Finding the Equivalence Point (calculation method) • Strong Acid vs. Strong Base – 100 % ionized! pH = 7 No equilibrium! • Weak Acid vs. Strong Base – Acid is neutralized; Need Kb for conjugate base equilibrium • Strong Acid vs. Weak Base – Base is neutralized; Need Ka for conjugate acid equilibrium • Weak Acid vs. Weak Base – Depends on the strength of both; could be conjugate acid, conjugate base, or pH 7 Exactly 100 mL of 0.10 M HNO2 are titrated with 100 mL of a 0.10 M NaOH solution. What is the pH at the equivalence point ? start (moles) 0.01 0.01 HNO2 (aq) + OH- (aq) NO2- (aq) + H2O (l) end (moles) 0.0 0.0 0.01 Final volume = 200 mL NO2- (aq) + H2O (l) Initial (M) Change (M) 0.01 = 0.05 M 0.200 OH- (aq) + HNO2 (aq) [NO2-] = 0.05 0.00 0.00 -x +x +x x x Equilibrium (M) 0.05 - x [OH-][HNO2] x2 -11 = 2.2 x 10 Kb = = [NO2-] 0.05-x pOH = 5.98 0.05 – x 0.05 x 1.05 x 10-6 = [OH-] pH = 14 – pOH = 8.02 Complex Ion Equilibria and Solubility A complex ion is an ion containing a central metal cation bonded to one or more molecules or ions. Co2+ (aq) + 4Cl- (aq) Co(H2O)2+ 6 CoCl42- (aq) CoCl24 16.10 16.10 Complex Ion Formation • These are usually formed from a transition metal surrounded by ligands (polar molecules or negative ions). • As a "rule of thumb" you place twice the number of ligands around an ion as the charge on the ion... example: the dark blue Cu(NH3)42+ (ammonia is used as a test for Cu2+ ions), and Ag(NH3)2+. • Memorize the common ligands. Common Ligands Ligands Names used in the ion H2O NH3 aqua ammine OHClBrCNSCN- hydroxy chloro bromo cyano thiocyanato (bonded through sulphur) isothiocyanato (bonded through nitrogen) Names • Names: ligand first, then cation Examples: – tetraamminecopper(II) ion: Cu(NH3)42+ – diamminesilver(I) ion: Ag(NH3)2+. – tetrahydroxyzinc(II) ion: Zn(OH)4 2- • The charge is the sum of the parts (2+) + 4(-1)= -2. When Complexes Form • Aluminum also forms complex ions as do some post transitions metals. Ex: Al(H2O)63+ • Transitional metals, such as Iron, Zinc and Chromium, can form complex ions. • The odd complex ion, FeSCN2+, shows up once in a while • Acid-base reactions may change NH3 into NH4+ (or vice versa) which will alter its ability to act as a ligand. • Visually, a precipitate may go back into solution as a complex ion is formed. For example, Cu2+ + a little NH4OH will form the light blue precipitate, Cu(OH)2. With excess ammonia, the complex, Cu(NH3)42+, forms. • Keywords such as "excess" and "concentrated" of any solution may indicate complex ions. AgNO3 + HCl forms the white precipitate, AgCl. With excess, concentrated HCl, the complex ion, AgCl2-, forms and the solution clears. Coordination Number • Total number of bonds from the ligands to the metal atom. • Coordination numbers generally range between 2 and 12, with 4 (tetracoordinate) and 6 (hexacoordinate) being the most common. Some Coordination Complexes molecular formula Lewis Lewis acid base/ligand donor coordination atom number Ag(NH3)2+ NH3 Ag+ N 2 [Zn(CN)4]2- CN- Zn2+ C 4 [Ni(CN)4]2- CN- Ni2+ C 4 [PtCl6] 2- Cl- Pt4+ Cl 6 [Ni(NH3)6]2+ NH3 Ni2+ N 6