CBSE Class 12 Chemistry Chapter 3 Revision Notes

Chapter 3: Electrochemistry Revision Notes

• Oxidation is defined as an electron loss, whereas reduction is defined as an electron gain.
• Both oxidation and reduction reactions occur simultaneously in a redox reaction.
• Both oxidation and reduction reactions take place in the same vessel in a direct redox reaction.
• In a direct redox reaction, chemical energy is converted to heat energy.
• Indirect redox reaction: Oxidation and reduction take place in separate vessels.
• Chemical energy is converted into electrical energy in an indirect redox reaction.

Electrochemical Cells

• An electrochemical cell is a device that converts chemical energy into electrical energy.
• The oxidation half-cell is an electrochemical half cell in which oxidation takes place.
• The reduction half-cell is the half-cell in which reduction occurs.
• The anode, which is negatively charged, undergoes oxidation, while the cathode, which is positively charged, undergoes reduction.
• Electric current flows in the opposite direction as electrons are transferred from anode to cathode.
• A metal plate is dipped into an electrolytic solution of its soluble salt to create an electrode.
• A salt bridge is a U-shaped tube made of agar-agar and gelatine that contains an inert electrolyte.
• By completing the electrical circuit, a salt bridge maintains electrical neutrality and allows the flow of electric current.

Source

Electrode Potential

• The potential difference that develops between the electrode and its electrolyte is known as electrode potential.

• The potential difference between the metal and the solution of its ions is the result of charge separation at equilibrium. It’s a metric for how likely an electrode in a half cell is to lose or gain electrons.

• Potential of a standard electrode: The electrode potential is known as standard electrode potential when the concentration of all species involved in a half cell is unity. E is the symbol for it.

• Electrode potentials come in a variety of shapes and sizes. There are two kinds of electrode potentials: Oxidation potential and reduction potential

• The tendency of an electrode to lose electrons or become oxidized is known as oxidation potential.

• The tendency of an electrode to gain electrons or get reduced is known as reduction potential.

• The inverse of reduction potential is oxidation potential.

• The electrode with a higher reduction potential has a greater tendency to gain electrons, and thus functions as a cathode, whereas the electrode with a lower reduction potential functions as an anode.

• An electrode’s standard electrode potential cannot be measured in isolation.

• The Standard Hydrogen Electrode is used as a reference electrode and has a zero potential at all temperatures, according to convention.

• SHE: A standard hydrogen electrode is made up of a platinum wire that is sealed inside a glass tube and has a platinum foil on one end.

• The electrode is immersed in a beaker filled with an aqueous solution of an acid containing 1 mol of hydrogen ions.

• At 298 K, hydrogen gas is continuously bubbled through the solution at 1 bar pressure.

• The Platinum foil is where the oxidation or reduction takes place.

• Both anode and cathode functions are possible with a standard hydrogen electrode.

• A substance with a higher reduction potential value has a higher likelihood of being reduced. As a result, it’s a good oxidizer.

• A substance with a lower reduction potential value has a higher likelihood of being oxidized. As a result, it works well as a reducing agent.

• The cathode is the electrode with the highest reduction potential, while the anode is the electrode with the lowest reduction potential.

• Cell potential is the difference in potential between the two electrodes of a galvanic cell, and it is measured in Volts.

• The cell potential is the difference between the cathode and anode reduction potentials.

E cathode – E anode = E cell

• When no current is drawn through the cell, its potential is referred to as the electromotive force of the cell (EMF).

Nernst Equation

• Nernst looked at how an electrode’s electrode potential changed with temperature and electrolyte concentration.

• Nernst devised a formula for calculating the standard electrode potential EΘ and the electrode potential E.

• Electrode potential rises as the concentration of the electrolyte rises and the temperature falls.

• When applied to a cell, the Nernst equation aids in calculating the cell potential.

• Ecteell’s cell potential is zero at equilibrium.

• The equilibrium constant Kc and the standard cell potential EΘcell have the following relationship:

• The decrease in Gibbs energy equals the work done by an electrochemical cell.

Conductance of Electrolytic Solution

• Conductors are materials that allow electricity to pass through them.

• Every conducting material obstructs the flow of electricity in some way, which is referred to as resistance. It is measured in ohms and is denoted by the letter R.

• Any object’s resistance is proportional to its length l, and inversely proportional to its cross section area A.

• Where the term ρ is used to denote specific resistance or resistivity.

• Ohm meter is the SI unit for specific resistivity.

• Conductance, G, is the inverse of resistance.

• The ohm-1 or mho unit is used for conductance. It is also denoted by the letter S in Siemens.

• Conductivity is known as the inverse of resistivity. It is symbolized by the symbol

.

• Sm-1 is the SI unit for conductivity.

Cell constant = Conductivity = Conductance

• There are two issues with measuring the resistance of an ionic solution:

• First, passing direct current alters the solution’s composition.

• Second, unlike a metallic wire or a solid conductor, a solution cannot be connected to the bridge.

• The problem of measuring the resistance of an ionic solution can be solved by using an alternating current source, and the second problem can be solved by using a conductivity cell, which is a specially designed vessel.

• A conductivity cell is made up of two Pt electrodes that have been coated with Pt black.

• They are separated by a distance ‘l’ and have an area of cross section A.

• The molar conductivity of a solution is defined as: It’s the total conducting power of all the ions produced when 1 mole of an electrolyte is dissolved in solution.

• Molar conductivity is the ability of a molecule to conduct electricity.

• Conductivity equivalent: It is the conductivity of all the ions produced when one gram of an electrolyte is dissolved in solution. S cm2 (g equiv) -1 is the equivalent conductivity unit.

• Kohlrausch’s Law of independent migration of ions: According to this law, an electrolyte’s molar conductivity can be expressed as the sum of individual contributions from its individual ions at infinite dilution.

• Dissociation degree: It’s the ratio of molar conductivity at a particular concentration ‘c’ to molar conductivity at infinite dilution.

• The Faraday constant is defined as follows: It’s the same as the charge on one mol of electrons. It is approximately equal to 96500 C mol-1 or 96487 C mol-1.

• Faraday’s First Law of Electrolysis: During electrolysis, the amount of substance deposited is proportional to the amount of electricity passed.

• Faraday’s Second law of Electrolysis: When the same charge is applied to different electrolytes, the amount of substance deposited is proportional to their respective weights.

Product of Electrolysis

• The electrolysis products are influenced by a variety of factors.
• The nature of the electrolyte and electrodes to be electrolyzed.
• Inert electrodes, such as platinum or gold, do not participate in chemical reactions, meaning they do not lose or gain electrons.
• When electrodes are reactive, they participate in chemical reactions and produce different results than inert electrodes.
• The oxidizing and reducing species‘ electrode potentials.
• Some electrochemical processes, while feasible but slow at lower voltages, necessitate extra voltage, i.e. voltage above that at which these processes will occur.
• Electrolysis products differ in terms of molten state and electrolyte aqueous solution.

Batteries

• Primary cells are the cells that make up the body. A primary cell, such as a Daniel cell, a dry cell, or a mercury cell, is one in which the reaction in the cell produces electrical energy. It is not rechargeable.
• Secondary cells are those that are used to store electricity, such as a lead storage battery or a nickel-cadmium cell. They are rechargeable.
• Nickel cadmium cell is a type of secondary cell that lasts longer than a lead storage cell but is more expensive to produce.

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