Mastering Electrochemistry (Chemistry-Class XII)

Somak Chatterjee | March 7th, 2013 at 01:47pm

Type of cells:

1. Electrochemical cell / Galvanic cell- Convert chemical energy of a spontaneous redox reaction into electrical energy, example: Daniel cell

2. Electrolytic cell: Convert electrical energy into chemical energy, example: two copper strips dipped in an aqueous solution of copper sulphate. Copper is dissolved (oxidised) at the anode and is deposited (reduced) at the cathode.

Some important terms:

· Electrode potential − Potential difference developed between the electrode and the electrolyte

· Standard electrode potential − Electrode potential when the concentration of the species of the half-cell is unity

· Anode − Electrode where oxidation takes place

· Cathode − Electrode where reduction takes place

· Cell potential − Potential difference between the cathode and the anode

· Cell electromotive force (emf) − Potential difference between cathode and anode when no current is drawn

· SHE ( Standard Hydrogen Electrode): It has zero potential at all temperatures.

Nernst equation: For a general electrochemical reaction

Nernst equation is given by

Relation between cell potential and Gibbs energy: 

Relation between equilibrium constant and Gibbs energy:

Resistance(R) : Directly proportional to length (l) and inversely proportional to cross-sectional area (A), unit: Ohm

Resistivity : Resistance of a substance when it is 1 metre long and has 1 m² cross-sectional area

Conductance: Reciprocal of resistance

Specific conductance / Conductivity: Conductance of a substance when it is 1 m long and has 1 m² cross-sectional area, its SI unit is S m⁻¹, but is often expressed in S cm⁻¹.

Molar conductivity: Molar conductivity of a solution at a given concentration is the conductance of volume V of a solution containing 1 mole of the electrolyte, kept between two electrodes with the cross-sectional area of A and distance of unit length.  It is denoted as Λ_m.

Molar conductivity = Specific conductivity × V

Molar conductivity, 

Molar Conductivity for Strong Electrolytes:  

Molar Conductivity for Weak Electrolytes: The plot of against is given below.

Kohlrausch law of independent migration of ions: This law states that the limiting molar conductivity of an electrolyte can be represented as the sum of the individual contributions of its anion and cation.

Faraday’s law of electrolysis:

· First law − The amount of chemical reaction which occurs at any electrode during electrolysis by a current is proportional to the quantity of electricity passed through the electrolytic solution or melt.

· Second law − The amount of different substances liberated by the same quantity of electricity passing through the electrolytic solution is proportional to their chemical equivalent weights.

Q (coulomb) = I (ampere) × t (second)

· 1F (Faraday) = 96487 C mol⁻¹ ≈ 96500 C mol⁻¹

Types of electrodes:

1. Inert: They act only as source or sink for electrons instead of participating in the reaction. Example: Pt, Au

2. Active: They participate in the reaction.

Batteries: Battery is a galvanic cell in which chemical energy of the redox reaction is converted into electrical energy.

Types of batteries:

1. Primary Batteries- In primary batteries, reaction occurs only once and cannot be reused. Example: Dry cell (or Leclanche cell), Mercury cell

2. Secondary Batteries- Secondary batteries can be recharged again by passing current through them in the opposite direction. Example: Lead storage battery, Nickel-cadmium cell

Corrosion: It is the process of slow conversion of metals into their oxides (undesirable) by the action of moisture, oxygen and other gases of the atmosphere. In the process of corrosion, metals lose electrons to oxygen and get oxidised.

Hydrogen Economy- Use of hydrogen as a renewable and non-polluting source of energy

Questions asked previously

Q. Express the relation among cell constant, resistance of the solution in the cell and conductivity of the solution. How is molar conductivity of a solution related to its conductivity? (2012 Delhi Set 3)

Q. The molar conductivity of a 1.5M solution of an electrolyte is found to be 138.9 S cm² mol^-1. Calculate the conductivity of this solution? (2012 Delhi Set 3)

Q. The electrical resistance of a column of 0.05M NaOH solution of diameter 1 cm and length 50 cm is 5.55×10³ ohm. Calculate its resistivity, conductivity and molar conductivity? (2012 Delhi Set 3)

Q. Express the relation between conductivity and molar conductivity of a solution held in a cell? (2011 Delhi Set1)

Q. The chemistry of corrosion of iron is essentially an electrochemical phenomenon. Explain the reactions occurring during the corrosion of iron in the atmosphere? (2011 Delhi Set 1)

Q. Determine the values of ΔG^o for the following reaction:

Ni(s) + 2Ag⁺(aq) à Ni²⁺(aq) + 2Ag(s)   E^o = 1.05V ( 1F = 96500 C mol^-) (Delhi 2011 Set 1)

All the best,

Team Meritnation!