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