Electrochemical Series

What is Electrochemical Series?

Electrochemical series also sometimes referred to as activity series is a list that describes the arrangement of elements in order of their increasing electrode potential values. The series has been established by measuring the potential of various electrodes versus standard hydrogen electrode (SHE).

In electrochemical series, the electrodes (metals and non-metals) in contact with their ions are arranged on the basis of the values of their standard reduction or oxidation potentials. Standard electrode potential are obtained by measuring the voltage when the half cell is connected to the standard hydrogen electrode under standard conditions.

Electrochemical Series Chart

The Electrochemical series chart is a simple way of visualizing similar vs. dissimilar metals. For roofers, you need to know what compatibility issues exist in regards to material. In this chart, common roofing metals (as well as uncommon) are shown. In most basic terms, metals that exist further apart from each other on this scale will react with a higher propensity for corrosion than ones that are close together. (i.e. Zinc and Copper = far apart on the scale. This means you would never have a copper pipe draining water onto a zinc-coated roof).

Electrochemical Series Chart

Application of Electrochemical Series

1. Oxidising and Reducing Strengths

Electrochemical series helps us to identify a good oxidising agent or reducing agent. All the substance appearing on the top of the electrochemical series are a good oxidising agent i.e., they have positive Value of standard reduction potential whereas those appearing on the bottom of the electrochemical series are a good reducing agent i.e., they have a negative value of standard reduction potential. For Example, F2 electrode with the standard reduction potential value of +2.87 is a strong oxidising agent and Li+ with 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 cell: reduction half cell and oxidation half cell

Eocell = Eored + Eoox

By convention, the standard oxidation potential is always expressed in terms of reduction potential.

Thus,  standard oxidation potential (Eoox) = – standard reduction potential Eored

Therefore,

Eocell  = ( standard reduction potential of reduction half cell) – ( standard reduction potential of oxidation half cell)

As oxidation takes place at anode and reduction takes place at the cathode. Hence,  

Eocell = Eocathode – Eoanode

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 electron and Ag+ gains. Hence, oxidation half cell or anode is Cd.

Using the formula,

Eocell = Eocathode – Eoanode

             =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:

ΔGo = nFEo

Where n is the number of electrons involved, F is the Faraday constant and Eo is the cell emf. 

  • ΔGcan be negative if Eo is positive.
  • When Eo 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  Eo for redox reaction is important in predicting the stability of a metal salt solution when stored in another metal container.

Also Read: Gibbs Free Energy

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 oxidation reaction.

Ni(s) + Cu+2(aq) → Ni+2(aq) + Cu(s)

From the above reaction, it is clear oxidation terminal will be Ni electrode.

 Eocell = Eocathode – Eoanode

= 0.34 – (-0.25)

= 0.59 volt

As the emf comes out to be positive, it implies 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 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 oxidising 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,  H+ ion is preferentially deposited at cathode (reduction)  instead of Na+ being reduced, this is because 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 lower reduction potential will be oxidised. Therefore, OH- with standard reduction potential  0.40 volt will be oxidised in preference to Cl- with standard reduction potential of  1.36 volt.

Solved Problems

1. Predict whether the following reaction will occur spontaneously or not:

Fe+3 + 2Cl Fe+2 + Cl2

E0Fe+2/Fe = -0.440 volt ; E0Cl / Cl –= 1.36 volt

Solution:

Eocell = Eocathode – Eoanode

Since chlorine has higher reduction potential than iron therefore at cathode reduction of chlorine occurs and oxidation of iron occurs at the anode.

Eocell = 1.36 -(-0.440) = 1.80 volts

The positive value of E0Cell implies that reaction occurs spontaneously.

2. The standard reduction potential at 250C for the following half-reaction are given below:

Zn+2(aq) + 2e → Zn(s);       -0.76 volt

Cr+3 (aq) + 3e→ Cr(s);        -0.740 volt

Cu+2(aq) + 2e → Cu(s);       0.34 volt

Fe+3 + e → Fe+2  ;                 0.77 volt

Which is the strongest reducing agent

  1. Zn
  2. Cr
  3. Cu
  4. Fe+3

Solution : Option 1

Reducing agent is a chemical species that loses an electron to another chemical species in a redox chemical reaction. Since, reducing agent loses electron  so it is oxidised. Out of the following given half-reaction, the reduction of Zn+2 has the lowest reduction potential(-0.762). We know that

Oxidation  potential = -(reduction potential)

So in terms of standard oxidation potential Zinc will have the highest oxidation potential i.e, 0.762 volts. Therefore, zinc is the strongest reducing agent.

3. The standard oxidation potential, E0 for the half-reactions are as follows,

Cu Cu+2 + 2e;  E0 = -0.34 volts

Fe Fe+2 + 2e–   ; E0 = 0.41 volts

Calculate the emf of the cell,  Cu+2 + Fe Cu + Fe+2

Solution: 

Eocell = (standard reduction potential of reduction half cell) – (standard reduction  potential of oxidation half cell)

Eocell = -(standard oxidation  potential of reduction half cell) – (-standard oxidation  potential of oxidation half cell)

Eocell = -0.34-(-0.41)

Eocell = 0.07 volt.

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