA dissolved ion can be represented with varying number of water molecules.Fe3+in solution,for example,also can be expressed as Fe3+H2O,Fe3+2H2O or even as H2FeO3+or H4FeO23+.n nThis is because the choice of how many water molecules

45、are associated with the formula of an ion is arbitrary,reflecting the fact that the actual number of water molecules associated with any dissolved ion is really undefined.Hydrolysis-Trivalent ions第 31 页/共 77 页第三十一页，共77 页。n n Convention has it Convention has it that Fe that Fe3+3+is chosen and its hy

46、drolysis is chosen and its hydrolysis products expressed as products expressed as FeOH FeOH2+2+,Fe(OH),Fe(OH)2 2+,Fe(OH),Fe(OH)3 30 0 and Fe(OH)and Fe(OH)4 4-.However,.However,H H4 4FeO FeO2 23+3+could have been could have been chosen chosen instead of Fe instead of Fe3+3+and the hydrolysis products

47、 and the hydrolysis products expressed as expressed as H H3 3FeO FeO2 22+2+,H H2 2FeO FeO2 2+,HFeO HFeO2 20 0 and and FeO FeO2 2-.n n Matching up species of the same charge,it is seen that Matching up species of the same charge,it is seen that HFeO HFeO2 20 0 and Fe(OH)and Fe(OH)3 30 0 are the same

48、species.If a are the same species.If a water molecule formula unit is subtracted from the water molecule formula unit is subtracted from the latter and rearranged,the former results.latter and rearranged,the former results.n n Similarly,Similarly,Fe(OH)Fe(OH)4 4-must be FeO must be FeO2 2-.A general

49、ization can.A generalization can be made:You can add or subtract water molecule be made:You can add or subtract water molecule formula units from an aqueous species without altering formula units from an aqueous species without altering the species to which you are referring.the species to which you

50、 are referring.Hydrolysis-Trivalent ions第 32 页/共 77 页第三十二页，共77 页。n nNow,lets go back to B3+hydrolysis in water.H3BO30 can alternatively be expressed as B(OH)30 and H2BO3-as B(OH)4-.Convention has it that the former nomenclature for boron species is chosen rather than the latter.n nThats fine but whe