Chemistry For Class IX (New Book ) - Chapter No. 7-Electrochemistry - Long Questions And Answers

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Chapter No.7-Electrochemistry
Detailed Questions And Answers

SECTION- C: DETAILED QUESTIONS

1. Describe the dry cell with diagram.
Ans: DRY CELL:
It is also known as Leclanche cell. It is a type of primary cell (non-rechargeable cell) which produce electricity using redox reaction between their chemical substances placed in it.
It converts chemical energy into electrical energy. It is used in torches, clocks, calculators etc.

Construction:
Electrodes:

• Anode: Dry cell has Zinc (Zn) case. It uses zinc as anode. The zinc case acts as negative plate and negative terminal.
• Cathode: A graphite(C) rod through the center of the cell serves as the cathode. The positive terminal is on the central-graphite rod.

A mixture of Magnese dioxide (MnO₂) and graphite powder is packed around the graphite rod. A copper cap is fixed on the top of the carbon (graphite) rod for conduction of electricity.

Electrolytes:
Aqueous ammonium chloride (NH₄Cl) or zinc chloride (ZnCl₂) uses as electrolyte.

Working:
Zinc and graphite are then connected by a metal wire, electric current flows. As a result following chemical reactions take place.
Reaction At Anode:

Zn ⟶ Zn⁺² + 2e^-

Zn⁺²_(aq) ions go into the moist paste and electrons are left behind on the zinc case.

Reaction At Cathode:
It is a complex raection

2NH₄⁺ + 2MnO₂ + 2e^- ⟶ Mn₂O₂ + 2NH₃ + H₂O

Voltage: It produces a potential of 1.5 volt.

2. What is battery? How lead storage battery works? OR Describe how a battery produces electrical energy.
Ans: BATTERIES:
A battery is a device which produces electricity through electro chemical reactions. A battery consists of group of galvanic cells connected in a series. It stores more energy and supplies large (higher) current.
Batteries are classified as primary (non rechargeable) and secondary (rechargeable) batteries.
Examples: of batteries include:

• Dry cell, lead storage battery, mercury battery etc.
• We use lot of electrical devices having batteries as a source of electricity.
• Scientist are working for enhancing high energy, safety, cycle ability of batteries for mobile phones, transportation, computer technology etc.

LEAD STORAGE BATTERY:
Construction:

1. Lead storage battery is an example of secondary cell in which chemical changes can be reversed. It is used in motor vehicles (automobiles).
2. It has several (six) voltaic cells connected in series.
3. Electrodes:
   It contains two types of plates:
   (i) Anode: lead plates which serve as anode (negative plate) and
   (ii) Cathode: lead oxide (PbO₂) plate which acts as cathode (positive plate).
   These plates are separated bya n inert material (wood or glass fibre).
4. Electrolyte:
   These electrodes are immersed in electrolytic solution of dilute sulphuric acid (H₂SO₄). The acid acts as a source of H⁺ ions and SO₄^2- ions.

Working Of Lead Battery:
It consist of two processes:

1. Discharging Process
2. Charging Process

1. Discharging Process:

• During discharge the chemical energy is converted into electrical energy.
• When two electrodes are connected to external circuit, the lead storage battery produces electricity by discharging.
• The battery discharges or released or give out electrons at the anode flow through the external circuit to the cathode.
• As a result the lead and lead oxide on the electrodes are slowly start converted to soft lead sulphate.
• The reaction dilutes the acid because water is also produced.
• The electrode reactions that occur during discharge are:
  At Anode:
  
  Pb_(s) + SO₄^2-_(aq) ⟶ PbSO_4(s) + 2e^- (Oxidation)
  
  At cathode:
  
  PbO_2(s) + SO₄^2-_(aq) + 4H⁺_(aq) + 2e^- ⟶ PbSO_4(s) + 2H₂O_(l) (Reduction)

Recharging Process:

• During recharging, electrical energy is converted into chemical energy, and the storage battery act as the electrolytic cell.
• A lead- storage battery can be charged, when it is connected to an external source of electrical energy (i.e battery charger).
• Electrons are pushed back through the battery (secondary cell).
• This reserve the electrodes reactions; lead and lead oxide are built up to on the respective electrodes again. So these are the materials of fully charges cells.

Overall Process:
Chemical changes during charging and discharging processes can be shown as:

3. Explain process of electrolysis in electrolytic cell.
Ans: ELECTROLYSIS:
Electrolysis is a process of migration of ions towards cathode and anode when current passes through en electrolyte.

OR

The electrolysis involves redox reactions. It is the process of producing chemical changes in a molten compound or its aqueous solution by causing its oppositely charged ions to move in opposite directions under a potential difference.
It carried out in electrolytic cell.

Electrolytic Cell:
The type of cell which uses electricity for a non spontaneous reaction to occur is called electrolytic cell. It convert electrical energy into chemical energy.
An electrolytic cell consists of electrolyte in a vessel, electrodes and a battery.

Process Of Electrolysis:

• In electrolysis, Some molten compound or or aqueous solution (electrolyte) is taken in a glass vessel.
• Two metallic rods (act as electrode) are dipped into electrolyte.
• The two electrodes are connected to a source of electricity i.e. battery by two wires.
• The electrode connected to the negative terminal of the battery is called cathode.
• The electrode connected to the positive terminal of the battery is called Anode.
• When current passes through an electrolyte, electrolysis starts.
• A molten compound is broken down, due to this migration of positive and negative ions towards cathode and anode takes place. As a result ions are discharged at their respective electrodes.

Reaction At Cathode:
Electrons from battery enter through cathode at which positive ions are reduced by accepting electrons. It means at cathode reduction occurs.

M⁺ + e^- ⟶ M (Reduction Gain of electron)

Reaction At Anode:
At anode negative ions loses electrons and undergoes oxidation.

X^- ⟶ X + e^- (Oxidation loss of electron)

Overall Result:

M⁺ + X^- ⟶ MX

Thus, in electrolysis, during the passage of current, molten compound or aqueous solution is decomposed into its elements.

4. What is Alloy? Explain its classification with examples.
Ans: ALLOY:
An alloy is the mixture of metal with metal or metal with non metal. There are about 7000 alloys which are used for different purposes in the world.
Alloy Formation:
Alloy can be prepared by mixing elements in different proportions. In alloy it become difficult for layers of metal atoms to slide over each other. So alloy is harder and stronger than pure metal.

Example:

• Brass is an alloy of Copper (Cu) and Zinc (Zn)
• Steel is a alloy of iron and carbon
• Bronze is an alloy of Copper (Cu), Zinc (Zn) and Tin (Sn)
• Amalgam is an alloy of Mercury (Hg), Silver(Ag), Copper (Cu) and Zinc (Zn) etc.

Classification Of Alloys:
Based on the presence or absence of Iron, alloys are classified as:

1. Ferrous alloys:
   Contain Iron as a major component.
   Example: Steel is a alloy of iron and carbon.
   
   
2. Non-ferrous alloys:
   Do not contain Iron as a major component.
   Example: Brass is an alloy of Copper (Cu) and Zinc (Zn).

5. What is rusting? How it can be prevented.
Ans: RUSTING:
Definition:
Corrosion of iron is an electro chemical process. Iron under goes redox reaction in presence of air or water to form iron (III) oxide (Fe₂O₃. nH₂O) called rusting of iron.
Explanation:
Rusted surface of iron provide no protection to underlying iron and eventually convert whole iron into reddish brown rust. Rusting occurs at different places of metal surface. A metal surface area of less moisture act as anode and oxidizes iron in this region.

Fe_(s) ⟶ Fe⁺²_(aq) + 2e^-

Metal surface with high moisture content act as cathode and reduces atmospheric oxygen to OH^- ions.

O₂ + 2H₂O + 4e^- ⟶ 4OH^-

The Fe⁺² ions further react with oxygen to form rust iron (III) oxide (Fe₂O₃. nH₂O)

PREVENTION FROM CORROSION:
All metals can be prevented from corrosion by following methods.
1. Alloying:
Formation of alloy prevents metal from corrosion by reducing its ability of oxidation.
Example: Iron (Fe) can be changed into stainless steel by mixing with chromium (Cr) and Nickle (Ni). Thus iron (Fe) is prevented from corrosion.

2. Metallic Coating (Electroplating):
All metals can be protected from corrosion by coating its surface with other metal like tin (Sn) or zinc (Zn). The coating of metal at the surface of other metal by electrolytic process is called electroplating. Example: Metals like iron can be electroplated with chromium (Cr), Nickle (Ni) and silver (Ag).

3. Cathodic Protection:
It is applied to protect underground pipes tanks, oil rigs etc from corrosion by making these materials as cathode. The active metal like magnesium (Mg) or aluminum (Al) is used as Anode and connected with iron (Fe). These active metals itself oxidizes and prevent other metal from corrosion.

4. Coating with paint:
A metal is commonly coated with paint to protect it from corrosion. Paint prevents the reaction of metal with oxygen moisture and other harmful chemical agents.

6. What is electroplating? How steel object can be electroplated with Tin, Zinc and Silver? OR Explain electroplating of metals on steel (using examples of zinc, Tin and chromium plating).
Ans: ELECTROPLATING (METALLIC COATING):
All metals can be protected from corrosion by coating its surface with other metal like tin (Sn) or zinc (Zn). The coating of metal at the surface of other metal by electrolytic process is called electroplating.

OR

The process of deposition of metal at the surface of other metal through electrolysis is called electroplating.
Example: Metals like iron can be electroplated with chromium (Cr), Nickle (Ni) and silver (Ag).

Electroplating on Steel:
Steel can be electroplated with:

1. Tin
2. Zinc
3. Silver
4. Chromium

1. Tin Plating:

• Electrolyte:
  Steel spoon can be tin plated by using acidified tin sulphate as electrolyte.
• Electrodes:
  Anode: Tin (Sn) metal is used as anode.
  Cathode: Steel spoon is used as cathode.

Process:

• When current passes through electrolyte tin ions (Sn+2) deposits at cathode as tin (Sn) metal. Tin (Sn) electrode, is then changes into tin ion (Sn2+).
  
  SnSO₄ ⇌ Sn⁺⁺ + SO₄
• Reaction At Anode:
  
  Sn ⟶ Sn⁺⁺ + 2e^- (Oxidation)
• Reaction At Cathode:
  
  Sn⁺⁺ + 2e^- ⟶ Sn (Reduction)

2. Zinc Plating:
Galvanizing:
The process in which zinc is electrolytically coated at the surface of other base metal is called galvanizing.

• Electrolyte:
  Potassium zinc cyanide is used as electrolyte to produce zinc ions (Zn²⁺).
• Electrodes:
  Anode: Zinc (Zn) metal serves as anode.
  Cathode: Steel object is used as cathode.

Process:

• During electrolysis Zn⁺⁺ deposits at cathode and Zinc (Zn) anode is then changes into zinc ion Zn²⁺.
• Following reactions occur during zinc electroplating.
  Reaction At Anode:
  
  Zn ⟶ Zn⁺⁺ + 2e^- (Oxidation)
  Reaction At Cathode:
  
  Zn⁺⁺ + 2e^- ⟶ Zn (Reduction)

3. Electroplating of Silver:
Silver Plating:
In this process silver (Ag) is coated electrolytically at the surface of steel or other metal. It is called silver plating.

• Electrolyte:
  In this process aqueous solution of silver chloride (AgCl) is used as electrolyte to produce silver (Ag⁺) ions.
• Electrodes:
  Anode: Silver (Ag) metal is used as anode.
  Cathode: Steel object like spoon used as cathode.

Process:

• Silver (Ag⁺) ions are reduced at cathode by accepting electron. Silver anode loses electron and oxidized to silver (Ag⁺) ion.
• Following chemical changes takes place.
  
  AgCl ⇌ Ag⁺ + Cl⁺
• Reaction At Cathode:
  
  Ag⁺ + e^- ⟶ Ag (Reduction)
• Reaction At Anode:
  
  Ag ⟶ Ag⁺ + e^- (Oxidation)

4. Chromium Plating:
The process in which chromium (Cr) is coated electrolytically at the surface of other base metal is called chromium plating.

• Electrolyte:
  Acidified chromium sulphate Cr₂(SO₄)₃ is taken as electrolyte to produce chromium (Cr⁺³) ion.
• Electrodes:
  Anode: Chromium metal serves as anode.
  Cathode: Other metal object is used as cathode.

Process:

• Following chemical changes take place in chromium plating.
  
  Cr₂(SO₄)_3(aq) ⇌ Cr³⁺ + 3SO₄^2-_(aq)
• Reaction At Anode:
  
  Cr ⇌ Cr³⁺ + 3e^- (Oxidation)
• Reaction At Cathode:
  
  Cr³⁺ + 3e^- ⇌ Cr (Reduction)

Uses Of Chromium Plating:

• Chromium plated objects are used in Auto parts industries.

7. State and explain Faraday first and second law of electrolysis.
Ans: Faraday's law of Electrolysis:
Michel Faraday was a British chemist who greatly contributed in the field of electrochemistry. He conducted several experiments regarding electrolysis and put forward two laws of electrolysis based on his observation.

Faraday's First Law Of Electrolysis:

It states that:

"Amount of any substance that is deposited or liberated at an electrode during electrolysis is directly proportional to the quantity of electricity passed through the electrolyte."

Explanation:
If 'W' is the weight of the substance deposited or liberated at electrode, when 'A' ampere of current passed in time 't' second. The Faraday's First law of electrolysis can be expressed mathematically as:

W ∝ A x t

or W = ZAt

In this equation,
'Z' is a constant and is known as electrochemical equivalent.
If one ampere of current is passed for one second, i.e. A =1 amp, t = 1 sec

then W = Z

This means that the electrochemical equivalent is the weight of a substance liberated by 1 ampere current passing for one second (1 columb).

Faraday's Second law of Electrolysis:

It states that:

"The amount of different substances deposited or liberated due to passage of same quantity of current through different electrolytes are proportional to their chemical equivalent masses."

Explanation Or Example:

• Take three electrolytic solution of silver nitrate, copper sulphate and aluminum nitrate in three electrolytic cells which are connected in series.
• Same quantity of current (96,500 coulombs) is passed through them.
• The masses of Ag, Cu and Al deposited on their respective electrodes (cathode) would be directly proportional to their equivalent masses.
• For an element:
  
  
  
  
• As a result 108 gm of silver, 31.75 gm of copper and 9 gm of aluminum are collected at their respective electrodes.

More Long Questions Answers

Q.8: Define electrochemical cells? Describe types of electrochemical cells?
Ans: ELECTROCHEMICAL CELLS:
The device which convert chemical energy into electrical energy or vise versa using redox reaction are called electrochemical cells.
The electrochemical reactions are carried out in electrochemical cells.

Structure Of Electrochemical Cell:
An electrochemical cell consists of electrodes, a battery and electrolyte in a vessel.
1. Electrodes:
An electrode is an electrical conductor. Electrochemical cell consists of two electrodes at which redox reaction occurs.

• Anode:
  The electrode at which oxidation takes place is called Anode. Negative ions move towards Anode.
  
  
• Cathode:
  The electrode at which reduction occurs is called Cathode. Positive ions move towards cathode.

The reactions occurs at each electrode is called half cell reaction. The overall cell reaction is the combination of two half cell reactions.

2. Battery:
Each electrode is in contact with battery.

3. Electrolyte:
Electrolyte is present in cell (Vessels). Electrolyte consists of free moving ions and conduct electricity.

TYPES OF ELECTROCHEMICAL CELLS:
Electrochemical cells are of two types.

1. Electrolytic Cells
2. Galvanic Cells or Voltaic Cells

Electrolytic Cells:

• The type of cell which uses electricity for a non spontaneous reaction to occur is called electrolytic cell.
• It convert electrical energy into chemical energy.
• For Example:
  (i) Down cell
  (ii) Castner-Kellner cell

Galvanic Cells or Voltaic Cells:

• It convert chemical energy into electrical energy.
• For Example:
  (i) Daniell cell
  (ii) Batteries

Q.9: Define following with examples:

• Electrolyte
• Strong electrolyte
• Weak electrolyte
• Non-electrolyte

Ans: ELECTROLYTE:
An electrolyte consists of free moving ions and conduct electricity.
In other words,
It is a substance that can conduct electricity in aqueous solution or in the fused state with chemical change because of the presence of freely moving ions. The electrolytes are strong electrolytes or weak electrolytes.
Example:

• Acids, bases and salts in molten or in aqueous solution form (or ionic and polar compound) are electrolytes. e.g.
  HCl, PbI, NaCl, NH₄OH etc.

STRONG ELECTROLYTE:
Compounds (substances) that are exist in aqueous solution or in molten (fused) state and dissociate completely as ions, therefore they can conduct electric current readily are known as strong electrolysis.
Example:

• Acids: HCl, HNO₃, HI, H₂SO₄
• Bases: KOH, NaOH, LiOH
• Salts: KI, NaCl, CuSO₄

WEAK ELECTROLYTE:
Compounds (substances) that are partially dissociated into ions in aqueous solution or in molten (fused) state, therefore they can not conduct electric current readily are known as weak electrolysis.
Example:

• Acids: H₂S, H₂CO₃, CH₃COOH
• Bases: NH₄OH, C(aOH)₂, Mg(OH)₂
• Salts: PbI, KHCO₃, AgCl

NON-ELECTROLYTES:
The substances which are unable to conduct electricity in molten state or in aqueous solution form are called non electrolytes.

OR

Compounds (substances) that are exist as molecules and therefore do not conduct electric current in aqueous solution or are in the fused state are known as non-electrolysis.

OR

The substances that can not form ions in aqueous solution form are called non electrolytes.
Example:

• Benzene, Glucose, Sucrose and Urea etc are non- electrolytes.

Q.9: List the possible uses of an electrolytic cell.
Ans: Applications Or Uses of Electrolytic cells:
Important uses of electrolytic cell are given below.

1. It is used to prepare sodium metal from molten sodium chloride using the down's cell.
2. It is used to prepare caustic soda (NaOH) from aqueous sodium chloride by Nelson's cell. It is also used to obtain chlorine gas.
3. It is used to extract aluminum metal.
4. It is used in electro refining of copper.
5. Electrolytic cell are used for electro plating of metals.

Q.10: Name Alloys with their components and uses or applications?
Ans: Some Important Alloys With Their Components And Applications ares:

Name Of Alloy	Components	Applications
Bell Metal	Cu-Sn	Casting of bell
Brass	Cu-Zn	Door nobs and hand rails due to antibacterial nature, Hose nozzles, Stamping dies.
Bronze	Cu-Zn-Sn	Coins, medals, tools, etc.
Monel	Ni-Cu-Fe	Corrosion resistant containers
Duralumin	Al-Cu-Mg-Ni	Boat, Aircraft etc.
Solder	Sn-Pb-Cu-Sb	Joining electrical components into circuits
Alnico	Fe-Al-Ni-Co	Magnets used in loud speakers
Amalgam	Hg-Ag-Cu-Zn	Dental filling
Cupronickel	Cu-Ni-Mn	Coins
Pewter	Sn-Cu-Pb-Sb-Bi	Ornaments
Sterling silver	Ag-Cu	Cutlery set, Medical tools
White Gold (18 Karat)	Au-Pb-Ag-Cu	Jewelry
Red Gold (18 K)	Au-Cu	Jewelry
Yellow Gold (22 K)	Au-Ag-Cu-Zn	Jewelry

Q.11: Why iron is not use in electro-plating and tin and silver are used in electroplating and not iron?
Ans: Iron Is not Use In Electroplating:
Iron is reactive metal. It can react with food items and can spoil food. So It is not use in electroplating.

Tin Plating:
Tin is non toxic, less reactive and resistant to corrosion. Tin can not react with organic acids or salt present in food so tin plated cans are used for beverages and for storing foods.

Silver Plating:
Silver is lustrous white metal. Many metal objects are silver plated to enhance its beauty and strength against corrosion.
Thin layer of silver on metal surface form durable layers. Thick coating of silver on metal surface is soft and gradually turns black, due to formation of silver sulphide(Ag₂S).