NEET Chemistry Notes Electrochemistry – Nernst Equation
The relationship between electrode potential and concentration of solution is called Nernst equation.
Applications of Nernst Equation
There are two important applications of Nernst equation as given below:
Some Important Relationships in Electrochemistry
Relationship between Cell Potential and Gibbs Energy Change ∆G
In an electrochemical Cell, maximum work done is given by
where, F = Faraday’s constant
n = number of moles of electrons transferred
A galvanic cell does electrical work by transferring electrical charge through an external circuit. When small amount of current is drawn from the cell then
If ∆G of the system is positive then the process would be possible only if the surroundings do electrical work on the system (as in electrolysis).
If ∆G of the system is negative then the system does electrical work on the surroundings (as in galvanic cells). For a voltaic cell, the work is done on the surroundings, thus given a negative sign. Hence,
Relation between Cell Potential (E°cell), Free Energy ∆G° and Equilibrium constant (K)
The relation between cell potential (E°cell), free energy ∆G° and equilibrium constant (Kc) is given by,
where, n is the number of moles of electrons transferred in the balanced equation for the process to which you apply the Nernst equation.
Relation between Standard Potentials of Half-cells Containing a Metal in Different Oxidation States
If two half-reactions having potentials E1° and E2° are combined to give a third half-reaction having a potential E3°,then
In lead accumulator, the electrodes are made of lead and the electrolyte consists of dilute sulphuric acid. The electrodes are usually cast from a lead alloy containing 7-12% of antimony (to give increased hardness and corrosion resistance) and a small amount of tin (for better casting properties).
The electrodes are coated with,a paste of lead (II) oxide (PbO) and finely divided lead; after insertion into the electrolyte, a ‘forming’ current is passed through the cell to convert the PbO on the negative plate into a sponge of finely divided lead. On the positive plate, the PbO is converted to lead (IV) oxide (Pb02). The equation for the overall reaction during discharge is
The reaction is reversed during charging. Each cell gives an emf of about 2 V and in motor vehicles, a 12 V battery of six cells is usually used. The lead-acid battery produces 80-120 kj per kilogram.
A cell or a battery (arrangement of one or more cells connected in series) is basically a galvanic cell and used where the chemical energy of redox reaction is converted into electrical energy. There are two types of batteries:
The primary batteries are those in which the cell reaction occurs only once and the battery becomes dead after use over a period of time and cannot be reused again. Hence, primary batteries are not rechargeable., e.g. dry cells like Leclanche cell, mercury cell etc.
Dry cell or Leclanche cell is also called primary voltaic cell. In it, the electrode reactions are
The cell potential is 1.6 V.
Another type of dry cell is mercury cell. The electrode reactions for the cell are
These are also called reversible galvanic or voltaic cell. Secondary batteries are rechargeable because on charging, reaction becomes reverse, e.g. lead storage battery, nickel-cadmium cell etc.
In lead storage battery, a solution of sulphuric acid surrounds the plates and acts as an electrolyte, The battery consists of 6 cells, each contains lead anode and lead oxide cathode. The cell potential is 12 V. The half-cell reactions, when the battery is being used up are
These cells are another means by which chemical energy may be converted into electrical energy. Energy can be obtained indefinitely from a fuel cell as long as outside supply of fuel is maintained, e.g.H2-02 fuel cell.
The half-reactions are
Efficiency of the Fuel Cell
The efficiency of the fuel cell is the ratio of change in Gibbs energy ∆G to the heat of combustion ∆H and mathematically can be given as,
Uses of Fuel Cell
This cell was used as a primary source of eIectrical energy on the moon flights. The overall cell reaction produces water, which was used for drinking by the astronauts.
It is basically an electrochemical phenomenon. A metal is oxidised by loss of electrons to oxygen and form metal oxide, e.g. conversion of iron to rust [Fe203 • xHzO], the tarnishing of silver (due to formation
of Ag2S), development of a green coating on copper and bronze.
Corrosion of iron, known as rusting, occurs in the presence of water and oxygen.
Rusting of iron dap. be prevented by the following methods:
- Barrier protection through coating of paints or electroplating.
- Galvanisation or coating of surface with tin metal.
- By the use of anti-rust solutions.