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Science students encounter several common parameters for which absolute values are not, and in some cases cannot be, determined. These parameters include the Fahrenheit and Celsius temperature scales, potential energies, electrode potentials, and formation and reaction enthalpies. Handling these parameters is usually approached in the same way: a reference state is arbitrarily chosen and assigned some fixed and arbitrary value, which is usually, but not always, zero. Values of the parameter are then determined by measuring differences between a chosen state and the reference state.
Our experience has been that students generally have a limited understanding of this approach. Any doubts that may arise about the arbitrary selection of reference states and zero values are soon dispelled when they find that the unquestioned use of the approach leads to favorable grades. This lack of insight on the part of students is rarely apparent to instructors, possibly because they invariably question students only on the applications of such parameters. Instructors who doubt the validity of these assertions should try asking their students questions of the following type: "In principle, could any electrode other than the hydrogen electrode have been chosen as the standard reference electrode? Explain your answer."
"In principle, could the standard hydrogen electrode have been assigned any value other than zero volts? Explain your answer." "Imagine a table of E o values based on a standard Ag/Ag+ electrode with an arbitrary E value of 100 V. What would be the emf of the standard Daniell Zn/Cu cell on this scale? Explain your answer."
If we can, just once, give our students a clear understanding of the validity of this reference-state approach, it will stand them in good stead when it is used in subsequent sections of the syllabus. We find the following analogy to be particularly useful and we use it when introducing students to the electromotive series of electrode potentials.
Instead of a voltmeter, we use an "age meter". This imaginary instrument consists of a large, colorful cardboard cutout that looks like a voltmeter except that the units have been changed from volts to years. The scale has a central zero with negative values on the left and positive values on the right. The age meter has a large movable pointer and comes...