Electrolytic Cells, Electrolysis & Products of Electrolysis | Chemistry Class 12 - NEET PDF Download

What is an Electrolytic Cell?

An electrolytic cell can be defined as an electrochemical device that uses electrical energy to facilitate a non-spontaneous redox reaction. 

• Electrolytic cells are electrochemical cells that can be used for the electrolysis of certain compounds. 
• For example, water can be subjected to electrolysis (with the help of an electrolytic cell) to form gaseous oxygen and gaseous hydrogen. 
• This is done by using the flow of electrons (into the reaction environment) to overcome the activation energy barrier of the non-spontaneous redox reaction.

The three primary components of electrolytic cells are:

1. Cathode
   (which is negatively charged for electrolytic cells)
2. Anode
   (which is positively charged for electrolytic cells)
3. Electrolyte
   (The electrolyte provides the medium for the exchange of electrons between the cathode and the anode. Commonly used electrolytes in electrolytic cells include water (containing dissolved ions) and molten sodium chloride.

Components of Electrolytic Cell

Working of an Electrolytic Cell

An Electrolytic Cell is an electrochemical device that uses an external source of electrical energy to drive a non-spontaneous chemical reaction. It's the opposite of a galvanic or voltaic cell, which spontaneously generates electrical energy from a chemical reaction.

Here's how an electrolytic cell works:

• Molten sodium chloride (NaCl) can be subjected to electrolysis with the help of an electrolytic cell, as illustrated below.

Working of an Electrolytic Cell

• Here, two inert electrodes are dipped into molten sodium chloride (which contains dissociated Na+ cations and Cl– anions). 
• The metal strip at which positive current enters is called an anode; the anode is positively charged in an electrolytic cell. On the other hand, the electrode at which the current leaves is called the cathode. Cathodes are negatively charged.

• When an electric current is passed into the circuit, the cathode becomes rich in electrons and develops a negative charge. 
• The positively charged sodium cations are now attracted towards the negatively charged cathode. 
• This results in the formation of metallic sodium at the cathode. Simultaneously, the chlorine atoms are attracted to the positively charged cathode. 
• This results in the formation of chlorine gas (Cl₂) at the anode (which is accompanied by the liberation of 2 electrons, finishing the circuit). 
• The associated chemical equations and the overall cell reaction are provided below.

Reaction at Cathode: Na⁺ + e^– → Na

Reaction at Anode: 2Cl^– → Cl₂ + 2e^–

Cell Reaction: 2NaCl → 2Na + Cl^–

• Thus, molten sodium chloride can be subjected to electrolysis in an electrolytic cell to generate metallic sodium and chlorine gas as the products.

What is Electrolysis?

The decomposition of electrolyte solution by passage of electric current, resulting in deposition of metals or liberation of gases at electrodes is known as electrolysis.

Electrolysis

Key characteristics of electrolysis include:

• Electrolytic Cell: Electrolysis takes place in an electrolytic cell, which consists of two electrodes (an anode and a cathode) immersed in an electrolyte. The electrodes are usually made of inert materials like platinum or graphite.
• External Electrical Energy: An external power source, such as a battery or direct current (DC) power supply, is connected to the electrodes. This power source provides the necessary electrical potential (voltage) to drive the electrolysis process.
• Redox Reactions: Electrolysis involves redox (reduction-oxidation) reactions. At the anode (positive electrode), oxidation occurs as electrons are removed from the species present, leading to the generation of cations or other products. At the cathode (negative electrode), reduction occurs as electrons are gained by the species present, leading to the formation of anions or other products.
• Ions in the Electrolyte: The electrolyte is a solution or molten compound that contains ions. These ions are involved in the electrochemical reactions at the anode and cathode. For example, in the electrolysis of water, the electrolyte contains H⁺ and OH⁻ ions.
• Migration of Ions: Positive ions (cations) migrate toward the cathode, and negative ions (anions) migrate toward the anode within the electrolyte. This movement of ions helps maintain charge neutrality during the electrolysis process.
  
  Migration of Ions: Electrolysis
• Product Formation: The chemical reactions at the anode and cathode result in the production of new substances. These substances can be collected or used for various purposes. For example, in the electrolysis of water, hydrogen gas is produced at the cathode, and oxygen gas is produced at the anode.

Faraday's Laws of Electrolysis

These laws describe the relationship between the amount of substance deposited or produced during an electrolysis process and the quantity of electricity (electric charge) passed through the system.

• Faraday's Laws of Electrolysis are two fundamental principles in electrochemistry established by the English scientist Michael Faraday in the early 19th century. 
• Faraday's Laws laid the foundation for the quantitative understanding of electrolysis and electrochemical reactions. There are two Faraday's Laws of Electrolysis:

(i) First law of electrolysis: 

The amount of substance deposited or liberated at an electrode is directly proportional to the amount of charge passed (utilized) through the solution.

where

• W = weight liberated
• Q = charge in coulomb
• Z = electrochemical equivalent

when Q = 1 coulomb, then W = Z

Thus, weight deposited by a 1-coulomb charge is called electrochemical equivalent.

Let 1-ampere current is passed till 't' seconds.

Then, Q = It => W = ZIt

1 Faraday = 96500 coulomb = Charge of one mole electrons

One faraday is the charge required to liberate or deposit one gm equivalent of a substance at the corresponding electrode. 

Let 'E' is equivalent weight then 'E' gm will be liberated by 96500 coulombs.

1 Coulomb will liberate  gm ; By definition, Z =

W =

When gas is evolved at an electrode, then the above formula changes as,

V =

where V = volume of liberated gas, V_e = equivalent volume of gas.

Equivalent volume may be defined as:

The volume of gas liberated by 96500 coulombs at STP.

Hence, According to Faraday's first law of electrolysis:

Where: 

• m = mass of substance
• E = equivalent weight of substance
• I = current in amperes
• t = time required
• F = 96,500 Coulombs

(ii) Second law of electrolysis:

When the same amount of charge is passed through different electrolyte solutions connected in series then the weight of substances deposited or dissolved at anode or cathode is in the ratio of their equivalent weights. i.e. 

Solved Examples on Faraday's Laws of Electrolysis  

Q.1. Electrolysis of dilute aqueous NaCl solution was carried out by passing 10 milliampere current. The time required to liberate 0.01 mol of H₂ gas at the cathode is:

a) 9.65 x 10⁴ s 

b) 28.95 x 10⁴ s 

c) 19.3 x 10⁴ s

d) 38.6 x 10⁴ s

Solution:

Mass of H₂ = m = 0.01 mol x 2 g mol^-1

Equivalent weight of  H₂ = E = Atomic weight / valence = 1 g mol^-1 / 1  = 1 g mol^-1

Current in ampere = I = 10 milliamperes = 10 x 10^-3 amperes.

Conclusion: The correct option is 'c'. 

Q.2. A 4.0 molar aqueous solution of NaCl is prepared and 500 mL of this solution is electrolyzed. This leads to the evolution of chlorine gas at one of electrodes (relative atomic mass of Na = 23, Hg = 200; 1 Faraday constant = 96500 Coulombs mol^-1) :

The total number of moles of chlorine gas evolved is:
a) 3.0
b) 2.0
c) 0.5
d) 1.0 

Solution: 

First we have to calculate the amount of NaCl present in 4.0 molar solution.

The number of moles of NaCl in 500 mL of 4.0 molar solution = Molarity x Volume (in L) = 4.0 x 500 x 10^-3 = 2.0 moles.
The following reaction occurs during electrolysis of aqueous solution of NaCl.

Dissociation of NaCl: 2 NaCl(aq) ------> 2Na⁺(aq) + 2Cl^-(aq)

At cathode: 2H₂O + 2e- -------> H₂ + 2OH^-

At anode: 2Cl^-(aq) ---------> Cl₂(g) + 2e^-

Complete reaction: 2NaCl(aq) + 2H₂O ------>H₂ + Cl₂(g) + 2Na+(aq) + 2OH^-(aq)

That means, two moles of NaCl gives one mole of Cl₂ gas.

Since there are 2.0 moles of NaCl present in the solution, one mole of Cl₂ gas will be evolved at anode upon complete electrolysis.

Conclusion: The correct option is 'd'.

Electrolysis of Molten Sodium Chloride 

Electrolysis of molten sodium chloride (NaCl) is a chemical process that involves the decomposition of sodium chloride into its constituent elements, sodium (Na) and chlorine (Cl), using electricity. Here's how the electrolysis of molten sodium chloride works:

• Setup: The process typically takes place in an electrolytic cell, which consists of a container with two electrodes (conducting rods or plates) immersed in molten sodium chloride. The container itself should be made of a material that can withstand high temperatures and the corrosive nature of molten salt.

• Electrolyte: In this case, the molten sodium chloride serves as the electrolyte. In the molten state, the sodium and chloride ions are free to move and carry electric charge.
• Electrodes: There are two electrodes, one called the cathode and the other called the anode. The cathode is usually made of a material that won't react with sodium, such as graphite, and the anode is typically made of a material that won't react with chlorine, such as titanium.
• Electrolysis: When an electric current is passed through the molten sodium chloride, several reactions occur at the electrodes:
  
  
• At the cathode: Sodium ions (Na⁺) migrate to the cathode and gain electrons. This results in the formation of sodium metal (Na) at the cathode:
  2Na⁺(l) + 2e⁻ → 2Na(l)
• At the anode: Chloride ions (Cl⁻) migrate to the anode and lose electrons. This leads to the formation of chlorine gas (Cl₂) at the anode:
  2Cl⁻(l) → Cl₂(g) + 2e⁻

Na⁺ accumulates at negatively charged electrode and Cl^- ions will surround positively charged electrode

Overall Reaction: The net result of the electrolysis is the decomposition of sodium chloride into sodium metal and chlorine gas:
2NaCl(l) → 2Na(l) + Cl₂(g)

• Collection of Products: Sodium metal is less dense than the molten sodium chloride, so it rises to the surface and can be collected. Chlorine gas is released at the anode and can be collected as well.
• Electrolysis with Ionic Electrolytes: If the electrolyte contains more than one type of cation and anion, such as in an aqueous solution of an ionic electrolyte, the specific ions discharged depend on various factors like size, mass, positive charge, and negative charge. Qualitatively predicting which ion will be discharged first is challenging, as one factor may enhance the ability to discharge while another factor may hinder it.

• Standard Potential (SRP) Values: To determine which ion is discharged preferentially, the concept of standard potential is used. Standard reduction potential (SRP) values for cations are compared, with the cation having a higher SRP value being discharged preferentially, provided the ions are at a concentration of 1 M.

• Standard Oxidation Potential (SOP) Values: For anions, standard oxidation potential (SOP) values are compared. Anions with higher SOP values are preferentially discharged, again assuming a 1 M concentration for each ion. The standard potential values are determined under specific conditions: 1 M concentration, 1 atm pressure for gases, and a temperature of 25°C.

Solved Examples on Electrolysis

1. Na, Cl₂
2. _{Na, O2}
3. H₂, Cl₂
4. H2, O2

Answer: H₂, Cl₂ 
Solution:

Q.2. What is the product formed at the cathode in the electrolysis of aqueous CuSO4?

a) Copper metal

b) Oxygen gas

c) Hydrogen gas

d) Sulphur

Answer: a 
Solution: In the electrolysis of aqueous CuSO₄, Cu²⁺, SO₄²⁺, H⁺ and OH^– are the ions formed after dissociation. Copper ions have much higher reduction potential than water. Hence, these ions are easily reduced and deposited as Cu at the cathode

Products of Electrolysis

1. Products of electrolysis depend on the material being electrolyzed. In other words, the nature of electrolyte governs the process of electrolysis. The process is fast for a strong electrolyte whereas for a weak electrolyte an extra potential better known as overpotential is required.
Products of electrolysis depend on upon the value of this over potential too.

2. Products of electrolysis depend on the nature of electrodes too. That is, in the case of the inert electrode (say gold, platinum), it doesn’t participate in the reaction whereas if the electrode used is reactive in nature it takes part in the reaction.

3. Various oxidising and reducing species present in the electrolytic cell do affect the products of electrolysis.

4. The products of electrolysis depend on standard electrode potentials of the different oxidizing and reducing species present in the electrolytic cell.

5. In the case of multiple reactions, the product of electrolysis depends on the standard electrode potential of various reactions taking place. For example, electrolysis of an aqueous solution of sodium chloride. Out of the multiple reduction reactions taking place, the reduction reaction which has the highest value of standard electrode potential takes place at the cathode. Similarly, out of the multiple oxidation reactions, the oxidation reaction which has the lowest value of standard electrode potential takes place at the anode.

The SRP Values at 25º C for some of the reduction half-reaction are given in the table below. 

• When a solution of an electrolyte contains more than one type of cations and anions at concentrations different than 1 M, the discharge of an ion does not depend solely on standard potentials but also depends on the concentration of an ion in the solution.
• The value is referred to as potential, called reduction potential for cation and oxidation potential for the anion. The relation between reduction potential and standard reduction potential is given by the Nernst Equation, as

• Where E_RP = Reduction potential of cation and Eº_RP = Standard reduction potential of the cation. 
• Thus, it is possible that a cation (A⁺ ) with lower standard reduction potential getting discharged in preference to cation (B⁺) having higher standard reduction potential because their concentration might be such that the reduction potential of A  is higher than that of B⁺.
• When two metal ions in the solution have identical values of their reduction potentials, the simultaneous deposition of both the metals will occur in the form of an alloy.

Some Important Questions 

Q.1.  What is the product formed at the cathode in the electrolysis of molten NaCl?
a) Chlorine gas
b) Sodium metal
c) Hydrogen gas
d) Oxygen gas

Answer: b
Explanation: In the electrolysis of NaCl, if the electrolyte is molten NaCl, then the only ions formed after dissociation are Na+ and Cl– ions. The cathode being a negatively charged electrode attracts the positive Na+ ions and neutralizes it to form Sodium metal.

Q.2.  What is the product formed at the cathode in the electrolysis of aqueous Na₂SO₄?
a) Sodium metal
b) Oxygen gas
c) Hydrogen gas
d) Sulphur

Answer: c
Explanation: Na₂SO₄ dissociates into Na⁺ and SO₄^2- ions in the electrolysis of aqueous Na₂SO₄. Na⁺ has much lower reduction potential than water and hence Na⁺ ions are not reduced at the cathode. Instead, reduction of water occurs giving out hydrogen gas at the cathode.

Q.3. What is the product formed at the cathode in the electrolysis of aqueous CuSO4?

a) Copper metal
b) Oxygen gas
c) Hydrogen gas
d) Sulphur

Answer: a
Explanation: In the electrolysis of aqueous CuSO₄, Cu²⁺, SO₄²⁺, H⁺ and OH^– are the ions formed after dissociation. Copper ions have much higher reduction potential than water. Hence, these ions are easily reduced and deposited as Cu at the cathode.

Q.4. What are the two electrodes used in Daniell cell?

a) Pt and Cu
b) Al and Zn
c) Al and Pt
d) Zn and Cu

Answer: d
Explanation: The two electrodes that are used in a Daniell cell are zinc (as anode) and copper (as cathode) electrodes which are dipped in a solution containing its own ions, generally zinc sulphate and copper sulphate.

Q.5. What is the electrolyte used in the electroplating of gold?

a) Molten gold
b) [AgCN₂]^–
c) AuCN
d) AuCl₃

_{Answer: c
Explanation: The electrolyte in electrolysis should contain the metal to be coated, gold in this case. AuCN is used because it is exceptionally stable and doesn’t resist the flow of Au⁺ ions from anode to cathode.}