大學化學(1)醫(yī)學基礎(chǔ)化學：chapter2 ELECTROLYTE SOLUTION

1、 2-1 Strong and Weak Electrolyte Solution 2-2 Theory of Acid-base 2-3 Acidity and Calculation of Solution 2-4 Equilibrium Between Dissolution and Precipitation 2-1.1 Theory of Strong Electrolyte SolutionIon-ion Interaction TheoryFigure 2-1 Ion atmosphere of strong electrolyte solutionIon Activity an

2、d Activity CoefficientActivity(a): Ion concentration, which can play a real action in solution is ionic effective concentration, is called ion activity. actual concentration of ion (c) multiply a correction factor - activity coefficient ( f ). a = f c (2-1) Generally, ac, 0f1 Activity coefficient ar

3、e influenced by ion concentration the electric-charge number of ion has nothing to do with the nature of ion. Ionic Strength ( I )Where, I is ionic strength; c is the amount-of-substance concentration of the ion i; z i is the charge number of the ion i.Note that the activity is for an ion; the ion s

4、trength is for a solution.2ii222211zczczcI21.Table 2-1 Ion activity coefficient and ion strength of solutionExample 2-1 25ml 0.02mol /L HCl mixed with 25ml 0.18mol /L KCl, calculate activity of H+ ?Solution: (1) calculate the ion strength of the solution: I =( 0.0112+0.0112+0.0912+0.0912)/2 =0.1 (2)

5、 look up the activity coefficient of ion has one charge: when I = 0.1, Z =1, f = 0.78 (3) calculate the activity of H+ (aH+) cH+ =0.02/2 =0.01mol /L, f = 0.78 So, aH+ = f c = 0.780.01 = 0.0078 (mol /L)Ionization Equilibrium of Weak Electrolyte Solution The law of Chemical Equilibrium (Equilibrium Co

6、nstant)a A + b B c C + d D CcDd K = - (2-3) AaBb Ionization Constant ( HAc + H2O H3O+ + Ac - or simply HAc H+ + Ac - The corresponding equilibrium-constant expression is H+Ac - K i = - (2-4) HAc Degree of Ionization ( )1. Definition: Number of ionized molecules = - -100% 100% total number of solute

7、molecules Number of ionized molecules = -100%100% ionized molecules + non- ionized molecules concentration of ionized weak electrolyte = - = -100%100% initial concentration of weak electrolyte the nature of solute: 18，0.1mol/L, HAc= 1.33%, H2S= 0.07%, HCN= 0.007% the initial concentration of solute:

8、 (the more dilute the solution, the greater the degree of ionization). temperature:Dilution Law HA H+ + A - c Initial c c 0 0Equilibrium c c c cc c concentration H+A - cc c2 KHA = - = - = - HA c- c 1- For Ka is very small, is very small, 1- 1 KHA = c2 cKHAorPhysical meaning:Note that above dilute la

9、w only is for some given conditions : (1) the weak electrolyte must be monoprotic (2) 5% 5% The Common Ion Effect and Salt Effect1. Common ion effect The ionization of a weak electrolyte is markedly decreased by the adding to the solution an ionic compound containing one of the ion of the weak elect

10、rolyte, this effect is called the common ion effect.For example,HAcAc - + H+ NaAc Ac + Na+shift the equilibrium from right to left, decreasing the H+.2. Salt effect The ionization of a weak electrolyte is increased by adding to the solution an soluble strong electrolyte which not contains the common

11、 ion with the weak electrolyte. This effect is called salt effect. For example: 0.1mol/L H Ac = 1.33%, adding NaCl, NaCl= 0.1mol/L, = 1. 68%Example: There is a solution of c(HAc) =0.1mol/L, if we add NaAc, when c(NaAc)=0.1mol/L. Calculate the of HAc.Solution: HAcH+ + Ac -Initial c 0.1 0 0Equilibrium

12、 c 0.1-H+ H+ 0.1+ H+ 0.1 0.1 H+Ac- H+0.1 Ka= - = - = H+ = 1.810-5 H Ac 0.1 = H+/HAc = 0.018% 1.33%2-2.2 Bronsted-Lowry Acids and Bases 1. Definition of acid and base Acid - is a substance capable of donating a proton. HCl, NH4+, HSO4-, H2O Base - is a substance capable of accepting a proton. Cl-, NH

13、3, HSO4-, OH-lHClbaseacidConjugate acid-base pairH+ + Cl-lH2CO3H+ + HCO3-lHCO3-H+ + CO32-lNH4+H+ + NH3lH3O+H+ + H2OlH2OH+ + OH-lAcid or base may be a molecule, atom, or ion.lSome molecules or ions are capable of donating a proton, and also accepting a proton, which named ampholyte.lThere are no conc

14、epts of salt in acid-base proton theory.Essence: proton transfer reaction.For example: HCl(g) + NH3(g)H+conjugateNH4+ + Cl-A1 + B2A2 + B1(1) Compare by K a or Kb HAc + H2O H3O+ + Ac- H3O+ Ac- K a = HAc H+The smaller the value for K a， the weaker the acid；The greater the value for K a， the stronger t

15、he acid.Ac- + H2O HAc + OH- HAcOH-K b = Ac - H+The smaller the value for Kb， the weaker the base；The greater the value for Kb， the stronger the base. The relationship between Ka and Kb: Ka Kb = K w(2) The relationship between acid-base strength and solvent H2O weak acidNH3 strong acidHNO3 H2O strong

16、 acidHAc weak acidH2SO4 base substanceHAc 4. The Leveling Effect and Differentiating EffectThe leveling effect: The inability of a solvent to differentiate among the relative strengths of all acids stronger than the solvents conjugate acid is known as the leveling effect. Because the solvent is said

17、 to level the strengths ofthese acids, making them seen identical.leveling solvent:Strong acid such as HClO4, HCl, HNO3, H2SO4 will appear to be of equal strength in aqueous solution.Strong acids-hydrochloric acid (HCl), nitric acid (HNO3), perchloric acid (HClO4), and sulfuric acid (H2SO4), for exa

18、mple- are all strong electrolytes. They may be assumed to be completely ionized in water. HCl(a q) + H2O(1) H3O+(aq) + Cl - (aq) HNO3(a q) + H2O(1) H3O+(aq) + NO3-(aq) HClO4(a q) + H2O(1) H3O+(aq) + ClO4 - (aq) H2SO4(a q) + H2O(1) H3O+(aq) + HSO4 - (aq)we can not determine their strength, because H3

19、O+ is the strongest acid that can exist in aqueous solution.The differentiating effect:The ability of a solvent to differentiate among therelative strengths of all acids stronger than the solventsconjugate acid is known as the differentiating effect.If we use a more weakly basic solvent like acetic

20、acid, acetic acid can function as a base by accepting a proton. Since acetic acid is a much weaker base than water, it isnot as easily protonated. Thus there are appreciabledifferences in the extent.llllIn acetic acid solvent, their relative strength increase as follows: HNO3 H2SO4 HCl HClO42-3 Acid

21、ity and Calculation of Solution2-3.1 Autoionization of WaterH2O(l) + H2O(l)H3O+ (aq) + OH-(aq)Water is capable of acting as a proton donor and protonacceptor toward itself. The process by which this occursis called autoionization of water. Kw = H3O+OH-Where Kw is the equilibrium constant for water (

22、unitless) , is called ion product of water or autoionization equilibrium constant. At 25, Kw = H3O+OH - = 1.0 10 -14 H+ OH- in acid solutions H+ OH- in basic solutions H+ = OH- in neutral solutions2-3.2 Acidity of solution pH = - log aH+ = -log H3O+ pOH = - log aOH- = -logOH- For, H+OH- = Kw= 110-14

23、 So, pH + pOH =pKw= 14.00 In neutral solutions, pH7pOH, In acid solutions, pH 7 pOH In basic solutions, pH 7 pOH2-3.3 Calculation of Acidity of Solution For Strong Acids and Bases pH = - logH+ = -logacid pOH = - logOH- = -logbaseExample:Monoprotic Weak Acids and H AcSolve this equation: H+ = - Ka /2

24、 + Ka2 /4 +Ka c H+ + Ac initial c 0 0 equilibrium c- H+ H+ Ac - = H+ H+ Ac - H+2 Ka= - = - H Ac c-H+ H+2 + KaH+ Kac = 0When: c/Ka 103, or 5%, c - H+ cthus,cKHaSimilarly, for weak base , there is an equation: cKOHbNote that above equation is limited for some given condition:Example 4-5: P412-4 Equili

25、brium between Dissolution and Precipitation2-4.1 Solubility Product Constant (Ksp) Ksp = Ag+Cl- AgCl(s)dissolutionprecipitationAg+ + Cl-solubility product constant Mg(OH) 2 (s)Mg2+ + 2OH-Ksp = M g2+OH-2 Ag2CrO4 (s)Ksp = A g+2CrO42- 2Ag+ + CrO42- Fe(OH) 3 (s)Fe3+ + 3OH- Ksp = Fe3+OH-3 AmBn(s)mAn+ + n

26、Bm- Ksp = An+m Bm-n AB (s , ksp) spKs AB(s)A + BIn saturated solution: A=B=s (mol/L) Ksp = A B= s2 AB2(s)34spKs A + 2BIn saturated solution: A=s (mol/L) B=2s(mol/L) Ksp = A B2 = s(2s)2= 4s3 or34spKs The Ksp for CaF2 is 3.910-11. What is its solubility in water, in grams per liter? Example2-4: s = 2.1 10- 4 mol /L Solution: 2.1 10 - 4 mol /L 78.1 g /mol = 1.6 10-2 g / L Qi (ion product quotient): the product of the ion concentration in solution when the system is under any situation ( at equilibrium or not ). Qi (AgCl)