1. Cell Potentials and Good Batteries

2. OUTCOME QUESTION(S): C12-6-07: Vocabulary & Concepts
Calculate standard cell potentials and predict spontaneity of reactions.
Vocabulary & Concepts
Electrode potential

3. Spontaneous reactions occur without added energy
2 Ag+(aq) + Cu(s) →
2 Ag(s) + Cu2+(aq)
exothermic
Ag(s) + Cu2+(aq) →
no reaction
*endothermic
Ag+ ions CAN oxidize Cu metal
Cu2+ could NOT oxidize Ag metal
Spontaneous reactions can make good batteries and release energy

4. Cell potential (Eocell)
measured in voltage
sum of individual electrode potentials
Remember voltage measures energy per group of electrons
Electrode potentials (Eo)
voltage measured by placing a substance in a cell to compete against hydrogen
2H+(aq) + 2e– ↔ H2(g)
Hydrogen was chosen as the “standard” to compare against

5. X – oxidized Hydrogen – reduced
(+) Eo potential:
Element was reduced and is stronger than H
X – reduced Hydrogen – oxidized
H2(g) / H+(aq) // Cu2+(aq) / Cu(s)
Eo = V
(-) Eo potential:
Element was oxidized and is weaker than H
X – oxidized Hydrogen – reduced
Zn(s) / Zn2+(aq) // H+(aq) / H2(g)
Eo = V

6. Reduction Potential Chart
NO3¯ + 4 H+ + 3e¯  NO(g) + 2 H2O
+0.96
Hg2+ + 2e¯  Hg(l)
+0.85
Ag+ + e¯  Ag(s)
+0.80
1/2 Hg22+ + e¯  Hg(l)
NO3¯ + 2 H+ + e¯  NO2(g) + H2O
+0.78
Fe3+ + e¯  Fe2+
+0.77
I2(s) + 2e¯  2 I¯
+0.53
Cu+ + e¯  Cu(s)
+0.52
Cu2+ + 2e¯  Cu(s)
+0.34
SO42¯ + 4 H+ + 2e¯  SO2(g)+ 2 H2O
+0.17
Sn4+ + 2e¯ Sn2+
+0.15
S + 2 H+ + 2e¯  H2S(g)
+0.14
2 H+ + 2e¯  H2(g)
0.00
Fe3+ + 3e¯  Fe(s)
–0.04
Pb2+ + 2e¯  Pb(s)
–0.13
Sn2+ + 2e¯  Sn(s)
–0.14
Ni2+ + 2e¯  Ni(s)
–0.25
Co2+ + 2e¯  Co(s)
–0.28
Cd2+ + 2e¯  Cd(s)
–0.40
Se + 2 H+ + 2e¯  H2Se(g)
Fe2+ + 2e¯  Fe(s)
–0.44
Cr2+ + 2e¯  Cr(s)
–0.56
Ag2S + 2e¯  2 Ag(s) + S2¯
–0.69
Cr3+ + 3e¯  Cr(s)
–0.74
(+) means a good “potential” to be reduced – substances likely to take electrons
(-) means a bad “potential” to be reduced – substances likely to give electrons

7. Eocell = Eoox + Eored Cell potential (Eocell)
sum of electrode potentials of each half-cell
Eocell = Eoox + Eored
Your table lists electrode reduction potentials
Oxidation potentials are the reverse – we need to switch the sign
If the sum is…
(+) Eocell – spontaneous reaction
(-) Eocell – non - spontaneous reaction

8. Eocell = Eoox + Eored = + 0.46 - 0.34 + 0.80
What is the cell potential for a silver-copper cell?
Ag+(aq) + 1e– → Ag(s)
E° = V
red
Cu2+(aq) + 2e– →
Cu(s)
E° = V ox + –
The substance with the lowest reduction potential will be oxidized  reverse the reaction and switch the sign on the potential
Eocell = Eoox + Eored =
+ 0.46
spontaneous

9. For a cell of zinc and gold metal as electrodes:
a) What is the cathode and what is the anode? b) What is the cell potential?
c) What is the net reaction? d) What is the line notation for the cell?

10. + 2.26 [ ] ×2 Au3+(aq) + 3e– → Au(s) E° = +1.50 V red [ ] ×3
[ ] ×2
Au3+(aq) + 3e– → Au(s)
E° = V
red
[ ] ×3  
Zn2+(aq) + 2e– →
Zn(s)
E° = V ox + -
Eocell =
+ 2.26
To write the net reaction you will need to balance the electrons lost and gained
Do not multiple the potentials
2 Au3+(aq) + 3 Zn(s) → 2 Au(s) + 3 Zn2+(aq)
Zn(s) / Zn2+(aq) // Au3+(aq) / Au(s)
(oxidized) (reduced)

11. 2 Ag+(aq) + Cu(s) → 2 Ag(s) + Cu2+(aq) Ag(s) + Cu2+(aq) → no reaction
Eoc = +0.46
no reaction
Eoc =
You should be able to predict spontaneity by doing the math for the cell potential
Hg2+ + 2e¯  Hg(l)
+0.85
Ag+ + e¯  Ag(s)
+0.80
1/2 Hg22+ + e¯  Hg(l)
NO3¯ + 2 H+ + e¯  NO2(g) + H2O
+0.78
Fe3+ + e¯  Fe2+
+0.77
I2(s) + 2e¯  2 I¯
+0.53
Cu+ + e¯  Cu(s)
+0.52
Cu2+ + 2e¯  Cu(s)
+0.34
SO42¯ + 4 H+ + 2e¯  SO2(g)+ 2 H2O
+0.17
Sn4+ + 2e¯ Sn2+
+0.15
S + 2 H+ + 2e¯  H2S(g)
+0.14
2 H+ + 2e¯  H2(g)
0.00
Fe3+ + 3e¯  Fe(s)
–0.04
Pb2+ + 2e¯  Pb(s)
–0.13
Sn2+ + 2e¯  Sn(s)
–0.14

12. Will tin react in hydrochloric acid?
spontaneous
Sn(s) + H+(aq) → ?
Eoc = +0.14
Sn(s) + H+(aq) → Sn2+(aq) + H2(g) 2
The chlorine ion (Cl-) in the acid is too electronegative to react – it is not part of this reaction
Hg2+ + 2e¯  Hg(l)
+0.85
Ag+ + e¯  Ag(s)
+0.80
1/2 Hg22+ + e¯  Hg(l)
NO3¯ + 2 H+ + e¯  NO2(g) + H2O
+0.78
Fe3+ + e¯  Fe2+
+0.77
I2(s) + 2e¯  2 I¯
+0.53
Cu+ + e¯  Cu(s)
+0.52
Cu2+ + 2e¯  Cu(s)
+0.34
SO42¯ + 4 H+ + 2e¯  SO2(g)+ 2 H2O
+0.17
Sn4+ + 2e¯ Sn2+
+0.15
S + 2 H+ + 2e¯  H2S(g)
+0.14
2 H+ + 2e¯  H2(g)
0.00
Fe3+ + 3e¯  Fe(s)
–0.04
Pb2+ + 2e¯  Pb(s)
–0.13
Sn2+ + 2e¯  Sn(s)
–0.14

13. CAN YOU / HAVE YOU? C12-6-07: Vocabulary & Concepts
Calculate standard cell potentials and predict spontaneity of reactions.
Vocabulary & Concepts
Electrode potential