Electrochemistry JEE Notes

Electrochemistry is the branch of chemistry that deals with the changes caused in matter by passing an electric current, and converting chemical energy to electrical energy and vice versa. The cells and the batteries are examples of devices that convert chemical energy into electrical energy.

Table of Contents

• Conductors and Non-Conductors
• Distinguish between Metallic and Electrolytic Conduction
• Electrolytes
• Electrode
• Electrolysis
• Arrhenius Theory of Electrolytic Dissociation
• Electrolytic Conductance
• Kohlrausch’S Law
• Electrochemical Cell
• Standard Electrode Potential
• Electrochemical Series
• Table of Electrochemical Series
• Nernst Equation
• Relation between Gibbs Free Energy and Emf
• Solved Examples
• Practice Problems
• Frequently Asked Questions

Conductors and Non-Conductors

Substances can be classified as conductors and non-conductors based on their ability to conduct electricity.

Conductors: Substances that allow electric current to flow through them are called conductors. For example, Plastic, Wood, etc.

Non-Conductors: Non-conductors are insulators that do not allow electricity to pass through them. For example, Copper, Iron, etc.

Types of Conductors

Conductors are divided into two groups: Metallic conductors and Electrolytes.

Metallic Conductors: These conductors conduct electricity by the movement of electrons without any chemical change during the process. This type of conduction happens in solids and in the molten state.

Electrolytes: They conduct electricity by the movement of the ions in the solutions. It is present in the aqueous solution.

Distinguish between Metallic and Electrolytic Conduction

Electrolytes

(a) Substances whose aqueous solutions allow the conductance of electric current and are chemically decomposed are called electrolytes.

(b) The positively charged ions furnished by the electrolyte are called cations, while the negatively charged ions furnished by the electrolyte are called anions.

Types of Electrolytes

(a) Weak electrolytes: Electrolytes that are decomposable to a very small extent in their dilute solutions are called weak electrolytes. For example, organic acids, inorganic acids and bases etc.

(b) Strong electrolytes: Electrolytes that are highly decomposable in aqueous solution and conduct electricity frequently are called electrolytes. For example, mineral acid and salts of strong acid.

Electrode

For the electric current to pass through an electrolytic conductor, the two rods or plates called electrodes are always needed. These plates are connected to the terminals of the battery to form a cell. The electrode through which the electric current flows into the electrolytic solution is called the anode, also called the positive electrode, and anions are oxidised here.

An electrode through which the electric current flows out of the electrolytic solution is called the cathode, also called the negative electrode, and cations are reduced there.

Electrolysis

Electrolysis is the process of chemical deposition of the electrolyte by passing an electric current. Electrolysis takes place in an electrolytic cell. This cell will convert the electrical energy to chemical energy.

The product of electrolysis will depend on the following factors:

(i) The nature of the electrolyte

(ii) The nature of the electrodes

(iii) The concentration of the ions in the substance

(iv) The amount of current passed

Faraday established the relation between the amount of material deposited at the electrode or liberated and the amount of current passed through the electrolyte.

Faraday’s laws of electrolysis – First law

The amount of ion oxidised or reduced at either electrode during the passage of current is proportional to the quantity of electricity passed.

w ∝ Q

or w ∝ (I x t)

w = ZIt

Where,

w is the mass of the material deposited on the electrode or liberated

Q is the amount of charge utilised

t is the time for which current was passed through the electrolyte

I is the strength of the current in amperes

Z is the electrochemical equivalent

Faraday’s laws of electrolysis – Second law

During electrolysis, when the same quantity of electricity passes through the electrolytic solution, a number of different substances liberated are proportional to their chemical equivalent weights (Equivalent weight is defined as the ratio of the atomic mass of metal and the number of electrons required for reducing the cation).

W ∝ E

W/E = F (constant)

F = 96500 C per mole = Faraday constant

Application of electrolysis

(a) In the preparation of chemicals

(b) In the extraction of metals

(c) Preparation of organic compounds

(d) Corrosion and their prevention

Examples of electrolysis

Arrhenius Theory of Electrolytic Dissociation

The properties of the electrolytic solutions were explained by the Arrhenius theory of electrolytic solution. The important points of the theory are as follows:

• When the electrolyte is dissolved in water, it will break into positive charges and negative charges. These charged particles are called ions. The negatively charged ions are called anions, and the positively charged ions are called cations.
• Ionisation is the process by which the molecules are split into ions in an electrolyte.
• The degree of dissociation or degree of ionisation is given by the equation,

α = Number of molecules dissociated into ions/Total number of molecules

• When electricity is passed through the electrolytic solution, the cations will move towards the cathode, and the anions move towards the anode.
• The ions are constantly reuniting to form molecules; hence, there is a dynamic equilibrium between the ionised and non-ionised states.
• The electrolytic solution will always have a neutral charge.
• The properties of the electrolytic solutions will be the properties of the ions present in the solution.
• The conductivity of the electrolytic solution will depend on the number of ions and their nature.

Electrolytic Conductance

The conductance is the property of the conductor which permits the flow of electricity through it. It is equal to the reciprocal of the resistance.

Conductance = 1/Resistance = 1/R

(a) Ohm’s law: According to this law, the current flowing through a conductor at a given standard temperature is directly proportional to the potential difference (V) and inversely proportional to the resistance (R).

I = V/R

(b) Specific resistance (ρ): The resistance between two opposite faces of one cm cube of metal is called the specific resistance. It is also called resistivity.

ρ = R.a/l

The unit of specific resistance is the ohm. cm

(c) Specific conductance or conductivity (k): It is the reciprocal of specific resistance. It is called specific conductance.

K = 1/ρ

(d) Electrical conductivity (C) :

It is the reciprocal of resistance which is expressed as ohm^-1 or mho

C = 1/R

The unit of electrical conductivity is siemens.

(e) Equivalent conductivity

The conducting power of all the ions furnished by one equivalent of an electrolyte in any solution is called equivalent conductivity

The equivalent conductivity is expressed as

Equivalent conductivity (Λ_eq) = Conductivity (K)/Concentration in equivalents per unit volume (C_eq).

(d) Molar conductivity

The conducting power of all the ions furnished by one mole of an electrolyte in any solution is called molar conductivity.

Λ_m = K/C_m

Relation between molar conductivity and equivalent conductivity

According to the definition,

Λ_m = K/C_m ——–(1)

And Λ_eq = K/C_eq ——–(2)

Using equations (1) and (2), we get

C_m/C_eq = Λ_eq /Λ_m

Λ_eq /Λ_m = 1/z

Λ_m = z x Λ_eq

Where z can have values equal to 1,2,3,…..

Kohlrausch’s Law

Kohlrausch’s law states that at infinite dilution, the conductivity of an electrolyte solution is equal to the addition of the conductivity of both ions (which are in the given electrolyte).

Cell

A combination of two electrodes is called the cell.

Types of Cells

Cells are divided into the following two types:

Electrolytic cells: In this type of cell, electrical energy is converted into chemical energy.

Electrochemical cells: In this type of cell, chemical energy is converted into electrical energy.

Electrochemical Cell

It is a device that produces electricity as a result of a chemical reaction.

The cells work on oxidation-reduction reaction.

Anode

The electrode of negative polarity at which the oxidation occurs.

Cathode

The electrode of positive polarity at which the reduction occurs.

Salt Bridge

The salt bridge completes the circuit of an electrochemical cell, thereby allowing the flow of current through it. It also helps maintain the overall electrical neutrality of the cell. Generally, a U-tube containing a solution of KCl or NH₄NO₃ sets the Agar Agar powder in the form of a jelly-like structure.

The simplest form of the electrochemical cell is Daniel Cell.

The reaction that occurs at the two electrodes are as follows:

At anode: Zn (s) ⟶ Zn²⁺(aq) + 2e^–

At cathode: Cu²⁺(aq) + 2e^– ⟶ Cu (s).

Electrode Potential

When a metal is placed in a solution of its ions, the metal acquires either a positive or negative charge with respect to the solution. On account of this, a definite potential difference is developed between the metal and the solution. This potential difference is called electrode potential. For example, if a zinc plate is placed in a solution containing Zn²⁺ ions, it will become negatively charged with respect to the solution. This will create a potential difference between the plate and the solution. This is called the electric potential of zinc.

Oxidation potential: When the electrode is charged negatively with respect to the solution, i.e., when the electrode acts as an anode, oxidation occurs.

M → Mⁿ⁺ + ne^–

Reduction potential: When the electrode is charged positively with respect to the solution, i.e., when the electrode acts as a cathode, reduction occurs.

Mⁿ⁺ + ne^– → M

EMF of the cell

The EMF of the cell is equal to the sum of the potential on the two electrodes. It is the sum of the oxidation potential of the anode and the reduction potential of the cathode.

Standard Electrode Potential

The potential difference developed between a metal electrode and the solution of its ion of unit molarity (1M) at 25⁰ C is called the standard electrode potential.

Electrochemical Series

By measuring the potential of various electrodes versus hydrogen electrodes (SHE), a series of standard electrode potentials has been established.

When the electrode is in contact with its ions, it is arranged on the basis of the values of their standard reduction potentials or standard oxidation potentials. The resulting series is called the electrochemical or electromotive, or activity series of the elements.

Table of Electrochemical Series

Characteristics of Electrochemical Series

(a) A negative sign of standard reduction potential indicates that an electrode, when joined with SHE, acts as an anode, and oxidation occurs on this electrode. Similarly, the positive sign of SRP indicates that an electrode, when joined with SHE, acts as the cathode, and reduction occurs on this electrode.

(b) The substances which are stronger reducing agents than hydrogen are placed above hydrogen in the series.

(c) The substances which are stronger oxidising agents than H⁺ ions are placed below hydrogen in the series.

(d) The metals on top are called active metals, and activity decreases from top to bottom.

Application of Electrochemical Series

(i) Reactivity of metals

(a) Alkali metals and alkaline earth metals have high negative values of SRP, which are chemically active. These metals react with cold water and evolve hydrogen, and readily dissolve in acids.

(b) Metals like Fe, Pb, Sn, Ni etc., do not react with cold water but react with steam to evolve hydrogen.

(c) Metals U, Cu, Ag, Au etc., which lie between hydrogen, are less reactive and do not evolve hydrogen from water.

(ii) The electropositive character of metals

The electropositive character of metals decreases from top to bottom.

(iii) Displacement reactions

To predict whether a given metal will displace another from its salt solution, the metal having low SRP will displace the metals from its salt solution, which has a higher value of SRP.

(iv) Reducing power of metals

Reducing nature decreases from top to bottom in the electrochemical series.

(v) Oxidising nature of non-metals

Oxidising nature increases from top to bottom in the electrochemical series.

(vi) Thermal stability of metallic oxides

The thermal stability of metal oxide decreases from top to bottom.

(vii) Products of electrolysis

The ion, which is a stronger oxidising agent, is discharged first at the cathode.

K⁺, Ca²⁺, Na⁺, Mg⁺², Al⁺³, Zn⁺², Fe⁺², H⁺, Cu⁺², Ag⁺, Au⁺³

Increasing order of deposition

(viii) Corrosion of metals

Corrosion is defined as the deterioration of a substance because of its reaction with its environment. The corrosion tendency decreases from top to bottom.

(ix) Extraction of metals

Ag and Au are extracted by the cyanide process.

Nernst Equation

Nernst equation is an equation relating the cell potential to the standard potential and to the activities of the electrically active species. The standard cell potential is related to the effective concentrations of the components.

Consider the reaction,

Mⁿ⁺_(aq) + ne^– → M_(s)

According to the Nernst equation,

E_cell = E₀ – [RT/zF] ln Q

Where,

E_cell = cell potential of the cell

E₀ = cell potential under standard conditions

R = universal gas constant

T = temperature

z = number of electrons transferred in the redox reaction

F = Faraday constant

Q = reaction quotient

Q = [a]ⁿ/[b]ⁿ

a is the concentration of the products

b is the concentration of the reactants

n is the number of moles

So, the Nernst equation becomes,

On simplifying, we get

Relation between Gibbs Free Energy and EMF

The Gibbs free energy can be calculated by multiplying the total charge driven through the cell and the potential difference. Thus,

-ΔG = Total charge x EMF of the cell

-ΔG = nF x E_cell

The negative sign shows a decrease in free energy. As the EMF of the cell becomes more and more positive, the Gibbs free energy will become more and more positive.

Points to remember

• Electrochemistry is the branch of Chemistry that deals with the changes caused in matter by passing an electric current and converting chemical energy to electrical energy and vice versa.
• Substances whose aqueous solutions allow the conductance of electric current and are chemically decomposed are called electrolytes.
• For the electric current to pass through an electrolytic conductor, the two rods or plates called electrodes are always needed.
• Electrolysis is the process of chemical deposition of the electrolyte by passing an electric current.
• Faraday established the relation between the amount of material deposited at the electrode or liberated and the amount of current passed through the electrolyte.
• The properties of the electrolytic solutions were explained by the Arrhenius theory of electrolytic solution.
• The reciprocal of specific resistance is called specific conductance.
• Kohlrausch’s law states that at infinite dilution, the conductivity of an electrolyte solution is equal to the addition of the conductivity of both ions.
• An electrochemical cell is a device that produces electricity as a result of a chemical reaction.
• The potential difference between the electrode and the electrolytic solution is called the electric potential.
• The standard cell potential is related to the effective concentrations of the components by the Nernst equation.

Solved Examples

1) Read the following statements and predict the corresponding law. At infinite dilution, when dissociation is complete, each ion makes a definite contribution towards the total equivalent conductance of the electrolyte, irrespective of the nature of the ion.

1) Ostwald’s-dilution law

2) Kohlrausch’s law

3) Nernst equation

4) Ohm’s law

Answer: 2) Kohlrausch’s law

Solution:

Ostwald’s dilution law: It gives the relation between the dissociation constant (Kd) and the degree of dissociation (∝) of a weak electrolyte.

Kohlrousch’s law: At infinite dilution, when dissociation is complete, each one makes a definite contribution towards the total equivalent conductance of the electrolyte, irrespective of the nature of the ion.

Nernst equation: In electrochemistry, the Nernst equation is an equation that relates the reduction potential of a half-cell (or the total voltage, i.e., the emf of the full cell) at any point in time to the standard electrode potential, temperature, activity and reaction quotient of the underlying reaction and species used.

Ohm’s law: Ohm’s law states that the current through a conductor between two points is directly proportional to the voltage across the two points.

2) A solution of CuSO₄ is electrolysed for 10 minutes with a current of 1.5 A. What is the mass of copper deposited at the cathode?

1) 2.096 g

2) 0.296 g

3) 3.029 g

4) 2.096 g

Answer: 2) 0.296 g

Solution:

The reaction is,

Cu²⁺ + 2e^– → Cu(s)

Q = It

I = 1.5 Ampere

Time t = 10 x 60 = 600 sec

Applying the equation Q = It,

= 1.5 x 600

= 900 C

2 x 96500 is the amount of electricity required to deposit copper of 63.5 g at the cathode.

900 C will deposit

=[(63.5 x 900) /(2 x 96500)]

=0.296 g

3) Give the products available on the cathode and the anode, respectively, during the electrolysis of an aqueous solution of MgSO₄ between inert electrodes.

1) H₂ (g) and O₂ (g)

2) O₂ (g) and H₂ (g)

3) O₂ (g) and Mg (s)

4) O₂ (g) and SO₂ (g)

Answer: 1) H₂ (g) and O₂ (g)

Solution:

The electrolysis of an aqueous solution of magnesium using inert electrodes produces hydrogen at the cathode and oxygen at the anode, and a neutral solution of magnesium sulphate remains unaltered by the electrolysis.

Cathode reaction: 4H₂O + 4e^– → 2H₂ + 4OH^–

Anode reaction: 2H₂O → O₂ + 4H⁺ +4e^–

The overall reaction is,

6H₂O → 2H₂ + O₂ + 4H⁺ + 4OH^–

4) The standard emf of a cell involving one electron change is found to be 0.591 V at 25°C. The equilibrium constant of the reaction is (F = 96500 C mol^-1)

1) 1.0 × 10¹

2) 1.0 × 10⁵

3) 1.0 × 10¹⁰

4) 1.0 × 10³⁰

Answer: 3) 1.0 × 10¹⁰

Solution:

For a cell reaction in equilibrium at 298 K,

E⁰_cell = (0.0591/n) logK_c

log K_c = (E⁰_cell x n)/0.0591

= (0.591 x 1)/0.0591

= 10

K_c = antilog 10

= 1 x 10¹⁰

5) In which of the following pairs are the constant quantities not mathematically related to each other?

1) Gibbs free energy and standard cell potential

2) Equilibrium constant and standard cell potential

3) Rate constant and activation energy

4) Rate constant and standard cell potential

Answer: 4) Rate constant and standard cell potential

Solution:

1. ΔG⁰ = -nFE⁰_cell
2. E⁰_cell = (0.0591/n) logK_c
3. K = Ae ^-(Ea/RT)
4. Rate constant and standard cell potential are not related to each other

Practice Problems

1) The standard emf of a galvanic cell involving cell reaction with n = 2 is found to be 0.295 V at 25C°. The equilibrium constant of the reaction will be

(Given, F = 96500 C mol^-1, R = 8. 314 JK^-1 mol^-1 )

1) 2.0 × 10¹¹

2) 4.0 × 10¹²

3) 1.0 × 10²

4) 1.0 × 10¹⁰

2) When an electric current is passed through acidified water for 1930 s, 1120 mL of H₂ gas is collected (at STP) at the cathode. What is the current passed in amperes?

1) 0.05

2) 0.50

3) 5.0

4) 50

3) Which of the following metals cannot be obtained by electrolysis of the aqueous solution of their salts?

1) Ag and Mg

2) Ag and Al

3) Mg and Al

4) Cu and Cr

4) The ionic conductance of Ba²⁺ and Cl ^– are 127 and 76Ω^– cm², respectively, as infinite dilutions. The equivalent conductance (in Ω^-1 cm²) of BaCl₂ at infinite dilution will be

1) 330

2) 203

3) 139.5

4) 51

5) Resistance of a conductivity cell filled with a solution of an electrolyte of concentration 0.1 M is 100 Ω. The conductivity of this solution is 1.29 Sm^-1. The resistance of the same cell, when filled with 0.2 M of the same solution, is 520Ω. The molar conductivity of 0.02 M solution of the electrolyte will be

1) 124 × 10^-4 S m² mol^-1

2) 1240 × 10^-4 S m² mol^-1

3) 1.24 × 10^-4 S m² mol^-1

4) 12.4 × 10^-4 S m² mol^-1

Electrochemistry – Important Topics

Electrochemistry – Important Questions