Assignment 20 A1

Assignment 20 A
1- Which of the following oxidation numbers in the species given is incorrect?
a) SO32−; S +4, O −2
b) ClO3−; Cl +5, O −2
c) H2C2O4; H 1+, C 3+, O 2−
d) H2O2; H +1, O −1
e) Ca(ClO)2; Ca +2, Cl +2, O −2
f) Zn(NH3)6Cl2; Zn 2+, N 3−, H 1+, Cl 1−
(Cl is +1 in this compound. Ca: 1 × (+2) = +2. O: 2 × (−2) = −4.
Charge on the compound = 0; therefore, (+2) + (−4) + 2 × Cl = 0. The charge on Cl is 1+.)
2- A voltaic cell is constructed. One electrode compartment consists of a zinc strip placed in an aqueous solution of Zn(NO3)2, and the other has a nickel strip placed in an aqueous solution of NiCl2.
E°red(Zn2+/Zn) = −0.76 V
E°red(Ni2+/Ni) = −0.28 V
Which one of the following occurs at the anode and which at the cathode?
(i) Ni2+(aq) + 2e-  Ni(s)
(ii) Zn2+(aq) + 2e-  Zn(s)
(iii) Ni(s)  Ni2+(aq) + 2e(iv) Zn(s)  Zn2+(aq) + 2ea) i anode, iv cathode
b) i cathode, iii anode
c) i anode, iii cathode
d) i anode, ii cathode
e) i cathode, iv anode
(Zinc is more easily oxidized than nickel.)
3- Complete and balance the following equation. (All stoichiometric coefficients must be integers.)
MnO4−(aq) + Cl−(aq)  Mn2+(aq) + Cl2(g) (acidic solution)
How many hydrogen ions are needed and on which side of the equation must they appear?
a) 16, on the left
b) 16, on the right
c) 8, on the left
d) 8, on the right
(There are also two permanganate ions and 10 chloride ions on the reactant side of the balanced equation.) 4- Complete and balance the following equation. (All stoichiometric coefficients must be integers.)
Pb(OH)42−(aq) + ClO−(aq)  PbO2(s) + Cl−(aq) (basic solution)
How many hydroxide ions, OH−(aq), are needed and on which side of the equation must they appear?
a) 2, on the left
b) 4, on the right
c) 4, on the left
d) 5, on the right
e) 2, on the right
(There is one water molecule produced per formula unit of Pb(OH)42− as well.)

5- Write the net ionic equation for the redox reaction that occurs in the voltaic cell with a Cu(s)/Cu2+(aq) electrode and a Ag(s)/Ag+(aq) electrode.
E°red(Cu2+/Cu) = +0.34 V
E°red(Ag+/Ag) = +0.80 V
a) Cu(s) + Cu2+(aq)  2Ag+(aq) + Ag(s)
b) 2Ag+(aq) + Ag(s)  Cu(s) + Cu2+(aq)
c) Cu(s) + 2Ag+(aq)  Cu2+(aq) + 2Ag(s)
d) Cu2+(aq) + 2Ag(s)  Cu(s) + 2Ag+(aq)
(Two electrons are transferred in this redox reaction. You correctly recognized that the silver cation has a more positive standard reduction potential than the copper (II) cation.)
6- How many electrons are transferred in the following reaction?
2Al(s) + 6H+(aq)  2Al3+(aq) + 3H2(g)
a) 1
b) 5
c) 6
d) 2
e) 3
(Three electrons are transferred from each aluminum, and there are a total of two moles of Al(s).)
7- For a reaction that is spontaneous at standard conditions
a) G° > 0, E° < 0, and K > 1
b) G° = 0, E° = 0, and K > 1
c) G° > 0, E° < 0, and K < 1
d) G° < 0, E° > 0, and K > 1
e) G° < 0, E° = 0, and K >> 1
(The equilibrium favors the products of the reaction and the voltage for the electrochemical cell is positive.)
8- What is the cathode material in an alkaline battery?
a) Zinc
b) graphite and MnO2(s) mixture
c) Lead
d) Magnesium
(A mixture of graphite and MnO2(s) is used in alkaline batteries.)
9- The value of E° for the following reaction is 0.63 V. What is the value of E for this reaction when the concentration of Zn2+(aq) is 0.00020 M and the concentration of Pb2+(aq) is 1.0 M?
Pb2+(aq) + Zn(s)  Zn2+(aq) + Pb(s)
a) 0.85 V
b) −0.11 V
c) 0.74 V
d) 0.52 V
(E = 0.63 – 0.0592/2 * log[0.00020]/[1.0])= 0.74 V)

10- The following cell could be used to provide a measure of the pH in the cathode compartment.
Zn(s) + 2H+(aq)  Zn2+(aq) + H2(g)
Calculate the pH of the cathode compartment solution if the cell potential is measured to be 0.720 V when
[Zn2+] = 0.10 M and PH2 = 1.0 atm. The standard potential is 0.763 V.
a) −1.23
b) 1.2
c) 2.5
d) 0.059
e) 0.73
(The concentration of hydrogen ions is 0.06 M.)
11- An external potential of 2.2 V is applied to a cell undergoing a two-electron redox reaction. If 2.0 moles of electrons are used in the reaction, what is the minimum amount of energy that must be supplied to the cell?
a) 4.2 × 105 J
b) 2.2 J
c) 2.1 × 105 J
12- Cu metal displaces Ag+(aq) from an aqueous solution. Which of the following is correct?
a) Cu2+ is a better oxidizing agent than Ag+.
b) Ag is a better reducing agent than Cu.
c) Ag+ is easier to reduce than Cu2+.
d) Ag is easier to oxidize than Cu.
(The standard reduction potential for Ag+/Ag is + 0.799 V, and Cu2+/Cu is + 0.337 V.)
13- A voltaic cell that uses the reaction
Tl+3(aq) + 2Cr+2(aq)  Tl+(aq) + 2Cr+3(aq) has a measured standard cell potential of 1.19 V. What is E°red for the reduction of Tl3+(aq) to Tl+(aq)? The
E°red for Cr3+/Cr2+ is −0.41 V.
a) −0.78 V
b) 0.78 V
c) 1.60 V
d) 1.19 V
e) 0.59 V
(1.19 V + (− 0.41 V) = 0.78 V. The chromium half-reaction occurs at the anode because it is an oxidation.) 14- Suppose that an alkaline dry cell was manufactured using cadmium metal rather than zinc. What effect would this have on the cell potential? The standard reduction potential for Cd2+(aq) to Cd(s) is −0.403 V while the standard reduction potential for Zn2+(aq) to Zn(s) is −0.763 V.
a) The cell emf would decrease by 0.360 V.
b) no change
c) The voltage would increase by 0.360 V.
d) It would not work.
(The cadmium standard reduction potential is −0.403 V whereas the zinc standard reduction potential of −0.763 V.)

15- Using standard reduction potentials, predict which of the following metal(s) would react with a solution of
Sn(II) ions: iron, copper, zinc, sodium.
E°red Zn2+(aq)/Zn(s) =−0.763 V.
E°redCu2+(aq)/Cu(s) =+0.34 V.
E°redFe2+(aq)/Fe(s) =−0.440 V.
E°redNa1+(aq)/Na(s) =−2.71 V.
E°redSn2+(aq)/Sn(s) =−0.136 V.
a) iron only
b) zinc only
c) copper only
d) iron, zinc, and sodium.
e) iron and zinc only
(These all have lower reduction potentials and would react to reduce the tin.)
16- Given the following standard reduction potentials,
E°redAl3+/Al = −1.66 V.
E°redCr3+/Cr = −0.74 V calculate the equilibrium constant for the following reaction at 298 K:
Al(s) + Cr3+(aq)  Al3+(aq) + Cr(s)
a) 4.0 × 1046
b) 2.4 × 10−47
c) 47
d) 3.5 × 1015
(E° = +0.92 V, which is placed into the equation[log K = nE°/0.0592] to obtain the equilibrium constant.) 17- An electrolytic cell is set up for the production of aluminum, which involves the reduction of Al 3+ to Al.
The external source passes a current of 11.2 A through the cell with an emf of 6.00 V. How long does it take for the cell to produce 454 g of aluminum metal?
a) 226 hr
b) 1.35 × 103 hr
c) 40.3 hr
d) 121 hr
e) 3.26 × 103 hr
(There are 16.8 moles of Al(s) produced, 50.5 moles of electrons transferred with a total charge of 4.87 × 106 C.)
18- What mass in grams of copper metal will be deposited from a solution of Cu2+ by a current of 2.50 A in
2.00 hr?
a) 5.93 g
b) 0.187 g
c) 11.9 g
d) 23.7 g
(18,000 C of charge is produced, which is equal to 0.187 moles of electrons. 2 moles of electrons are required for each mole of copper ions reduced.)

19- What is G° at 298 K for the reaction Hg(l) + 2Fe3+(aq)  Hg2+(aq) + 2Fe2+(aq)?
The E°red for Fe3+/Fe2+ = +0.771 V, while the E°red for Hg2+/Hg = +0.854 V.
a) −16 kJ
b) +314 kJ
c) 16 kJ
d) −0.083 J
e) −314 kJ
(This reaction is not spontaneous under standard state conditions for all components in the reaction.)