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

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Chapter No.7-Electrochemistry
Short Questions And Answers

SECTION- B: SHORT QUESTIONS:

1. Define oxidation, reduction reactions with examples.

OR

Define oxidation and reduction in terms of loss or gain of oxygen or hydrogen.

OR

Explain oxidation and reduction in terms of loss or gain of electrons.

OR

Describe the nature of electrochemical processes.
Ans: Oxidation and reduction reactions OR Redox reactions:
Oxidation and reduction reactions are electrochemical reactions. They are also called redox reaxtion. In electrochemistry oxidation and reduction reactions involve transfer of electrons.

OXIDATION REACTION:
(i) In terms of loss or gain of oxygen or hydrogen:
"Oxidation may involve introduction of oxygen or removal of Hydrogen from a chemical substance."
Example:

C + O₂ ⟶ CO₂ (burning of coal)

2C₈H₁₆ + 25O₂ ⟶ 16CO₂ + 18H₂O (combustion of octane)

N₂H₄ + O₂ ⟶ N₂ +2H₂O (removal of Hydrogen from Hydrazine)

(ii) In terms of loss or gain of electrons:
"The electrochemical reaction in which atom molecule or ion loses electron and its oxidation number increases is called oxidation reaction."
Example:

Cu ⟶ Cu⁺⁺ + 2e^- (Oxidation reaction)

REDUCTION REACTION:
(i) In terms of loss or gain of oxygen or hydrogen:
"Reduction may involve addition of Hydrogen or removal of oxygen from a chemical substance."
Example:

CuO + C ⟶ Cu + CO (reduction of copper oxide)

Fe₂O₃ 2Al ⟶ Al₂O₃ + 2Fe + Heat (Reduction of Iron oxide)

(ii) In terms of loss or gain of electrons:
"The electrochemical reaction in which atoms, molecule or ion accepts electron and its oxidation number decreases is called reduction reaction."
Example:

S + 2e^- ⟶ S^2- (Reduction reaction)

2. Why ionic compounds conduct electricity in molten or in aqueous solutions only?
Ans: Ionic compounds conduct electricity when in molten (liquid) or in aqueous solution (dissolved in water)because they contain charged particles called cations and anions. These ions are free to move from place to place and conduct electricity. In solid state, onic compounds cannot conduct electricity, because the ions are held together in fixed positions by strong electrostatic force and cannot move freely.

3. What is electrolytic cell? Explain with diagram.
Ans: 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.
For Example:
(i) Down cell
(ii) Castner-Kellner cell

Electrolytic Cell

Explanation:
When an electric current is passed through electrolyte, electrolysis start and following reaction takes place in electrolytic cell.
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)

4. Define oxidizing and reducing agent with examples.
Ans: Oxidizing Agent:
Oxidation occurs due to oxidizing agent. Thus the oxidizing agents are substances that accept electrons.
Example:

Zn + Cl₂ ⟶ ZnCl₂

Chlorine undergoes reduction by accepting electrons and act as oxidizing agent.
Other oxidizing agents are: H₂SO₄, HNO₃, KMnO₄, K₂Cr₂O₇, Cl₂, Br₂, I₂ etc.

Reducing agent:
Reducing agent is responsible for reduction. Thus the reducing agent are substances which loses electrons.
Example:

Zn + Cl₂ ⟶ ZnCl₂

Zinc undergoes oxidation by losing electrons and it act as reducing agent.
Other reducing agents are: Alkali metals, Al, H₂S, ZnNaH, KH etc.

5. Examine the following chemical equations and identify.
(i) Oxidizing agent
(ii) Reducing agent
(iii) Substance undergoes oxidation
(iv) Substance undergoes reduction
1. Zn + C1₂ ⟶ ZnCl₂
2. Br₂ + H₂S ⟶ 2HBr + S
3. 2Ca + O₂ ⟶ 2CaO
4. 2Li + S ⟶ Li₂S

Ans: 1. Zn + C1₂ ⟶ ZnCl₂
Above reaction occur as:

Zn ⟶ Zn²⁺ + 2e^-

C1₂ + 2e^- ⟶ 2Cl^-

Zn²⁺ + 2Cl^- ⟶ ZnCl₂

1. Oxidizing agent (Gain of electron): Chlorine act as oxidizing agent
   
2. Reducing agent (Loss of electron): Zinc act as reducing agent
   
3. Substance undergoes oxidation (Loss of electron occur): Zinc undergoes oxidation by losing electrons.
   
4. Substance undergoes reduction (Gain of electron occur): Chlorine undergoes reduction by accepting electrons

2. Br₂ + H₂S ⟶ 2HBr + S
Above reaction occur as:

H₂ ⟶ 2H⁺ + 2e^-

Br₂ + 2e^- ⟶ 2Br^-

2Br^- + 2H^- ⟶ 2HBr

1. Oxidizing agent (Gain of electron): Bromine act as oxidizing agent.
   
2. Reducing agent (Loss of electron): Hydrogen act as reducing agent.
   
3. Substance undergoes oxidation (Loss of electron occur): Hydrogen undergoes oxidation by losing electrons.
   
4. Substance undergoes reduction (Gain of electron occur): Bromine undergoes reduction by accepting electrons

3. 2Ca + O₂ ⟶ 2CaO
Above reaction occur as:

2Ca ⟶ 2Ca²⁺ + 4e^-

O₂ + 4e^- ⟶ 2O^2-

2Ca²⁺ + 2O^2- ⟶ 2CaO

1. Oxidizing agent (Gain of electron): Oxygen act as oxidizing agent.
   
2. Reducing agent (Loss of electron): Calcium act as reducing agent.
   
3. Substance undergoes oxidation (Loss of electron occur): Calcium undergoes oxidation by losing electrons.
   
4. Substance undergoes reduction (Gain of electron occur): Oxygen undergoes reduction by accepting electrons

4. 2Li + S ⟶ Li₂S
Above reaction occur as:

2Li ⟶ 2Li⁺ + 2e^-

S + 2e^- ⟶ S^2-

2Li⁺ + S^2- ⟶ Li₂S

1. Oxidizing agent (Gain of electron): Sulphur act as oxidizing agent.
   
2. Reducing agent (Loss of electron): Lithium act as reducing agent.
   
3. Substance undergoes oxidation (Loss of electron occur): Lithium undergoes oxidation by losing electrons.
   
4. Substance undergoes reduction (Gain of electron occur): Sulphur undergoes reduction by accepting electrons

6. Identify the Alloy.

Components	Alloy
Cu – Zn	Brass
Cu—Al—Mg—Ni	Duralumin
Cu—Zn—Sn	Bronze

More Short Question And Answer

Q.7: Define electrochemistry and electrochemical reaction?
Ans: ELECTROCHEMISTRY:
It can be defined as:
"The branch chemistry which deals with electro chemical reactions, electrolyte and electrochemical cells is called electrochemistry."

OR

"It deals with the conversion of electrical energy into chemical energy and chemical energy into electrical energy."
Example:
Extraction of metals like aluminum, copper and electro plating of metals are few applications of electrochemistry.

ELECTROCHEMICAL REACTIONS:
The chemical reactions in which chemical energy changes into electrical energy or Vice Versa are called electrochemical reactions.

Q.8: Define redox reaction with examples with oxidizing and reducing agents?
Ans: Oxidation-Reduction Reaction or Redox Reaction:
Oxidation-reduction or redox reaction is a reaction that involves the transfer of electrons between chemical species (the atoms, ions, or molecules) involved in the reaction.

Example:
(i) Photosynthesis is an example of redox reaction.
Photosynthesis produces glucose:

6CO₂ + 6H₂O + sun light ⟶ C₆H₁₂O₆ + 6O₂

Water (H₂O) under goes oxidation by losing electrons and form hydrogen ions and it act as reducing agent. Carbon dioxide (CO₂) undergoes reduction by accept electrons and hydrogen ions to form glucose (C₆H₁₂O₆ + 6O₂) and act as oxidizing agent.

(ii) Formation of Zinc Chloride from Zinc and Chlorine:

Zn + Cl₂ ⟶ ZnCl₂

It is also a redox reaction. Chlorine undergoes reduction by accepting electrons and act as oxidizing agent. Zinc undergoes oxidation by losing electrons and it act as reducing agent.

Q.9: Write down contribution of Micheal Faraday?
Ans: CONTRIBUTION OF MICHEAL FARADAY:

• Michel Faraday was a British chemist who greatly contributed in the field of electrochemistry.
• He observed the quantitative relationship between current and the amount of substance collected at electrodes.
• He conducted several experiments regarding electrolysis and put forward two laws of electrolysis based on his observation.

Q.10: Define the following:

• Coulombs
• Faraday
• Electrochemical equivalent

Ans: COULOMB:
Coulomb is the S.I unit for charge and is represented by (C). It is define as:
"The electric charge which passes when a current of one ampere flows for one second."
 Therefore, Ampere (A) x time (t) = Coulomb

FARADAY:
Faraday is define as:
"The quantity of charge which deposits or liberates 1 gm equivalent weight of substance is called 1 Faraday. It is represented by symbol (F).

1 F = 96500 Coulombs

ELECTROCHEMICAL EQUIVALENT:
Electrochemical equivalent of a substance is defined as:
"The weight of the substance collected at the electrodes when one coulomb of electric currents is passed through the electrolyte."
Its symbol is"Z", and S.I unit is Kg/C (Kilogram per coulomb)

Q.11: Which gold is 100% pure? Name alloy of gold with their components.
Ans: PURE GOLD:
24 karat gold is called 100% pure gold.

Alloys Of Gold:
Addition of metals to gold form different colour.

1. Yellow gold (22 K) Alloy contains 91.67% gold with Ag, Cu, Zinc as other component.
2. Red gold (18 K) contains 75% gold with Cu as other component.
3. White gold (18 K) contains 75% gold with Cu, Ag as other component.

Test Yourself

1. Identify the oxidizing and reducing agents from the following.

1. Al
2. Na
3. H₂S
4. H₂SO₄
5. KMnO₄
6. Zn

Ans: Oxidizing Agent:

1. H₂SO₄
2. KMnO₄

Reducing Agent:

1. Al
2. Na
3. H₂S
4. Zn

2. Identify the oxidizing and reducing agents for the following reaction.

H₂ + Cl₂ ⟶ 2HCl

Ans: The reaction takes place as:

H₂ ⟶ 2H⁺ + 2e^- (Oxidation)
Cl₂ + 2e^- ⟶ 2Cl^- (reduction)
2H⁺ + 2Cl^- ⟶ 2HCl

• Hydrogen undergoes oxidation by losing electrons and it act as reducing agent
• While chlorine undergoes reduction by accepting electrons and act as oxidizing agent.

3. Define 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 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.

4. What are strong electrolytes?
Ans: 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

5. What are non-electrolytes?
Ans: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.

6. Identify strong and weak electrolytes from the following
1. HCl_(aq),
2. KI_(aq),
3. NaOH_(aq),
4. H₂S_(aq),
5. CH₃COOH_(aq),
6. NH₄OH_(aq)
Ans: Strong And Weak Electrolytes

S.NO.	Compounds	Type Of Electrolyte
1.	HCl(aq)	Strong
2.	KI(aq)	Strong
3.	NaOH(aq)	Strong
4.	H2S(aq)	Weak
5.	CH3COOH(aq)	Weak
6.	NH4OH(aq)	Weak

Q.7(i): Sketch electrolytic cell for electrolysis of molten potassium chloride.
Ans: Diagram Of Electrolytic cell For Electrolysis Of Molten Potassium Chloride:

Q.7(ii): Identify cathode and anode, oxidation, reduction reaction, movement of electron from the following sketch of electrolytic cell.
Ans: Overall reaction

2KCl_(l) ⟶ 2K_(l) + Cl_2(g)

(i) Cathode And Anode:

• Cathode: The cathode is connected o the negative terminal of the battery and hence, its negatively charged electrode.
• Anode: The anode is connected o the positive terminal of the battery and hence, its positively charged electrode.

(ii) Oxidation, Reduction reaction:

• At cathode:
  Reduction occurs at cathode because Potassium ions (K⁺ or Cation-Positive) gain electron and deposited on cathode.
  
  K⁺ + e^- ⟶ K_(l)
  
  
• At Anode:
  Oxidation occurs at anode because chloride ions (Cl^- or Anion-negative) lose electron and form chlorine gas, which is evolved at the anode.
  
  2Cl^- ⟶ Cl_2(g) + 2e^-

(iii) Movement of electrons and ions:

• The electrodes act as external conductors and provide surfaces where electron transfer will take place. The power source or battery serves as an electron “pump".
• Movement of Anion: At anode anion (negative ions or Cl^-) loses electrons and undergoes oxidation.
  
  2Cl^- ⟶ Cl_2(g) + 2e^-
• Movement of cation: These electrons from battery enter through cathode at which positive ions (K⁺) are attracted and reduced by accepting electrons.
  
  K⁺ + e^- ⟶ K_(l)
• The reactions occurs at each electrode is called half cell reaction. It means at cathode reduction half-reaction occurs and oxidation half-reaction takes place at anode.
• The migration of ions and electrons in the electrolyte solution completes the electrical circuit.

Q.8: What is Corrosion of metal? OR Define corrosion?
Ans: CORROSION:
Metals react with oxygen in presence of moisture and can form harmful metal oxide. These metal oxide layers are porous and expose metal for further reaction with oxygen to form harmful metal oxide. It is called Corrosion of metal.

Q.9: Name the methods which are used to protect metal from corrosion.
Ans:Methods Used to protect metals:

1. Alloying
2. Metallic Coating (Electroplating)
3. Cathodic Protection
4. Coating with paint

Q.10: How cathodic protection prevent metal from corrosion?
Ans: 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.

Q.11: Sketch a Danniell cell, labeling the cathode, the anode, and the direction of flow of the electrons.
Ans: DANNIELL CELL:

Differences

Q.12: Differentiate between:

• Oxidation and reduction reaction
• Electrolytic and Galvanic cells
• Primary Cell and Secondary cell
• Cell and Battery
• Electrolytes and Non-electrolytes
• Strong electrolytes and Weak electrolytes

Ans: Difference Between Oxidation And Reduction Reaction:

S.NO.	Oxidation Reaction	Reduction reaction
1.	Addition of oxygen	Addition of hydrogen
2.	Removal of hydrogen	Removal of oxygen
3.	Loss of electron by a substance	gain of electron by a substance
4.	Increase in oxidation number of a substance	Decrease in oxidation number of a substance
5.	e.g. C + O2 ⟶ CO2 Cu ⟶Cu++ + 2e-	e.g. CuO + C ⟶ Cu + CO S + 2e- ⟶ S2-

Difference Electrolytic and Galvanic Cells:

S.NO.	Electrolytic Cells	Galvanic Cells
1.	The type of electrochemical cell which uses electrical energy for a non spontaneous redox reaction to occur is called electrolytic cell.	The type of electrochemical cell in which a spontaneous redox reaction occur to generates electrical energy is called galvanic cell.
2.	It converts electrical energy into chemical energy.	It converts chemical energy into electrical energy.
3.	For Example:  Down cell Castner-Kellner cell	For Example: Daniell cell Batteries

Difference Between Primary Cell And Secondary Cell:

S.NO.	Primary Cell	Secondary Cell
1.	It is used to convert chemical energy into electrical energy and it is an irreversible cell.	It is a reversible cell which can be restored to its original condition.
2.	It can not be recharged.	It can be recharge repeatedly.
3.	It has a high internal resistance and hence it produces a small current.	It has a low internal resistance and so it can produce a large current.
4.	e.g. A dry cell	e.g. A lead storage cell.
5.	It is used in most flash lights, calculators, clocks, transistors and in portable electronic devices	It is used in automobiles.

Difference Between Cell and Battery

S.NO.	Cell	Battery
1.	A cell usually consists of two different electrodes immersed (dipped) in an electrolytes.	A battery is made up of two or more cells connected in series.
2.	A cell stores less energy and supplies a small current.	A battery stores more energy and supplies large current.
3.	A cell can not be recharged	A secondary battery can be recharged.
3.	e.g. dry cell. a simple voltaic cell.	e.g. lead storage battery.

Difference Between Electrolytes and Non-electrolytes

	Electrolytes	Non-Electrolytes
Ionization	These are compounds which ionize or dissociate into negative and positive ions in aqueous solution or molten state.	These are compounds which do not ionize or dissociate into ions in aqueous solution or molten state.
Conductivity	They conduct electricity in aqueous solution or molten state.	They cannot conduct electricity.
Nature	They are mostly ionic compounds.	They are organic compound.
Example	HCl, H2SO4, NaCl, NaOH etc.	Sugar (C12H22O11), Glucose (C6H12O6), alcohol, Benzene etc.

Difference Between Strong Electrolytes And Weak Electrolytes

	Strong Electrolytes	Weak Electrolytes
Ionization	These are compounds ionize to a large extent in dilute aqueous solution.	These compounds ionize to small extend in dilute aqueous solution.
Conductivity	They conduct electricity in aqueous solution or molten state.	They cannot conduct electricity.
Nature	They conduct electric current to a large extent.	They conduct electric current to a small extent.
Example	HCl, NaOH, H2SO4, NaCI, NaNO3 etc.	Acetic acid (CH3COOH) and benzoic acid.