Question

Discuss the main applications of electrochemical series.

Accepted Answer

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. $E °$ is obtained by measuring the voltage when the half cell is connected to the standard hydrogen electrode under standard conditions.

(1)Electropositive and Electronegative Elements

Elements (other than hydrogen) that display a greater tendency to lose electrons to their solution are usually categorised as electropositive. Similarly, elements that acquire electrons are said to be electronegative. They are usually below the element hydrogen in the series. In any case, if we look at the electrochemical series, we can figure out the order in which metals will replace one another from their salts. So, electropositive metals generally replace hydrogen from acids.

(2)Any redox reaction would occur spontaneously if the free energy change $({ΔG}^{0})$ ) 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 E^o is the cell emf. ${ΔG}^{o}$ can be negative if $E^{o}$ 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 than 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.

(3) Calculation of Standard emf ( $E^{0}$ ) 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

$E^{o} cell = E^{o} red + E^{o} ox [in term of oxidation potentiqal standard oxidation potential = - standard reduction potential (E^{o} ox) = - E^{o} red E^{o} cell = E^{o} ox - E^{o} red$

(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 oxidizing agent (high value of standard reduction potential) is discharged first at the cathode.

$(a) Electrolysis of conc molten sodium chloride anode - chlorine gas; cathode - sodium metal Electrolysis of aqueous sodium chloride anode - chlorine gas; cathode - hydrogen gas and sodium hydroxide . (b) electrolysis of copper sulphate with copper electrode (c) electrolysis of copper sulphate with platinum electrode$

Thus, when an aqueous solution of Sodium Chloride containing ${Na}^{+}, {Cl}^{-}, H^{+} and {OH}^{-}$ ions are electrolysed, Hydrogen ion is preferentially deposited at cathode (reduction) instead of Sodium cation 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 oxidized. Therefore, hydroxide ion with standard reduction potential $0.40 volt$ will be oxidized in preference to Chloride with standard reduction potential of $1.36 volt .$

Discuss the main applications of electrochemical series.

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