1) Ch.17 & 20 equilibrium III & electrochemistry (Partners: Ralph, Karen & Gloria) 1. What is the pH of a 45mL aqueous solution containing 25 mM sodium acetate and 35mM acetic acid after the addition of __. a. 10 mL of water While addition of water changes [ ], it doesn’t change ratio of [ ] in H-H solution. pH = pKa + log([A-]/[HA]) = -log(1.8x10^-5) + log(25mM/35mM) = 4.745 – 0.146 = 4.60 b. 10 mL of 50. mM HCl i) nHCl = [ ]V = 50mmol/10^3mL(10mL) = 0.50 mmol nAc- = [ ]V = 25mmol/10^mL(45mL) = 1.125 mmol nHA = [ ]V = 35mmol/10^3mL(45mL) = 1.575 mmol ii) H+ + Ac HAc I(mmol) 0.5 1.125 1.575 C(mmol) -0.5 -0.5 +0.5 E(mmol) 0 0.625 2.075 iii) pH = pKa + log([A]/[HA]) = 4.745 + log(0.625/2.075) = 4.22 c. 10 mL of 50 mM KOH i) nOH= [ ]V= 50mmol/10^3mL(10mL) = 0.5 mmol ii) OH+ HAc AcI(mmol) 0.5 1.575 1.125 C(mmol) -0.5 -0.5 +0.5 E(mmol) 0 1.075 1.625 iii) pH = pKa + log([Ac-]/[HAc]) = 4.745 + log(1.625/1.075) = 4.92 2. What is the minimum volume of a 0.1 mM NaCl solution to be added to a 25.0x10^-6 M 50.0mL solution of silver nitrate to produce a precipitate? (free points) Ppt @ Q > Ksp [Cl-][Ag+] > Ksp [Cl-]VCl-/(Vtotal) x [Ag+]VAg/(Vtotal) > Ksp 0.1x10^-3mol/10^3mL(VCl-)/(50mL + VCl-) x (25x10^6mol/10^3mL)(50mL)/(50mL + VCl-) > Ksp 10^-7x(1.25x10^-6)/(50+x)2 > Ksp 1.25x10^-13x/(2500+100x+x2) > 1.8 x 10^-10 1.25*10^-13x > 4.5x10^-7 + 1.8x10^-8x + 1.8*10^-10 (x2 ) 1.8*10^-10x2 + 1.8*10^-8 (x) + 4.5x10^7 X < -50mL ? 3. What is the relative solubility of AgCl in water versus the solubility of AgCl in 0.1 mM NaCl? Rationalize/justify your answer without doing any calculations. Solubility in H2O > solubility in NaCl. 4. How much time(in seconds) would it take to deposit 125 mg of zinc onto a carbon electrode with a current of 275mA? Zn+2 + 2e- Zn i) 125mgZn(1g/10^3mg)(1molZn/63.55gZn)(2molee-/1molZn)(96,500 Coulombs/1mole e-) = 380 coulombs ii) I = #coulombs/time 275mA (A/10^3mA) = 380 coulombs/time Time= 1.38x10^3 sec 5. Sketch the voltaic cell for the redox reaction: Zn + Cu(NO3)2(aq) Cu + Zn(NO3)2(aq). Clearly label (i) the identity of the electrode(s), (ii) electrolyte(s) in the cell, (iii) the half-reactions at the electrode, (iv) direction of the current, and (v) cathode versus anode in the electrochemical cell. Note: current flows in the opposite direction of e- flow, so current flows to the left. 6. Is the following redox reaction spontaneous(under standard conditions): ___; justify/rationalize your answer. a. Zn + HCl(aq) [hint: it’s a single replacement reaction] i) Zn Zn+2 + 2e- E° = +0.763V 2e- + 2H+ H2 E° = 0 E°cell = 0.763 V ii) ΔG° = -nFE° E° > 0 ΔG ° < 0 Rx spontaneous b. Cu + HCl(aq) Cu+2 i) Cu Cu+2 + 2e2e- + 2H+ H2 ii) E° = -0.337 E° = 0 E°cell = -0.337 V ΔG° = -nFE° E° < 0 ΔG°> 0 Rx not spontaneous ************************************************************ Gloria Lau and Karen Wong Equilibrium III & Electrochemistry 1. What is the pH of a 45mL aqueous solution containing 25 mM sodium and 35 mM acetic acid after the addition of __.[20 points] a. 10mL of water addition of water change [ ]; doesn’t change rxn of [ ] in H-H equ pH = pKe + log = -log (1.8E-5) + log b. 10mL of 50.0 mM HCl = 4.60 i. nHCl = [ ]V = 10mL = 0.50 mM nAc = [ ]V = 45mL = 1.123 mM nHA = [ ]V = H+ + Ac- ii. 45mL = 1.575 mM HAc I 0.5 1.125 1.575 C -0.5 -0.5 +0.5 E 0 0.625 2.075 pH = pKe + log iii. = 4.22 c. 10mL of 50mM KOH nHCl = [ ]V = i. OH- + HAc ii. 10mL = 0.50mM Ac- I 0.5 1.575 1.125 C -0.5 -0.5 +0.5 E 0 1.075 1.625 pH = pKe + log iii. = 4.92 2. What is the minimum volume of a 0.1mM NaCl solution to be added to a 25.0 solution of silver nitrate to produce a precipitate? [10 points] ppt @ Q > Ksp > Ksp 50.0mL [Cl-][Ag+] > Ksp > 1.8E-10 > Ksp x > -90mL 3. What is the relative solubility of AgCl in water versus the solubility of AgCl in 0.1 mM NaCl? ratioalize/ justify your answer without doing any calculations. [10 points] solubility in H2O > solublity in NaCl 4. How much time (in sec) would it take to deposit 125mg of Zinc onto a carbon electrode with a current of 275 mA? [10 points] Zn2+ + 2e- Zn 125mg Zn i. I= ii. = 380 cal = 275mA = t = 1.38E3 sec 5. Sketch the voltaic cell for the redox reaction: Zn + Cu(NO3)2 Cu + Zn(NO3)2. Clearly label (i) the identity of the electrode, (ii) electrolyte(s) in the cell, (iii) the half-reaction at the electrode, (iv) direction of the current, and (v) cathode versus anode in the electrochemical cell. [10 points] Zn2+ + 2e- Zn Cu2+ + 2e- anode Cu cathode 6. Is the following redox reaction spontaneous (under standard condition): __; justify/ rationalize your answer. [10 points] a. Zn + HCl(aq) i. [hint: it’s a single replacement reaction] Zn2+ +2e- Zn 2e- + 2H+ Eo = +0.763 Eo = 0 H2 Eotot = 0.763 o ii. = -nFEo Eo. > 0 <0 Rx spontaneous b. Cu + HCl(aq) i. Cu Cu2+ + 2e- 2e- + 2H+ Eo = -0.337 Eo = 0 H2 Eotot = -0.337 ii. = -nFEo Eo = < 0 >0 Rx not spontaneous 7. Identify the species (i.e. “atom”) that is oxidized or reduced in the following redox reaction and its oxidation number. [10 points] a. Decomposition of hydrogen peroxide b. Cu + K2Cr2O7(aq) + H+(aq) Cr3+(aq) + H2O + Cu2+(aq) 8. Old topic: describe and rationalize how to determine the Ka of benzoic acid based in __. [10 points] a. the pH of the benzoic acid solution i. ii. HBz H+ + BzI HBz 0 0 C -x -x +x E HBz - x -x x Ka = b. Benzoic acid titration curve using a strong base, NaOHVb = Vb pH = pKe + log pH = pKa [H+] = Ka