Presentation on theme: "Types of Electrochemical Cells Electrolytic Cells: electrical energy from an external source causes a nonspontaneous reaction to occur Voltaic Cells (Galvanic."— Presentation transcript:
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
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
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 – 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 http://www.chembio.uoguelph.ca/educmat/chm19 105/galvanic/galvanic1.htm http://www.chembio.uoguelph.ca/educmat/chm19 105/galvanic/galvanic1.htm http://www.youtube.com/watch?v=0oSqPDD2rM A http://www.youtube.com/watch?v=0oSqPDD2rM A
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 0.000 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
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 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 +4 + 2 Cr 3 Sn +2 + 2 Cr +3 Calculate G° and K eq