2. Write the chemical formula(s) of the product(s) and balance the following spontaneous reactions.

Transcription

1 1. Using the Activity Series on the Useful Information pages of the exam write the chemical formula(s) of the product(s) and balance the following reactions. Identify all products phases as either (g)as, (l)iquid, (s)olid or (aq)ueous. Soluble ionic compounds should be written in the form of their component ions. If no reaction occurs write NR. a) Mg(s) + Pb(NO 3 ) 2 (aq) Mg 2+ (aq) + Pb(s) + 2NO 3 (aq) Pb 2+ (aq) + 2e Pb(s) E = 0.13 v Mg 2+ (aq) + 2e Mg(s) E = 2.37 v E cell = E cathode E anode = 0.13 v 2.37 v) = v b) 3Fe 2+ (aq) + 2Cr(s) 3Fe(s) + 2Cr 3+ (aq) 3 ( Fe 2+ (aq) + 2e Fe(s) ) E = 0.44 v 2 ( Cr 3+ (aq) + 2e Cr(s) ) E = 0.74 v E cell = E cathode E anode = 0.44 v ( 0.74 v) = v 2. Write the chemical formula(s) of the product(s) and balance the following spontaneous reactions. a) Sn 2+ (aq) + Ag + (aq) Ag + (aq) + e Ag(s) E = v Sn 4+ (aq) + 2e Sn 2+ (aq) E = v Sn 2+ (aq) + 2e Sn(s) E = 0.14 v Ag + (aq) must be reduced so Sn 2+ (aq) must be oxidized, so the reaction of Sn 2+ (aq) being reduced can not be used. E cell = E cathode E anode = v (+0.13 v) = v b) 2Al(s) + 6H 2 O(l) 2Al 3+ (aq) + 3H 2 (g) + 6OH 3 ( 2H 2 O(l) + 2e H 2 (g) + 2OH ) E = 0.83 v 2 ( Al 3+ (aq) + 3e Al(s) ) E = 1.66 v E cell = E cathode E anode = 0.83 v ( 1.66 v) = v

2 3. Write the chemical formula(s) of the product(s) and balance the following reactions. Identify all products phases as either (g)as, (l)iquid, (s)olid or (aq)ueous. Soluble ionic compounds should be written in the form of their component ions. If no reaction occurs write NR. a) Zn(s) + 2Ag + (aq) 2Ag(s) + Zn 2+ (aq) 2 ( Ag + (aq) + e Ag(s) ) E = v 1 ( Zn 2+ (aq) + 2e Zn(s) ) E = 0.76 v E cell = E cathode E anode = v ( 0.76 v) = v b) HC 3 H 5 O 2 (aq) + NaOH(aq) C 3 H 5 O 2 (aq) + H2 O(l) + Na + (aq) c) Fe(s) + 2HCl(aq) H 2 (g) + Fe 2+ (aq) + 2Cl (aq) 2H + (aq) + 2e H 2 (g) E = volts Fe 2+ (aq) + 2e Fe(s) E = 0.44 v E cell = E cathode E anode = v ( 0.44 v) = v 4. Complete and balance the following reactions. Identify all product's phases as either (g)as, (l)iquid, (s)olid or (aq)ueous. If no reaction occurs, write NR. a) Zn(s) + Cl 2 (g) Zn 2+ (aq) + 2Cl (aq) Cl 2 (aq) + 2e 2Cl (aq) E = volts Zn 2+ (aq) + 2e Zn(s) E = 0.76 v E cell = E cathode E anode = v ( 0.76 v) = v b) 3Cu(s) + 8H + (aq) + 2NO 3 (aq) 3Cu 2+ (aq) + 2NO(aq) + 4H2 O(l) volts v 2 ( NO 3 (aq) + 4H + (aq) + 3e NO(aq) + 2H 2 O(l) ) E = 3 ( Cu 2+ (aq) + 2e Cu(s) ) E = E cell = E cathode E anode = v (+0.34 v) = v 5. Complete and balance the following reactions. Identify all product's phases as either (g)as, (l)iquid, (s)olid or (aq)ueous. If no reaction occurs, write NR.

3 a) Mg(s) + Br 2 (l) Mg 2+ (aq) + 2Br (aq) Br 2 (aq) + 2e 2Br (aq) E = volts Mg 2+ (aq) + 2e Mg(s) E = 2.37 v E cell = E cathode E anode = v ( 2.37 v) = v b) Zn(s) + 2H + (aq) + ClO 4 (aq) Zn 2+ (aq) + ClO3 (aq) + H 2 O(l) volts ClO 4 (aq) + 2H + + 2e ClO 3 (aq) + H2 O(l) E = Zn 2+ (aq) + 2e Zn(s) E = 0.76 v E cell = E cathode E anode = v ( 0.76 v) = v 6. Balance the following oxidation-reduction reaction using the half-reaction method. a) KMnO 4 (aq) + H 2 SO 4 (aq) + NaCl(aq) Cl 2 (aq) + NaHSO 4 (aq) + K 2 SO 4 (aq) + MnSO 4 (aq) acidic solution 1 ( 2KMnO 4 (aq) + 3H 2 SO 4 (aq) + 10H + (aq) + 10e K 2 SO 4 (aq) + 2MnSO 4 (aq) + 8H 2 O(l) ) 5 ( 2H 2 SO 4 (aq) + 2NaCl(aq) Cl 2 (aq) + 2NaHSO 4 (aq) + 2H + (aq) + 2e ) 2KMnO 4 (aq) + 13H 2 SO 4 (aq) + 10NaCl(aq) 5Cl 2 (aq) + 10NaHSO 4 (aq) + K 2 SO 4 (aq) + 2MnSO 4 (aq) + 8H 2 O(l) identify the oxidizing agent NaCl identify the reducing agent KMnO 4 7. Balance the following oxidation-reduction reactions using the half-reaction method. a) Br (aq) + SO 4 2 (aq) Br2 (aq) + SO 2 (g) acidic solution 2Br (aq) Br 2 (aq) + 2e SO 4 2 (aq) + 4H + (aq) + 2e SO 2 (g) + 2H 2 O(l) 2Br (aq) + SO 2 4 (aq) + 4H + (aq) Br2 (aq) + SO 2 (g) + 2H 2 O(l)

4 b) Na 2 O 2 (aq) + CoCl 2 (s) Co(OH) 3 (s) + Cl (aq) + Na + (aq) basic solution Na 2 O 2 (aq) + 4H + (aq) + 2e 2Na + (aq) + 2H 2 O(l) CoCl 2 (s) Co(OH) 3 (s) + 2Cl (aq) Na 2 O 2 (aq) + CoCl 2 (s) Co(OH) 3 (s) + Cl (aq) + Na + (aq) Identify the oxidizing agent identify the reducing agent 8. Balance the following oxidation-reduction reactions using the half-reaction method. (a) N 2 H 4 (aq) + CO 3 2 (aq) N2 (g) + CO(g) basic solution N 2 H 4 (aq) N 2 (g) + 4H + (aq) + 4e 2(2e + 4H + (aq) + CO 3 2 (aq) CO(g) + 2H2 O(l) ) 4H 2 O(l) 4H + (aq) + N 2 H 4 (aq) + 2CO 3 2 (aq) N2 (g) + 2CO(g) OH + 4 OH 4H 2 O(l) + 4OH (aq) 4H 2 O(l) + N 2 H 4 (aq) + 2CO 3 2 (aq) N2 (g) + 2CO(g) + identify the oxidizing agent CO 3 2 identify the reducing agent N 2 H 4 (aq) (b) Hg 2 HPO 4 (s) + Cl (aq) + Au(s) Hg(l) + H 2 PO 4 (aq) + AuCl4 (aq) acidic solution 3 ( Hg 2 HPO 4 (s) + H + (aq) + 2e 2Hg(l) + H 2 PO 4 (aq) ) 2 ( 4Cl (aq) + Au(s) AuCl 4 (aq) + 3e ) 3Hg 2 HPO 4 (s) + 3H + (aq) + 8Cl (aq) + 2Au(s) 3Hg(l) + 3H 2 PO 4 (aq) + 2AuCl 4 (aq)

5 Au(s) identify the oxidizing agent Hg 2 HPO 4 (s) identify the reducing agent 9. Balance the following oxidation-reduction reactions using the half-reaction method. (a) Zn(s) + MnO 4 (aq) Zn 2+ (aq) + Mn 2+ (aq) solution acidic 5 ( Zn(s) Zn 2+ (aq) + 2e ) 2 ( MnO 4 (aq) + 8H + (aq) + 5e Mn 2+ (aq) + 4H 2 O(l) ) 5Zn(s) + 2MnO 4 (aq) + 16H + (aq) 5Zn 2+ (aq) + 2Mn 2+ (aq) + 8H 2 O(l) identify the oxidizing agent MnO 4 identify the reducing agent Zn(s) (b) P 4 (s) PH 3 (g) + H 2 PO 2 (aq) basic solution 1 ( P 4 (s) + 12H + (aq) + 12e 4PH 3 (g) ) 3 ( P 4 (s) + 8H 2 O(l) 4H 2 PO 2 (aq) + 8H + (aq) + 4e ) 4P 4 (s) + 24H 2 O(l) 4PH 3 (g) + 12H 2 PO 2 (aq) + 12H + (aq) 12OH (aq) + 12OH (aq) 4P 4 (s) + 24H 2 O(l) + 12OH (aq) 4PH 3 (g) + 12H 2 PO 2 (aq) + 12H 2 O(l) 4P 4 (s) + 12H 2 O(l) + 12OH (aq) 4PH 3 (g) + 12H 2 PO 2 (aq) identify the oxidizing agent P 4 (s) identify the reducing agent P 4 (s) 10. Balance the following oxidation-reduction reactions using the half-reaction method. (a) HCOOH(aq) + MnO 4 (aq) CO2 (aq) + Mn 2+ (aq) solution acidic 5 ( HCOOH(aq) CO 2 (aq) + 2H + (aq) + 2e )

6 2 ( MnO 4 (aq) + 8H + (aq) + 5e Mn 2+ (aq) + 4H 2 O(l) ) 5HCOOH(aq) + 2MnO 4 (aq) + 6H + (aq) 5CO2 (aq) + 2Mn 2+ (aq) + 8H 2 O(l) HCOOH(aq) identify the oxidizing agent MnO 4 (aq) identify the reducing agent (b) HXeO 4 (aq) XeO6 4 (aq) + Xe(g) basic solution 3 (HXeO 4 (aq) + 2H2 O(l) XeO 6 4 (aq) + 5H + (aq) + 2e ) 1 (HXeO 4 (aq) + 7H + (aq) + 6e Xe(g) + 4H 2 O(l) ) 4HXeO 4 (s) + 2H2 O(l) 3XeO 6 4 (aq) + Xe(g) + 8H + (aq) 8OH (aq) + 8OH (aq) 4HXeO 4 (s) + 2H2 O(l) + 8OH (aq) 3XeO 6 4 (aq) + Xe(g) + 8H 2 O(l) 4HXeO 4 (s) + 8OH (aq) 3XeO6 4 (aq) + Xe(g) + 6H2 O(l) HXeO 4 (aq) identify the oxidizing agent HXeO 4 (aq) identify the reducing agent 11. Given the electrochemical cell shown in Figure I. Figure I

7 a) Write the balanced net ionic equation for the spontaneous reaction that occurs as the cell operates. b) 0.44 v 1.18 v Fe 2+ (aq) + 2e Fe(s) E = Mn 2+ (aq) + 2e Mn(s) E = b) Determine the standard cell voltage, E. E cell = E cathode E anode E cell = 0.44 v ( 1.18 v) = v c) Describe (use a diagram if you like) what is happening at the submicroscopic/atomic level on the surface of the anode in the cell. d) At the anode electrode surface manganese atoms lose two electrons to produce manganese(ii) ions in solution. c) Indicate the direction of flow of the ions in the salt bridge. (Be sure the anode and cathode compartments are identified.) Anions flow towards the anode compartment to balance the positive charged manganese cations that are being formed from the oxidation of the manganese atoms, and cations flow towards the cathode compartment to replace the cations that are removed from solution when iron cations are reduced at the cathode surface to produce iron atoms.

8 12. Short answer. a) The standard reduction potential for the reduction of RuO 4 to RuO 4 2- is volts. Which of the following substances can oxidize RuO 4 2- to RuO 4 - under standard condition? H 2 SO 4 (aq), Pb 2+ (aq), Cr 2 O 7 2- (in acidic solution) Write the balanced chemical equation to support your choice volts volts volts 0.13 v Cr 2 O 7 2 (aq) + 14H + (aq) + 6e 2Cr 3+ (aq) + 7H 2 O(l) E = RuO 4 (aq) + 1e RuO 4 2 (aq) E = SO 2 4 (aq) + 2H + (aq) + 2e SO 2 (g) + 2H 2 O(l) E = Pb 2+ (aq) + 2e Pb(s) E = The only substance that can oxidize RuO 4 2- to RuO 4 - is Cr 2 O 7 2 (aq) in acidic solution. The balanced reaction is, Cr 2 O 2 7 (aq) + 14H + (aq) + 6RuO4 2 (aq) 2Cr 3+ (aq) + 7H2 O(l) + 6RuO 4 (aq) E cell = E cathode E anode E cell = v (+0.59 v) = v 13. A standard chlorine half-cell Cl 2 (g) Cl - (aq) is coupled with a metal(s) metal ion(aq) half-cell. The voltage of the cell reads 3.73 volts. a) Identify the metal(s) metal ion(aq) combination that is involved. The standard reduction potential for chlorine is; Cl 2 (aq) + 2e 2Cl (aq) E = volts To calculate the reduction potential for the metal; E cell = E cathode E anode v = v E anode

9 E anode = v (+3.73 v) = 2.37 v This reduction cell potential corresponds to; Mg 2+ (aq) + 2e Mg(s) E = 2.37 v So the metal is Mg(s) and the ion is Mg 2+ (aq). b) Write the balanced chemical equation for the overall oxidation-reduction reaction. Mg(s) + Cl 2 (aq) Mg 2+ (aq) + 2Cl (aq) c) Identify the oxidizing agent and the reducing agent in the reaction in part (b). oxidizing agent Cl 2 (aq) reducing agent Mg(s) d) Predict what will happen to the cell potential (more positive, less positive, remain constant) if the [Cl - ] is raised and all other species amounts remain at standard state conditions. Without doing any calculations use the Nernst equation to explain your answer. You must explain, do NOT do any calculations. log [Mg2+ ][Cl ] 2 P Cl2 E cell = E n log [Mg2+ ][Cl ] 2 P E cell = Cl2 2 If the Cl (aq) ions concentration is raised, while the concentration of Mg 2+ (aq) and the partial pressure of Cl 2 remain constant, then the numerator of the Nernst equation will get larger, and the quotient, [Mg 2+ ][Cl ] 2 P, will be larger than one. The log of a number greater than Cl2 one is positive so the term log [Mg2+ ][Cl ] 2 P Cl2 is positive and is subtracted from the standard cell potential. This make E cell smaller (less positive). 14. The following materials are available for the construction of an electrochemical cell.

10 Figure I a) Three different electrochemical cells can be constructed from the solutions, metals and materials illustrated above. Write a balanced net-ionic equation for the reaction that occurs in the cell that has the largest positive value of E (electrochemical potential). (4) The three reduction reactions from the table of standard reduction potentials are; Ag + (aq) + e Ag(s) E = v Ni 2+ (aq) + 2e Ni(s) E = 0.25 v Fe 2+ (aq) + 2e Fe(s) E = 0.44 v To calculate the most positive cell potential, we use the equation; E cell = E cathode E anode To get the most positive cell potential Ag + (aq) must be reduced and Fe(s) must be oxidized. So the balanced net ionic reaction is; Fe(s) + 2Ag + (aq) Fe 2+ (aq) + 2Ag(s) b) Identify the reducing agent in your reaction. (2)

11 The reducing agent is the species that is oxidized. Fe(s) is oxidized in the reaction, so Fe(s) is the reducing agent. Remember metals like to lose electrons, so they are excellent reducing agents. c) Determine the standard cell voltage, E for the equation written part a. (4) E cell = E cathode E anode E cell = v ( 0.44 v) = v d) A cell is constructed based on the reaction in part a). Identify the metal used for the anode and indicate the ion(s) in the cathode compartment. Clear indicate your answers. (6) Since Fe(s) is oxidized in the reaction the anode material is Fe(s). The cathode compartment must contain Ag + (aq) and an anion that forms a soluble salt. The best anion would be NO 3 (aq). e) Which of the following compounds is most appropriate to use in a salt bridge, NaOH, CuS, or NaNO 3? Justify your answer with a brief explanation why you selected the particular compound, and why you did not select the other compounds. (6) The best choice is NaNO 3. NaNO 3 is soluble in water and neither cation ion or anion will form insoluble ionic solids when migrating into the anode or cathode compartments. NaOH is not a good choice because the anion hydroxide, OH will form an insoluble solid in the presence of Fe 2+ (aq). Since Fe 2+ (aq) is formed in the anode compartment and OH ions will migrate towards the anode compartment a precipitate of Fe(OH) 2 will form, reducing the concentration of Fe 2+ (aq) in the solution and changing the cell potential. CuS is not a good choice because it is insoluble in water, so a salt bridge solution prepared by adding CuS to water contains so few ions the cell will not discharge and no reaction will occur. f) If the concentration of the solution in the cathode is reduced from 1.0 M to 0.25 M, calculate the new cell potential, E cell. (10) E cell = E n log [Fe2+ ] [Ag + ] 2

12 The cathode compartment contains Ag + (aq) ions, that appear in the denominator of the Nernst equation. Reducing the concentration to 0.25 M means the denominator is a small number, and the quotient will be larger than one. The log of a number greater than one is positive so the term n log [Fe2+ ] [Ag + ] 2 is positive and is subtracted from the standard cell potential. This make E cell smaller (less positive). 15. Short Answer a) In an electrochemical cell Zn is oxidized at the anode. i. Identify the ions in the anode compartment; (4) j. Since the only half reaction that contains Zn is; Zn 2+ (aq) + 2e Zn(s) E = 0.76 v The ions in the anode compartment (where oxidization occurs) are Zn 2+ (aq) ions. ii. The salt bridge is a saturated solution of NH 4 NO 3. Which ion migrates towards the anode? Explain why that ion migrates towards the anode. (4) The NO 3 (aq) ions migrate towards the anode compartment. Since oxidation occurs in the anode compartment, Zn metal is oxidized to form Zn 2+ (aq) ions, increasing the concentration of cations in the anode compartment. To maintain a charge balance NO 3 (aq) ions in the salt bridge must migrate towards the anode compartment. iii. iv. An unknown metal ion is reduced at the cathode. The E for the reaction between Zn and the unknown metal ion is v. What is the unknown metal ion? (3) E cell = E cathode E anode v = E cathode ( 0.76 v) E cathode = v + ( 0.76 v) = v Look up a standard reduction potential of volts and the reduction reaction is,

13 Ag + (aq) + e Ag(s) E = v v. Write the half-reaction that occurs at the cathode.(2) Ag + (aq) + e Ag(s) E = v vi. What is the electrode metal for the cathode? (2) vii. The cathode is made of silver solid. b) In the reaction 5Fe(s) + 2MnO 4 (aq) + 16 H + (aq) 5Fe 2+ (aq) + 2Mn 2+ (aq) + 8H2 O(l) i. Calculate E for the reaction. (4) Fe 2+ (aq) + 2e Fe(s) E = 0.44 volts MnO 4 (aq) + 8H + (aq) + 2e Mn 2+ (aq) + 4H 2 O(l) E = volts E cell = E cathode E anode = v v ( 0.44 v) = v ii. If the ph of the solution is raised to 3, and assuming all other ions in the solution remain 1 M, will the E cell increase, decrease or remain the same? Briefly, explain your answer. (NOTE: No calculation is required.) (4) E cell = E n log [Fe2+ ] 5 [Mn 2+ ] 2 [MnO 4 ] 2 [H + ] 16 Raising the ph to 3 means the [H + ] is reduced to 1 x 10 3 M. So the denominator now has a value of 1 x The ratio is now larger than 1 so the log of a number larger than one is positive, so the n log [Fe2+ ] 5 [Mn 2+ ] 2 [MnO 4 ] 2 [H + ] 16 term is positive and will be subtracted from the positive E, so E cell will decrease. 16. Given the following reactions and information about the spontaneous direction, prepare the corresponding activity series showing the four reduction half-reactions (V e V: Hg e Hg: Sn e Sn: Cd e Cd) in the order of most reactive metal ion (least reactive metal) at the top of the list to least reactive metal ion (most reactive metal) at the bottom of the list. V(s) + Cd(NO 3 ) 2 V(NO 3 ) 2 + Cd Hg + Sn(NO 3 ) 2 no reaction

14 Sn(s) + Cd(NO 3 ) 2 no reaction From reaction 1) we know that Cd 2+ is more likely to be reduced compared to V 2+ ; Cd e Cd V e V From reaction 3) Sn can not reduce Cd 2+ ; Sn e Sn Cd e Cd V e V From reaction 2) Hg cannot reduce Sn 2+ ; Hg e Hg Sn e Sn Cd e Cd V e V 17. A galvanic cell is constructed using a chromium electrode in a 1.00-molar solution of Cr(NO 3 ) 3 and a copper electrode in a 1.00-molar solution of Cu(NO 3 ) 2. Both solutions are at 25 C. (a) Write a balanced net ionic equation for the spontaneous reaction that occurs as the cell operates. Identify the oxidizing agent and the reducing agent. (6) 2Cr(s) + 3Cu 2+ (aq) 2Cr 3+ (aq) + 3Cu(s) Cu 2+ (aq) + 2e (aq) Cu(s) Cr 3+ (aq) + 3e Cr(s) E = volts E = volts To get a positive cell potential E = E cathode E anode v = 0.34 v ( 0.74 v) = (b) A partial diagram of the cell is shown below.

15 (i) Which metal is the cathode? (2) Cu metal is the cathode (ii) What additional component is necessary to make the cell operate? (3) A salt bridge is needed. (iii) What function does the component in (ii) serve? (4) The ions in the salt bridge migrate towards the electrode compartments to balance the charge. As oxidation occurs in the anode compartment, cations are produced, so the anions in the salt bridge must migrate towards the anode compartment. In the cathode compartment, reduction occurs and cations are removed, so cations in the salt bridge migrate towards the cathode compartment. (c) How does the potential of this cell change if the concentration of Cr(NO 3 ) 3 is changed to 3.00-molar at 25 C? Explain. (4) E cell = E n log [Cr3+ ] 2 [Cu 2+ ] 3 Since [Cr 3+ ] is in the numerator in the ratio, a higher concentration will make the ratio greater than one. The log of a number larger than one is positive so the n log [Cr3+ ] 2 [Cu 2+ ] 3 term must be subtracted from the standard cell potential, and the cell potential decreases. (d) Calculate K and G for the reaction you wrote in part a). (14) E = n log K

16 +1.08 = log K = log K = 10 log K K = 2.88 x G = nfe J G = 6 96,500 mol volt v = 625,000 J mol 1 kj 1000 J = 625 kj mol e) In another Galvanic cell Cr 3+ is reduced at the cathode by the metal, X. The anode electrode, composed of the metal X, is immersed in a solution of 1.0 M X(NO 3 ) 2. The measured standard cell potential for the reaction between X and Cr(NO 3 ) 3 is v. Calculate the standard reduction potential for X 2+. (4)

17 17B. Which of the following is an oxidation reduction reaction? A) Na 2 CO 3 (s) + 2HCl(aq) 2NaCl(aq) + 2H 2 O(l) + CO 2 (g) B) H 2 SO 4 (aq) + Ba(OH) 2 (aq) BaSO 4 (s) + 2H 2 O(l) C) H + (aq) + OH (aq) H 2 O(l) D) Pb 2+ (aq) + Ni(s) Pb(s) + Ni 2+ (aq) E) NaCl(s) Na + (aq) + Cl (aq) 18. Mercury batteries, like those used in electric watches, furnish a voltage of 1.35 v. If the overall oxidation-reduction reaction taking place in basic solution is Zn(s) + HgO(s) + 2H 2 O(l) Zn(OH) 2 (s) + Hg(l) the anode reaction must be A) HgO(s) + 2H 2 O(l) + 2e Hg(l) + 2OH (aq) B) Zn(s) + 2OH (aq) Zn(OH) 2 (s) + 2e C) Hg(l) + 2OH (aq) HgO(s) + 2H 2 O(l) + 2e D) Zn(OH) 2 (s) + 2e Zn(s) + 2OH (aq) 19. Aluminum oxide may be electrolyzed at 1000 C to furnish aluminum metal. The cathode reaction is Al e Al To prepare 5.12 kg of aluminum metal by this method would require A) 5.49 x 10 7 C of electricity B) 1.83 x 10 7 C of electricity C) 5.49 x 10 4 C of electricity D) 5.49 x 10 1 C of electricity = 5.49 x 10 7 C 5.12 kg 1000 g 1 kg 1 mol Al 27.0 g 3 mol e 1 mol Al coulomb 1 mol e 20. The cell potential, E cell, for the reaction

18 Zn 2+ (0.500 M) Zn 2+ ( M) is A) v B) 0.00 v C) v D) can not be determine without additional information E cell = E n log [Zn 2+ ]( dilute ) [Zn 2+ ]( concentrated ) Note: Since the half-reactions must be; Zn 2+ (0.500 M) Zn (s) + 2 e Zn (s) + 2e Zn 2+ ( M) 2 electrons must be transferred. E cell = E log E cell = 0.00 volts [Zn 2+ ]( dilute ) [Zn 2+ ]( concentrated ) log E cell = 0.00 volts ( volts) = volts 21. Given the following half-reactions E Mg 2+ (aq) + 2e Mg(s) H + (aq) + 2e H 2 (g) The measured cell potential for the spontaneous reaction Mg(s) + 2H + (aq) Mg 2+ (aq) + H 2 (g) under standard conditions is v. The ph of the solution in the cathode compartment must be

19 A) 0 B) 1 C) 7 D) Draw a diagram of the cell in which the following reaction occurs. Cr(s) + Sn 2+ (aq) Cr 3+ (aq) + Sn(s) Label the anode and cathode, the anode and cathode electrode material, the halfreaction at each electrode, the ions in the anode and cathode compartments and salt bridge, the direction of electron flow and the direction of ion movement, and calculate E. 23. A voltaic cell is constructed to carry out the reaction, 2Cr 2+ (aq) + Pb 2+ (aq) 2Cr 3+ (aq) + Pb(s) If the [Cr 3+ ] = M, [Pb 2+ ] = M and the [Cr 2+ ] = M, calculate E cell at 25 C. E cell = E E cell = E E cell = E n [0.0300] log 2 [0.250] 2 [0.150] [0.0300] log 2 [0.250] 2 [0.150] [Cr log 3+ ] 2 [Cr 2+ ] 2 [Pb 2+ ] 24. Calculate K and G for the reaction in Problem #5. E = n G = nfe log K

20 25. A voltaic cell consists of an anode compartment with a nickel electrode immersed in a NiSO 4 solution and a cathode compartment with a copper electrode immersed in a CuSO 4 solution. A salt bridge connects the two cells. a. Write a balanced equation for the cell reaction. Ni(s) + Cu 2+ (aq) Ni 2+ (aq) + Cu(s) b. A current of 1.50 amps is observed to flow for a period of 2.00 hours. How much charge passes through the circuit during this time? How many moles of electrons is this charge equivalent to? 1.50 amps 2 hrs 3600 sec 1 hr 1 coulomb 1 amp-sec 1.50 amps 2 hrs 3600 sec 1 hr 1 coulomb 1 amp-sec 1 mol e coulomb c. Calculate the change in mass at the copper electrode. d. Calculate the change in mass at the nickel electrode. e. At which of the two electrodes does the mass increase?

21 26. Write the half-reactions which occur at each electrode when the following solutions are electrolyzed between inert electrodes. If more than one halfreaction can occur at the anode or cathode, write all possible half-reactions and briefly explain which half-reaction will occur. a. AlCl 3 (l) ANODE: 2Br Br 2 + 2e CATHODE: K + + 1e K b. CuBr 2 (aq) ANODE: 2H 2 O(l) O 2 (g) + 4H + (aq) + 4e 2Br Br 2 (l) + 2e CATHODE: Cu e Cu(s) 2H 2 O(l) + 2e 2OH (aq) + H 2 (g)

22 27. Which of the following species is the strongest reducing agent? (Circle your answer.) H 2 (g) : Cl (aq) : Zn(s) : Mg 2+ (aq) The strongest reducing agent (the species that will be oxidized) is the species with the most negative reduction potential. So must look for the species on the right side of the table of standard reduction potentials. Cl 2 (g) + 2e- 2Cl- (aq) v 2H+ (aq) + 2e- H 2 (s) 0.00 v Zn 2+ (aq) + 2e- Zn(s) v Mg 2+ (aq) + 2e- Mg(s) v We can eliminate Mg 2+ (aq) immediately because it cannot be oxidized. Of the remaining three species Zn has the most negative 28. Will Sn(s) displace Ag + (aq) from solution? Ag+ (aq) + 1e- Ag(s) v Sn 2+ (aq) + 2e- Sn(s) v Yes, the cell potential for the reaction (Sn(s) + Ag+ (aq) Sn 2+ (aq) + Ag(s)) is positive. 29. Will gold dissolve in HCl(aq)? Au 3+ (aq) + 3e- Au(s) 1.50 v 2H+ (aq) + 2e- H 2 (g) 0.00 v No, the cell potential for the reaction (Au + 2H + (aq) H 2 (g) + Au(s) is negative. 30. From the following information estimate the E for M 2+ (aq) + 2e- M(s)

23 knowing the metal, M, dissolves in 1 M HCl. It will displace Fe 2+ (aq), but not Al 3+ (aq).

24 31. Write the half-reactions and determine E for the electrochemical cells as described below, in part b include the overall chemical equation. a. 2Al(s) + 3Ni 2+ (aq) 2Al 3+ (aq) + 3Ni(s) 32. A voltaic cell is constructed that the reaction is, 2Cr(s) + 3Sn 4+ (aq) 2Cr 3+ (aq) + 3Sn 2+ (aq) If the [Cr 3+ ] = 1.00 M, [Sn 4+ ] = M and the [Sn 2+ ] = 0.025, calculate E cell at 25 C. 33. Calculate [Cr 3+ ] for the reaction in Problem #5 if the [Sn 2+ ] is 1.00 M and [Sn 4+ ] is 1.00 M and the measured cell potential is volts.

25 34. Write the half-reactions which occur at each electrode when the following solutions are electrolyzed between inert electrodes. If more than one halfreaction can occur at the anode or cathode, write all possible half-reactions and briefly explain which half-reaction will occur. a. KBr(l) ANODE: CATHODE: b. NiCl 2 (aq) ANODE: CATHODE:

26 35. When a dilute solution of H 2 SO 4 is electrolyzed, O 2 (g) is produced at the anode and H 2 (g) is produced at the cathode. a. Calculate the coulombs of charge produced when an average current of 8.50 amps passes through the cell for 15.0 minutes. b. Calculate the mass of O 2 and H 2 produced when the cell operates for 15.0 minutes at an average current of 8.50 amps. 36. Calculate E cell for the reaction in Problem #9a if the [Zn 2+ ] is 1.20 M and [Cr 3+ ] is 1.00 x 10 3 M.

27 37. Using the table of standard reduction potentials, calculate K sp for PbSO 4.

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29 Standard Reduction Potentials at 25 C Half-Reaction E F 2 (g) + 2e- 2F- (aq) v H 2 O 2 (aq) +2H+ (aq) +2e- 2H 2 O(l) v MnO 4 - (aq)+8h+ (aq)+5e- Mn 2+ (aq) + 4H 2 O(l) v Cl 2 (g) + 2e- 2Cl- (aq) v Cr 2 O 7 2- (aq)+14h+ (aq)+6e- Cr 3+ (aq) + 7H 2 O(l) v O 2 (g) + 4H+ (aq) + 4e- 2H2O(l) v Br 2 (l) + 2e- 2Br- (aq) v NO 3 - (aq) +4H+ (aq)+3e- NO(g) + 2H 2 O(l) v Ag+ (aq) + 1e- Ag(s) v I 2 (s) + 2e- 2I- (aq) v Cu 2+ (aq) + 2e- Cu(s) v AgCl(s) + 1e- Ag(s) + Cl- (aq) v 2H+ (aq) + 2e- H 2 (g) 0.00 v Pb 2+ (aq) + 2e- Pb(s) v Sn 2+ (aq) + 2e- Sn(s) v Ni 2+ (aq) + 2e- Ni(s) v Cr 3+ (aq) + 3e- Cr(s) v

30 Zn 2+ (aq) + 2e- Zn(s) v 2H 2 O(l) + 2e- H 2 (g) + 2-OH(aq) v Mg 2+ (aq) + 2e- Mg(s) v Na+ (aq) + 1e- Na(s) v Ca 2+ (aq) + 2e- Ca(s) v K+ (aq) + 1e- K(s) v Li+ (aq) + 1e- Li(s) v

31 (12) 1. Complete and balance the following reactions. Identify all product's phases as either (g)as, (l)iquid, (s)olid or (aq)ueous. If no reaction occurs, write NR. a) Zn(s) + Cu(NO 3 ) 2 (aq) Zn(NO 3 ) 2 (aq) + Cu(s) b) Ni(s) + HCl(aq) NiCl 2 (aq) + H 2 (g) c) 3Cu(s) + 2NO 3 (aq) + 8H + (aq) 3Cu 2+ (aq) + 2NO(g) + 4H 2 O(l) d) Mg(s) + KF(aq) NR (18) 2. Balance the following oxidation-reduction reactions using the half-reaction method. (a) N 2 H 4 (aq) + CO 3 2 (aq) N2 (g) + CO(g) basic solution N 2 H 4 (aq) N 2 (g) + 4H + (aq) + 4e 2(2e + 4H + (aq) + CO 3 2 (aq) CO(g) + 2H2 O(l) ) 4H 2 O(l) 4H + (aq) + N 2 H 4 (aq) + 2CO 3 2 (aq) N2 (g) + 2CO(g) OH + 4 OH 4H 2 O(l) + 4OH (aq) 4H 2 O(l) + N 2 H 4 (aq) + 2CO 3 2 (aq) N2 (g) + 2CO(g) + identify the oxidizing agent CO 3 2 identify the reducing agent N 2 H 4 (aq) (b) Hg 2 HPO 4 (s) + Cl (aq) + Au(s) Hg(l) + H 2 PO 4 (aq) + AuCl4 (aq) acidic solution

32 2(4Cl (aq) + Au(s) AuCl 4 (aq) + 3e ) 3(2e + H + + Hg 2 HPO 4 (s) 2Hg(l) + H 2 PO 4 (aq) ) 3H + + 3Hg 2 HPO 4 (s) + 8Cl (aq) + 2Au(s) 2AuCl 4 (aq) + 6Hg(l) + 3H2 PO 4 (aq) Au(s) identify the oxidizing agent Hg 2 HPO 4 (s) identify the reducing agent

33 (8) 3. Write the half-reactions and determine E for the electrochemical cells as described below, in part b include the overall chemical equation. a. 3Pb(s) + SO 4 2 (aq) + 8H + (aq) 3Pb 2+ (aq) + S(s) + 4H2 O(l) 3(Pb Pb e ) 6e + SO 4 2 (aq) + 8H + (aq) S(s) + 4H2 O(l) v v 3Pb(s) + SO 4 2 (aq) + 8H + (aq) 3Pb 2+ (aq) + S(s) + 4H2 O(l) v = E b. Ni(s) Ni 2+ (aq) Cl 2 (g), Cl - (aq) Pt(s) Ni(s) Ni 2+ (aq) + 2e 2e + Cl 2 (g) 2Cl (aq) v v Ni(s) + Cl 2 (g) Ni 2+ (aq) + 2Cl (aq) v = E (14) 4. Given the following data Substance H kj mol S J mol K G kj mol FeO(s) C(graphite) Fe(s) CO(g) a. Using the above data, calculate G, S, and H for the reaction,

34 FeO(s) + C(graphite) Fe(s) + CO(g) H rxn = ( H f (CO)) ( H f (FeO)) = kj ( kj) = kj S rxn = (S (Fe) + S (CO)) (S (FeO) + S (C)) = (27.28 J K J K ) (57.49 J K J K ) = +162 J K G rxn = ( G f (CO)) ( G f (FeO)) = kj ( kj) = +108 kj b. Determine at what temperature is the reaction spontaneous. G = H T S G = 0 H S = T = J J K = 964 K

35 (8) 5. A voltaic cell is constructed to carry out the reaction, 2Cr 2+ (aq) + Pb 2+ (aq) 2Cr 3+ (aq) + Pb(s) If the [Cr 3+ ] = M, [Pb 2+ ] = M and the [Cr 2+ ] = M, calculate E cell at 25 C. 2Cr 2+ (aq) 2Cr 3+ (aq) + 2e ) Pb 2+ (aq) + 2e Pb(s) v v E = v E cell = E n log Q = E log [Cr 3+ ] 2 [Cr 2+ ] 2 [Pb 2+ ] = log [0.03] 2 [0.25] 2 [0.15] = log (0.096) = ( 1.018) = = v

36 (8) 6. Calculate K and G for the reaction in Problem #5. G = nfe = 2 96,500 J mol v ( v) = 54.4 kj E =.059 n log K v = log K = log K K = 3.49 x 10 9

37 (12) 7. A voltaic cell consists of an anode compartment with a nickle electrode immersed in a NiSO 4 solution and a cathode compartment with a copper electrode immersed in a CuSO 4 solution. A salt bridge connects the two cells. a. Write a balanced equation for the cell reaction. Ni(s) Ni 2+ (aq) + 2e Cu 2+ (aq) + 2e Cu(s) Cu 2+ (aq) + Ni(s) Cu(s) + Ni 2+ (aq) b. A current of 1.50 amps is observed to flow for a period of 2.00 hours. How much charge passes through the circuit during this time? How many moles of electrons is this charge equivalent to? 1.50 amps 2.00 hours 60 min 1 hr 60 s 1 min 1 mol C = mol e c. Calculate the change in mass at the copper electron mol e 1 mol Cu 2 mol e 63.5 g 1 mol = 3.56 g Cu d. Calculate the change in mass at the nickle electrode mol e 1 mol Ni 2 mol e 58.7 g 1 mol = 3.29 g Cu e. At which of the two electrodes does the mass increase? mass increases at the cathode

38 (8) 8. Write the half-reactions which occur at each electrode when the following solutions are electrolyzed between inert electrodes. If more than one halfreaction can occur at the anode or cathode, write all possible half-reactions and briefly explain which half-reaction will occur. a. AlCl 3 (l) ANODE: 2Cl Cl 2 + 2e CATHODE: Al e Al b. CuBr 2 (aq) ANODE: 2H 2 O(l) O 2 (g) + 4H + (aq) + 4e 2Br Br 2 (l) + 2e CATHODE: Cu e Cu(s) 2H 2 O(l) + 2e 2OH (aq) + H 2 (g) Short Answer: (12 points)

39 9. Which of the following species is the strongest reducing agent? (Circle your answer.) H 2 (g) : Cl (aq) : Zn(s) : Mg 2+ (aq) 10. Will Sn(s) displace Ag + (aq) from solution? YES 11. Will gold dissolve in HCl(aq)? NO 12. From the following information estimate the E for M 2+ (aq) + 2e- M(s) knowing the metal, M, dissolves in 1 M HCl. It will displace Fe 2+ (aq), but not Al 3+ (aq). Mn or Zn

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