Chapter 18 problems (with solutions)

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Jacob Hensley
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1 Chapter 18 problems (with solutions) 1) Assign oxidation numbers for the following species (for review see section 9.4) a) H2SO3 H = +1 S = +4 O = -2 b) Ca(ClO3)2 Ca = +2 Cl = +5 O = -2 c) C2H4 C = -2 H = +1 d) H3PO4 H = +1 P = +5 O = -2 e) Cl2 Cl = 0 f) BrO4 - Br = +7 O = -2 g) KMnO4 K = +1 Mn = +7 O = -2 h) NaOH Na = +1 O = -2 H = +1 i) H2O2 H = +1 O = -1 j) H4P2O7 H = +1 P = +5 O = -2 2) Balance each redox reaction occurring in acid aqueous solution. In all cases we will use the procedure discussed in class. We will first find balanced half-cell oxidation and reduction half-reactions. We will multiply one or both reactions by an integer value so that the number of electrons transferred will cancel when the half-reactions are combined. Finally, we will combine the half-reactions and cancel any substances appearing as both reactants and products to obtain the final balanced cell reaction. a) I - (aq) + NO2 - (aq) I2(s) + NO(g) ox 2 I - (aq) I2(s) + 2 e - red NO2 - (aq) + 2 H + (aq) + e - NO(g) + H2O( ) x 2 2 I - (aq) + 2 NO2 - (aq) + 4 H + (aq) I2(s) + 2 NO(g) + 2 H2O( ) b) NO3 - (aq) + Sn 2+ (aq) Sn 4+ (aq) + NO(g) ox Sn 2+ (aq) Sn 4+ (aq) + 2 e - x 3 red NO3 - (aq) + 4 H + (aq) + 3 e - NO(g) + 2 H2O( ) x 2 3 Sn 2+ (aq) + 2 NO3 - (aq) 8 H + (aq) 3 Sn 4+ (aq) + 2 NO(g) + 4 H2O( ) c) S2O3 2- (aq) + I2(aq) S4O6 2- (aq) + I - (aq) ox 2 S2O3 2- (aq) S4O6 2- (aq) + 2 e - red I2(aq) + 2 e - 2 I - (aq) 2 S2O3 2- (aq) + I2(aq) S4O6 2- (aq) + 2 I - (aq) 1

2 3) Balance each redox reaction occurring in base aqueous solution. Balancing oxidation-reduction reactions for bas conditions is exactly the same as balancing the reactions for acid conditions except that there is one additional step where OH - ions are added, if necessary, to convert all H + ions into water molecules. a) MnO4 - (aq) + Br - (aq) MnO2(s) + BrO3 - (aq) ox Br - (aq) + 3 H2O( ) BrO3 - (aq) + 6 H + (aq) + 6 e - red MnO4 - (aq) + 4 H + (aq) + 3 e - MnO2(s) + 2 H2O( ) x 2 add OH - Br - (aq) + 2 MnO4 - (aq) + 2 H + (aq) BrO3 - (aq) + 2 MnO2(s) + H2O( ) 2 OH - (aq) 2 OH - (aq) Br - (aq) + 2 MnO4 - (aq) + H2O( ) BrO3 - (aq) + 2 MnO2(s) + 2 OH - (aq) b) NO2 - (aq) + Al(s) NH3(g) + AlO2 - (aq) ox Al(s) + 2 H2O( ) AlO2 - (aq) + 4 H + (aq) + 3 e - x 2 red NO2 - (aq) + 7 H + (aq) + 6 e - NH3(g) + 2 H2O( ) add OH - 2 Al(s) + NO2 - (aq) + 2 H2O( ) 2 AlO2 - (aq) + NH3(g) + H + (aq) OH - (aq) OH - (aq) 2 Al(s) + NO2 - (aq) + H2O( ) + OH - (aq) 2 AlO2 - (aq) + NH3(g) 4) Explain the difference between a voltaic (or galvanic) electrochemical cell and an electrolytic one. In a voltaic cell (also called a galvanic cell or a battery) a chemical reaction is used to generate a voltage. In an electrolytic cell an external voltage is used to force a chemical reaction to occur in a particular direction. 5) Explain the purpose of a salt bridge in an electrochemical cell. The salt bridge is used to complete the circuit. It provides a way in which charge can pass from one part of the cell to the other part without having the two parts of the cell directly react chemically. 2

3 6) How can Table 18.1 be used to predict whether or not a metal will dissolve in HCl? In HNO3? Any metal below the H + /H 2 half-cell reduction potential, at 0.00 v, would be expected to dissolve in HCl (hydrochloric acid) for standard conditions. This is because when the half-cell oxidation potential for the metal (found by reversing the reaction and changing the sign of the potential) is combined with the half-cell reduction potential for H + /H2, a positive voltage will result, indicating a spontaneous reaction. The same reasoning applies for HNO3 (nitric acid) except that now there is a different reduction half-reaction to use, that for NO3 - /NO at 0.96 v. Metals lying below this half-cell reduction potential would be expected to dissolve in HNO3. So, for example, Ni (nickel) will dissolve in both HCl and HNO3, Cu (copper) will dissolve in HNO3 but not HCl, and Au (gold) will not dissolve in either acid. 7) Give the half cell oxidation reaction, the half cell reduction reaction, and the cell reaction for the following galvanic cells. a) Pb(s) Pb 2+ (aq) Cu 2+ (aq) Cu(s) ox Pb(s) Pb 2+ (aq) + 2 e - red Cu 2+ (aq) + 2 e - Cu(s) Pb(s) + Cu 2+ (aq) Pb 2+ (aq) + Cu(s) b) Pt, H2(g) H + (aq) Cl - (aq) AgCl(s), Ag(s) ox H2(g) 2 H + (aq) + 2 e - red AgCl(s) + e - Ag(s) + Cl - (aq) x 2 H2(g) + 2 AgCl(s) 2 H + (aq) + 2 Cl-(aq) + 2 Ag(s) 8) Calculate the standard cell potential for each of the following electrochemical cells. Also, represent each cell as a cell diagram using standard notation. a) Ni 2+ (aq) + Mg(s) Ni(s) + Mg 2+ (aq) Mg(s) Mg 2+ (aq) Ni 2+ (aq) Ni(s) ox Mg(s) Mg 2+ (aq) + 2 e - E = v red Ni 2+ (aq) + 2 e - Ni(s) E = v Ni 2+ (aq) + Mg(s) Ni(s) + Mg 2+ (aq) E = v 3

4 b) 2 H + (aq) + Fe(s) H2(g) + Fe 2+ (aq) Fe(s) Fe 2+ (aq) H + (aq) H2(g), Pt ox Fe(s) Fe 2+ (aq) + 2 e - E = v red 2 H + (aq) + 2 e- H2(g) E = 0.00 v 2 H + (aq) + Fe(s) H2(g) + Fe 2+ (aq) E = v c) 2 NO3 - (aq) + 8 H + (aq) + 3 Cu(s) 2 NO(g) + 4 H2O( ) + 3 Cu 2+ (aq) Cu(s) Cu 2+ (aq) NO3 - (aq) NO(g), Pt ox 3 Cu(s) 3 Cu 2+ (aq) + 6 e - E = v red 2 NO3 - (aq) + 8 H + (aq) + 6 e - 2 NO(g) + 4 H2O( ) E = v 2 NO3 - (aq) + 8 H + (aq) + 3 Cu(s) 2 NO(g) + 4 H2O( ) + 3 Cu 2+ (aq) E = v 9) Which of the redox reactions do you expect to occur spontaneously in the forward direction (for standard conditions)? a) Ni(s) + Zn 2+ (aq) Ni 2+ (aq) + Zn(s) b) Ni(s) + Pb 2+ (aq) Ni 2+ (aq) + Pb(s) c) Al(s) + 3 Ag + (aq) Al 3+ (aq) + 3 Ag(s) d) Pb(s) + Mn 2+ (aq) Pb 2+ (aq) + Mn(s) There are two ways in which we can do this sort of problem. We could calculate the cell potential, and then use the fact that for spontaneous processes the cell potential will be positive. Alternatively, we can use the position of the half-cell reactions in Table If the substance being reduced in the reaction is higher in Table 18.1 than the substance being oxidized then the reaction is spontaneous. In this problem I will use the second method, but I will also give the cell potential (although not the oxidation, reduction, and cell reactions). a) Zn 2+ /Zn is below Ni 2+ /Ni, and so not spontaneous. E = v b) Pb 2+ /Pb is above Ni 2+ /Ni, and so spontaneous. E = v c) Ag + /Ag is above Al 3+ /Al, and so spontaneous. E = v d) Mn 2+ /Mn is below Pb 2+ /Pb, and so not spontaneous. E = v 4

5 10) Which metal cation is the best oxidizing agent? a) Pb 2+ b) Cr 3+ c) Fe 3+ d) Sn 2+ The best oxidizing agent will be the species that has the largest (most positive) reduction potential. Based on Table 18.1 Pb 2+ /Pb E = Cr 3+ /Cr E = v Fe 2+ /Fe E = v Sn 2+ /Sn E = v So Pb 2+ is the best oxidizing agent. 11) Which metal is the best reducing agent? a) Mn b) Al c) Ni d) Cr The best reducing agent will be the species that has the largest (most positive) oxidation potential. Note that to get the oxidation potential we have to reverse the reduction reaction, and so change the sign of the potential. Mn/Mn 2+ E = v Al/Al 3+ E = v Ni/Ni 2+ E = v Cr/Cr 3+ E = v So Al is the best reducing agent. 12) Use tabulated electrode potentials to calculate G rxn for each reaction at 25. C. Then calculate the equilibrium constant for each reaction. To find G rxn and K (the thermodynamic equilibrium constant) we will use the expression G rxn = - nfe cell lnk = nfe cell/rt Recall that (1 C) (1 v) = 1 J. Values for E are given in the Table a) O2(g) + 2 H2O( ) + 2 Cu(s) 4 OH - (aq) + 2 Cu 2+ (aq) ox 2 Cu(s) 2 Cu 2+ (aq) + 4 e - E = v red O2(g) + 2 H2O( ) + 4 e - 4 OH - (aq) E = v O2(g) + 2 H2O( ) + 2 Cu(s) 4 OH - (aq) + 2 Cu 2+ (aq) E = v n = 4 G rxn = - (4) (96485 C/mol) ( v) = kj/mol ln K = (4) (96485 C/mol) ( v) = 9.34 K = e 9.34 = 1.1 x 10 4 (8.314 J/mol. K) (298.2 K) 5

6 b) Br2( ) + 2 I - (aq) 2Br - (aq) + I2(s) ox 2 I - (aq) I2(s) + 2 e - E = v red Br2( ) + 2 e - 2 Br - (aq) E = v Br2( ) + 2 I - (aq) 2 Br - (aq) + I2(s) E = v n = 2 G rxn = - (2) (96485 C/mol) ( v) = kj/mol ln K = (2) (96485 C/mol) ( v) = K = e = 1.7 x (8.314 J/mol.K) (298.2 K) 13) A voltaic cell employs the redox reaction 2 Fe 3+ (aq) + 3 Mg(s) 2 Fe(s) + 3 Mg 2+ (aq) Calculate the cell potential at 25. C under each set of conditions a) Standard conditions. We first need to find E. ox 3 Mg(s) 3 Mg 2+ (aq) + 6 e - E = v red 2 Fe 3+ (aq) + 6 e - 2 Fe(s) E = v 2 Fe 3+ (aq) + 3 Mg(s) 2 Fe(s) + 3 Mg 2+ (aq) E = v Ecell = E cell - RT ln Q Q = [Mg 2+ ] 3 n = 6 nf [Fe 3+ ] 2 For standard conditions Ecell = E cell = v b) [Fe 3+ ] = 1.0 x 10-3 M; [Mg 2+ ] = 2.50 M Q = (2.50) 3 = 1.56 x 10 7 (1.0 x 10-3 ) 2 Ecell = v - (8.314 J/mol. K) (298.2 K) ln(1.56 x 10 7 ) = v (6) (96485 C/mol) c) [Fe 3+ ] = 2.00 M; [Mg 2+ ] = 1.5 x 10-3 M Q = (1.5 x 10-3 ) 3 = 8.44 x (2.00) 2 Ecell = v - (8.314 J/mol. K) (298.2 K) ln(8.44 x ) = v (6) (96485 C/mol) 6

7 14) Copper can be electroplated at the cathode of an electrolytic cell by the half reaction Cu 2+ (aq) + 2 e - Cu(s) How much time would it take for 325. mg of copper metal to be plated at a current of 5.6 A? MW(Cu) = g/mol t = 325. x 10-3 g Cu 1 mol Cu 2 mol e C 1 s = 176. s g Cu 1 mol Cu 1 mol e C 15) What mass of aluminum metal can be produced per hour in the electrolysis of molten aluminum salt (containing Al 3+ ion) by a current of 25. A? The reduction for aluminum (MW(Al) = g/mol) is Al 3+ Al g Al = 1 hr 3600 s 25 C 1 mol e - 1 mol Al g Al = 8.39 g Al 1 hr 1 s C 3 mol e - 1 mol Al 16) The Ksp of Zn(OH)2 is 1.8 x Find E cell for the half reaction Zn(OH)2(s) + 2 e - Zn(s) + 2 OH - (aq) The key to doing this problem is to find a combination of an oxidation and a reduction reaction that add up to the reaction of interest, which in this case is Zn(OH)2(s) Zn 2+ (aq) + 2 OH - (aq) The cell potential can be found using E cell = (RT/nF) ln K = (8.314 J/mol. K) (298.2 K) ln(1.8 x ) = v (2) (96485 C/mol) ox Zn(s) Zn 2+ (aq) + 2 e - E = v red Zn(OH)2(s) + 2 e - Zn(s) + 2 OH - (aq) E =? Zn(OH)2(s) Zn 2+ (aq) + 2 OH - (aq) E cell = v So the half-cell reduction potential for the reaction in question is 0.76 v + E = v ; E = v v = v 7

8 17) A metal forms from the fluoride compound MF3. Electrolysis of molten MF3 by a current of 3.86 A for 16.2 minutes deposits 1.25 g of metal. What is the molar mass of the metal? Based on the formula MF3 the reduction reaction for the metal is M e - M We know how many grams of metal are produced. To find the number of moles of metal we use mol M = 16.2 min 60 s 3.86 C 1 mol e - 1 mol M = x 10-2 mol M 1 min 1 s C 3 mol e - So the molecular mass is M = 1.25 g = 96.4 g/mol x 10-2 mol 18) (18.77) Calculate the emf of the following concentration cell at 25. C Cu(s) Cu 2+ (0.080 M) Cu 2+ (1.20 M) Cu ox Cu(s) Cu 2+ (aq) + 2 e - red Cu 2+ (aq) + 2 e - Cu(s) Cu(s) + Cu 2+ (aq) Cu(s) + Cu 2+ (aq) E cell = 0.00 v Since the oxidation and reduction reactions are the same reaction, we know the standard cell potential will be 0.00 v. However, since the two electrodes have a different concentration of Cu 2+ ion, a voltage can still be generated. Ecell = E cell - RT ln Q Q = [Cu 2+ ]left = 1.20 = 15 n = 2 nf [Cu 2+ ]right Ecell = 0.00 v - (8.314 J/mol K) (298.2 K) ln(15) = v (2) ( C/mol) 8

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