Aqueous Equilibria, Part 2 Name: ____________________ AP Chemistry Lecture Outline The Common-Ion Effect Suppose we have a weak acid and a soluble salt of that acid. CH3COOH CH3COOH NaCH3COO CH3COO– + H+ Since NaCH3COO is strong, adding it to the solution… By Le Chatelier… The result is that… This illustrates the common-ion effect: “The dissociation of a weak electrolyte decreases when a strong electrolyte – that has an ion in common with the weak electrolyte – is added to the solution.” EX. Find the pH of a solution containing 0.085 M HNO2 (Ka = 4.5 x 10–4) and 0.10 M KNO2. Buffered Solutions (“buffers”) --- contain a weak conjugate acid-base pair, i.e., -- contain an acidic species and a basic species that do NOT consume each other through neutralization e.g., For a CH3COOH-and-CH3COO– buffer, you could use… For an NH4+-and-NH3 buffer, you could use… 1 buffer capacity: the amount of acid or base the buffer can “neutralize” before the pH begins to change appreciably -- EX. Find the pH of a buffer that is 0.12 M lactic acid, HC3H5O3 (Ka = 1.4 x 10–4) and 0.10 M sodium lactate. Alternately, you could use the Henderson-Hasselbalch equation: pH pK a log base acid -- Addition of Strong Acids or Bases to Buffers -- Reactions between strong acids/bases and weak bases/acids proceed to completion. -- We assume the strong acid/base is completely consumed. -- When adding a “strong” to buffered solutions… (1) (2) 2 EX. A buffered solution of pH 4.74 contains 0.30 mol CH3COOH (Ka = 1.8 x 10–5) and 0.30 mol NaCH3COO. Calculate the pH after 0.020 mol NaOH is added. Ignore volume changes. Acid-Base Titrations equivalence point: Strong Acid – Strong Base Titrations pH curve for HCl titrated with NaOH Any indicator whose color change begins and ends along the vertical line is okay. -- phenolphthalein (pH 8.3-10.0) pH base = acid = 7 -- methyl red (pH 4.2-6.0) base = acid = mL of NaOH added 3 EX. Find pH when 24.90 mL of 0.10 M HNO3 are mixed with 25.00 mL of 0.10 M KOH. pH curve for CH3COOH titrated with NaOH Weak Acid – Strong Base Titrations The equivalence point is when, say, 50.0 mL of 0.10 M NaOH have been pH added to 50.0 mL of 0.10 M CH3COOH, but pH is > 7 at that point because... 7 mL of NaOH added pH curve for H2CO3 Titration curves for polyprotic acids pH (e.g., H2CO3) look something like -- they have... mL of base added 4 EX. Calculate the pH when 10.0 mL of 0.050 M NaOH are added to 40.0 mL of 0.0250 M benzoic acid (C6H5COOH, Ka = 6.3 x 10–5). EX. Calculate the pH at the equivalence point when 40.0 mL of 0.0250 M C6H5COOH (Ka = 6.3 x 10–5) are titrated with 0.050 M NaOH. 5 Solubility Equilibria -- involve the dissolution or precipitation of ionic compounds Consider a saturated solution of barium sulfate: For this case, the solubility-product constant is equal to: -- Ksp is the equilibrium constant between undissolved and dissolved ionic solute in a saturated aqueous solution EX. Write the solubility-product constant expression for calcium fluoride. EX. Copper (II) azide has Ksp = 6.3 x 10–10. Find the solubility of Cu(N3)2 in water, in g/L. 6 Factors That Affect Solubility 1. For solids, as temperature increases, solubility... 2. common-ion effect Use Le Chatelier’s principle. For example, with... Ca2+(aq) + 2 F–(aq) CaF2(s) 3. pH and solubility Compounds with anions exhibiting basic properties (e.g., Mg(OH)2 / OH–, CaCO3 / CO32–, CaF2 / F–) _______________ in solubility as solution becomes more acidic. 4. presence of complex ions: metal ions and the Lewis bases bonded to them e.g., AgCl(s) + 2 NH3(aq) Ag(NH3)2+(aq) + Cl–(aq) In general, the solubility of metal salts ______________ in the presence of suitable Lewis bases (e.g., NH3, CN–, OH–) if the metal forms a complex ion with the bases. 5. amphoterism Many metal hydroxides and oxides are amphoteric. They are insoluble at pH ~ 7, but will dissolve in strongly acidic or strongly basic solutions. (Recall that many metal ions act like acids in solution.) As an example... Al(H2O)63+(aq) + OH–(aq) Al(H2O)5(OH)2+(aq) + H2O(l) If we continue adding more and more OH–(aq), we’ll get... Al(H2O)5(OH)2+(aq) + OH–(aq) Al(H2O)4(OH)2+(aq) + H2O(l) Al(H2O)4(OH)2+(aq) + OH–(aq) Al(H2O)3(OH)3(s) + H2O(l) + OH–(aq) Al(H2O)2(OH)4–(aq) + H2O(l) Al(H2O)2(OH)4–(aq) + OH–(aq) Al(H2O)(OH)52–(aq) + H2O(l) Al(H2O)(OH)52–(aq) + OH–(aq) Al(OH)63–(aq) Al(H2O)3(OH)3(s) Often, the H2O is left out. 7 + H2O(l) Precipitation and Separation of Ions For BaSO4(s) Ba2+(aq) + SO42–(aq)... Ksp = [ Ba2+ ] [SO42– ] We could reach equilibrium from the left... or from the right... At any given time, the ion product Q = [ Ba2+ ] [SO42– ] If Q > Ksp... If Q < Ksp... If Q = Ksp... selective precipitation: using the different solubilities of ions to separate them Consider a solution with Ag+ and Cu2+. -- Add HCl. The Ksp of AgCl = 1.8 x 10–10, but CuCl2 is soluble (i.e., Cu2+ doesn’t precipitate). -- Then... EX. Will a precipitate form from mixing 0.10 L of 8.0 x 10–3 M Pb(NO3)2(aq) and 0.40 L of 5.0 x 10–3 M Na2SO4(aq)? 8 Qualitative Analysis for Metallic Elements Steps: (1) (2) aqueous solution containing “god-knowswhat” metal ions Add HCl. insoluble chlorides (AgCl, Hg2Cl2, PbCl2) ppt out. Filter and test for a specific ion. Bubble H2S(g) in. acid-insoluble sulfides (CuS, Bi2S3, CdS, PbS, HgS, As2S3, Sb2S3, SnS2) ppt out. Filter and test for a specific ion. Add (NH4)2S. base-insoluble sulfides and hydroxides (Al(OH)3, Fe(OH)3, Cr(OH)3, ZnS, NiS, CoS, MnS) ppt out. Filter and test for a specific ion. Add (NH4)2PO4. insoluble phosphates (Ba3(PO4)2, Ca3(PO4)2, MgNH4PO4) ppt out. Filter and test for a specific ion. 9