Importance of Electrochemical Series

• A series in which the reduction electrode potentials of various electrodes have been arranged in the increasing order (downwards) is called Electrochemical Series .
• The standard reduction potential of hydrogen is zero.The electrodes above hydrogen have negative reduction potential while those place below it have positive reduction potential and vice-versa.
• To understand the importance and application of Electrochemical series we have to study about Oxidation,Reduction and Standard Reduction Potential.

What is Electrochemistry?

Electrochemistry is the branch of chemistry which deals with the study of the chemical changes which occur on passing electric current into certain chemical systems and also with the generation of electricity by carrying chemical reactions which are redox reactions in nature. And Redox reactions are those reactions in which both oxidation and reduction taking place.

Electronic Concept of Oxidation and Reduction

According to the electronic concept, oxidation may be defined as the process in which an atom or ion loses one or more electrons. The loss of electrons is also called de-electronation. As a result, there is increase in positive valency or decrease in negative valency of the species.

The species which lose electrons during oxidation are called Reducing agents.

According to the electronic concept, reduction may be defined as the process in which an atom or ion gains one or more electrons. The gain of electrons is also called electronation. As a result, there is increase in negative valency or decrease in positive valency of the species.

The species which gain electrons during reduction are called Oxidising agents.

Electrode Potential

It is the potential difference between the electrode and its ions in solution.

The value of Electrode potential depends upon:

(i) Nature of the metal

(ii) Concentration of the ions in solution.

(iii) Temperature.

Types of Electrode Potential

• Oxidation Potential:-

It is the tendency of the electrode to lose electrons and as a result it gets oxidized.

• Reduction Potential:-

It is the tendency of the electrode to accept electrons and as a result,it gets reduced.

Measurement of Standard Electrode Potential (Eo):-

The standard electrode potential(electron releasing or electron accepting tendency) of an electrode in contact with its electrolyte in a half cell cannot be measured experimentally due to the following reasons:-

A half cell whether oxidation or reduction half cell cannot work of its own and can work only when connected to the other half cell.

The electron realeasing or accepting tendency of an electrode is only relative tendency and not absolute tendency.

Thus we cannot determine the absolute standard electrode potential of an electrode.To solve the problem,a reference electrode is needed and an arbitrary electrode potential must be assigned to it.The commonly used reference electrode is standard hydrogen electrode (SHE) also called normal hydrogen electrode(NHE) and its standard electrode potential (oxidation as well as reduction) is taken as zero.

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We have stated that a standard hydrogen electrode acts as the reference electrode and it helps in measuring the standard electrode potential of an electrode.An electrochemical cell is set up in which the metal electrode under consideration is kept in one half cell and the standard hydrogen electrode acts as the other half cell. The potential difference developed as a result of the redox reaction is measured with the help of high resistance voltmeter(or beter by potentiometer). Since the electrode potential of the hydrogen electrode under standard conditions is taken zero, the reading of the voltmeter will therefore,give the standard electrode potential of the electrode under consideration.

The deflection of the voltmeter in the cell circuit represents the flow of current. The flow of current is towards opposite side.If it is towards the metal electrode, this means that the flow of electrons is towards the standard hydrogen electrode. Therefore, metal electrode will act as anode and standard hydrogen electrode as cathode. In case, the deflection is towards the hydrogen electrode, this means that the flow of electrons is from hydrogen electrode towards the metal electrode. In such a case, hydrogen electrode will act as anode and the metal electrode as the cathode.

In General, Eo cell = Eo cathode – Eoanode

Where Eo cell value represents the standard reduction potential of the electrode.

Electrochemical Series or EMF Series :-

The standard electrode potential (Eo cell) of a large number of electrodes are determined with respect to the standard hydrogen electrode acting as a reference electrode. The standard reduction potential of hydrogen is zero. The electrodes above hydrogen have negative reduction potential while those place below it have positive reduction potential and vice-versa. They are arranged in decreasing strength of reducing agent to form a series known as Electrochemical Series.

Application of Electrochemical Series:-

To predict the relative oxidizing and reducing powers:-

The electrochemical series helps to pick out substances that are good oxidizing agents and those which are good reducing agents.In an electrochemical series the species which are placed above hydrogen are more difficult to be reduced and their standard reduction potential values are negative. The Li : Li+ (aq) electrode has the least Eo value and therefore, it is reduced with more difficulty. Therefore, Li+ cannot accept electrons easily and so loses electrons to behave as a reducing agent. Li is the strongest reducing agent.

The species which are easily reduced than hydrogen are palced below it in electrochemical series and their Eo value are positive. The F2 : 2F-(aq) electrode has the highest Eo value and therefore, F2 has the greatest tendency to get reduced,it is consequently the strongest oxidizing agent. In general, oxidizing agents have + Eo values.

Higher the positive value, stronger will be the oxidizing agent and reducing agents have -Eo values, higher the negative value, stronger will be the reducing agent.

For Example:

Increasing order of reducing power of metal is

Ag+/Ag(+0.80V) < Cr3+/Cr(-074V) < K+/K (-2.93V)

Calculation of the EMF of the Cell

The following steps determine the reduction potential of the cathode and anode:

Step I

The two half-cell reactions are written in such a way that the reaction taking place at the left hand electrode is written as an oxidation reaction and that taking place at the right electrode is written as reduction reaction.

Step II

The number of electrons in the two equations are made equal by multiplying one of the equations if necessary by a suitable number. However, electrode potential values (E°) are not multiplied.

Step III

The electrode potentials of both the electrodes are taken to be reduction potentials and so the EMF of the cell is equal to the difference between the standard potential of the right hand side and the left hand side electrode.

Eo cell = Eo R – EoL

Step IV

If the EMF of the cell is +ve, the reaction is feasible in the given direction and the cell is correctly represented, i.e., oxidation occurs at left electrode (anode) and reduction occurs at the right electrode (cathode). If it is -ve, the cell reaction is not feasible in the given direction and the cell is wrongly represented. Thus, to get positive value for the EMF the electrodes must be reversed.

To predict whether a metal will react with acids to give H2 gas:-

Metals above hydrogen in Electrochemical series have great tendency for oxidation,so they displace hydrogen from acids.All metals having negative electrode potentials (negative E° values) show greater tendency of losing electrons as compared to hydrogen. So, when such a metal is placed in an acid solution, the metal gets oxidized, and H+ (hydrogen) ions get reduced to form hydrogen gas. Thus, the metals having negative E° values liberate hydrogen from acids.

metal having negative E° value

For example, metals such as Mg (E (Mg2+ Mg) = – 2.37 V),

Zn (E (Zn2+ Zn) = – 0.76 V), Iron (E (Fe2+ Fe) = – 0.44 V) etc., can displace hydrogen from acids such as HCl and HSO4. But metals such as Copper, (E (Cu2+ Cu) = + 0.34V), silver (E (Ag+ Ag) = + 0.80V) and gold (E (Au3+ Au) = +1.42 V) cannot displace hydrogen from acids because of their positive reduction potential value.

To predict the Feasibility of Redox Reaction:-

From the E° values of the two electrodes one can find out whether a given redox reaction is feasible or not. A redox reaction is feasible only if the species which has higher potential is reduced i.e., accepts the electrons and the species which has lower reduction potential is oxidized i.e., loses electrons.

The electrochemical series gives the increasing order of electrode potentials (reduction) of different electrodes on moving down the table. This means that the species, which accept the electrons (reduced) must be lower in the electrochemical series as compared to the other which is to lose electrons. (oxidized). For example,

From the electrochemical series E° value of Cu = +0.34 V and that of Ag = +0.80 V since the reduction potential of Ag is more than that of Cu, this means that silver has greater tendency to get reduced in comparison to copper. Thus, the reaction occurs more readily than the reaction

The reduction potential of copper is less than that of Ag, this means that copper will be oxidized or will go into solution as ions in comparison to Ag. Thus, the reaction, occurs more readily than

Therefore, silver will be reduced and copper will be oxidized and the above reaction is not feasible. Rather the reverse reaction, can occur. Thus a metal will displace, any other metal, which occurs below it in the electrochemical series from its salt solution. When a metal having lower E° value is placed in a solution, containing ions of another metal having higher E° value, then the metal having lower E° value gets dissolved and the ions of the metal having higher E° value get precipitated.

Problems

Q:- Write the half-cell reaction and the overall cell reaction for the electrochemical cell:

Calculate the standard emf for the cell if standard electrode potentials (reduction) Pb2+ Pb and Zn2+ Zn electrodes are -0.126V and -0.763 V respectively.

Solution

Zn electrode acts as anode while Pb electrode acts as cathode and, therefore oxidation occurs at zinc electrode and reduction occurs at lead electrode. The half cell reactions are:

Q:- Iodine (I2) and bromine (Br2) are added to a solution containing iodide (I-) and bromide (Br-) ions. What reaction would occur if the concentration of each species is 1 M? The electrode potentials for the reactions are:

Solution

Since the reduction potential of Br2 is more than that of I2, it means that bromine can be readily reduced. Therefore, I- will be oxidized to I2 and this reaction should be written as oxidation. Therefore, the following reactions will occur:

Since for the feasibility of the reaction, the emf should be +ve, and to get + ve value for the cell reaction, subtract the equation representing lower value of E° from the equation representing the higher value of E°.

Q:-. What will be the spontaneous reaction between the following half-cell reactions?

Calculate Ecell.

Solution

Since the reduction potential of reaction (ii) is more than that of reaction (i); reaction (ii) will occur as reduction. Therefore, reaction (i) should be written as oxidation. To obtain the net reaction, we multiply the reactions by appropriate coefficients so that electrons get cancelled.

Ecell = Esubstance reduced – Esubstance oxidized

= 1.28 – (- 0.74) = 2.02V

To predict the spontaneity of any redox reaction:-

For any spontaneous reaction (deltaG) should be negative.Since

deltaG = -nFE cell

Hence E cell should be positive for spontaneous reaction. E cell is the emf of the cell and is calculated from the standard redox potentials by using the reaction.

E cell = Ecathode – Eanode

If E cell is positive, the cell reaction is spontaneous, otherwise not.

To predict the Replacement tendency :-

The relative ease with which the various species of metals and ions may be oxidized or reduced is indicated by the reduction potential values. The metals with lower reduction potential are not reduced easily but are easily oxidized to their ions losing electrons. These electrons would reduce the other metals having higher reduction potentials. In other words, a metal having smaller reduction potential can displace metals having larger reduction potentials from the solution of their salt.For example, copper lies above silver in the electrochemical series, therefore, if copper metal is added to AgNO3 solution, silver is displaced from the solution. In general a metal occupying higher position in the series can displace the metals lying below it from the solutions of their salts and so are more reactive in displacing the other metals. Thus, Li is the most electropositive element in solutions and fluorine is the most electronegative element.

To predict the correct Metallurgical Methods :-

Eo values of Cu,H2O and Al are +0.34V,-0.83V and -1.66V.It means Cu gets more easily reduced than water and water gets more easily reduced than aluminium.Hence copper can be produced by the electrolysis of aqueous copper sulphate but not aluminium.this is due to the fact that when Al3+(aq) is electrolysed,the H2O will be electrolysed but not Al3+(aq).

For calculation of Equilibrium Constant :-

Therefore measurement of E o enables the determination of the equilibrium constant for the electrode reaction.