Ch 5Electrochemistry — Class 12 Chemistry, concept-first.

Dry cells power our everyday electrical and electronic equipment because a chemical reaction inside them generates electricity — in a dry cell, chemical energy is converted into electrical energy.

Electric current represents a transfer of charge. A charge transfer — a flow of electricity — occurs through substances called conductors.

Electrical conduction through a metal involves a direct flow of electrons from one point to the other.

Electrolytic conduction involves conduction of electric current by the movement of ions of an electrolyte.

Measurement of the conductivities of solutions provides two kinds of information:

According to Ohm's law, the electrical resistance of a conductor equals the electric potential difference divided by the electric current :

We have seen that and that is directly proportional to length and inversely proportional to cross-sectional area. It therefore follows that is directly proportional to and inversely proportional to :

The electrolytic conductivity is not suitable for comparing conductivities of different solutions: the conductivity of a solution depends on the number of ions present in unit volume of the solution,…

Conductivity is the electrical conductance of of solution. If is the volume of solution in containing 1 mole of dissolved electrolyte, its electrical conductance is .

The variation of conductivity with concentration follows from what conductivity measures:

The molar conductivity behaves differently from conductivity on dilution:

The variation of molar conductivity with concentration is qualitatively different for strong and weak electrolytes.

Kohlrausch's law of independent migration of ions states that at infinite dilution each ion migrates independently of its co-ion, and contributes to the total molar conductivity of the electrolyte irr…

The degree of dissociation () of a weak electrolyte is related to its molar conductivity at a given concentration by the equation

The conductivity of a solution can be determined from resistance measurements by the Wheatstone bridge.

An electrochemical cell consists of two metal plates or carbon (graphite) rods. These electronic conductors are dipped into an electrolytic or ionic conductor — an aqueous electrolyte solution or a mo…

The chemical reaction occurring in an electrochemical cell involves transfer of electrons from one species to the other — it is a redox reaction, which we learnt in Standard XI (Chapter 6).

Electrodes are the surfaces on which the oxidation and reduction half reactions take place. Electrodes may or may not participate in the reactions; the electrodes which do not take part in the reactio…

There are two types of electrochemical cells.

An electrolytic cell consists of a container in which an electrolyte is placed. Two electrodes are immersed in the electrolyte and connected to a source of direct current.

Construction of cell : The electrolytic cell consists of a container in which fused (molten) NaCl is placed.

Electrolysis of an aqueous NaCl solution can be carried out in the cell used for the electrolysis of molten NaCl, with inert electrodes (Fig.

a. The mass of reactant consumed or the mass of product formed at an electrode during electrolysis can be calculated by knowing the stoichiometry of the half reaction at the electrode.

In galvanic or voltaic cells, electricity is generated through the use of spontaneous chemical reactions.

In a galvanic cell the two solutions are connected by a salt bridge. It is a U tube containing a saturated solution of an inert electrolyte, such as KCl or , and 5 % agar solution.

A galvanic cell is represented by a formula or short notation that includes electrodes, aqueous solutions of ions and other species which may or may not be involved in the cell reaction.

The cell reaction corresponding to a cell notation is written on the assumption that the right hand side electrode is the cathode (+) and the left hand side electrode is the anode (-).

A galvanic cell is composed of two half cells, each consisting of electronic (metal plates) and electrolytic (solution of ions) conductors in contact.

The electrode potential and the cell potential depend on the concentrations of solutions, the pressures of gases and the temperature.

The standard cell potential tells us whether or not the reactants in their standard states form the products in their standard states spontaneously.

The heading groups two closely related results: how the electrical work obtainable from a galvanic cell relates to the Gibbs energy change of the cell reaction (5.8.1), and how the standard cell poten…

The electrical work done in a galvanic cell is the electricity (charge) passed multiplied by the cell potential:

The relation between the standard Gibbs energy change of a cell reaction and the standard cell potential is given by Eq. (5.28): .

Every oxidation needs to be accompanied by reduction — the occurrence of only oxidation or only reduction is not possible (refer to the Std. XI Chemistry Textbook, Chapter 6).

Construction : SHE consists of a platinum plate coated with platinum black, used as the electrode. This plate is connected to the external circuit through a sealed narrow glass tube containing mercury…

Voltaic (or galvanic) cells in common use can be classified as primary and secondary cells.

The dry cell is a cell without a liquid component — but the electrolyte is not completely dry: it is a viscous aqueous paste.

The lead accumulator stores electrical energy due to the regeneration of the original reactants during recharging. It functions as a galvanic cell and as an electrolytic cell as well.

The nickel-cadmium cell is a secondary dry cell — in other words, it is a dry cell that can be recharged.

The mercury battery consists of a zinc anode amalgamated with mercury. The cathode is a paste of Hg and carbon. The electrolyte is strongly alkaline, made of a paste of KOH and ZnO.

The functioning of fuel cells is based on the fact that combustion reactions are of redox type — and hence can be used to generate electricity.

In the - fuel cell, the fuel is hydrogen gas and oxygen gas is the oxidising agent. The energy of the combustion of hydrogen is converted into electrical energy.

The standard potentials of a number of electrodes have been determined using the standard hydrogen electrode.

[!NOTE] The book's own numbering of this group runs i.-vi. and then jumps directly to viii. - no item vii. is printed. The numbering here follows the book exactly.

Sample papers and previous-year board questions for this subject.