Study Portal 15 – Chapter 15

Electochemistry

 

Q1.      Fill in the blanks with appropriate words.

  1. i) Reduction is the process which involves Gain of electron.
  2. ii) In the free state of an element, the oxidation state has always zero

iii)     The galvanic cell produces Electric current using redox reaction.

  1. iv) Coating with paints prevents
  2. v) Dry cell are also a type of
  3. vi) Electroplating brings both the Decoration and resistance to
  • The food cans are electroplated with
  1. Caustic soda is industrially prepared by Electrolytic / Nelson’s
  2. K2Cr2O7 is a strong oxidizing
  3. Collectively the oxidation and reduction process are called Redox Reactions.

Q2.      Choose the correct answer.

  1. i) The semi conductors are the species that:

(a) conduct electricity easily

(b) do not conduct electricity easily

(c) conduct electricity but not heat

(d) conduct heat but not electricity

  1. ii) Oxidation means:

(a) addition of electrons                (b) removal of electron

(c) addition of protons                   (d) removal of protons

iii)     Oxidation state is charge:

(a) apparent +ive charge    (b) apparent –ive charge

(c) apparent charge                      (d) not related with charge

  1. iv) The electroplating prevents:

(a) oxidation                                  (b) reduction

(c) both oxidation and reduction (d) neither oxidation nor reduction

  1. v) Steel is the alloy of:

(a) iron and mercury          (b) mercury and copper

(c) iron and copper            (d) iron, chromium and nickel.

Q3.      Answer the following questions in short.

  1. i) Define electrochemistry also state its importance.

Ans.  Electrochemistry is the branch of chemistry that deals with the relationship between electricity and chemical reactions. It involves redox reaction. Electrochemistry plays a vital role on industrial level for the production of industrial products.

  1. ii) What is oxidation? Illustrate with an example.

Ans.  Oxidation is defined as addition of oxygen or removal of hydrogen during a chemical reaction let us elaborate it with example.

  • Addition of oxygen:- When carbon burns in the presence of oxygen to give carbon monoxide.

C + O2      CO

  • Removal of hydrogen:- Hydrogen sulphides is oxidized by bromine water to sulphur.

H2S + Br2            2HBr + S

Here hydrogen has been removed from hydrogen sulphides, so H2S is being oxidize.

iii)           Explain the term reduction with an example.

Reduction is the process, which involves the addition of hydrogen or the removal of oxygen thus reduction is reverse of oxidation.

Examples:-

Addition of hydrogen:-

2H2 + O2            2H2O

N2 + 3H2             2NH3

In these reactions, O2 and N2 molecules gain hydrogen and forms H2O and NH3.

Removal of oxygen:-

Fe2O3 + 3 CO       2Fe + 3 CO2

Zn + CuO             ZnO + Cu

In these examples Fe2O3 and CuO loses oxygen and forms Fe and Cu respectively.

  1. iv) Draw a sketch of an electrolytic cell showing its different parts.
  2. v) What are oxidizing and reducing agents? Explain with examples.

Ans.  An oxidizing agent is the species that oxidizes a substance by taking electrons from it. The substance which is reduced itself by gaining electrons is called oxidizing agent. Non-metals are oxidizing agents because they accept electrons being more electronegative elements e.g. H2O2, KMnO4 and K2Cr2O4 etc.

Reducing agent is the species that reduce a substance by donating electron to it. The substance which is oxidized by losing electrons is also called reducing agent. Almost all metals are good reducing agents because they have the tendency to lose electrons e.g. C, SO2, H2, HBr, H2S etc.

  1. vi) What is difference between weak and strong Electrolytes? Explain with examples.

Ans.  Strong electrolytes:- The electrolytes which ionize completely in solution and produce more ions are called strong electrolytes. Examples of strong electrolytes are aqueous solution of NaCl, NaOH and H2SO4.

Weak electrolytes:- The substance which ionize to a small extent when dissolved in water and could not produce more ions are called weak electrolytes. Active acids (CH3COOH) and Ca(OH)2 when dissolved in water, ionize to a small extent and are good examples of weak electrolytes e.g. ionization of acetic acid in water produces less ions. As a result the weak electrolyte is a poor conductor of electricity.

vii)          What is electroplating? Also describe its importance.

Ans.  Electroplating is a process in which a metal is deposited on another metal electrolytically.

Importance of Electroplating:-  There are three main objectives of electroplating.

Decoration:-  The inferior metals are electroplated by expensive metals such as gold and silver to increase the beauty.

Protection:-  Electroplating is used to protect the inferior metal such as iron from rusting and reaction of organic acids.

Repairing:- The broken parts of the machinery can be welded by electrode position of metals Generally copper, silver, nikel, chromium and gold are electroplated.

viii)         What is electrolysis?

Ans.  A chemical change caused by the passage of an electric current through an electrolyte such as fused ionic compound or its aqueous solution. It breaks into its components the process is called electrolysis.

  1. ix) Describe importance of Alloy formation in industries.

Ans.  Two or some time more metals are mixed to obtain a mixture of these metals called alloy, which is very resistant to corrosion e.g. the steel is an alloy of iron, chromium and nickel, brass is an alloy of copper and zinc. Alloys are very resistant to corrosion. Alloying of iron with other metals has proved to be very successful technique against rusting.

  1. x) Explain why the food articles are packed in tin plated cans rather than chrome plated cans.

Ans.  Tin is a malleable, ductile, highly crystalline, silvery, white metal. This metal resists corrosion from distilled sea, or soft tap water, but can be attacked by strong acids, alkalis and by acid salts. Tin platting is used extensively to protect both ferrous and non ferrous surfaces metals plated using tin is useful for the food processing industry. Since they are non-toxic, ducite and corrosion resistant. While chrome platting is applied top metal objects when there is an increased opportunity for corrosion to occur.

 

Q4.      Answer the following question with reasoning.

  1. i) Differentiate between oxidation number and valency.

Ans.  The combining power of atom of one element with other is called valency or number of unpaired electron in the outer most shelf of an atom is called valency of the atom. Oxidation number is the apparent charge assigned to an atom of an element in a molecule or in an ion, e.g. in HCl, oxidation number of H is +1 and that of Cl is -1.

Rules for assigning oxidation numbers:-

  • The oxidation number of all elements in the Free State is zero.
  • The oxidation number of an ion consisting of a single element is the same as the charge on the ion.
  • The oxidation number of different elements in the periodic table is in Group first it is +1, in group 2nd it is +2 and in group 3rd it is +3.
  • In any substance the more electro negative atom has the negative oxidation number.
  • In neutral molecules, the algebraic sum of the oxidation number of the entire element is zero.
  • In ions, the algebraic sum of oxidation number equals the charge on the ion.
  1. ii) How electricity is produced from chemical reactions?

Ans.  The oxidation-reduction reaction takes place at the electrodes when fused ionic compounds are electrolyzed. The reverse process may also be done in other words. Oxidation-reduction reactions can be used to produce electricity. This can be understood with a very simple process. Take a plate of zinc metal and dip it in an aqueous solution of copper sulphate. After some time a pink layer of copper metal will appear on the zinc plate and the blue color of the solution will become light if the quantity of zinc is enough and sufficient time is allowed. The solution will become colorless. In this process electrons are transferred from zinc to copper ions. Every copper ion gets two electrons from zinc atom to get deposited as a metallic copper atom. A zinc atom goes into the aqueous solution as Zn2+.

Zn + Cu2+           Zn2+ + Cu

In other words zinc is oxidized to Zn2+ while Cu2+ is reduced to copper metal. This reaction can be made a basis for producing electric current.

iii)     Explain how oxidation and reduction goes side by side in chemical processes.

Ans.  The oxidation and reduction go side by side in a reaction. A substance which oxidizes the other is itself reduced and vice versa.

For example:

Fe2O3 + 3CO             2Fe + 3 CO2

                                                                                  Oxidation

Another example is the reaction between the H2 and F2

H2 + F2     2HF

Here, the H2 loses two electrons which are accepted by the F2.

H2                      2H + 2 e     (oxidation)

Fe + 2 e            2F                (reduction)

H2 + F2   2HF               (overall reaction)

As seen, the overall reaction results in the formation of hydrogen fluoride.

  1. iv) Explain how NaCl yields to different products i.e. Na-metal and NaOH by the same process of electrolysis.

Ans.  The fused NaCl produce Na+ and Cl ions which migrate to their respective electrodes on the passage of electric current? The electrodes are separated by steel gauze to prevent the contact between the products. The Cl ions are oxidized to give Cl2 gas at the anode. It is collected over the anode with in an inverted cone-shaped structure while Na+ are reduced at cathode and molten Na-metal floats on the denser molten salt mixture from where it is collected in a side tube following reactions take place during the electrolysis of the molten sodium chloride

2NaCl           2Na+ + 2Cl

2Cl        Cl2 + 2e

2Na+ + 2 e             2Na

2NaCl       Cl2 + 2Na         (overall process)

Manufacture of NaOH from Brine:-

On industrial scale NaOH is produced in Nelson’s cell by the electrolysis of aqueous solution of NaCl called brine.

Aqueous solution of sodium chloride consists of Na+, Cl, H+ and OH ions. These ions move towards their respective electrodes and redox reaction take place at these electrodes. When

electrolysis takes place Cl ions are discharged at anode and Cl2 gas rises into the Dom at the top of the cell. The H+ ions are discharged at cathode and H2 gas escapes through a pipe. The sodium hydroxide solution slowly percolates into a catch basin brine ionizes to produce ions.

2NaCl       2Na+ + 2Cl

Reaction at anode (oxidation)

2Cl        Cl2 + 2e

Reaction at cathode (reduction)

2H2O + 2e         H2 + 2OH

Overall cell reaction

2NaCl + 2 H2O               H2 + Cl2 + 2NaOH

  1. v) How you can prevent a piece of iron from corrosion?

Ans.  Corrosion is a general term but corrosion of iron is called rusting.

Prevention of corrosion

Removal of stains:- The regions of stains in an iron rod act as the site for corrosion. If the surface of iron is properly cleaned and stains is removal. It would prevent corrosion.

Paints and greasing:- Polishing or painting of the surface can prevent the corrosion of iron.

Alloy:-  Alloy is a homogeneous mixture of one metal with one or more other metals. Alloying of iron with other metals has proved to be very successful technique against rusting.

Metallic coating:- The best method for protection against the corrosion of metals is coating the metal. Corrosion resistant metals like Zn, Sn and Cr are coated on the surface of iron to protect it from corrosion.

 

Q5.      Distinguish between the oxidation and reduction processes.

Ans.  There are three different ways of expressing of oxidation and reduction reactions.

  1. Loss or gain of oxygen and hydrogen
  2. Loss or gain of electrons
  3. Oxidation number decreases or increases
  4. i) Redox is stand for reduction- oxidation process simultaneously observed in same chemical reaction. These reactions play an important role as follow.

In steel mils, iron ores usually oxides of iron are converted to the pure metal on industrial level by the following reaction in the blast furnace.

C + O2      CO2+heat (Exothermic Reaction)

CO2 + C   2CO (Endothermic Reaction)

Fe2O3 + 3 CO        2Fe + 3 CO2

In this reaction Fe2O3 loss oxygen which is gains by CO and oxidizes into CO2. Thus Fe2O3 is reduced into Fe and CO oxidized into CO2.

  1. ii) Coal is burned in thermal power station to produce electricity. The following reaction occurs. When it burns:

C + O2       CO2 + heat.

Reduction is defined as the gain of hydrogen atoms e.g.

2H2 + O2      2H2O

N2 + 3H2      2NH3

Reduction is also defined as the loss of oxygen atoms e.g.

Fe2O3 + 3CO       2Fe + 3CO2

Zn + CuO      ZnO + Cu

iii)     The element that shows an increase in oxidation number e.g.

Nao                   Na+1 + e

Fe     Fe+2 + 2e

Fe+2              Fe+3 + 3e

The elements that show a decrease in oxidation number are reduced and the process will be called reduction e.g.

Clo + e       Cl1

Oo + 2e     O2

H + e        H1

H2 + Cl2     2HCl

 

Q.6.  What is the oxidation number? Describe the rules to calculate the oxidation number.

Ans.  Oxidation number is the apparent charge assigned to an atom of element in a molecule or in an ion, e.g. in HCl oxidation number of H is +1 and Cl is -1

Rules for assigning oxidation number.

  1. The oxidation number of all elements in the Free State is zero.
  2. The oxidation number of an ion consisting of a single element is the same as the charge on the ion.
  3. The oxidation number of different element in the periodic table is in group -1 it is +1, in group -2 it is +2 etc.
  4. The oxidation number of oxygen in all its compounds is -2 but it is -1 in peroxides and +2 in OF2.
  5. The oxidation number of hydrogen in all its compounds is +1,but in metal hydrides it is -1.
  6. In any substance the more electro negative atom has the negative oxidation number.
  7. In neutral molecules, the algebraic sum of the oxidation numbers of all the elements is zero.
  8. In ions, the algebraic sum of oxidation number equals the charge on the ion.

 

Q7.      Calculate oxidation number of Mn in KMnO4.

Ans.  KMnO4:- Since KMnO4 is a neutral molecule there fore; its overall oxidation number is zero.

(Oxidation number of K) + (Oxidation number of Mn) + 4(Oxidation number of Oxygen) = zero

Oxidation number of K  = +1

Oxidation number of O  = -2

Oxidation number of Mn            = x

By putting values in above equation

(+1) + x + 4(-2)   = 0

1 + x -8               = 0

x                         = 8 – 1

x                         = +7

So, oxidation number of Mn in KMnO4 is +7.

Q8.      What do you know about the electrochemical industries?

Ans.  Electrical energy is extensively consumed by the chemical process industries. Electrochemical industries use electricity to bring chemical change and produce wide Varity of substances. Such industries are called electrochemical industries. These industries produce substance that can not be made economically in another way, e.g. aluminum and calcium carbide, sodium hydroxide, magnesium and hydrogen. These industries also include electrolytic refining and electroplating industries. Electricity is also used to produce heat that produces high temperature required in the electro thermal chemical industries. Here are some important electrochemical industries.

  1. Manufacture of sodium metal from fused sodium chloride.
  2. Manufacture of NaOH from brine.
  3. Electrolytic refining of copper.
  4. Electroplating of zinc, tin and chromium on steel

Q.9.  What is corrosion? How can you present the corrosion process?

Ans.  The eating away of metals by environmental gases and water is called corrosion. It is a natural redox process that oxidizes metal to their oxides and sulphides under atmospheric conditions.

Prevention of Corrosion:-  Prevention of corrosion is an important way of conserving our natural resources. Following methods have been divided to protect metals from corrosion.

  1. Coating with paint:- Corrosion can be prevented by painting the metal, so that it does not come in contact with oxygen and moisture and other harmful agents paint is cheap and easily applied. Paint is used to protect many everyday steel objects such as cars, trucks, train, Bikes, Bridge etc.
  2. Alloying:- The tendency of iron to oxidize can be greatly reduced by alloying it with other metals e.g. stainless steel is an alloy of iron, chromium and nickel. It is protected from corrosion by an outer layer of Cr2O3.
  3. Coating with a thin layer of another metal:– Metals that readily corrode can be protected by coating with a thin layer of an other metal that resists corrosion this can be done by (i) Tinning (ii) Galvanizing (iii) Electroplating
  4. Cathodic protection:- Cathodic protection is the process in which the metal that is to be protected from corrosion is made cathode and is connected to metals such as magnesium or aluminum. These metals are more active than iron, so they act as anode and iron as cathode. The more active metal themselves oxidize and save iron from corrosion.

 

Q.10.   Describe the working of an electrolytic cell, How sodium metal can be purified by using electrolytic cell?

Ans.  On the large scale sodium metal is produced by the electrolysis of fused NaCl. An electrolytic cell called Down’s Cell is used for this purpose.

The electrodes are iron cathode and graphite anode chlorine is obtained as by-product in molten NaCl. Na+ ions are free to move about under the influence of electric current, Na+ ions move towards the cathode and Cl- ions towards the anode. At electrodes following reactions occur.

At anode 2Cl        Cl2 + 2 e (oxidation)

At cathode          2Na+ + 2e           2Na (reduction)

Molten sodium is collected into a sodium collecting ring, from where it is periodically drained. Where as, chlorine gas is collected into the funnel at the top of the cell.

 

Q11. What is electrochemical cell? Discuss the different types of electrochemical cells?

Ans.  An arrangement which consists of electrodes dipped into an electrolyte in which chemical reactions uses or generates electric current is called electrochemical cells e.g. electrolytic cells (Down’s and Nelson’s cell) Galvanic cells (Daniel’s and Ni-Cd cell)

  1. Electrolytic cell:- The cell in which a reaction occurs with the help of electric current is called electrolytic cell e.g. Down’s cell.

2NaCl              2Na + Cl2

Nelson’s cell

2NaCl + H2O       2Na+ + 2OH+ H2 + Cl2

Non-spontaneous oxidation reduction reactions take place.

  1. Galvanic or Voltaic cell:- The cell in which a reaction generates electric current is called voltaic or Galvanic cells e.g. Daniel’s cell, lead storage battery, fuel cell etc.

Spontaneous oxidation reaction takes place.

 

Q12. Discuss the importance of caustic soda? How it is prepared on industrial scale?

Ans.  Sodium hydroxide is one of the most important chemicals and is used extensively.

  1. It is used in the refining of petroleum and vegetable oils.
  2. It is used as a cleaning agent in the washing powder for cleaning machines and metal sheet.
  3. It is used in the manufacture of soap, paper, viscous rayon, which is artificial silk.
  4. It is used for preparation of organic dyestuffs, sodium metal and many other chemicals.
  5. It is used for mercerizing cotton.
  6. It is used as a reagent in the laboratory.
  7. It is used in the reclining of sulphur.

Preparation   of   caustic   soda:-    Electrolysis   of   brine,  a

concentrated aqueous solution of sodium chloride is used for the industrial production of sodium hydroxide. Electrolysis of brine produces simultaneously three important industrial chemicals, chlorine gas, hydrogen gas and sodium hydroxide. The electrolytic cell called Nelson’s cell.

The solution contains four types of ions i.e. Na+, Cl-, H+, OH- ions.

During electrolysis chloride ions move towards anode. At anode chlorine ions are oxidized to produce chlorine gas. Sodium ions move towards cathode However sodium ions do not reduce to sodium metal in this process. This is because water molecules are more easily reduced than sodium ions. The reduction of water molecule produces hydroxide ions and hydrogen gas. Thus the electrolyte in solution becomes sodium hydroxide. Following reactions occur in the electrolytic cell.

At anode (oxidation)       2Cl          Cl2 + 2e

At cathode (reduction)    2 H2O + 2e           H2 + 2 OH

2 Na+ + 2e           2Na

Overall reaction:

2Na+ + 2Cl + 2 H2O   Cl2 + H2 + 2OH + 2Na+

2Na+ + 2OH     2NaOH

The solution contains Na+ and OH ions. Evaporation of water from this solution produces relatively pure solid sodium Hydroxide. H2 and Cl2 are obtained as useful by products.

 

Q.13.   How the electricity is prepared by chemical reaction?

Ans.  The oxidation-reduction reaction takes place at the electrodes when fused ionic compounds are electrolyzed. The reverse process may also be done in other words. Oxidation-reduction reactions can be used to produce electricity. This can be understood with a very simple process. Take a plate of zinc metal and dip it in an aqueous solution of copper sulphate. After some time a pink layer of copper metal will appear on the zinc plate and the blue color of the solution will become light if the quantity of zinc is enough and sufficient time is allowed. The solution will become colorless. In this process electrons are transferred from zinc to copper ions. Every copper ion gets two electrons from zinc atom to get deposited as a metallic copper atom. A zinc atom goes into the aqueous solution as Zn2+

Zn + Cu2+            Zn2+ + Cu

In other words zinc is oxidized to Zn2+ while Cu2+ is reduced to copper metal. This reaction can be made a basis for producing electric current.

Q.14.   Write a note on electroplating process.

Ans.  In this process the object to be electroplated is cleaned with sand and washed with caustic soda solution. The anode is made of the metal which is to be deposited like Cr, Ni and Silver. The cathode is made up of the object that is to be electroplated like same sheet made up of iron. The electrolyte in this system is a salt of the metal being deposited. The electrolytic tank is made of cement, glass or wood in which anode and cathode are suspended. These electrodes are connected with a battery. When the current is passed, the metal from anode dissolves in the solution and metallic ions migrate to the cathode and discharge or deposition on the cathode (object). As a result of this discharge, a thin layer of metal deposits on the object which is then pulled out and cleaned.

Activity:- Calculate the oxidation number of “N” in following (i) HNO3 (ii) NO2 (iii) NOCl.

HNO3:       Since HNO3 is a neutral molecule; therefore its overall oxidation number is zero.

(Oxidation number of H) + (oxidation number of N) + 3 (oxidation number of O)           = Zero

By putting values in above equation

(+1) + x + 3(-2)     = 0

1 + x – 6                = 0

x – 5                                  = 0

x                            = +5

NO2:          Since NO2 is a neutral molecule; therefore its overall oxidation number is zero.

(Oxidation number of N) + 2(oxidation number of O)      = 0

By putting values in above equation

x + 2 (-2)   = 0

x – 4                      = 0

x                = 4

NOCl:        Since NOCl is a neutral molecule, therefore its overall oxidation is zero.

(Oxidation number of N) + (oxidation number of O) + (oxidation number of Cl) = 0

By putting the value in above equation

(x) + (-2) + (-1)    = 0

x – 2 – 1                = 0

x – 3                                  = 0

x                                  = 3

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