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Gatewaylectrochemistry: Voltaic Cells and Voltage

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Table of Contents

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Ox amp; Red


Voltaic Cells

Cell Voltage

Calc. Potentials


Electrolytic Cells

Goal: to understand how potentials are defined and generated in a voltaic cell

Working Definitions:

'click' here for another definition. A coulomb is the charge associated with the movement of 6.24 x 1018 electrons past a given point in one second.

'click' here for another definition. A volt is the unit of electrical potential and equals one joule per coulomb.

'click' here for another definition. The standard hydrogen electrode (SHE) is the electrode customarily chosen for comparing electrochemical cell potentials. The voltage between this electrode and its solution is arbitrarily defined as exactly zero volts.

Moving electrons can operate a load. Redox reactions are all about moving electrons. Voltaic cells, a type of electrochemical cell, take advantage of these moving electrons by allowing them to flow through an external circuit.

**What causes these electrons to flow and how much voltage is generated when they do?**

To begin to answer these important questions, let's consider a single zinc electrode immersed in a 1.0 M aqueous solution of zinc nitrate. An equilibrium is established with some zinc atoms shedding their valence electrons and diffusing into the solution. Due the movement of those electrons, a potential difference is formed between the solution and the zinc electrode. Alas, having no where to go, the electrons recombine with the errant ions and reduce them to zinc atoms. This equilibrium expression is shown below.

Zn(electrode)  Zn2+(solution)   +   2 e(electrode)

Will increasing the concentration of zinc nitrate in the beaker affect the potential difference? (hint: Le Chatelier may have something to say about this...)

Yes, it will cause the potential difference to increase Yes, it will cause the potential difference to decrease No, it will not affect the potential difference

Increasing the concentration of zinc ions will shift the equilibrium left, suppressing ionization. How will this affect the charge separation and potential difference?

Good! Increasing the concentration of zinc ions will shift the equilibrium left, suppressing ionization and charge separation; this will decrease the potential difference between the electrode and the solution.

Now consider a silver electrode immersed in a 1.0 M aqueous solution of silver nitrate; here too, an equilibrium is established where a few of the silver atoms shed their single valence electron and go into solution as ions. This leaves the electrode slightly negative and the solution slightly positive and likewise a potential is created across the electrode:

Ag(electrode)  Ag+(solution)   +   e(electrode)

sour power!Note: At home we commonly make use of arbitrary standards. For example, we may define how "sour" something is by comparing it to the taste of a lemon. When we are comparing the "taste" of potentials generated on a single electrode, our lemon is the standard hydrogen electrode.

Two different electrodes in two different solutions generating two different potentials. Which potential is greater? There is no sure way for measuring the potential of a single electrode. Instead, we can measure the potential differences of each electrode relative to a third whose electrical potential has been arbitrarily defined. The electrode customarily used for this purpose is the standard hydrogen electrode (SHE) and is assigned a potential of exactly 0 V (zero volts). The equilibrium that is established on this single electrode involves hydrogen gas and is represented by the following equation:

H2 (Pt electrode)  2H+(solution)   +   2 e(electrode)

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