lectrochemistry: Voltaic Cells and Voltage

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

Working Definitions:

A coulomb is the charge associated with the movement of 6.24 x 10¹⁸ electrons past a given point in one second.

A volt is the unit of electrical potential and equals one joule per coulomb.

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

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)

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:

H_{2 (Pt electrode)}  2H⁺_(solution)   +   2 e^–_(electrode)

Electrochemical Cells and Voltage