Electrode Potentials And Cells

Question 1

What is importance of a salt bridge

Answer 1

Completes circuit
Allows movement of ions

Question 2

What makes a reaction feasible

Answer 2

If EMF>0

Question 3

Conventional cell representation

Answer 3

Oxidation on left
Reduction on right
Species with highest oxidation state in middle
| shows state change
, shows same change
|| is salt bridge

Question 4

Non rechargeable cells

Answer 4

Alkaline batteries, dry cells
Negative electrode: Zn -> Zn2+ + e-
Positive electrode: MnO2 + H2O + e- -> MnO(OH) + OH-
Overall reaction
Zn + 2MnO2 + 2H2O -> Zn2+ + 2MnO(OH)2 +2OH-
Cell leaks after a long time as zinc is used up

Question 5

Nickel cadmium cell

Answer 5

Negative electrode: Cd +2OH- -> Cd(OH)2 + 2e-
Positive electrode: NiO(OH) + H2O +e- -> Ni(OH)2 + OH-
Overall reaction
Cd + 2NiO(OH) + 2H2O -> Cd(OH)2 + 2Ni(OH)2
Reaction reversed when recharging

Question 6

Lead acid cell

Answer 6

Negative electrode: Pb + HSO4- -> PbSO4 + H+ + e-
Positive electrode: PbO2 + 3H+ + HSO4- + Pb -> 2PbSO4 + 2H2O
Overall reaction
PbO2 + 2H+ + 2HSO4- + Pb -> 2PbSO4 + 2H2O
Formation of Pb(ii) dangerous as can build up so cannot be recharged

Question 7

Lithium ion cell

Answer 7

Positive electrode: Li+ + CoO2 + e- -> Li+[CoO2]-
Negative electrode: Li -> Li+ + e-
Overall reaction
Li + CoO2 -> Li[CoO2]
No water as it reacts with lithium

Question 8

Hydrogen fuel cell

Answer 8

Acidic
Negative electrode: H2 -> 2H+ + 2e-
Positive electrode: O2 + 4H+ + 4e- -> 2H2O
Alkaline
Negative electrode: H2 + 2OH- -> 2H2O
Positive electrode: O2 + 2H2O + 4e- -> 4OH-
Porous platinum electrodes used to increasesurface area

Question 9

Ethanol fuel cell

Answer 9

Negative electrode: C2H5OH + 3H2O -> 2CO2 + 12H+ + 12e-
Positive electrode: 12H+ + 3O2 + 12e- -> 6H2O