A series in which the decrease electrode potencies of assorted electrodes have been arranged in the increasing order ( downwards ) is called Electrochemical Series.
The standard decrease potency of H is zero.The electrodes above H have negative decrease potency while those topographic point below it have positive decrease potency and vice-versa.
To understand the importance and application of Electrochemical series we have to analyze about Oxidation, Reduction and Standard Reduction Potential.
What is Electrochemistry: –
Electrochemistry is the subdivision of chemical science which deals with the survey of the chemical alterations which occur on go throughing electric current into certain chemical systems and besides with the coevals of electricity by transporting chemical reactions which are redox reactions in nature.And Redox reactions are those reactions in which both oxidization and decrease taking topographic point.
Electronic Concept of Oxidation and Reduction: –
Harmonizing to the electronic construct, oxidization may be defined as the procedure in which an atom or ion loses one or more electrons.The loss of negatrons is besides called de-electronation.As a consequence, there is addition in positive valency or lessening in negative valency of the species.
The species which lose negatrons during oxidization are called Reducing agents.
Harmonizing to the electronic construct, decrease may be defined as the procedure in which an atom or ion additions one or more electrons.The addition of negatrons is besides called electronation.As a consequence, there is addition in negative valency or lessening in positive valency of the species.
The species which gain negatrons during decrease are called Oxidising agents.
Electrode Potential: –
It is the possible difference between the electrode and its ions in solution.
The value of Electrode possible depends upon: –
( I ) Nature of the metal
( two ) Concentration of the ions in solution.
( three ) Temperature.
Types of Electrode Potential: –
Oxidation Potential: –
It is the inclination of the electrode to lose negatrons and as a consequence it gets oxidized.
Reduction Potential: –
It is the inclination of the electrode to accept negatrons and as a consequence, it gets reduced.
Measurement of Standard Electrode Potential ( Eo ) : –
The standard electrode potency ( electron releasing or negatron accepting inclination ) of an electrode in contact with its electrolyte in a half cell can non be measured by experimentation due to the undermentioned grounds: –
A half cell whether oxidization or decrease half cell can non work of its ain and can work merely when connected to the other half cell.
The negatron realeasing or accepting inclination of an electrode is merely comparative inclination and non absolute inclination.
Therefore we can non find the absolute criterion electrode potency of an electrode.To solve the job, a mention electrode is needed and an arbitrary electrode potency must be assigned to it.The normally used mention electrode is standard H electrode ( SHE ) besides called normal H electrode ( NHE ) and its standard electrode potency ( oxidization every bit good as decrease ) is taken as nothing.
We have stated that a standard H electrode acts as the mention electrode and it helps in mensurating the standard electrode potency 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 H electrode acts as the other half cell.The possible difference developed as a consequence of the redox reaction is measured with the aid of high opposition voltmeter ( or beter by potentiometer ) .Since the electrode potency of the H electrode under standard conditions is taken nothing, the reading of the voltmeter will therefore, give the
standard electrode potency of the electrode under consideration.
The warp 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 negatrons is towards the criterion H electrode.Therefore, metal electrode will move as anode and standard H electrode as cathode.In instance, the warp is towards the H electrode, this means that the flow of negatrons is from H electrode towards the metal electrode.In such a instance, H electrode will move as anode and the metal electrode as the cathode.
In General, Eo cell = Eo cathode – Eoanode
Where Eo cell value represents the standard decrease potency of the electrode.
Electrochemical Series or EMF Series: –
The standard electrode potency ( Eo cell ) of a big figure of electrodes are determined with regard to the standard H electrode moving as a mention electrode. The standard decrease potency of H is zero.The electrodes above H have negative decrease potency while those topographic point below it have positive decrease potency and vice-versa.They are arranged in diminishing strength of cut downing agent to organize a series known as Electrochemical Series.
Table of Standard Reduction Potential: –
Application of Electrochemical Series: –
To foretell the comparative oxidizing and cut downing powers: –
The electrochemical series helps to pick out substances that are good oxidising agents and those which are good cut downing agents.In an electrochemical series the species which are placed above H are more hard to be reduced and their standard decrease potency values are negative.The Li: Li+ ( aq ) electrode has the least Eo value and hence, it is reduced with more trouble. Therefore, Li+ can non accept negatrons easy and so loses negatrons to act as a cut downing agent.Li is the strongest reduction agent.
The species which are easy reduced than H are palced below it in electrochemical series and their Eo value are positive.The F2: 2F- ( aq ) electrode has the highest Eo value and hence, F2 has the greatest inclination to acquire reduced, it is accordingly the strongest oxidizing agent.In general, oxidising agents have + Eo values.
Higher the positive value, stronger will be the oxidizing agent and cut downing agents have -Eo values, higher the negative value, stronger will be the cut downing agent.
For Example:
Increasing order of cut downing power of metal is
Ag+/Ag ( +0.80V ) & A ; lt ; Cr3+/Cr ( -074V ) & A ; lt ; K+/K ( -2.93V )
Calculation of the EMF of the Cell
The undermentioned stairss determine the decrease potency of the cathode and anode:
Measure I
The two half-cell reactions are written in such a manner that the reaction taking topographic point at the left manus electrode is written as an oxidization reaction and that taking topographic point at the right electrode is written as decrease reaction.
Measure II
The figure of negatrons in the two equations are made equal by multiplying one of the equations if necessary by a suited figure. However, electrode possible values ( E & A ; deg ; ) are non multiplied.
Measure III
The electrode potencies of both the electrodes are taken to be decrease potencies and so the EMF of the cell is equal to the difference between the standard potency of the right manus side and the left manus side electrode.
Eo cell = Eo R – EoL
Measure IV
If the EMF of the cell is +ve, the reaction is executable in the given way and the cell is right represented, i.e. , oxidization occurs at left electrode ( anode ) and decrease occurs at the right electrode ( cathode ) . If it is -ve, the cell reaction is non executable in the given way and the cell is wrongly represented. Therefore, to acquire positive value for the EMF the electrodes must be reversed.
To foretell whether a metal will respond with acids to give H2 gas: –
Metallic elements above H in Electrochemical series have great inclination for oxidization, so they displace H from acids.All metals holding negative electrode potencies ( negative E & A ; deg ; values ) show greater inclination of losing negatrons as compared to hydrogen. So, when such a metal is placed in an acerb solution, the metal gets oxidized, and H+ ( H ) ions get reduced to organize H gas. Therefore, the metals holding negative E & A ; deg ; values liberate H from acids.
metal holding negative E & A ; deg ; value
For illustration, 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 H 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 ) can non displace H from acids because of their positive decrease possible value.
To foretell the Feasibility of Redox Reaction: –
From the E & A ; deg ; values of the two electrodes one can happen out whether a given oxidation-reduction reaction is executable or non. A redox reaction is executable merely if the species which has higher potency is reduced i.e. , accepts the negatrons and the species which has lower decrease potency is oxidized i.e. , loses negatrons.
The electrochemical series gives the increasing order of electrode potencies ( decrease ) of different electrodes on traveling down the tabular array. This means that the species, which accept the negatrons ( reduced ) must be lower in the electrochemical series as compared to the other which is to lose negatrons. ( oxidized ) . For illustration,
From the electrochemical series E & A ; deg ; value of Cu = +0.34 V and that of Ag = +0.80 V since the decrease potency of Ag is more than that of Cu, this means that Ag has greater inclination to acquire reduced in comparing to Cu. Therefore, the reaction
occurs more readily than the reaction
The decrease potency of Cu is less than that of Ag, this means that Cu will be oxidized or will travel into solution as ions in comparing to Ag. Thus, the reaction,
occurs more readily than
Therefore, Ag will be reduced and Cu will be oxidized and the above reaction is non executable. Rather the rearward reaction,
can happen. Thus a metal will displace, any other metal, which occurs below it in the electrochemical series from its salt solution. When a metal holding lower E & A ; deg ; value is placed in a solution, incorporating ions of another metal holding higher E & A ; deg ; value, so the metal holding lower E & A ; deg ; value gets dissolved and the ions of the metal holding higher E & A ; deg ; value get precipitated.
Problems
Q: – Write the half-cell reaction and the overall cell reaction for the electrochemical cell:
Calculate the standard voltage for the cell if standard electrode potencies ( decrease ) Pb2+ Pb and Zn2+ Zn electrodes are -0.126V and -0.763 V severally.
Solution
Zn electrode Acts of the Apostless as anode while Pb electrode Acts of the Apostless as cathode and, hence oxidization occurs at Zn electrode and decrease occurs at lead electrode. The half cell reactions are:
Q: – Iodine ( I2 ) and Br ( Br2 ) are added to a solution incorporating iodide ( I- ) and bromide ( Br- ) ions. What reaction would happen if the concentration of each species is 1 M? The electrode potencies for the reactions are:
Solution
Since the decrease potency of Br2 is more than that of I2, it means that Br can be readily reduced. Therefore, I- will be oxidized to I2 and this reaction should be written as oxidization. Therefore, the undermentioned reactions will happen:
Since for the feasibleness of the reaction, the voltage should be +ve, and to acquire + ve value for the cell reaction, deduct the equation stand foring lower value of E & A ; deg ; from the equation stand foring the higher value of E & A ; deg ; .
Q: – . What will be the self-generated reaction between the following half-cell reactions?
Calculate Ecell.
Solution
Since the decrease potency of reaction ( two ) is more than that of reaction ( one ) ; reaction ( two ) will happen as decrease. Therefore, reaction ( one ) should be written as oxidization. To obtain the net reaction, we multiply the reactions by appropriate coefficients so that negatrons get cancelled.
Ecell = Esubstance reduced – Esubstance oxidized
= 1.28 – ( – 0.74 ) = 2.02V
To foretell the spontaneousness of any redox reaction: –
For any self-generated reaction ( deltaG ) should be negative.Since
deltaG = -nFE cell
Hence E cell should be positive for self-generated reaction. E cell is the voltage of the cell and is calculated from the standard oxidation-reduction potencies by utilizing the reaction.
E cell = Ecathode – Eanode
If E cell is positive, the cell reaction is self-generated, otherwise non.
To foretell the Replacement inclination: –
The comparative easiness with which the assorted species of metals and ions may be oxidized or reduced is indicated by the decrease potency values. The metals with lower decrease potency are non reduced easy but are easy oxidized to their ions losing negatrons. These negatrons would cut down the other metals holding higher decrease potencies. In other words, a metal holding smaller decrease potency can displace metals holding larger decrease potencies from the solution of their salt.For illustration, Cu prevarications above Ag in the electrochemical series, hence, if Cu metal is added to AgNO3 solution, Ag is displaced from the solution. In general a metal busying higher place 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. Therefore, Li is the most positively charged component in solutions and F is the most negatively charged component.
To foretell the right Metallurgical Methods: –
Eo values of Cu, H2O and Al are +0.34V, -0.83V and -1.66V.It means Cu gets more easy reduced than H2O and H2O gets more easy reduced than aluminium.Hence Cu can be produced by the electrolysis of aqueous Cu sulfate but non aluminium.this is due to the fact that when Al3+ ( aq ) is electrolysed, the H2O will be electrolysed but non Al3+ ( aq ) .
For computation of Equilibrium Constant: –
Therefore measuring of E O enables the finding of the equilibrium invariable for the electrode reaction.
