Types of Electrochemical Cells Electrolytic Cells: electrical energy from an external source causes...
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Transcript of Types of Electrochemical Cells Electrolytic Cells: electrical energy from an external source causes...
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