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

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 – NOTa voltaic cell 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 ---> ELECTRIC CURRENT To obtain a useful current, we separate the oxidizing and reducing agents so that electron transfer occurs thru an external wire. To obtain a useful current, we separate the oxidizing and reducing agents so that electron transfer occurs thru an external wire. This is accomplished in a GALVANIC or VOLTAIC cell. This is accomplished in a GALVANIC or VOLTAIC cell. A group of such cells is called a battery. A group of such cells is called a battery.

Voltaic Cell links 105/galvanic/galvanic1.htm 105/galvanic/galvanic1.htm A A

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 – Cu Cu 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. H 2 gas is bubbled at 1 atm over the electrode Assigned a potential of V 2 H + (aq, 1 M) + 2e- H 2 (g, 1 atm) E° = 0.000V H 2 (g, 1 atm 2 H + (aq, 1 M) + 2e- E° = 0.000V H 2 (g, 1 atm 2 H + (aq, 1 M) + 2e- E° = 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 = V Therefore, reduction potential = 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 e - Cu E° = 0.34 V Zn Zn e - E° = 0.76 V Therefore, 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 K eq  G° = -nF E°  G =  G° + RT ln Q  G° = -RT ln K eq

“Triangle of Truth” E° cell K eq G°G° nFE° = RT ln K eq  G° = -nFE°  G° = -RT ln K eq

Calculate 3 Sn Cr 3 Sn Cr +3 Calculate  G° and K eq