Electrodics
Electrodics
Electrochemistry is the study of chemical effects of electric current. It involves the mechanism of either chemical reactions that are produced by the passage of electric current or production of electric reactions that are produced by passage of electric current or production of electric current by chemical reactions. A substance or material which allows the passage of electric current is known as conductor, one which does not do so means that that do not conduct current is called an insulator. Generally conductors are classified in to metallic conductors and electrolytic conductors. The conductions of electric current through a metallic conductor is due to the movement of free electrons. There will not be any change in the physical or chemical state of the metallic conductor during the passage of electricity. Passage of electric current through an electrolytic medium is due to the movement of free ions in solution medium, and there is migration or transfer of substance from one place to another and electrolysis takes place. Hence a chemical change takes place during electrolytic conduction. Therefore it can be generally stated that conductance of electricity through a metallic conductor is due to the flow of electrons and it is due to the movement of free ions in the case of electrolytic conductors.
Conductance
Generally resistance of a substance is its ability to oppose the flow of electric current through it. Conductance means the ability of the material to allow the passage of electricity through it. Conductance means the ability of the material to allow the passage of electricity through it. Otherwise conductance of a substance is defined as the case with which the electric current passes through it. Therefore it is the inverse property of resistance.
Conductance C = 1/R
The unit of conductance is mho.
According to the principle of ohm’s law, the resistance (R) of any conductance varies directly with its length (l) and varies inversely with its area of cross section (a)
And R is directly proportional to l/a
i.e., R = Sl/a
where S is the specific resistance or resistivity of the material.
When l is cm and a = 1 cm*cm, R = S
Therefore we define specific resistance or resistivity or resistivity as the resistance of a material of 1 cm in length and 1 cm*cm in area of cross section.
Specific conduction (K or Kappa)
Specific conductance is a term usually represented for conductance of a solid material. It is reciprocal of specific resistance
i.e., K = 1/S
i.e., K = 1/R*l/a
Where l/a is called cell constant.
Therefore specific conductance K = conductance (c) * cell constant
When l =1 cm and a = cm*cm, then K = C
Hence specific conductance can be defined as the conductance of a material of 1 cm in length and 1 cm*cm in cross section. Otherwise, generally it is the conductance of 1 cc of any material.
Unit of specific conductance K = 1/ohm * cm/cm*cm = 1/ohm*cm or mho/cm.
Equivalent conductance (λ)
In order to consider the conductance of an electrolyte, a more precise term, equivalent conductance is used. Equivalent conductance is defined as the conducting power of all the ions produced by one gm. Equivalent of an electrolyte in a given volume of the solution.
Equivalent conductance = 1000 k/C
Where k is the specific conductance and C is the concentration of the solution in normality. Unit of equivalent conductance in mho cm*cm/eq
Molar conductance (μ)
It is the conducting power of all the ions produced by one gram mole of an electrolyte in a given volume of the solution
Μ = 1000 k/C
Where k is the specific conductance and C is the concentration of the solution of the solution in molarity.
Measurement of conductance in an electrolyte
Wheat stone bridge method
Wheatstone bridge is as shown in the figure. The assembly consists of two resistance arms. In the first arm a resistance box is included where a standard resistance R can be introduced. The solution whose resistance and in turn the conductance is to be measured is taken in a conductivity cell and introduced in the second arm. The wheat stone bridge is fed with an alternating source of current through a key. If a direct current is fed with an alternating source of current through a key. If a direct current is used, electrolysis will occur at the conductivity cell and this will produce a back e.m.f. due to the accumulation of products at the electrode. This will produce a back e.m.f. due to accumulation of product at the electrode. This also changes the concentration of the accumulation of products at the electrode. This also changes the concentration of the solution near the electrode. The middle of the arm is connected to a head telephone which in turn is connected to a sliding contact ‘J’ which can move along the wire AB which is uniform wire of high resistance. The length of the wire can be read from the scale fixed below it. When the connections are made, a sound in the head telephone is heard. On moving the sliding contact along the wire AB at a particular point a minimum sound is heard which indicates the null point i.e. the resistance in both arms will be equal. At this point
R is directly proportional to AJ and
Rx is directly proportional to BJ
Standard resistance R/resistance of the solution in conductivity cell Rx = AJ/BJ
Therefore resistance of the solution = std. resistance R * BJ/AJ
Rx = R * BJ/AJ
The conductance of the electrolyte solution
C = 1/Rx
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