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Standard Electrode Potentials

Before we discuss standard electrode potential, we will talk about electromotive force (emf). The electromotive force of a cell is the potential difference between the two electrodes. This can be measured using a voltmeter. The maximum voltage of a cell can be calculated using experimentally determined values called standard electrode potentials. By convention, the standard electrode potentials are usually represented in terms of reduction half-reactions. The standard electrode potential values are set under ideal and standard-state conditions (1atm pressure and 25oC temperature). From the MCAT point of view, you can assume that the conditions are standard, unless stated otherwise. Table 12-1 shows a list of standard electrode potentials (in aqueous solution) at 25oC.
The standard electrode potential at the above mentioned standard state conditions is denoted by E o. For the MCAT, the values of the standard electrode (reduction) potentials will be given to you if you are required to solve such a question. Do not try to memorize those values. The standard electrode potentials are based on an arbitration with reference to standard hydrogen electrode. The standard hydrogen electrode potential is considered to be 0 volt.

image\Ch 12 table 21-1.png

Finding the emf of a Cell

The emf of a cell can be calculated from the standard electrode potentials of the half-reactions. In order to find the emf, we have to look at the two half-reactions involved in the reaction. Then, set up the two half-reactions so that when they are added we will get the net reaction. Once we have set the equations properly and assigned the proper potentials to those half-reactions, we can add the standard electrode potentials. A common mistake that students make is that they forget the fact that the standard electrode potentials are given in terms of reduction reactions. Redox reactions involve both oxidation and reduction. If one half-reaction is reduction, the other should be oxidation. So we must be careful about the signs of the half-reaction potentials, before we add the two half-reaction potentials to get the emf value. Do the next example.

Example 12-1

  Calculate the emf of the cell, based on the following net reaction.


image\25293 ch 12.png

Standard Electrode Potentials of the half-cells


image\25303 ch 12.png



First, we have to write the half-reactions as indicated below.


image\25313 ch 12.png

From Table 12-1, we can take the standard electrode potential values. The cell containing the copper electrode has a standard potential value of 0.34 V. For the other half-cell, the reaction is oxidation. Since the value given in the table is in terms of reduction half-reactions, we have to reverse the sign of the standard electrode potential given. The correct value for the oxidation half-cell is +0.76 V instead of –0.76 V. Now, you can add the two values to get the emf of the whole setup.


emf = 0.34 + 0.76 = 1.10 V


The answer is 1.10 V.

Table: Electrolytic cell & Galvanic cell - A comparison

image\Ch 12 table 12-2 4.png

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