Types of Electrochemical Cells

Electrolytic Cells: electrical energy from an external source causes a nonspontaneous reaction to occur

Voltaic Cells (Galvanic Cells): spontaneous chemical reactions produce electricity and supply it to an external circuit

Electrical Conduction

Electric current represents charge transfer Charges conducted through:

1. liquid electrolytes

2. metals – metalic conduction Ionic Conduction – conduction of an electric

current through motion of ions in solution

Ionic Conduction

+Migrate

Neg. Electrode

-Migrate

Pos. Electrode

Electrodes

Surfaces upon which oxidation and reduction half reactions occur

May or may not participate in the reaction Inert Electrodes – do not participate

Ex. Pt, C, Pd Reduction at cathode Oxidation at anode

Electrodes

RED CAT

And

AN OX

Ted Talk

http://ed.ted.com/lessons/electric-vocabulary

Voltaic or Galvanic Cells

Spontaneous oxidation – reduction reactions produce electrical energy

Two halves of redox reaction are separated Half cell – contains the oxidized and reduced

forms of an element or other complex species

Voltaic or Galvanic Cells

Salt bridge – completes circuit between the two half cells

Salt bridge is any medium through which ions can flow

Agar + Salt Gelations

1. Allows electrical contact between two solutions

2. Prevents mixing of electrode solutions

3. Maintains electrical neutrality

Redox Reaction

Redox Reaction

Redox reaction – NOTa voltaic cell

Zn metal

Cu2+ ions

Zn metal

Cu2+ ions

With time, Cu plates onto the Zn metal strip, and Zn strip disappears

• Electrons are transferred from Zn to Cu2+, but there is no useful electric current.

CHEMICAL CHANGE --->CHEMICAL CHANGE --->ELECTRIC CURRENTELECTRIC CURRENT To obtain a useful To obtain a useful

current, we separate current, we separate the oxidizing and the oxidizing and reducing agents so that reducing agents so that electron transfer occurs electron transfer occurs thru an external wire.thru an external wire.

This is accomplished in a This is accomplished in a GALVANICGALVANIC or or VOLTAICVOLTAIC cell.cell.

A group of such cells is called A group of such cells is called a a batterybattery..

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Zn

Zn2+ ions

Cu

Cu2+ ions

wire

saltbridge

electrons

Voltaic Cell links

http://www.chembio.uoguelph.ca/educmat/chm19105/galvanic/galvanic1.htm

http://www.youtube.com/watch?v=0oSqPDD2rMA

Cu - Ag Cell

Sn – Cu cell

Summary of Zn, Cu, Ag Zn – Cu

Cu electrode – cathode

Cu+2 is more easily reduced than Zn+2

Zn is a stronger reducing agent than Cu

Ag – CuCu electrode – anode

Ag+ is more easily reduced than Cu+2

Cu is a stronger reducing agent than Ag

Cathode – Anode are dictated by species present

Summary of Zn, Cu, Ag

Strength as oxidizing agents

Zn+2 < Cu+2 < Ag+

Strength as reducing agents

Zn > Cu > Ag

Standard Electrode Potentials

Magnitude of a cell’s potential measures the spontaneity of its redox reaction

Higher cell potentials indicate a greater driving force

Want to separate total cell potentials into individual potentials of the two half reactions

Determine tendencies for redox reactions

Standard Hydrogen Electrode

“Every oxidation needs a reduction”

e- must go somewhere Therefore it is impossible to determine

experimentally the potential of a single electrode Establish an arbitrary standard electrode

Standard Hydrogen Electrode, SHE

Standard Hydrogen Electrode

Metal coated with Pt immersed in a 1.0 M H+

solution. H2 gas is bubbled at 1 atm over the electrode

Assigned a potential of 0.000 V

2 H2 H++(aq, 1 M) + 2e- <----> H(aq, 1 M) + 2e- <----> H22(g, 1 atm) E(g, 1 atm) E° = 0.000V° = 0.000V

HH22(g, 1 atm <----> 2 H(g, 1 atm <----> 2 H++(aq, 1 M) + 2e- E(aq, 1 M) + 2e- E° = 0.000V° = 0.000V

Cu – SHE Cell

Zn – SHE Cell

Zn – Cu Cell

Electromotive Series

Can develop series of standard electrode potentials

When involve metals in contact with their ions – electromotive series

Zn: Std. oxidation potential = +0.763 V

Therefore, reduction potential = -0.763 V

Electromotive Series

International convention is to use reduction half reactions

Indicates tendencies of electrodes to behave as cathodes toward SHE

If E° < 0.0 V, then electrode acts as anode versus SHE

Uses of the Electromotive Series

Predict the spontaneity of redox reactions

Question: Will Cu+2 oxidize Zn to Zn+2 or will Zn+2 oxidize Cu?

Write half reactions and make sure E° is positive.Cu+2 + 2 e- Cu E° = 0.34 V Zn Zn+2 + 2 e- E° = 0.76 VTherefore, Cu+2 will oxidize Zn to Zn+2

Will Cr+3 oxidize Cu to Cu+2 or will Cu+2 oxidize Cr to Cr+3?

Nernst Equation

Use when you do not have standard state conditions

Problem

Calculate E for Fe+3/Fe+2 electrode if the [Fe+2] is 5 times that of [Fe+3].

Problem

Calculate E for a Al – Cu cell in which the temperature is 20.5 °C and the [Cu+2] = 0.25 M and [Al+3] is 0.75 M.

Relationship of E° to G° and Keq

G° = -nF E°

G = G° + RT ln Q

G° = -RT ln Keq

“Triangle of Truth”

E°cell Keq

G°

nFE° = RT ln Keq

G° = -nFE°G° = -RT ln Keq

Calculate

3 Sn+4 + 2 Cr 3 Sn+2 + 2 Cr+3

Calculate G° and Keq