Electrochemistry 2

t fluorine gas (F2 ) has the maximum tendency to get reduced to fluoride ions (F– ) and therefore fluorine gas is the strongest oxidising agent and fluoride ion is the weakest reducing agent. reduction potential of F2= +2.87 V As the reduction potential becomes for +ve, the tendency to get reduced increases and hence it gains electrons easily. The oxidising tendency increases, of the species on leftt hand side while the reducing tendency decreases of the species on the right hand side.

Lithium has the lowest electrode potential indicating that lithium ion is the weakest oxidising agent while lithium metal is the most powerful reducing agent in an aqueous solution. As the value becomes more negative, the tendency to gain electrons ie get reduced decreases. Hence more negative electrode potential makes better reducing agents The reducing tendency increases of the species in the right hand side, while the oxidising tendency decreases of the species in the left hand side electode potential of Li+ is -3.05V

• calculate standard cell potential of cell • ## predict spontaneity of a reaction

It is the equation wich relates the electrode potential values and concentration. It can be seen that E(cell) depends on the concentration of both Cu2+ and Zn2+ ions. It increases with increase in the concentration of Cu2+ ions and decrease in the concentration of Zn2+ ions.

If the circuit in Daniell cell (Fig. 2.1) is closed then we note that the reaction Zn(s) + Cu2+(aq) ® Zn2+(aq) + Cu(s) (2.1) takes place and as time passes, the concentration of Zn2+ keeps on increasing while the concentration of Cu2+ keeps on decreasing. At the same time voltage of the cell as read on the voltmeter keeps on decreasing. After some time, we shall note that there is no change in the concentration of Cu2+ and Zn2+ ions and at the same time, voltmeter gives zero reading. This indicates that equilibrium has been attained.

Electrical work done in one second is equal to electrical potential multiplied by total charge passed. If we want to obtain maximum work from a galvanic cell then charge has to be passed reversibly. The reversible work done by a galvanic cell is equal to decrease in its Gibbs energy and therefore, if the emf of the cell is E and nF is the amount of charge passed and DrG is the Gibbs energy of the reaction, then DrG = – nFE(cell) It may be remembered that E(cell) is an intensive parameter but DrG is an extensive thermodynamic property and the value depends on n.

The electrical resistance is represented by the symbol ‘R’ and it is measured in ohm (W) . It can be measured with the help of a Wheatstone bridge with which you are familiar from your study of physics. The electrical resistance of any object is directly proportional to its length, l, and inversely proportional to its area of cross section, A. That is, The constant of proportionality, r (Greek, rho), is called resistivity (specific resistance). Its SI units are ohm metre (W m) and quite often its submultiple, ohm centimetre (W cm) is also used. IUPAC recommends the use of the term resistivity over specific resistance

The inverse of resistance, R, is called conductance, G, and we have the relation: The SI unit of conductance is siemens, represented by the symbol ‘S’ and is equal to ohm–1 (also known as mho) or W –1. The inverse of resistivity, called conductivity (specific conductance) is represented by the symbol, k (Greek, kappa). The SI units of conductivity are S m–1 but quite often, k is expressed in S cm–1. Conductivity of a material in S m–1 is its conductance when it is 1 m long and its area of cross section is 1 m2 . It may be noted that 1 S cm–1 = 100 S m–1 .

Materials are classified into conductors, insulators and semiconductors depending on the magnitude of their conductivity. Metals and their alloys have very large conductivity and are known as conductors. Certain non-metals like carbon-black, graphite and some organic polymers* are also electronically conducting. Substances like glass, ceramics, etc., having very low conductivity are known as insulators.

Substances like silicon, doped silicon and gallium arsenide having conductivity between conductors and insulators are called semiconductors and are important electronic materials. Certain materials called superconductors by definition have zero resistivity or infinite conductivity. Earlier, only metals and their alloys at very low temperatures (0 to 15 K) were known to behave as superconductors, but nowadays a number of ceramic materials and mixed oxides are also known to show superconductivity at temperatures as high as 150 K

Electrical conductance through metals is called metallic or electronic conductance and is due to the movement of electrons. The electronic conductance depends on (i) the nature and structure of the metal (ii) the number of valence electrons per atom (iii) temperature (it decreases with increase of temperature). As the electrons enter at one end and go out through the other end, the composition of the metallic conductor remains unchanged. The mechanism of conductance through semiconductors is more complex.

when the temperature increases, the kinetic energy of the atoms increases and hence they vibrate more faster. this obstructs the smoooth flow of electrons and hence retards the flow of current.

When electrolytes are dissolved in water, they furnish their own ions in the solution hence its conductivity also increases. The conductance of electricity by ions present in the solutions is called electrolytic or ionic conductance. Passage of direct current through ionic solution over a prolonged period can lead to change in its composition due to electrochemical reactions. The ions get discharged and hence deposited/liberated.

The conductivity of electrolytic (ionic) solutions depends on: (i) the nature of the electrolyte added (ii) size of the ions produced and their solvation (iii) the nature of the solvent and its viscosity (iv) concentration of the electrolyte (v) temperature (it increases with the increase of temperature).

Firstly, passing direct current (DC) changes the composition of the solution. Secondly, a solution cannot be connected to the bridge like a metallic wire or other solid conductor. The first difficulty is resolved by using an alternating current (AC) source of power. The second problem is solved by using a specially designed vessel called conductivity cell.

Basically it consists of two platinum electrodes coated with platinum black (finely divided metallic Pt is deposited on the electrodes electrochemically). These have area of cross section equal to ‘A’ and are separated by distance ‘l’. Therefore, solution confined between these electrodes is a column of length l and area of cross section A. T

The quantity l/A is called cell constant denoted by the symbol, G*. It depends on the distance between the electrodes and their area of cross-section and has the dimension of length–1 and can be calculated if we know l and A. Measurement of l and A is not only inconvenient but also unreliable. The cell constant is usually determined by measuring the resistance of the cell containing a solution whose conductivity is already known. For this purpose, we generally use KCl solutions whose conductivity is known accurately at various concentrations

Once the cell constant is determined, we can use it for measuring the resistance or conductivity of any solution. The Wheatstone bridge is fed by an oscillator O (a source of a.c. power in the audio frequency range 550 to 5000 cycles per second). P is a suitable detector (a headphone or other electronic device) and the bridge is balanced when no current passes through the detector. These days, inexpensive conductivity meters are available which can directly read the conductance or resistance of the solution in the conductivity cell.

The conductivity of solutions of different electrolytes in the same solvent and at a given temperature differs due to charge and size of the ions in which they dissociate, the concentration of ions or ease with which the ions move under a potential gradient. therefore molar conductivity is defined Λm=κ/C if k is expressed in S m–1 and the concentration, c in mol m–3 then the units of Lm are in S m2 mol–1 Λm=κ x 1000/M Scm2mol-1 1 S m2mol–1 = 10^4 S cm2mol–1