Chem 1B
Dr. White
1
Worksheet 19 Key Corrosion, Concentration and Electrolytic Cells
1. A nickel concentration cell was prepared with the following concentrations: 1.00 M
Ni2+(aq) and 1.00 ×10-3 M Ni2+(aq). Which is the cathode and which is the anode solution?
What is the cell potential at 298K?
cathode: 1.00M solution anode: 0.00100M solution
E cell
€
0.0257 1.00x10 −3
=0−
ln
= 0.0888V
2
1.00
2. If the two metals Fe and Ni are in contact under such conditions that corrosion could
occur, which metal will act as the anode and which as the cathode? Show the net reaction
that would take place in this case.
Ni is higher on Table 21.2 than Fe, or Ni+2 has a higher reduction
potential than Fe+2 so Ni is the inactive cathode and Fe the active
anode. So Fe oxidizes, Ni just moves electrons.
Fe (s)
Fe+2 (aq) + 2e-
Oxidation
Conditions for corrosion include presence of oxygen (O2 gas), this will be
reduced to water in the presence of H+.
O2 (g) + 4H+ (aq) + 4e-
2H2O
Reduction
3. If the two metals Fe and Mn are in contact under such conditions that corrosion could
occur, which metal will act as the anode and which as the cathode? Show the net reaction
that would take place in this case.
Same idea here as in last problem, but now the Fe+2 has the higher
reduction potential so Fe will be the inactive cathode, Mn (s) will be
the anode
Mn (s)
2e- + Mn+2 (aq)
O2 (g) + 4H+ (aq) + 4e2 Mn (s) + O2 (g) + 4H+ (aq)
2H2O
Reduction
2H2O (l) + 2 Mn+2 (aq)
4. What will happen to iron plumbing connected directly to copper plumbing without any
electrical insulation between the two?
The iron plumbing will preferentially corrode or oxidize, as it has a more negative
reduction potential than copper.
5. During the reconstruction of the Statue of Liberty, Teflon spacers were placed between
the iron skeleton and the copper plates that cover the statue. What purpose do these
spacers serve?
Chem 1B
Dr. White
2
Teflon acts acts to keep iron and copper from making electrical contact. If
they were in contact, copper , being the less active metal, will act as an
inactive cathode and increase the likelihood of iron oxidizing
6. Magnesium metal is produced by electrolysis of molten magnesium chloride using inert
electrodes.
a. Sketch the cell, label the anode and the cathode, show the direction of electron and
ion flow.
b. Write the balanced equation for the anode, cathode and overall cell reactions.
a) Could do this all in one container, don’t need separate half-cells
e-
Battery
- Cathode
+ Anode
B(s)
A(s)
Cl2
(g)
b) Anode Reaction: 2 Cl- (l)
Cathode Reaction: Mg+2 (l) + 2eNet Reaction: 2 Cl-(l) + Mg+2 (l)
Cl-
Mg+2
Mg
(s)
Cl2 (g) + 2eMg (l)
Mg (l) + Cl2 (g)
7. Predict the anode, cathode and overall cell reactions when a molten mixture of LiF and
CaCl2 is electrolyzed.
Both Li+ and Ca+2 could be reduced, I would guess Li metal has higher EA
(it’s smaller) so Li+ should want to be reduced more –
Li+ (aq) + eLi (s)
this will occur at cathode
F- and Cl- could both be oxidized, Cl is lower on PT than F so it has a lower
IE, so Cl- gives up electrons easier -2Cl- (aq)
Cl2 (g) + 2e-
this will occur at anode
Chem 1B
Dr. White
3
8. How many grams of silver can be obtained when an aqueous silver nitrate solution is
electrolyzed for 20.0 min with a constant current of 2.40 A?
⎛ 1 mole e- ⎞⎛ 1 mole Ag ⎞⎛ 108 g Ag ⎞
⎛1 C/s ⎞⎛ 60 s ⎞
⎟⎟⎜
⎜
⎟⎟ = 3.22 g Ag
2.40 A ⎜
⎟⎜
⎟(20 min)⎜⎜
4
- ⎟⎜
⎝ 1 A ⎠⎝ 1 min ⎠
⎝ 9.65x10 C ⎠⎝ 1 mole e ⎠⎝ 1 mole Ag ⎠
9. How many hours are required to produce 1.00 x 103 kg of sodium by electrolysis of molten
NaCl with a constant current of 3.00x104 A? How many liters of Cl2 at STP will be obtained
as a by-product?
2Na+ (l) + 2 e- 2 Na (l)
2 Cl- (l)
Cl2 (g) + 2e⎛ 1 mole Na ⎞⎛ 1 mole e- ⎞⎛ 9.65x10 4 C ⎞⎛
⎞⎛ 1 hr ⎞
1 sec
6
1.00 x 10 g Na ⎜
⎜
⎟⎜
⎟ = 38.8 hr
⎜
⎟
⎜
⎟
⎟
⎝ 22.99 g Na ⎠⎝1 mole Na ⎠⎝ 1 mole e- ⎠⎝ 3.00 x 10 4 C ⎠⎝ 3600 sec ⎠
€
⎛ 1 mole Na ⎞⎛ 1 mole Cl2 ⎞
⎟⎟⎜
1.00 x 106 g Na ⎜⎜
⎟ = 2.17 x 10 4 mol Cl2
⎝
⎝ 23 g Na ⎠ 2 mole Na ⎠
nRT
2.17 x 10 4 mol Cl 2 (0.0821 L•atm/mol•K)(298 K)
V=
=
= 5.32 x 105 L Cl 2
P
1 atm
10. What constant current would be required to produce 4.9 g of Cr from molten CrO3 in one
hour?
Cr+6 (l) + 6 eCr (l)
⎛1 mole Cr ⎞⎛ 6 mole e- ⎞⎛ 9.6485x10 4 C ⎞⎛ 1 ⎞⎛ 1 hr ⎞⎛ 1 A ⎞
4.9 g Cr⎜
⎟⎜
⎟⎜
⎟ =
⎟⎜
⎟⎜
⎟⎜
⎝ 52.0 g Cr ⎠⎝1 mole Cr ⎠⎝ 1 mole e- ⎠⎝1 hr ⎠⎝ 3600 s ⎠⎝1 C/s ⎠
15 A if it's exactly 1 hour; 20 A if it's ~ 1hr (1 sig fig)
€