5. ELECTROCHEMISTRY

5.1 . Electronic concept of Oxidation and Reduction

a) Oxidation : Oxidation may be defined as a process in which an atom or an ion loses one or more electrons .It is also known as de-electronation.

The loss of electron either increase the positive charge or decrease the negative charge of the atom or the ion.

For example ,

1) Loss of electron results in increase on positive charge . Na → Na+ + e-

Mg → Mg2+ + 2e-

Fe 2+ → Fe3+ + e-

Sn2+ → Sn4+ + 2e-

2) Loss of electrons results in decrease in negative charge .

MnO42- → MnO-

4 + e-

S2- → S + 2e-

2Cl- → Cl2 + 2e-

b) Reduction : Reduction may be defined as the process in which an atom or an ion gains one or more electrons.The gain of electron either decrease the positive charge or increase the negative charge of the atom or the ion.For example : 1) Gain of electron results in decrease in positive charge :

Fe 3+ + e- → Fe2+

2 Hg2+ + e- → Hg+2

Sn4+ + e- → Sn2+

2) Gain of electrons results in increase in negative charge : Cl2 + 2e- → 2Cl-

MnO-4 + e- → MnO2-

4

S +2e-→ S2-

5.2 Electrolyte And Non electrolyte:

1) Electrolyte : These are those substances which allow the electricity to pass through them in either molten state or in the form of their aqueous solution and undergo decomposition . e.g. Acids ,Salts and Bases.

2) Non-electrolyte: These are those substances which do not conduct electricity in their molten state or through their aqueous solution .e.g. All covalent compound except graphite such as sugar ,glucose etc.

3) Electrolysis : It may be defined as the process of decomposition of an electrolyte by the passage of electric current through its molten state or through its aqueous solution .

For example :

5.3 Faraday`s laws of electrolysis.

1) Faraday`s first law of electrolysis : According to this law “The mass of any substance deposited or liberated at any electrode is directly proportional to the quantity of electricity passed .”

If w is the mass of substance liberated on passing Q coulombs of charge , then

W @ Q

W@ I ˣ t ( Q = CURRENT ˣ TIME )

or W = ZˣIˣt

where Z is proportionality constant and called electrochemical equivalent .

the value of Z can be calculated as follows :

when Q =1C , OR I=1Ampere and t=1 second then

W =Z

2)Faraday`s second law of electrolysis :

According to this law “ when the same quantity is passed through different electrolyte solutions connected in series , the weights of different substances produced at the electrodes are proportional to their equivalent weights .

This law can be illustrated by passing the same quantity of electricity through two electrolytic cell of CuSO4 ,and AgNO3 Connected in series .According to this law Mass of Cu deposited /Mass of Ag deposited = Eq. mass of Copper /Eq. mass of silver This law can be drawn from 1st law of electrolysis .According to 1st law of electrolysis W = ZˣQ

Then for first electrolyte W1 = Z1 ˣQ1 ………….(1)& For second electrolyte W2 = Z2ˣQ 2 …………..(2) According to second law of electrolysis Q1 = Q2 On dividing 1 by 2 , we get W1/W2 = Z1/Z2= E1/96500/E2/96500= E1/E2

Where E1 & E2 their equivalent masses. It is clear the electro chemical equivalent is directly proportional to equivalent mass. Thus E1 @ Z1

And E2 @ Z2 In general E @ ZOR E = F ˣZ Where F= constant of proportionality and known as Faradays law constant

IMPORTANCE OF FARADAY`S LAW: 1) The first law is helpful in finding the electro-chemical equivalent of a substance or ion .2) The second law helps in the determination of equivalent weights .

Industrial application of electrolysis :

1) Electroplating : The process of depositing a superior metal over an inferior metal with the help of electric current is called electroplating .Purpose : it is done to achieve the following objectives .i) Decoration : To make the things more beautiful or attractive ,electroplating of superior metal is done over the inferior metal e.g. ornamentals, fancy articles , frames etc. are electroplated with gold,silver , nickel etc.

ii) Protection from corrosion : To protect the metal like iron from corrosion these metals are electroplated with tin , nickel , or chromium.

iii) Repairing : Sometime broken parts or worn out parts of machinery are repairing by electroplating by deposition of metal in between the broken or at the defective part .

A) Method of electroplating 1. Firstly throughly clean the surface of article to be electroplated by mechanically rubbing the surface with sand etc.2. Then it is treated with hot alkali or soap solution to remove greases and dilute acid to remove dust or layer of oxide from its surface.

3. The cleaned article is made cathode and connected to the negative terminals where as the metal which is electroplated is made anode and connected to positive terminal of battery.4. The cathode and anode are placed in electrolytic tank made of glass or wood or enamelled iron or cement etc.5. Soluble salt of anode is taken as the electrolyte .A.1) Silver plating : in this cathode are the articles which are electroplated and anode is pure silver metal . Sodium or Potassium argentocyanide dissolved in water is taken as article and Following reaction take place during electroplating . K[Ag(CN)]2 ↔ K+ +[Ag(CN)]-

[Ag(CN)]- ↔ Ag+ + 2CN-

At cathode: Ag+ + e- →AgAt anode : Cyanide ion cause an equivalent amount of silver from the anode to dissolve forming AgCN and finally k[Ag(CN)2.

A.2) Condition for good deposit : it has been found experimentally that the deposited metal is in the form of fine crystal . Therefore , finer the crystal , the smoother and brighter is the deposited. Hence for bright and smooth crystals , following condition are necessary :1. The electrolytic cell should be worked at low temperature .2. High current density should be employed .3. High metal concentration is required .

ELETROREFINING: The process of removal of impurities from crude metal by electrolysis is called electro-refining. Refining of various metals like Cu, Ag, Au ,Al etc. are now a days done electrolytically.

Process :

The crude metal (impure metal ) is made the anode and thin plate of same metal in pure form is made the cathode. Electrolyte is the suitable salt of the anode metal dissolved in water. On passing electric current , pure metal from electrolyte deposits on cathode and an equivalent amount of metal dissolves from the anode. The impurities is deposited at the bottom as anode mud.

FOR EXAMPLE :

Electrorefining of copper : Crude Cu metal is made anode and pure Cu metal is made cathode. An aqueous solution of copper sulphate acidified with sulphuric acid is taken as an electrolyte . On passing electric current , copper from electrolyte gets deposited on cathode and an equivalent amount of copper from copper anode goes into the electrolyte . The impurities are deposited in the form of anode mud.

Reaction of the process:

CuSO4 ↔ Cu2+ + SO42-

At cathode :

Cu2+ +2e- → Cu

At anode :

Cu → Cu2+ + 2e-

These Cu2+ ions combine with SO42- ions to give CuSO4 , the electrolyte .

Cu2+ ↔ CuSO4

This process continue till the whole strip of crude metal is purified and deposited over the cathode leaving behind the impurities in the form of anode mud .

EXERCISE

Q.1 Define Cathode and Anode.

Q.2 Define electro-chemical equivalent .

Q.3 Define electrolysis,discuss the process of electrolysis with example .

Q.4 State and explain Faraday`s law of electrolysis.

Q.5 What are the different industrial application of electrolysis, explain?

6. General Principal of Extraction of Metal: The element known till today have been divided into three groups:

i) Metalii) Non-Metaliii) Metalloids

i. Metal : The elements which form positive ions by losing electrons and hard ,ductile,good conductors of electricity and have high melting and boiling are called metals.e.g. Iron , copper , gold etc.ii) Non-Metals : The element which form negative ions by gaining electrons and are soft , dull in appearance , poor conductor of electricity , electro-negative in character and have low melting & boiling point are called non-metals. e.g. Sulphur , phosphorous etc.iii) Metalloids : The element which have properties of both metal and non-metals are called metalloids . e.g. Arsenic , Antimony , Bismuth etc.

6.1 Minerals and ore of iron , aluminium , and copper

MINERALS : The metals which exist in nature in combination with metals

ORES OF ALUMINIUM.

(a) Corrundum Al2O3

(b) Bauxite Al2O3.2H2O (c) Cryolite Na3AlF6

ORES OF IRON(a) Haematite Fe2o3

(b) Magnetite Fe3O4

(c) Iron Pyrite FeS4

ORES OF COPPER:(a) Chalcopyrite CuFeS2

(b) Chalcocite Cu2S

METALLURGY AND ITS TYPES:Metallurgy :The process of extraction of metals from their ores in pure states is called metallurgy.

TYPES OF METALLURGY :There are three types :1. Pyrometallurgy: Pyrometallurg is the process of extraction of metal from their ores by heating them to high temperature.e.g Extraction of Iron from its ore hematite Fe2o3

2. Electrometallurgy : Electrometallurgy is the process of extraction of metals from their ores by passing an electric current through their solutions.For example ,sodium and potassium are extracted from their ores by electro metallurgy.3. Hydrometallurgy : Hydrometallurgy is the process of extraction of metal from its concentrated ore by dissolving the ore in a suitable reagent.For example ,Extraction of silver and gold is done by hydro metallurgy..

GENERAL STEPS OF METALLURGY:A. Crushing and grinding of the ore.B. Pulverization of the ore.C. Concentration of the ore.D. Extraction of metal from concentrated ore.E. Purification or refining of the metal.

A : CRUSHING AND GRINDING :Ores found in nature are usually bulky in size .It is therefore, crushed to small pieces of 3-5 cm in size with the help of Jaw Crushers.

B: PULVERIZATION : The small pieces of the ore obtained above are then powered with the help of stamp mills. This process is known as Pulverization.

C . CONCENTRATION OF THE ORE :The process of increasing the percentage of metal in the ore by removing impurities is known as concentration of ore.

→ Froth Floatation process :

This method is used for the concentration of sulphide ores.For this process , Pulverised ore is taken is a tank .It is mixed with water and pine oil. This mixture is then stirred vigorously by passing compressed air through a pipe. Oil will form a forth with air. The sulphide ore particles wetted by oil get stick to the forth and become lighter and come to the surface where as impurities wetted by water become heavy and settle down at the bottom of the tank and removed. The froth carrying ore at the top overflows into the settling tank and settles down.For example, Galena , Zinc blende , copper pyrites are concentrated by this method.

D. OXIDATION OF ORE:(a) Roasting : The process of heating the concentrated ore in excess of air at a temperature below its melting point. This method is generally used in case of sulphide ores. The following changes takes place during roasting.

I. Removal of moisture.II. Removal of volatile impurities such as sulphur , arsenic, phosphorus etc .as their

oxides. S + O2

∆ ͢ SO2↑

P4 + 5O2 ͢∆ 2P2O5 ↑

III. Oxidation of sulphide ores into their oxides. 2ZNS + 302 ͢∆ 2ZnO + 2SO2↑

(ZINC SULPHIDE ) Iv. Charge become process.

Refining of Metal:

(a) Electrolytic refining : In this method, the impure metal act as the anode and thin sheet of pure metal act as cathode . A water soluble salt of the metal to be refined is taken as electrolyte .On passing the electric current ,the impure metal dissolves from the anode and goes into the electrolyte solution , and pure metal from the electrolyte deposited on the cathode. The impurities are left behind in the solution ,below the anode . copper and aluminium are purified by this method.

Following reaction take place :

At cathode :Cu2+ + 2e- → Cu

At anode : Cu → Cu2+ + 2e-

The metallurgyical process can be shown by flow diagram as shown below :

ALLOY Definition : An alloy is a homogeneous mixture of two or more metals or metals and non metals.e.g. sodium-amalgam is an alloy of sodium and mercury. Alloy can be classified in two categories :1. Ferrous alloys2. Non-Ferrous alloys Classification of Alloys :1. Ferrous Alloys : The alloys which contains iron as one of the major component are called ferrous alloys .e.g. Stainless steel ,nickel steels2. Non-ferrous Alloys : The alloys which do not have iron as one of the main component are called non-ferrous alloys.e.g. brass ,bronzePurpose of Making Alloys:

i. To harden the metal.ii. To lower the melting point of metals.

iii. To increase tensile strength.iv. To reduce Malleability and ductility.v. To modify chemical activity.

vi. To make the metal more resistant .vii. To modify colour.

EXERCISEI. What are metals and non metals ?

II. What is meant by ore concentration ? Name three common process of ore concentration.

III. Explain Froth Floatation Process for the concentration of sulphide ores.

IV. What is an alloy? Explain different types and properties.V. Describe different methods used for refining of metals.

FUELS

Fuels can be defined as any combustible substance which during combustion gives large amount of heat which can be used economically for domestic and industrial purpose.

1. High calorific value2. Low moisture content 3. Readily available 4. Low cost5. Moderate ignition temperature6. Low non combustion material 7. Low cost of storage 8. Not so much of smoke 9. Product of combustion should be harmful10. Easy to transport

Classification of fuels:

1. On the basis of physical state 2. On the basis of source

On the basis of physical state:i) SOLID FUELS ii) LIQUID FUELSiii) GASEOUS FUELS

On the basis of source :

I) NATURAL FUEL OR PRIMARY FUELSII) ARTIFICAL FUELS OR SECONDARY FUELS

Calorific value of a fuel:

It is defined as the total quantity of heat produced when a unit mass of the fuel is burn completely.

Gross calorific value:

The total heat produces when one unit mass of the fuel has been burnt completely and products of combustion are cooled to room temperature.

Its unit in C.G.S.is Cal/gm

Its unit in M.K.S.is Kca/kg

Gaseous fuel:

(a) BIOGAS :

Biogas is produced by the degradation of biological matter by the bacterial action (of anaerobic bacteria) in the absence of free oxygen.e.g. i) Natural gasii) Gobar gasiii)H2- 5-10%( a combustible gas).

Uses of biogas:

1. For cooking food. 2. As a fuel to run engines. 3. As an illuminant in village.

Advantages of biogas:i) Biogas production is very economical.ii) The gas has all advantages of gaseous fuel like cleanliness , absence of smoke , flexibility.iii) It does not contain poisonous gas ,CO,as an ingredient.iv) It provides simultaneously excellent yield of good manure.

(b) LIQUEFIED PETROLEUM GAS (LPG): LPG or bottled gas or refinery gas is obtained as a by product , during the cracking of heavy oils or from natural gas.Composition of LPG:i) n-butane ii) Isobutane iii) Butylene and Propane

USES:

The largest use of LPG at present is as domestic fuel and industrial fuel. There is increasing trend to use LPG as motor fuel.ADVANTAGES OF LPG:1 .LPG is very neat and clean domestic fuel.2. LPG does not produce any poisonous gases on burning.3. LPG has high calorific value. So it is a good fuel.4. LPG burns with a smokeless flame and does not cause pollution.5. LPG is easy to Handle and convenient to store.ADVANTAGES OF GASEOUS FUEL OVER SOLID AND LIQUID FUEL:1. The gaseous fuel do not produce ash on burning and burn freely in the presence of air.2. They can be supplied anywhere with the help of pipe line and this eliminates the manual labour.3. The supply of gas to the burners can be easily controlled.4. The gaseous fuel burn with more efficiency and a high flame is obtained in no time.5. The gaseous fuel are economical than those of solid and liquid ones.6. They can be easily lighted.7. They are clean to use.8. They burn without smoke and are ashless.

9. They have high calorific value.