Electrochemical Series Explained: Definition, Standard Electrode Potentials, Applications, and Trends in Redox Reactions

The electrochemical series is also called the “activity series”. It is a method of arranging elements in

• The electrochemical series indicates how electropositive or electronegative an element or ion is compared to the Standard Hydrogen Electrode (SHE). Each element/ion pairing is referred to as a half-cell.
• A more electropositive metal loses electrons more readily than hydrogen in the Standard Hydrogen Electrode (SHE), while a more electronegative element gains electrons more easily. In general, electronegative elements tend to accept electrons from electropositive ones. Thus, the electrochemical series can be viewed as a measure of an element’s electronegative character.
• Hydrogen, by definition, has an electrode potential of 0.00 V and serves as the reference in the Standard Hydrogen Electrode (SHE). All other electrode potentials are measured relative to it. Half-cells with highly positive electrode potentials are strong oxidizing agents and appear high in the electrochemical series. In contrast, half-cells with negative electrode potentials act as reducing agents—the greater the reducing strength, the more negative the potential.

In general, metals are electropositive, while non-metals are electronegative. The most reactive metals are found at the bottom of the electrochemical series, whereas the most reactive non-metals appear at the top. Reactivity is lowest near the middle of the series. Metals higher in the series can be reduced by those lower down, while non-metals higher in the series can oxidize both metals and non-metals that are lower in the series.

Applications of Electrochemical Series

•          Calculation of Cell EMF:

Each half-cell undergoes a reaction—one involving oxidation and the other reduction—each with its own oxidizing or reducing potential. The cell EMF (electromotive force) is the sum of these potentials and indicates the spontaneity of the overall cell reaction. It also reflects the amount of work the cell can perform. The electrochemical series allows us to calculate the cell EMF by using the standard electrode potentials of the half-cells and combining them appropriately.

                                                  E∘cell=E∘red–E∘ox

Here, E∘red is the Standard Reduction Potential for the reduction half-cell, and

 E∘ox is the Standard Reduction Potential for the oxidation half-cell.

•           Measuring Spontaneity of a Reaction:

The feasibility or spontaneity of a redox reaction is directly linked to the cell EMF of the reaction:

      a. A positive EMF indicates a spontaneous reaction.

b. A negative EMF indicates a non-spontaneous reaction.

  Therefore, by analyzing the reactants and products, we can determine whether a redox reaction will occur spontaneously.

    Estimating Gibbs Free Energy:

Gibbs free energy (ΔG∘cell) is another measure of the spontaneity of a reaction. It is related to the cell EMF (E∘cell) as follows. ΔG∘cell=−nFE∘cell     

Where:

n = number of electrons involved in the reaction

F = Faraday’s constant = 96,485 C·mol⁻¹

The relationship between cell EMF and Gibbs Free Energy (ΔG) is as follows:

If cell EMF is negative, then ΔG is positive, and the reaction is non-spontaneous.

If cell EMF is positive, then ΔG is negative, and the reaction is spontaneous.

•     Predicting End-Products of a Redox Reaction:

If only the reactants are given, the products of a redox reaction can be predicted using the electrochemical series:

1. List the Standard Electrode Potential (E°) values for each reactant from the electrochemical series.

2. Identify the species with the highest and lowest reduction potentials.

   The species with thehighest reduction potential is reduced at the cathode.

  The species with the lowest reduction potential is oxidized at the anode.

3. The oxidized and reduced species determine the final products of the reaction.

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