Preferential Discharge Theory: Redox Reaction and Electrode Potential, Preferential Discharge Theory, Electrochemical Series, Products of Electrolysis, Factors Affecting Product of Electrolysis, Practice Problems and FAQs:

Imagine you are the chief selector of the Indian cricket team. You are asked to select 5 batsmen for an international match. How would you choose and what are the criteria that you will follow for the selection?

I am pretty sure that instead of five batsmen you must have thought of 7-8 batsmen who are the top quality batsmen for the desired position. I know it’s tough but you have to choose only 5.

In this scenario we choose players based on their current performance.

Similarly during the process of electrolysis, some times there are more than one type of cation and anion present in the solution who are willing to move towards the cathode or anode but at a time only one type of cation or anion can move to the respective cathode or anode. Hence, there should be some selection criteria.

Yes there’s a method of selecting these ions which is understood through preferential discharge theory. Let’s try to understand the concept of this theory.

Table of content

• Redox Reaction and Electrode Potential
• Preferential Discharge Theory
• Definition of Electrochemical Series
• Product of Electrolysis
• Practice Problems
• Frequently Asked Questions

Redox Reaction and Electrode Potential

• The electrode potential is the tendency of an electrode to lose or gain electrons when it is in contact with the solution of its ions in a half-cell. It's measured in volts.
• Oxidation is the loss of electrons whereas reduction is gain of electrons.
• The cell potential is the difference between the cathode and anode electrode potentials (reduction potentials). When no current is drawn through the cells, it is referred to as the standard electromotive force (emf).

Preferential Discharge Theory

According to this hypothesis, if more than one type of ion is attracted to a single electrode, the ion that has the most positive potential gets discharged in preference.

Definition of Electrochemical Series

Electrochemical series, also referred to as activity series, is a list that describes the arrangement of elements in order of their increasing electrode potential values. Here substances are arranged in the order

of highest standard reduction potential to the lowest reduction potential.

• The standard electrode potentials of a large number of electrodes have been determined using standard hydrogen electrode as the reference electrode, for which the electrode potential has been arbitrarily fixed as zero.
• Also, as already mentioned, it is a common practice to express the electrode potentials as the reduction potentials. Since reduction half reaction is just the reverse of oxidation half reaction, the reduction potential of any electrode is obtained from its oxidation potential by simply changing the sign.
• Further, according to the latest convention of sign, the electrode at which reduction takes place with respect to standard hydrogen electrode has reduction potential which is given a positive sign; the electrode at which oxidation takes place with respect to standard hydrogen electrode has a positive oxidation potential or expressed as reduction potential, it will have a negative sign.

Product of Electrolysis

• Electrolysis is the breaking of chemical bonds in compounds to produce elemental substances. The process involves passing electric current at specific voltage between two electrodes, through the compound (electrolyte) to be decomposed. The electrolytes are composed of positively charged (cations) and negatively charged anions). Positively charged anode attracts anions. Anions, on reaching anode, release their electrons to it and get reduced to neutral atoms. Negatively charge cations, attract cations and discharge their positive charge to neutral atoms.

• The voltage required for the process depends on the ability of the ions to receive or release electrons. This ability of several elements, molecules, and ions has been measured in aqueous solutions of them and available as a standard reduction potentials. Standard reduction potential is the potential (ability) in volts, of receiving electrons, at standard conditions of 1molar aqueous solution of the electrolyte, measured at 298K and one atmospheric pressure.

For example, the ability of cupric ions to take two electrons to become neutral copper atom is, given by a standard reduction potential of 0.34V

$C{u}^{2+} \left(aq\right)+2e^{-} \to Cu \left(s\right)$

• Oxidizing ability (losing electrons) of the substance is opposite to the reduction process. So, the oxidizing potential of a substance is generally of same magnitude but opposite in sign to the reduction potential. Oxidation potential of copper atom to cupric ion is -0.34V.
• Higher the reduction potential, the greater the feasibility of reduction in aqueous solution. Similarly, higher the oxidation potential (smaller the reduction potential), higher the probability to get oxidized.
• Electrolysis is a process involving reduction at cathode and oxidation at anode. So, feasibility of electrolysis depends upon the individual redox potentials of the electrolyte ions.
• Moreover, in aqueous solutions, water also gets involved. Water can be, oxidized and reduced. Unlike other ions and their elemental atoms, redox potential is different. Redox reaction of water, also need a higher voltage than predicted by the thermodynamic values.
• Electrolysis reactions in aqueous solutions, involves one of the redox reactions-

At Cathode:
Reduction of

• cations to neutral atom and
• water to hydrogen

At anode:
Oxidation of

• anions to elemental atoms or molecules,
• active anode metal to metal ions and
• water to oxygen

Products of electrolysis, mostly depends on the redox potentials of the ions present in the electrolyte. The products of electrolysis for the same electrolyte, is also affected by factors like -

i) Nature and state of the electrolyte

ii) Nature and electrode potential of ions present in the electrolyte

iii) Nature of the electrode and

iv) Overvoltage at the electrodes.

i) Nature and State of the Electrolyte

• Electrolysis involves movement of ions towards the oppositely charged electrodes. Naturally, the electrolyte should have mobile ions. In solids, ions are in specific positions and cannot move at ordinary temperatures. Hence solids are unsuitable for electrolysis.
• For electrolysis, electrolyte should be in the liquid form- molten or in solution with a suitable polar solvent. Sodium chloride will undergo electrolysis in the molten state or in aqueous solution.

ii) Nature and Electrode Potential of Ions Present in the Electrolyte

Electrolysis of electrolytes of two elemental ions is straight forward giving the two elements on

electrolysis. Molten sodium chloride gives sodium atoms and chlorine molecule.

Electrolysis of radical ions do not give the elemental atoms.

Electrolytes containing more than one ionic compound, depends on the relative redox potentials.

Electrolysis of aqueous solutions of electrolytes is examples. Water molecules also can undergo redox

reactions and will compete with redox reactions of the electrolyte ions.

Electrolysis of molten sodium chloride gives sodium and chlorine. But electrolysis of aqueous sodium

chloride gives hydrogen and chlorine and not sodium.

iii) Nature of the Electrode

For the same electrolyte, the nature of the electrolyte may give different products.

When aqueous copper sulphate solution is electrolyzed, the following redox reactions are possible.

At cathode: Reduction at pH =7

$C{u}^{2+}+2e^{-}\to Cu \left(s\right): E^0=0.34 V$

$2H_{2}O+2e^{-}\to H_2+2O{H}^{-}: E^0=-1.02 V$

At anode: Oxidation at pH = 7

$Cu\left(s\right)\to C{u}^{2+} \left(aq\right)+2e^{-}: E^0= -0.34 V$

$2H_{2}O\to O_2\left(g\right)+4H^{+}+4e^{-}: E^0= -1.4V$

At cathode, out of the two electrodes, reduction potential of copper ions is more positive than

reduction of water. So, irrespective of electrode, copper ions from the electrolyte will be reduced and

deposited on the cathode, increasing its mas. But the reaction at anode depends on the electrode.

Electrolysis with inert electrodes like Platinum, graphite etc.

Inert electrodes does not react with the electrolyte or the products and so does not undergo any

changes. Since oxidation of water has more positive potential, oxygen will be evolved at the anode.

But, if the copper is used as anode, copper will react with the sulphate ion to retain the electrolyte

concentration. So, there will not be any gas evolution. Instead the anode mass slowly decreases going

into the solution.

iv) Overvoltage at the Electrodes.

Redox potential of electrolyte ions, decide the electrolysis reactions and products. Sometimes, redox potentials of some half reactions during the electrolysis is more than the thermodynamic potentials. This excess voltage (over voltage) of the half-reaction may make the reaction unfavorable and change the product of electrolysis.

In the hydrolysis of aqueous sodium chloride, at the anode, two oxidation reactions can take place. Reduction potential of water and chloride is +0.82V and 0.1.36V respectively.

$2H_{2}O\to O_2+4H^{+}+4e^{-} E^0= -0.82 V$

$2C{l}^{-}\to C{l_2+2e^{-} E^0= -1.36 V}^{}$

Oxidation of water being more positive, is more feasible and so, evolution of oxygen gas should happen at anode. But, evolution of oxygen from water has an over voltage of -0.6V making the voltage for the oxidation of water as -1.42V. Chloride oxidation is more positive than the net voltage of water oxidation.

So, chloride is, oxidized to chlorine at anode. Chlorine is liberated and not oxygen because of over voltage.

Understanding Product Formation from Electrolysis

1. A molten mixture of NaCl and AlF₃ is electrolyzed with inert electrodes. Identify the products.

Possible cathode half-reactions are-

$N{a}^{+}+e^{-}\to Na E^0= -2.71V$

$A{l}^{3+}+3e^{-}\to Al E^0= -1.66V$

Because of the higher reduction potential (or higher electro negativity) aluminum ions is reduced in

preference to sodium at cathode.

Possible anode half-reactions are:

$2C{l}^{-}\to C{l}_2+2e^{-} E^0= -1.36 V$

$2F^{-}\to F_2+2e^{-} E^0= -2.87V$

Because of the higher oxidation potential (or smaller electronegativity) chloride ions is reduced in

preference to fluoride at anode.

So, the products of electrolysis of the mixture are aluminum and chlorine.

2. List the products at cathode and anode in the following electrolytic cells:

Products of electrolysis-

i) Silver deposits on cathode. Mass of anode will decrease. Concentration of silver nitrate remains

constant.

Ii) Silver deposits on cathode. Oxygen evolves at anode.

iii) Hydrogen liberation at cathode and oxygen liberation at anode

iv) Copper deposits at cathode and chlorine liberation at anode.

Practice Problems

Q1. When a molten mixture of NaCl and AlF₃ is electrolyzed with inert electrodes which ion will reduce at cathode?

A. Na⁺
B. Cl^-
C. Al³⁺
D. F₂

Answer: (C)

Solution: When a molten mixture of NaCl and AlF₃ is electrolyzed with inert electrodes

Possible cathode half-reactions are:

$N{a}^{+}+e^{-}\to Na E^0= -2.71V$

$A{l}^{3+}+3e^{-}\to Al E^0= -1.66V$

Because of the higher reduction potential (or higher electro negativity) aluminum ions is reduced in

preference to sodium at cathode.

Q2. Which of the following won’t have any impact on product of electrolysis?

A. Nature and state of the electrolyte
B. Nature and electrode potential of ions present in the electrolyte
C. Nature of the electrode and
D. None of the above

Answer: (D)

Solution: Product of electrolysis is affected by the following factors

• Nature and state of the electrolyte
• Nature and electrode potential of ions present in the electrolyte
• Nature of the electrode and
• Overvoltage at the electrodes.

Q3. If reduction potential is higher then,

A. Greater will be the feasibility of reduction
B. Lesser will be the feasibility of reduction
C. Greater will be the feasibility of oxidation
D. Both A & C

Answer: (A)
Solution: Higher the reduction potential, the greater the feasibility of reduction in aqueous solution. Similarly, higher the oxidation potential (smaller the reduction potential), higher the probability to get oxidized.

Q4. Which of the following statement is true about overvoltage?

A. redox potentials of some half reactions during the electrolysis is more than the thermodynamic potentials.
B. redox potentials of some half reactions during the electrolysis is less than the thermodynamic potentials.
C. redox potentials of some half reactions during the electrolysis is euqal to the thermodynamic potentials.
D. None of the above

Answer: (A)

Solution: Redox potential of electrolyte ions, decide the electrolysis reactions and products. Sometimes, redox potentials of some half reactions during the electrolysis is more than the thermodynamic potentials. This excess voltage (over voltage) of the half-reaction may make the reaction unfavorable and change the product of electrolysis.

Frequently Asked Questions

Q 1. The process of electrolysis is based on which theory?
Answer: The mechanism of electrolysis is based on Ionization theory.

Q 2. Why NaCl conducts electricity only in molten state?
Answer: NaCl is an ionic compound. It doesn’t conduct electricity in its solid form because the ions Na⁺ and Cl^- have a fixed position in solid-state but in molten state, these ions become mobile and can conduct electricity.

Q 3. Is there a difference in how metallic and ionic substances conduct electricity?
Answer: In metallic substances, the flow of electricity is due to the flow of electrons while in the case of ionic substances in the aqueous solution or molten state, the flow of electricity is due to the flow of ions.

Q4. What kinds of metals can be electrolyzed to purify them?
Answer: The purification and electroplating of less reactive metals like Pb, Cu, and Ag utilize the electrolysis concept extensively. The cathode is where pure metal is deposited, while the anode, which is formed of impure metal, dissolves with the passage of electricity.