VoltaicCells

Harnessing the power of Redox

Get out your notes +

Chem Data Booklet

Redox Basics• Redox Reactions

happen when – one substance loses

electrons (oxidized) and – one gains electrons

(reduced)• Electrons are flowing Cl2

2Na2Cl-

2Na+

2 Electrons

Half equation #1: 2Na 2Na+ + 2e-

Half equation #2: Cl2 + 2e- 2Cl-

nett: 2Na + Cl2 2Na + 2Cl-

Example Reaction

Zn + Cu2+ Cu + Zn2+

Gains 2 electrons- reduced

Loses 2 electrons- oxidized

Cu2+

Zn

Zn2+

CuZn Zn2+ = +0.76

Cu2+ Cu = +0.34

Overall = +1.10

Harness Electrons• If the reactants are separated

• The electrons cannot travel directly between reactants

• Those electrons can be used to do work

• Note: the electrons will always flow in the direction which gives a +Eo

cell

• Eocell = Eo(red)-Eo(ox)

SO42-

Zn2+

CuZn

Reaction:

Zn + CuSO4 Cu + ZnSO4

Build a Cell 1 (Ch 20, sect 3)

SO42- Cu2+

Electrode-

carries electro

ns, may or

may not be part of

the reactio

n

Reaction:

Zn + Cu2+ Cu + Zn2+

Zn2+

CuZn

Reaction:

Zn + Cu2+ Cu + Zn2+

Electrons!

Build a Cell 2

Cu2+

Zn2+

CuZn

Reaction:

Zn + Cu2+ Cu + Zn2+

Electrons!

Build a Cell 3

Cu2+

Salt BridgeKCl

K+Cl-Cu

Zn

Build up of + ions

Build up of - ions

Terminology• Electrode: conductor which allows electrons to

enter or leave a cell (any conductive but unreactive material, sometimes part of the redox reaction)

– Anode: oxidation electrode, negative (Zn)– Cathode: reduction electrode, positive (Cu)

• Half-cell: half of the electrochemical cell where ox. or red. takes place.

• Electrolyte: salt solution in a half-cell, always contains oxidized and reduced form

Cl-Zn2+

H2Zn

Reaction:

Zn + 2H+ H2 + Zn2+

Electrons!

Other Cells 1

SO42- H+

Pt

Salt bridge

PtPt

Reaction:

2I- + 2Fe3+ I2 + 2Fe2+

Electrons!

Other Cells 2

I- Fe3+

I2Fe2+

e- e-

Salt bridge

PtC

Reaction:

Cl- + MnO4- Cl2 + Mn2+

10Cl- + 2MnO4- + 16H+ 5Cl2 + 2Mn2++ 8H2O

Electrons!

Other Cells 3

Cl-

MnO4-/H+

Cl2 Mn2+

e- e-

Salt bridge

Try it!• Draw a cell schematic to show the reaction:

Cu Cu2+ + 2e-

I2 + 2e- 2I- (platinum electrode)

Cu + I2 Cu2+ + 2I-

• Label the anode, cathode, and direction of electron flow

• Calculate the voltage

PtCu

Cu + I2 Cu2+ + 2I-

Electrons!

Try It! Answer

Salt bridge

Cu2+

I2

Anode- oxidationCathode- reduction

Voltage = -0.34 + 0.54 = 0.20V

Videos• Galvanic cells (2 min)• Animation (24 sec)• Li Batteries (5.5 min)• Potentials (2 min)• Song (3.5 min, This guy is seriously prolific)