Types of Electrochemical Cells n n Electrolytic Cells

Types of Electrochemical Cells n n 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 n 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 n n n 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 n http: //ed. ted. com/lessons/electric-vocabulary

Voltaic or Galvanic Cells n n n 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 n n n 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 n With time, Cu plates onto the Zn metal strip, and Zn strip disappears • Electrons are transferred from Zn to Cu 2+, but there is no useful electric current.

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

Voltaic Cell links n n http: //www. chembio. uoguelph. ca/educmat/chm 19 105/galvanic 1. htm http: //www. youtube. com/watch? v=0 o. Sq. PDD 2 r. M A

Cu - Ag Cell

Sn – Cu cell

Summary of Zn, Cu, Ag n Zn – Cu Cu electrode – cathode Cu+2 is more easily reduced than Zn+2 Zn is a stronger reducing agent than Cu n Ag – Cu Cu electrode – anode Ag+ is more easily reduced than Cu+2 Cu is a stronger reducing agent than Ag n Cathode – Anode are dictated by species present

Summary of Zn, Cu, Ag n Strength as oxidizing agents Zn+2 < Cu+2 < Ag+ n Strength as reducing agents Zn > Cu > Ag

Standard Electrode Potentials n n 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 n n n “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 n n 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) + 2 e- <----> H 2(g, 1 atm) E° = 0. 000 V H 2(g, 1 atm <----> 2 H+(aq, 1 M) + 2 e- E° = 0. 000 V

Cu – SHE Cell

Zn – SHE Cell

Zn – Cu Cell

Electromotive Series n Can develop series of standard electrode potentials When involve metals in contact with their ions – electromotive series Zn: Std. oxidation potential = +0. 763 V n Therefore, reduction potential = -0. 763 V n n

Electromotive Series n n n 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 n 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 n 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 DG° and Keq n DG° = -n. F E° n DG = DG° + RT ln Q n DG° = -RT ln Keq

“Triangle of Truth” DG° = -RT ln Keq DG° = -n. FE° E°cell n. FE° = RT ln Keq

Calculate 3 Sn+4 + 2 Cr Calculate DG° and Keq 3 Sn+2 + 2 Cr+3