Presentation on theme: "Chapter 17 Additional Aspects of Aqueous Equilibria"— Presentation transcript:
1Chapter 17 Additional Aspects of Aqueous Equilibria Chemistry, The Central Science, 10th editionTheodore L. Brown; H. Eugene LeMay, Jr.; and Bruce E. BurstenChapter 17 Additional Aspects of Aqueous EquilibriaJohn D. BookstaverSt. Charles Community CollegeSt. Peters, MO 2006, Prentice Hall, Inc.
2March 11 COMMON ION EFFECT 17.11, 13, 15 BUFFER PROBLEMS 17, 19, 23, 25TITRATIONS 31 to 39 odd, 43
3The Common-Ion Effect Consider a solution of acetic acid: If acetate ion is added to the solution, Le Châtelier says the equilibrium will shift to the left.HC2H3O2(aq) + H2O(l)H3O+(aq) + C2H3O2−(aq)
4The Common-Ion Effect“The extent of ionization of a weak electrolyte is decreased by adding to the solution a strong electrolyte that has an ion in common with the weak electrolyte.”
5The Common-Ion EffectCalculate the fluoride ion concentration and pH of a solution that is 0.20 M in HF and 0.10 M in HCl.Ka for HF is 6.8 10−4.[H3O+] [F−][HF]Ka == 6.8 10-4
6The Common-Ion Effect HF(aq) + H2O(l) H3O+(aq) + F−(aq) Because HCl, a strong acid, is also present, the initial [H3O+] is not 0, but rather 0.10 M.[HF], M[H3O+], M[F−], MInitially0.200.10Change−x+xAt Equilibrium0.20 − x 0.20x 0.10x
9Buffers: Solutions of a weak conjugate acid-base pair. They are particularly resistant to pH changes, even when strong acid or base is added.
10ExampleWrite the reaction that occurs in a buffer solution containing HF/F- when:H+ is added H+ + F- HFOH- is added OH- + HF F- + H2O
11BuffersIf a small amount of hydroxide is added to an equimolar solution of HF in NaF, for example, the HF reacts with the OH− to make F− and water.
12BuffersIf acid is added, the F− reacts to form HF and water.
13Buffer CalculationsConsider the equilibrium constant expression for the dissociation of a generic acid, HA:HA + H2OH3O+ + A−[H3O+] [A−][HA]Ka =
14Buffer Calculations Rearranging slightly, this becomes [HA]Ka =[H3O+]Taking the negative log of both side, we getbase[A−][HA]−log Ka =−log [H3O+] + −logpKapHacid
15Buffer Calculations So pKa = pH − log [base] [acid] Rearranging, this becomespH = pKa + log[base][acid]This is the Henderson–Hasselbalch equation.
16MARCH 14 BUFFER PROBLEMS Calculation of the ph of a buffer Buffer capacityAddition of strong acid or bases to buffersBUFFER PROBLEMS 17, 19, 23, 25TITRATIONS a)Strong acid and Strong baseb ) Weak acid – Strong BaseTitration of polyprotic AcidsTITRATIONS problems 31 to 39 odd, 43
17Henderson–Hasselbalch Equation What is the pH of a buffer that is 0.12 M in lactic acid, HC3H5O3, and 0.10 M in sodium lactate? Ka for lactic acid is1.4 10−4.
19Buffer CapacityThe amount of acid of base the buffer can neutralize before the pH begins to change to an appreciable degree. It depends on the amount of acid and base from which the buffer is made.The pH of the buffer depends on the Ka of the acid, and on the relative concentrations of acid and base.
20Example A solution 1M in HA, 1M in NaA will have its pH=Ka. A solution .1M in HA and .1M in NaA will have SAME pH (Ka) but the first one will have greater buffer capacity.
21pH RangeThe pH range is the range of pH values over which a buffer system works effectively.It is best to choose an acid with a pKa close to the desired pH.In practice if the concentrations of one component of the buffer is 10 times the concentration of the other component the buffering action is poor.
22Addition of Strong Acids or Bases to Buffers The amount of strong acid or base added results in a neutralization reaction:X- + H3O+ HX + H2OHX + OH- X- + H2O.By knowing how much H3O+ or OH- was added (stoichiometry) we know how much HX or X- is formed.With the concentrations of HX and X- (note the change in volume of solution) we can calculate the pH from the Henderson-Hasselbalch equation or the original equilibrium expression (ICE chart).
23When Strong Acids or Bases Are Added to a Buffer… …it is safe to assume that all of the strong acid or base is consumed in the reaction.
24Calculating pH Changes in Buffers A buffer is made by adding mol HC2H3O2 and mol NaC2H3O2 to enough water to make 1.00 L of solution. The pH of the buffer is Calculate the pH of this solution after mol of NaOH is added.
25Calculating pH Changes in Buffers Before the reaction, sincemol HC2H3O2 = mol C2H3O2−pH = pKa = −log (1.8 10−5) = 4.74
26Calculating pH Changes in Buffers The mol NaOH will react with mol of the acetic acid:HC2H3O2(aq) + OH−(aq) C2H3O2−(aq) + H2O(l)HC2H3O2C2H3O2−OH−Before reaction0.300 mol0.020 molAfter reaction0.280 mol0.320 mol0.000 mol
27Calculating pH Changes in Buffers Now use the Henderson–Hasselbalch equation to calculate the new pH:pH = log(0.320)(0. 200)pH =pH = 4.80
28Addition of Strong Acid or Base to a Buffer Determine how the neutralization reaction affects the amounts of the weak acid and its conjugate base in solution.Use the Henderson–Hasselbalch equation to determine the new pH of the solution.
29ExamplesConsider a buffer prepared by placing 0.60 moles of HF (Ka = 7.2 x 10-4) and 0.48 moles of NaF in a 1.00 L solution.a. Calculate the pH of the bufferb. Calculte the pH after the addition of 0.08 moles of HClc. Calculate the pH after the addition of a total of 0.16 moles of HCld. Calculate the pH after the addition of 0.10 moles of NaOH.
30Consider a buffer containing 0. 78 moles of HC2H3O2 (Ka = 1 Consider a buffer containing 0.78 moles of HC2H3O2 (Ka = 1.8 x 10-5) and 0.67 moles of NaC2H3O2 in 1.00 L of solution.a. Calculate the pH of the buffer solution.b. Calculate the pH after the addition of 0.10 moles of HNO3.c. Calculate the pH after the addtiion of a total of 0.20 moles of HNO3.d. Calculate the pH after the addition of 0.10 moles of NaOH.
31TitrationA known concentration of base (or acid) is slowly added to a solution of acid (or base).
32TitrationA pH meter or indicators are used to determine when the solution has reached the equivalence point, at which the stoichiometric amount of acid equals that of base.
33Titration of a Strong Acid with a Strong Base From the start of the titration to near the equivalence point, the pH goes up slowly.
34Titration of a Strong Acid with a Strong Base Just before and after the equivalence point, the pH increases rapidly.
35Titration of a Strong Acid with a Strong Base At theequivalence point,mol acid = mol base, the solution contains only water and the salt from the cation of the base and the anion of the acid.
36Titration of a Strong Acid with a Strong Base As more base is added, the increase in pH again levels off.
37Titration of a Weak Acid with a Strong Base Unlike in the previous case, the conjugate base of the acid affects the pH when it is formed.The pH at the equivalence point will be >7.Phenolphthalein is commonly used as an indicator in these titrations.
38Titration of a Weak Acid with a Strong Base At each point below the equivalence point, the pH of the solution during titration is determined from the amounts of the acid and its conjugate base present at that particular time.
39Titration of a Weak Acid with a Strong Base With weaker acids, the initial pH is higher and pH changes near the equivalence point are more subtle.
40Titration of a Weak Base with a Strong Acid The pH at the equivalence point in these titrations is < 7.Methyl red is the indicator of choice.
41Titrations of Polyprotic Acids In these cases there is an equivalence point for each dissociation.
42Solubility ProductsConsider the equilibrium that exists in a saturated solution of BaSO4 in water:BaSO4(s)Ba2+(aq) + SO42−(aq)
43Solubility ProductsThe equilibrium constant expression for this equilibrium isKsp = [Ba2+] [SO42−]where the equilibrium constant, Ksp, is called the solubility product.
44Solubility Products Ksp is not the same as solubility. Solubility is generally expressed as the mass of solute dissolved in 1 L (g/L) or 100 mL (g/mL) of solution, or in mol/L (M).
45Factors Affecting Solubility The Common-Ion EffectIf one of the ions in a solution equilibrium is already dissolved in the solution, the equilibrium will shift to the left and the solubility of the salt will decrease.BaSO4(s)Ba2+(aq) + SO42−(aq)
46Factors Affecting Solubility pHIf a substance has a basic anion, it will be more soluble in an acidic solution.Substances with acidic cations are more soluble in basic solutions.
47Factors Affecting Solubility Complex IonsMetal ions can act as Lewis acids and form complex ions with Lewis bases in the solvent.
48Factors Affecting Solubility Complex IonsThe formation of these complex ions increases the solubility of these salts.
49Factors Affecting Solubility AmphoterismAmphoteric metal oxides and hydroxides are soluble in strong acid or base, because they can act either as acids or bases.Examples of such cations are Al3+, Zn2+, and Sn2+.
50Will a Precipitate Form? In a solution,If Q = Ksp, the system is at equilibrium and the solution is saturated.If Q < Ksp, more solid will dissolve until Q = Ksp.If Q > Ksp, the salt will precipitate until Q = Ksp.
51Selective Precipitation of Ions One can use differences in solubilities of salts to separate ions in a mixture.