Standard Reduction Cell Potential (Ecell) •Cell potential (electromotive force, emf) is Potentials the driving force in a galvanic cell that pulls •The measured potential of a voltaic electrons from the oxidation in one cell is affected by changed in compartment to the reduction in the other. concentration of the reactants as the •The volt (V) is the unit of electrical reaction proceeds and by energy losses potential. due to heating of the cell and external Cell Potential •Electrical charge is measured in coulombs circuit. (C). • In order to compare the output of • different cells, the standard cell A volt is 1 joule of work per coulomb of o charge transferred: 1 V = 1 J/C. potential (E cell) is obtained at 298 K, 1 •A voltmeter is a device which measures cell atm for gases, 1 M for solutions, and the pure solid for electrodes. potential.

The Standard Electrode Thermodynamically favored

(Half-Cell) Potential (Ehalf-cell) reactions •As the potential increases in value (more •The Standard Hydrogen Electrode is •A standard electrode potential always refers to positive), the reaction is more likely to occur considered the reference half-cell the half-reaction written as a reduction. (Thermodynamically favored, spontaneouly - o electrode, with a potential equal to •Oxidized form + n e  reduced form E half-cell occurs). o 0.00 V. •E cell must be positive for the cell to produce •If you need the oxidation, you will have to • It is obtained when platinum is electricity. reverse the reaction immersed in 1 M H+(aq), through •A substance will have a thermodynamically •Reversing a reaction changes the sign of the favored or spontaneous reaction with another which H2(g) is bubbled. potential. o substance of a lower E cell. •Although some half-reactions must be o o o •E cell = E reduction + E oxidation manipulated with coefficients, NEVER o MULTIPLY THE E cell BY THE COEFFICIENT!!!

Calculating an Unknown Galvanic Cell Problems Line Notations. Eo from Eo . •The components of the anode compartment are •Consider a galvanic cell based on the following half-cell cell written to the left of the cathode compartment. reactions. For each give the balanced half cell •A voltaic cell based on the reaction between •Double vertical lines separate the half-cells and o reactions and calculate E cell. aqueous Br2 and vanadium (III) ions has represents the wire and salt bridge. •Eo = 1.39 V: • Within each half-cell, a single vertical line 2+ 2+ cell •Cu (aq) + Zn(s)  Zn (aq) + Cu(s) represents a phase boundary. A comma • 3+ separates half-cell components in the same •Fe3+ + Cu  Fe2+ + Cu2+ Br2(aq) +2V (aq) + 2H2O(l) (aq) (s) (aq) (aq)  2VO2+(aq) + 4H+(aq) +2Br-(aq) phase. •Half-cell components appear in the same order 2+ 3+ • •Al (s) + Cd (aq)  Cd(s) + Al (aq) as in the half-reaction, while electrodes appear •What is the standard electrode potential for at the extreme left and right of the notation. 2+ 3+ 2+ 3+ the reduction of VO to V ? •Mg(s) Mg (aq) Al (aq) Al(s) 2+ + - 2+ • Fe(s) Fe (aq) H , MnO4 (aq), Mn (aq) Pt(s)

1 Describing a Galvanic Cell. Diagramming Voltaic Cells

•A cell will always be thermodynamically •3. Designation of the anode and cathode. •In one compartment of a voltaic cell favored, or spontaneous, in the direction that •4. The nature of each electrode and the is an acidic solution of Cr(NO3)3, and produces a positive cell potential. ions present in each compartment. A a chromium electrode; in the other, a •A complete description of a galvanic cell chemically inert conductor is required if tin bar dips into a Sn(NO ) solution. always includes four items: none of the substances participating in the 3 2 half-reaction is a conducting solid. A KNO3 salt bridge joins the half- •1. The cell potential (always positive for a cells. galvanic cell) and the balanced cell reaction. •Diagram the cell, show balanced •2. The direction of electron flow, obtained equations, and write the cell notation. by inspecting the half-reactions and using o the direction that gives a positive E cell.

Description of a Galvanic Cell. •Describe completely the galvanic cell based on the following half-reactions under standard conditions: • + - o •Ag + e  Ag E cell = 0.80 V (1) 3+ - 2+ o •Fe + e  Fe E cell = 0.77 V (2) • •In addition, draw the cell and write the line notation.

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