2.2 Electric Units
Electric current density is the ratio of the current flowing through a conductor to the cross-sectional area of that conductor. Units are Asd, Asf, Asi, Asc.
Electrical resistance (R) is the quantity, analogous to friction, in a conductor, that determines the potential difference required to maintain a given electric current through it. The unit is the ohm.
Electric quantity (Q) is the amount of electricity present in any electric charge or passed through a circuit during any time interval by ,in electric current. The unit is the coulomb. So coulomb is the quantity of electricity transported in one second across any cross section of a circuit by a current of one ampere..
Electric resistivity (?) is the ratio of potential gradient in a conductor to the current density whereby produced as well as the specific resistance of a substance. The unit being ohm-cm.
Electric conductivity (k) is the specific electric conducting power of a substance or the reciprocal of electric resistance, the unit being mho.
2.3 Faraday s Law of Electrolysis
Conductors of electricity are divided into three classes:
1) metallic or electronic conductors such as metals, alloys, and other substances such
as carbon, where current passes through without affecting the structure of the matter.
2) electrolytic conductors which are the solutions of acids, bases, and salts, fused salts, some solid substances, and hot gases, where movement of the current is always associated with movement of matter.
3) mixed conductors (metallic and electrolyte), where the current passes partly in a metallic and partly in electrolytic manner.
For electrolytic conduction whatever the solution, the chemical action take place only at electrodes.
2.3.1 Electrolysis
Refers to the decomposition of a substance by an electric current.
The suffix -lysis comes from the Greek stem meaning to loosen or split up. Electrolysis literally uses an electric current to split a compound into its elements.
2NaCl(l) à 2Na(l) + Cl2(g)
the phenomena: the electrolyte consists of cations (positive ions) and anions (negative ions), and when the current passes the cations attracted toward the negatively charged cathode where their charge is neutralized and they are set free.
In the similar manner the anions move to and discharged at the anode.
Example: If two platinum plates or wires be dipped into a dilute solution of H2SO4 and connected either with two poles of a battery or with a source of direct current electrolysis will take place. At the platinum plate connected with the negative terminal of the battery H2 gas is given off, while at the plate connected with the positive pole of current source, oxygen is evolved. The gases are produced only at electrodes and not
along the path of the current through the electrolyte. If AgNO3 solution is electrolyze the silver will be deposited on the cathode and evolution of O2 at the anode.
Generally: whatever the solution the chemical action takes place just only at the electrodes, the points where the current enters and leaves the electrolyte.
current (amperes) is the rate of charge transport; 1 amp = 1 c/sec.
power (watts) is the rate of energy production or consumption;
1 w = 1 J/sec = 1 volt-amp; 1 kw-h = 3600 J.
Faraday s 1st law
The weights of substances formed at an electrode during electrolysis are directly
proportional to the quantity of electricity that passes through the electrolyte.
Faraday s 2nd law
The weights of different substances formed by the passage of the same quantity of electricity are proportional to the equivalent weight of each substance.
The equivalent weight of a substance is defined as the molar mass, divided by the number of electrons required to oxidize or reduce each unit of the substance.
2.3.2 Electrochemical equivalents
If taking silver as an example, from the electrochemical equivalents table:
symbol atomic weight valance mg/coulomb gm/hr
-------- ----------------- -------- -------------- -------
Ag 107.880 1 1.118 4.0245
It means one coulomb of electricity produce 0.001118 gm of silver from a solution of a silver salt:
Gram equivalents = atomic Wt / valence = 107.88 gm / 1 = 107.88
Coulomb = 107.88 gm / 0.001118 gm coulomb-1 = 96494 taken 96500
So, if one faraday be passed through a electrolytic conductor, one equivalent of some substance will be liberated at each electrode (the minimum quantity of electricity required for the production of a gram equivalent of a substance is 96500 coulomb).
Thus one mole of V3+ corresponds to three equivalents of this species, and will require
three faradays of charge to deposit it as metallic vanadium. Also 1F is required to
deposit 1mole of metallic silver and 2F required to deposit 1 mole of metallic copper from their solutions. In practice more than this is required, because:
1) There may be a separation of more than one substance at either electrode.
2) the products of electrolysis may suffer physical loss.
3) secondary reactions may take place at the electrodes.
4) current leaks, short circuits, and losses in the form of heat.
2.3.3 Current Efficiency ( CE)
The ratio of the theoretical to the actual quantity of current required or the amount of product formed from a definite amount of current divided by the theoretical amount.
Theoretical quantity of current Actual amount of a product formed
CE = -------------------------------------- = ------------------------------------------
Actual current Theoretical amount
In practice current efficiencies may vary from as low as:
25-30% in decomposition of certain fused salt and electrodeposition of chromium
92-95 % copper refining
95-98 % electroplating applications
100 % electrolytic oxidation
2.3.4 Current measurement (Coulometers)
Coulometric techniques can be used to determine the amount of matter transformed during electrolysis by measuring the amount of electricity required to perform the electrolysis. Coulometers are of two types:
1. Electronic coulometer
Is based on the application of the operational amplifier in the integrator-type circuit.
2. Electrochemical coulometers (voltameters)
There are two common types of coulometers based on electrochemical processes:
1.Copper coulometer is a one of the common application of the copper-copper(II) sulfate electrode. Such a coulometer consists of two identical copper electrodes immersed into the slightly acidic pH-buffered solution of the copper(II) sulfate. Passing of the current through the element leads to the anodic dissolution of the metal on anode and simultaneous deposition of the copper-ions on the cathode. These reactions have 100% efficiency over a wide range of the current densities. Amount of the quantity of electricity passed through the cell can be easily calculated by mass changes of any of the electrodes:
,
where Q is the quantity of electricity (coulombs); is the mass transported (gm); 63.546 is the atomic weight of copper, the factor 2 is due to the transport of divalent ions, and F is the Faraday constant (96500).
2.Mercury coulometer is an electroanalytical chemistry device using mercury to determine the amount of matter transformed (in coulombs) during the follow reaction:
Hg2+ +e = Hg
These oxidation/reduction processes have 100% efficienty with the wide range of the current densities. Measuring of the quantity of electricity (coulombs) is based on the changes of the mass of the mercury electrode. Mass of the electrode can be increased during cathodic deposition of the mercury-ions or decreased during the anodic dissolution of the metal.
where Q-quantity of electricity (coulombs); -mass changes (gr); F-Faraday constant (96500).
2.4 Electrolytic Dissociation
Electrolytic conductors which may be liquid solutions composed of a solute and a
solvent, fused salts, or some pure liquids, differ from electronic conductors in that the
carriers of electrical energy are charged particles of atomic or molecular size. Mass
transfer takes place. In a circuit composed of electronic conductors and electrolytic
conductors, chemical reaction occurs at each electronic-electrolytic boundary or at
the electrode interfaces. If the flow of electrons be toward the electrolytic conductor
or electrode, reduction occurs at the interface; if the electron flow be in the opposite
direction, oxidation will take place. An equal number of these types of boundaries or
interfaces will exist in any circuit. Oxidation and redaction will occur in any electrochemical cell, and the two are quantitatively equivalent.
Ionization involves the dissociation of molecules or ionic crystals into ions when the
substances are melted or dissolved in a suitable solvent.
The magnitude of the electrical charges carried by ions may be calculated. The
passage of one faraday of electricity through a solution of an electrolyte liberates one
gram equivalent of each ion at each electrode. One gram equivalent of an anion is
associated with 96,500 coulombs of negative electricity, and one gram equivalent of
a cation with the same quantity of positive electricity. Accordingly, the quantity of electricity carried by any gram ion is nF, where n is the valence of the ion. One gram ion contains Avogadro s number of ions, which is 6.023 x 1023.
The single ion charge or the quantity of negative electricity is called the "electron." It calculated to be 96500/6.023 x 1023 = 1.6 x 10-19 coulomb
Solutions of electrolytes in solvents other than water conduct the electric current. It may be inferred that electrolytic dissociation takes place, in these solvents. Substances which show conduction of the electric current in nonaqueous solutions are not necessarily dissociated in water. This solvent, however, is more effective in bringing about dissociation than most all others. Molten salts exhibit the same phenomena as solutions of electrolytes. Ions are present in this type of electrolytic conductor.
There is a rough proportionality existing between the dielectric constant of solvents and their dissociating power. Solvents with high dielectric constants, like water, possess a high dissociating power, while those of low dielectric constants dissociate dissolved material to a lesser degree.
2.5 Ionic Mobility
Faraday s law indicates that the electric current is carried by the migration of ions, it
states nothing about their relative or absolute velocity of movement. The relative
ionic velocity can be determined from the concentration changes that take place at the
electrodes. Ions are attracted to the oppositely charged electrode. This is true only in the very thin interfacial region near the electrode surface. Ionic motion throughout the bulk of the solution occurs mostly by diffusion, which is the transport of molecules in
response to a concentration gradient. Migration the motion of a charged particle
due to an applied electric field the transference number of an ion in a given electrolyte represents the fraction of the total current carried by the species of ion
during electrolysis. The ratios of the transference numbers of the anions and cations
are equal under the given conditions to the ratio of their respective mobilities.
Concentration differences may result near the electrodes, partly as the result of unequal rates of migration if electrolysis takes place in a solution not in motion. Such concentration differences are undesirable in that they increase the voltage required by the cell and may lead to impure products at the electrodes. Concentration changes are thus as far as possible destroyed by continuous circulation of the electrolyte or by mechanical mixing.
2.6 Conductance
In any industrial cell in which electrolysis is taking place, there are ohmic resistances
in portions of the system. These cause loss of energy and the liberation of heat.
2.6.1 Conductivity and Molar conductivity
The resistance of a sample of an electrolytic solution is defined by:
where l is the length of a sample of electrolyte and A is the cross sectional area. The symbol r is the proportionality constant and is a property of the solution. This property is called resistivity or specific resistance. The reciprocal of resistivity is called conductivity, k.
Since l is in cm, A in cm2 and R the total resistance in ohms W, the units of k are W-1cm-1 or Scm-1 (Siemens per cm).
Conductivity is measured with a cell consisting of two platinum electrodes a fixed distance apart. These electrodes are coated with finely divided electrodeposited platinum black. The ratio l /A is a property of the cell and is called the cell constant. It is the resistance that is measured with an alternating current to minimize electrolysis. The cell constant is not measured geometrically but by measuring the resistance of a sample solution of known conductivity, usually potassium chloride.
From Ohm s law E = IR, where E is electromotive force or voltage, I is the current,
and R is the resistance, and 1/R = I/E
the specific conductivity k equals k =l I/EA and is equal to the current flowing across
the cm3 of the electrolyte it a potential of one volt. With units mho/cm
The conductivity of a solution depends upon the electrolyte and its nature, the solvent, the concentration, and the temperature. Specific conductivity increases with
concentration to a maximum, after which the specific conductivity decreases. The
specific conductivity-temperature relation is almost linear, following the expression:
kt = k18[1 + b(t - 18)]
where b has values of 0.02 to 0.025 for salts and bases and 0.01 to 0.016 for acids, over a temperature range above 100°C.
If the conductivity k is in W-1cm-1 and the concentration C is in mol cm-3, then the molar conductivity ? is in W-1cm2 mol-1 and is defined by:
The variation of molar conductivity with concentration could be classified with respect to the behavior of the, strong and weak electrolytes, that have different characteristics. For strong electrolytes the empirical relationship of ? to C1/2
where the molar conductivity at infinite dilution ?° is arrived at by plotting the molar conductivity at various concentrations ?c vs. concentration and extrapolating to zero concentration.
weak electrolytes vary in a non-linear manner. They dissociate to a greater extent with increasing dilution. Using ethanoic acid as an example we have the equilibrium
with concentrations of the dissociated species expressed as aC and the undissociated acid as (1-a)C. The dissociation constant can be expressed as
where a is the degree of dissociation. Arrhenius showed that a can be given as
Combining the last two equations gives:
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .