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The Electrochemical Series
The electrochemical series is a list of reduction half reactions of common reduction-oxidation (redox) reactions and their corresponding standard potentials E°. The potentials were measured in Volts (V) relative to the standard hydrogen electrode (SHE), which has a potential of 0 V (highlighted in red). The standard potentials were measured under standard conditions, i.e., at a temperature of 298 K (25°C), a pressure of 1 atm, and a solution concentration of 1 M.
Potential values in following tables are from various sources available in public domains and school textbooks. They were then checked against the online textbook “Chemistry 2e” (https://openstax.org/details/books/chemistry-2e) and the “Physical Chemistry” textbook by P.W. Atkins (Physical Chemistry, P.W. Atkins, 6th Ed., Oxford University Press, New York). Potential values with higher precision were selected, and if values for a particular half-reaction are different from different sources, values from “Atkins’ Physical Chemistry” or “Chemistry 2e” were used, and denoted * for the former and ** for the latter.
1. Reduction Half Reaction (Half Cell) Standard Potentials in Decreasing Order of Oxidizing Strength
1.1. Standard potential range in descending order [+3.00 V … +1.229 V]
1.2. Standard potential range in descending order [+1.21 V … +0.591 V]
1.3. Standard potential range in descending order [+0.58 V … +0.142 V]
1.4. Standard potential range in descending order [+0.1 V … -0.399 V]
1.5. Standard potential range in descending order [-0.4030 V … -1.01 V]
1.6. Standard potential range in descending order [-1.04 V … -2.07 V]
1.7. Standard potential range in descending order [-2.09 V … -3.04 V]
2. Reduction Half Reaction (Half Cell) Standard Potentials in Alphabetical Order
2.1. Oxidant list between Ag+ and Ca(OH)2
2.2. Oxidant list between CdS and Cu2+
2.3. Oxidant list between Cu2+ and Hg2+
2.4. Oxidant list between Hg22+ and MnO4–
2.5. Oxidant list between MnO4– and PbO2
2.6. Oxidant list between PbS and Sn4+
2.7. Oxidant list between [SnFe]2- and Zr4+
Electrochemical series & electrochemical cells explained
In the electrochemical series, in each half reaction, the left side contains the oxidant of the redox reaction, while the right side contains its reductant counterpart. The stronger the oxidant is, the weaker its reductant form is. In the list in which the potentials are arranged in descending order, the oxidizing strength of the reactants on the left side of the equations decreases from top to bottom, while the reducing power of reactants on the right side increases in the same direction.
A spontaneous reaction will occur if an oxidant of a half reaction reacts with a reductant of another half reaction that has a lower standard potential; combining these two half reactions results in an overall equation of the redox reaction.
An electrochemical cell is formed if the two half reactions (called half cells) of a redox reaction are not in direct contact, but electrons from the oxidization half cell (called anode) are allowed to flow to the reduction half cell (called cathode). An anode is where the oxidation reaction takes place and electrons are produced, and a cathode is where reduction reaction takes place, and electrons are consumed. Whether the electrode of an electrochemical cell is negative or positive is determined by its relative potential. During a discharge process, electrons flow from the anode to the cathode via an external load. Therefore, the anode has a lower potential, hence it is a negative electrode, while the cathode has a higher potential, hence it is a positive electrode. During the recharge process, although anode and cathode sites swap, the negative and positive electrodes do not change, as their potential is determined by the DC source used to supply energy to the electrochemical cell. An example of negative/positive electrodes can be found here.
A salt bridge is commonly used to supply negative ions (anions) to anodes and positive ions (cations) to cathodes to balance the charges consumed or generated in the half cells.
The standard potential of an electrochemical cell is obtained by subtracting the standard potential at the anode from the standard potential at the cathode.
E°cell = E°cathode – E°anode
Note that the half-reaction that takes place at the anode is an oxidation reaction. To look up its E°anode in the Electrochemical Series, which is compiled for reduction half-reaction by convention, we look for the reverse of that half-reaction. For example, if the oxidation half-reaction at the anode is Mg(s) → Mg2+(aq) + 2e–, then the reverse of this reaction is the reduction half-reaction Mg2+(aq) + 2e– → Mg(s), which has a standard potential of -2.372 V; therefore, its E°anode is -2.372 V.
A redox reaction occurs spontaneously when an oxidant of a half reaction interacts with a reductant of another half reaction that has a lower standard potential. As a result, the standard potential E°cell of a spontaneous redox reaction has a positive value. In fact, if the cell’s potential Ecell is greater than 0, then the redox reaction still takes place spontaneously even the reaction is not in the standard conditions.