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Galvanic and Electrolytic Cells

lectrochemistry and electrochemical cells were introduced in our unit on diabetes monitoring. You can review that

ere. Electrochemical cells are in a circuit that allows the transfer of electrons through it. The cell includes:

an anode, the electrode where oxidation takes place a cathode, the electrode where reduction takes place an electrolyte, to allow conduction of ions within the solution in each half cell a salt bridge orsemipermeable membrane, to allow conduction of ions between half cells the external circuit that connects the two electrodes includes wires, a load, and meters

Galvanic Cells

n electrochemical cell that releases energy is called a galvanic cell. The electrochemical reaction has a negative vaf the Gibbs free energy and a positive cell potential difference.

elow is a galvanic cell in which the reaction between A+

and B is exothermic, with a G of -10 kJ/mol under

andard conditions and a value ofE0

of 0.10 V.

Hydrogen Electrode and Half Reactions

We havetablesthat show reduction half reactions. How are these obtained?

n electrochemical reaction in an electrochemical cell requires both a reduction half reaction and an oxidation half

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eaction. A voltmeter in the circuit shows the total cell voltage (Etot = Ered + Eox). We can determine a half cell potenty measuring the voltage of a cell that includes that half cell along with a standard half cell of known potential.

he accepted standard electrode is the hydrogen electrode. This consists of a half cell with an unreactive metal electr

2(g) at 1 atmosphere pressure, and a 1 M aqueous solution of acid.

n the reduction mode:

2H+

+ 2e-

H2

Ered = 0.00 V

n the oxidation mode:

H2 2H+

+ 2e-

Ered = 0.00 V

ou can see here that the potential of a half cell in which Cu+2

is reduced to Cu is 0.34 V.

reductionCu

+2+ 2e

-Cu

Ered = x

oxidationH2 2H

++ 2e

-

Eox = 0.00 V

netCu

+2

+ H2 Cu + 2H+

Etot = 0.34 V

lectrolytic Cells

n electrolytic cell has an endothermic chemical reaction. The reaction is not spontaneous so a power source is requi

nergy is stored in an electrochemical cell. That energy can be released when the cell is allowed to run in a galvanic

mode.

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bove right is the galvanic cell that we saw previously. The molecules A and B+

are produced and energy is released

he electrode on the left side of the cell is the cathode because A+

is reduced to A at that electrode.

bove left is the same cell in an electrolytic mode. An energy source causes the reaction to go in the direction oppos

the spontaneous direction. Now, A+

and B are produced. The electrode on the right is the cathode because B+

is

educed to B there.

Professor Patricia Shapley, University of Illinois, 2012