Chemical Effect of Current

8/2/2019 Chemical Effect of Current

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Chemical Effects of Current

The phenomenon of electrolysis is an important chemical effect of electric current. In a metallic

conductor, the electric current is due to the drifting of free electrons, there is no chemical or

physical change, only the generation of heat. However, in ionic solutions (electrolytes), theelectric current is due to the movement of ions and there are associated chemical changes. In a

typical experiment of electrolysis, two metal rods are immersed in an ionic solution andconnected to the two ends of a battery. The dissociated ions appear at the two plates. Michael

Faraday was the first to carry out quantitative investigations and summarise them in the now two

famous laws of electrolysis.

Sub Topics

Electrolytic Conduction Electrolysis

Electrolysis of Copper Sulphate solution Electrolysis of Silver Nitrate Solution

Electrolytic Conduction

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Electrolytes are substances that conduct electricity due to the drifting of ions. The positive ions

are called cations, and the negative ions anions. The common examples of electrolytes are

aqueous solutions of inorganic salts, acids and bases. Salts like NaCl, KCl are electrolytes intheir molten state. Solutions of organic compounds are poor conductors. Solid state electrolytes

(eg. AgI) with mobile ions are also present.

Here, we will see how aqueous solution of common salt conducts electricity. Solid crystalline

NaCl is made up of Na+

and Cl-ions bound by a strong force of attraction. The energy required

to separate Na+ and Cl- ions (i.e., dissociate them) is ~7.9eV per molecule. The thermal energy at

room temperature, is only 0.03eV per molecule, and thus cannot dissociate NaCl. However,

when NaCl is dissolved in water, the force of attraction is greatly reduced because of the high

dielectric constant (= 81) of water. In fact, the force reduces by a factor of 81, and the thermalenergy is sufficient to dissociate completely into Na+ and Cl- ions. This process is called

ionisation.

Electrolyte conductivity is smaller than that of metals by a factor of 10 -5 to 10-6 at room

temperature. This is due to the smaller number density of ions as compared to free electrons,greater viscosity of the medium in which they move and the larger mass of ions.

Electrolysis
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Electrolysis is the dissociation of a electrolyte into ions at the electrodes by the passage of

electric current.

That is, it is the conduction of electric current through an electrolyte together with the resultingchemical changes.

Electrolysis is carried out in an apparatus called voltameter or electrolytic cell. It consists of a

glass vessel containing an electrolyte and 2 metal plates called electrodes, connected to a battery.

The electrode connected to the positive terminal of the battery is called anode, and the oneconnected to the negative terminal is called cathode.

Electrolysis of Copper Sulphate solution

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Electrolysis of copper sulphate solution

It is found that copper is removed from the anode and is deposited on the cathode. Due toionization, the CuSO4 solution is dissociated.

When the source of emf is connected, a steady current flows in the circuit. Then, the following

happen

1. Electrons flow from the negative terminal of the battery via the wire to the cathode C.
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2. Electrode C is at a lower potential than electrode A. Therefore, the Cu2+

ions move towards C,

while the ions move towards A.

3. At the cathode C, the following reduction reaction occurs

These Cu atoms get deposited on the cathode.

4. At the anode A, the following oxidation reaction takes place. The Cu atoms are from the

anode.

5. The Cu2+

ions go into the solution. The released electrons flow back to the positive terminal ofthe battery via the wire.

Thus, copper gets deposited at the cathode, while the anode loses an equivalent amount of

copper. The concentration of CuSO4 in the solution remains unchanged.

Electrolysis of Silver Nitrate Solution

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Here, the electrolyte is AgNO3 and the electrodes are silver plates. The process of electrolysis isidentical to that of CuSO4 solution, except for one important difference. Copper has valency two,

while silver has valency one. The reactions at the electrodes are

So, silver gets deposited at the cathode, while the anode loses an equal amount of Ag. The

concentration of AgNO3 in the solution stays the same.

In the electrode position of silver, one electron circulated for depositing one silver atom on thecathode; but in the case of copper, two electrons circulate for the deposition of one copper atom.

Chemical Effects of Current (Contd..)
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Michael Faraday

On the basis of various experiments conducted, Faraday arrived at two laws of electrolysis,which are as follows:

1. The mass of a substance liberated or deposited at an electrode during electrolysis is directly

proportional to the quantity of charge passed through the electrolyte. i.e.,

m q

i.e. m It

or m = ZIt

where m is the mass of the substance, q is the charge (= It, a current I flowing for time t) passing

through the electrolyte.

Z is known as the electrochemical equivalent (E.C.E) of the substance.

If q = 1C, then m = Z.

Hence, electrochemical equivalent of a substance is the mass of the substance liberated or

deposited in electrolysis by the passage of 1 coulomb of charge. The S.I. Unit of Z is kg/coulomb

(kg C-1

).

2. When the same amount of charge is made to pass through any number of electrolytes, the

masses of the substances liberated or deposited at the electrodes are proportional to their

chemical equivalents.

i.e., if m1 and m2 are the masses and E1 and E2 are the chemical equivalents of the twosubstances, then


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Faraday's laws can be explained as follows. Suppose one mole of a substance of mass M and

valency p is deposited on an electrode, i.e., an Avogadro number N A of atoms gets deposited. Todeposit one atom, a charge pe is required.

Therefore, for a mole (of mass M), the charge required is NA Pe. Then, from the equation m =

Zq, we have

M = Z NA pe

The quantity M/p is a constant for a substance and is known as the chemical equivalent E. TheE.C.E (Z) of a substance is thus directly proportional to its chemical equivalent (E); this is

evident also from the following table.

Electrochemical and Chemical Equivalents

SubstanceElectro chemical equivalent,

Z(kg/C)

Atomic mass

(u)

Valency,

p

Chemical equivalent,

E(g/mol)

Cations

Hydrogen 1.045x10- 8

1.008 1 1.008

Copper 3.249x10- 7

63.57 2 31.78

Silver 1.118x10- 6

107.88 1 107.88

Zinc 3.387x10 - 7 65.39 2 32.695

Chromium 1.800x10 - 7 51.996 3 17.332

Aluminium 9.360x10 - 8 27.1 3 9.03

Gold 6.812x10- 7

197.2 3 65.73

Nickel 3.040x10 - 7 58.68 2 29.34

Anions

Oxygen 8.238x10 - 8 16 2 8

Chlorine 3.671x10- 7

35.46 1 35.46


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where F(=NAe) is a fundamental constant and is called Faraday's constant. Its value is 96487 Cmol-1.

Faraday's constant is equal to the amount of charge required to liberate the mass of a substanceduring electrolysis equal to its chemical equivalent (in gm).

Equation (1) is the combined form of Faraday's laws of electrolysis.

For a given electrolyte,

Faraday's laws have a lot of significance. They imply that to liberate one atom of a substance, the

charge required is That is, the charge per ion of any substance is Two importantresults follow from this:

1. The chemical concept of valency is related to electric charge.

2. Since p is an integer, all charges are multiples of an elementary charge

The quantitative significance is that the value of e can be calculated by using values of F fromelectrolysis experiments. The value of e comes out to be ~1.6 x 10 -19 C.

Sub Topics

Applications of Electrolysis Electroplating Extraction of metals from ores Purification of Metals Electrolytic Capacitors
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Applications of Electrolysis

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The phenomenon of electrolysis has many scientific and commercial applications.

Electroplating

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Electroplating of objects by nickel, silver and gold is very common. The conducting material to

be electroplated is made the cathode of an electrolytic cell. A strip of metal whose coating is

required on the cathode material is used as the anode, while a soluble salt of the same anode

material is taken as the electrolyte. Below figure shows an experimental set-up used for

electroplating. When the current is passed through the circuit, a thin film of the metal deposits onthe cathode. To make the electroplating uniform and firmly adherent, a suitable current strength

is used. If the current strength is very high, the plating may become brittle. For gold plating, we

need a current from 1V to 3V batteries; and for copper, current is drawn from a battery of 5V to10V.

Extraction of metals from ores

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Certain metals are extracted from their ores using electrolysis. For example, aluminium isobtained by passing an electric current through fused bauxite (Al2O3) and cryolite (Na3AlF6).

Active metals like sodium, calcium and magnesium are also extracted from their ores using

electrolysis.

Purification of Metals
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For this purpose, the impure metal is made the anode, and a pure metallic strip is used as

cathode. A soluble salt of pure metal is taken as the electrolyte. On passing current, the impure

metal anode dissolves but only the pure metal deposits on the cathode. Many metals like copperare purified up to 99.99% using electrolysis.

Electrolytic Capacitors

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These capacitors consist of two aluminium electrodes placed in an electrolytic mixture of

ammonium borate for sodium phosphate in glycerine. When a steady current is passed, a thinlayer of dielectric aluminium oxide (or hydroxide) is formed on the anode. Such very thin films

can offer large values of capacitance. Modern capacitors use electrolytes in the form of a paste ora solution soaked in paper placed between two aluminium foils. If the potential across the twoelectrodes becomes excessively high, this dielectric layer breaks down and temporarily ceases to

function. However, it is possible to regenerate this layer and repair the damage. Such capacitors

are very common in power circuits.

Electrochemical Cells

The fact that chemical reactions produce electrical effects was discovered accidentally in 1791by Luigi Galwani, professor of anatomy at the University of Bologna, Italy. He found that an

electric current flowed across two dissimilar metals between which was a moist substance. In hiscase, the moist substance was a frog and the passage of electrical current was detected by the

twitching of its leg. Galvani thought that this was a manifestation of animal electricity. Ten years

later, Alessandro Volta, a professor of natural philosophy at the nearby University of Pavia,successfully reproduced some of Galvani's results with inanimate things. He assembled a series

of silver and zinc discs in pairs separating each pair with a sheet of pasteboard soaked in

conducting liquid. When the top most disc of silver was connected by a wire with the lower zinc

disk, a steady current of electricity was produced. This is a voltaic pile. The basic reason for thiselectrochemical effect became clear rather slowly.

Sub Topics

Voltaic Cell Chemical Action Defects of Voltaic Cell Local Action Polarisation
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Daniel Cell Chemical Reactions Leclanche cell Chemical Reactions Dry cell

Voltaic Cell

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Electrodes: Copper (+ve) and Zinc (- ve)

Electrolyte: dil. H2SO4 (dilute sulphuric acid)

Chemical Action

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At cathode:
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At anode:

The Zn2+

repel the H+

in the solution towards the copper electrode.

They extract e-from the copper rod and become neutral hydrogen atoms and finally escapes as

gas. Due to this, the copper rod becomes positively charged and the zinc rod negatively charged.

This process continues and a maximum potential difference is reached between the two

electrodes dipped in the electrolyte. The emf is 1.08 volt. When connected externally throughany resistor or bulb, conventional current flows from Cu to Zn. Electric current can becontinuously drawn from the cell till all the zinc is dissolved or sulphuric acid is consumed.

Defects of Voltaic Cell

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Local Action

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This arises due to the presence of impurities in the zinc rod. These impurities form minute local

cells all along the zinc rod; small local currents are set up resulting in the wastage of zinc even

when the cell is not being used. This can be eliminated by rubbing mercury on the zinc rod (i.e.,

amalgamation).

PolarisationBack to Top

A layer of neutral hydrogen is formed near the copper rod thus weakening the action of the cellby increasing its internal resistance and also producing a back emf. This is removed by using a

depolariser like MnO2 or CuSO4, which oxidizes hydrogen into water.

In spite of the small emf and limited energy capacity, cells are used widely because of their

compactness and convenience. We discuss here a few. Types of electrochemical cells, mostly

those in common use.

Daniel Cell

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Electrodes: Copper vessel (+ve)

Zinc rod (-ve) (amalgamated)

Electrolyte: dil. H2SO4

Depolarizer: CuSO4 solution

The porous pot (of fired clay or porcelain) allows ions to pass between the two solutions, but

prevents the solutions from mixing. A perforated shelf containing CuSO4 crystals is placed near

the top of the Cu vessel to maintain the CuSO4 solution concentration.

Chemical Reactions

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The hydrogen ions so formed diffuse through the porous pot and interact with copper sulphate

forming H2SO4 and Cu2+

ions.
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Therefore, 2H+

+ CuSO4 ------> H2SO4 + Cu2+

.

The copper ions are deposited on the vessel and there is no depolarisation. The zinc rod, which is

amalgamated, avoids local action. The emf is 1.12 V.

Leclanche cell

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Electrodes: Carbon rod (+ve)

Zinc rod (-ve)

Electrolyte: Saturated NH4Cl solution

Depolariser: MnO2

Chemical Reactions

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The electrons thus released move to the zinc rod via the external circuit. The ammonia gas soproduced escapes. The hydrogen ions diffuse through the porous pot and interact with MnO 2.(The charcoal powder makes MnO2 electrically conducting.) Therefore,

These positive charges are given to the carbon rod increasing its positive potential with respect tothe electrolyte. The depolarisation action proceeds slowly. When current is drawn continuously,

a partial polarisation therefore sets in, because of which the current falls. So the circuit is

switched off till the hydrogen gas escapes. It is ready for use after sometime.

The Leclanche cell is used when intermittent currents are needed.

The emf of the cell is 1.45 V.

Dry cell

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This is a portable form of Leclanche Cell. A moist paste of NH4Cl and ZnCl2 is contained in a

zinc vessel acting as the negative electrode. ZnCl2 is added to the electrolyte NH4Cl. It keeps the

paste moist because it is highly hygroscopic. The top is sealed with shellac or pitch so that pastedoes not dry up. The carbon rod covered with a brass cap is placed in the middle of the vessel

surrounded by a paste of charcoal and MnO2 in a muslin bag acts as the positive electrode. A

small hole is provided in it to allow the gases formed by chemical action to escape.

The emf is ~ 1.5V
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The dry cell is useful in circuits requiring only intermittent current because it can provide only

0.25 A continuously. If more than this is drawn continuously, polarization occurs.

Electrochemical cells (Contd)The electrodes used in the cells discussed so far, deteriorate with the passage of current and

cannot offer a constant emf indefinitely. However, there was a few cells called standard cells,which can maintain a fairly constant emf over very long periods of time compared to the other

cells. The commonly used standard cell is the Weston Cell. This cell is usually in the form of an

H shaped tube. One leg of the tube contains mercury in contact with a paste of mercurous

sulphate (Hg2SO4) and is the cathode. The other leg of the tube contains an amalgam of cadmiumwith mercury, which acts as the anode of the cell. The electrolyte is a saturated solution of

cadmium sulphate. The mercurous sulphate paste serves as depolariser. Platinum wires are sealed

at the bottom of each leg to serve as terminals for connecting the cell to the external circuit. Theemf of cell is 1.0813 V at 20oC. This is independent of temperature over a considerable range.

Weston Standard cell

Sub Topics

Lead Accumulator Discharging Process Recharging Process Solid State cells
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Lead Accumulator

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This can be recharged by passing a current through it in the reverse direction. The chemical

processes that occur at the electrodes during discharge are reversed by this. Thus the cellrecovers its original state, except for some energy loss. Such cells are called secondary cells or

accumulators.

The lead-sulphuric acid cells is a common example. It was inverted by a French physicist,

Gaston plate, in 1859.

Electrodes: Alternate parallel plates of lead dioxide (+ve electrode) (oxidised from PbO)

Spongy lead (reduced from PbO) (-ve electrode)

These are kept separate by porous separators made of wood, plastic or glass fibre.

Electrolytes: Dilute sulphuric acid

Container: Glass or bakelite

Discharging Process

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Here stored chemical energy is converted to electrical energy or current is drawn from the cell.
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The hydrogen ions go to the +ve electrode and SO42-

to the -ve electrode. After giving their

charges they react with the electrodes and reduce the active material to lead sulphate.

Therefore, at the -ve electrode

At the +ve electrode,

Both plates (but only half of the active materials) are converted into PbSO 4 (whitish). Water is

formed thus lowering the specific gravity of H2SO4 (electrolyte).

The emf of the cell falls and sulphuric acid is consumed.

Recharging Process

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Current is passed through the two terminals in the reverse direction to that in which the cellprovided current. That is, the anode is connected to the positive terminal of the d.c. source, andthe cathode to the negative terminal.

The hydrogen ions move to the -ve electrode and sulphate ions to the +ve electrode.

At -ve electrode,

At the +ve electrode,
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Water is consumed and sulphuric acid is formed thus raising the specific gravity of the

electrolyte. In the charging process, the +ve electrode is coated with dark brown lead peroxideand the -ve electrode with grey spongy lead. The emf of the cell rises, and the electrical energy

supplied is converted into chemical energy which is stored in the cell.

The charging process is mentioned by measuring the specific gravity of the electrolyte. It variesfrom 1.28 when fully charted (sulphuric acid and water) to 1.12 when discharged (mostly water).

The emf of a full charged cell is ~2.1V. It should not be discharged to below 1.8V. Thissecondary cell has a low internal resistance that is, it can deliver a high current. It can be

recharged a very large number of times without any deterioration.

Solid State cells

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The cells discussed above use a liquid electrolyte, one cathode and an anode. Some of the major

disadvantages of these cells are leakage on long term storage, corrosion due to the use of liquidacidic/alkaline solution, short life, low weightage per kg mass of the cell and limit on

miniaturization.

Recently, some cells have been developed in which the electrolyte is a solid in which ions canmove (Solid state electrolytes). Such materials are available in the form of gels, polymers,

composites, polycrystalline solids or thin solid films. The basic geometry of a solid state cell is

given in Fig.4.13 using a solid electrolyte with mobile cation M+

and anion X-. Either one of

these ions or both can move.

In a lithium solid state cell, (Figure below) the basic electrochemical reaction with the electrode

(say, I2) is

A solid state cell

Some electrolyte is also mixed in the cathode or anode to decrease polarization. Many solid statematerials for use as cathode, anode and electrolyte have been recently developed. Some of the

Li+

batteries used in mobile phones are based on solid electrolytes. Some heart pacer batteries

also use Li-button cell in which the electrolyte is (LiI + AI2O3) composite or a similarelectrolyte. Polymer Li - batteries and H+ - batteries are in the advanced state of development for
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electrical cars. Some electrode materials like doped LiCoO2 or LiMnO2 have provided excellent

rechargeability to these cells.

Chemical Effect of Current

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