Electrochemical Series - JEE PDF Download

What is Electrochemical Series?

The Electrochemical Series, also known as the Activity Series, is a compiled list that organizes elements according to their increasing values of electrode potential. This series is constructed by comparing the potential of different electrodes with respect to the Standard Hydrogen Electrode (SHE).
In the Electrochemical Series, both metals and non-metals are arranged based on the values of their standard reduction or oxidation potentials. The standard electrode potential is determined by measuring the voltage when a half-cell is connected to the Standard Hydrogen Electrode under standard conditions.

Electropositive and Electronegative Elements

Electropositive elements are those (excluding hydrogen) that exhibit a greater tendency to relinquish electrons to their surroundings. On the other hand, electronegative elements are known for their affinity to acquire electrons. Typically, these elements are positioned below hydrogen in the elemental series. By referring to the electrochemical series, we can determine the sequence in which metals displace one another in the presence of their salts. Consequently, electropositive metals commonly replace hydrogen when exposed to acids.

Application of Electrochemical Series

1. Oxidizing and Reducing Strengths
The electrochemical series helps us to identify a good oxidizing agent or reducing agent. All the substances appearing on the top of the electrochemical series are good oxidizing agents, i.e., they have a positive value of standard reduction potential, whereas those appearing on the bottom of the electrochemical series are good reducing agents, i.e., they have a negative value of standard reduction potential. For example, the F2 electrode with the standard reduction potential value of +2.87 is a strong oxidizing agent, and Li+ with a standard reduction potential value of -3.05 volts is a strong reducing agent.

2. Calculation of Standard emf (E0) of Electrochemical Cell
The standard emf of the cell is the sum of the standard reduction potential of the two half cells: reduction half cell and oxidation half cell
E^ocell = E^ored + E^oox
By convention, the standard oxidation potential is always expressed in terms of reduction potential.
Thus, standard oxidation potential (E^o_ox) = – standard reduction potential E^o_red
Therefore,
E^o_cell  = (standard reduction potential of reduction half cell) – (standard reduction potential of oxidation half cell)
As oxidation takes place at the anode, and reduction takes place at the cathode. Hence,  
E^o_cell = E^o_cathode – E^o_anode
Example: For a reaction, 2Ag+ (aq) + Cd → 2Ag + Cd+2(aq)
The standard reduction potential given are: Ag+/ Ag =0.80 volt, Cd+2/ Cd = -0.40 volt From the reaction, we can see that Cd losses electrons and Ag+ gains. Hence, oxidation half cell or anode is Cd.
Using the formula,
E^o_cell = E^o_cathode – E^o_anode
= 0.80 -(-0.40)
= 1.20 volt 

3. Predicting the Feasibility of Redox Reaction
Any redox reaction would occur spontaneously if the free energy change (ΔG) is negative. The free energy is related to cell emf in the following manner:
ΔG^o = nFE^o
Where n is the number of electrons involved, F is the Faraday constant, and Eo is the cell emf.

• ΔGo can be negative if Eo is positive.
• When E^o is positive, the cell reaction is spontaneous and serves as a source of electrical energy.
• If it comes out to be negative, then the spontaneous reaction cannot take place.
• The resultant value of  E^o for redox reaction is important in predicting the stability of a metal salt solution when stored in another metal container.

For example, let us find out whether we can store copper sulphate solution in a nickel vessel or not.
Given: Ni⁺²/ Ni = -0.25 volt, Cu⁺²/Cu = 0.34 volt
Ni + CuSO₄ → NiSO₄ + Cu
We want to see whether Ni metal will displace copper from copper sulphate solution to give NiSO₄ by undergoing an oxidation reaction.
Ni(s) + Cu⁺²(aq) → Ni⁺²(aq) + Cu(s)
From the above reaction, it is clear the oxidation terminal will be the Ni electrode.
E^o_cell = E^o_cathode – E^o_anode
= 0.34 – (-0.25)
= 0.59 volt
As the emf comes out to be positive, it implies that copper sulphate reacts when placed in a nickel vessel and hence cannot be stored in it.

4. Predicting the Product of Electrolysis
In case two or more types of positive and negative ions are present in the solution, during electrolysis, certain metal ions are discharged or liberated at the electrodes in preference to others. In general, in such competition, the ion, which is a stronger oxidizing agent (high value of standard reduction potential), is discharged first at the cathode.
Thus, when an aqueous solution of NaCl containing Na+, Cl-, H+ and OH- ions is electrolysed, the H+ ion is preferentially deposited at the cathode (reduction) instead of Na+ being reduced; this is because the reduction potential of hydrogen (0.00 volt) is higher than the reduction potential of sodium (-2.71 volt). At the anode where oxidation takes place, the anion that has a lower reduction potential will be oxidized. Therefore, OH- with a standard reduction potential of 0.40 volt will be oxidized in preference to Cl- with a standard reduction potential of 1.36 volt.

Electrochemical Series Important Points

Here are some important points to remember from this lesson.

• In the electrochemical series, the reduction potential of an element is taken in reference to the hydrogen scale where Eo = zero. As per the definition, the standard reduction potential of an element is described as the measure of the tendency of an element to undergo reduction.
• The greater the reduction potential of an element, the more easily it will be reduced. Meanwhile, elements that have low reduction potential will get oxidized more quickly and easily.
• Alternatively, elements that give up electrons without any difficulty have negative or lower reduction potential. Elements that do not give up electrons easily accept electrons effortlessly and have positive or higher reduction potential.
• Stronger reducing agents that have negative standard reduction potential are usually situated below the hydrogen in the electrochemical series. On the other hand, weaker reducing agents with positive standard reduction potential are found above the hydrogen in the series.
• As we move down in the group, the reducing agent’s strength increases while the oxidizing agents’ strength decreases.
• Likewise, as we move from top to bottom in the series, the electro-positivity and activity of metals amplify or intensify. In the case of non-metals, it decreases.