chapter 7

Chapter 7

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1. How can you justify that a reaction between magnesium and oxygen is a redox reaction, while the reaction shows only addition of oxygen (oxidation)

Ans. This reaction between magnesium and oxygen (oxidation) by magnesium and gain of electrons (reduction) by oxygen. Hence, it is a redox reaction.

2. A reaction between carbon and oxygen involved only addition of oxygen (oxidation), but, it is called a redox reaction. Comment on this.Ans. Carbon is being oxidized as being reduced as its oxidation number decreases from 0 to reactions are taking place.That is why it is called redox reaction.

3. Oxidation and reduction proceed simultAns. where there is oxidation there is reduction. Both reactions proceed simultaneously. example,

In this reaction magnesium is oxidized and oxygen is reduced.

4. Identify which of the following is oxidation or

Ans.

(a) K involves lose of electron, so it is an (b) Br involves gain of electron, it is a(c) Cu involves lose of electron,(d) Iodide ion loses an electron,(e) Ferric ion loses an electron,

5. An element M reacts with another element X to form MXelectrons, identify the element which is oxidized and which is reducedAns.

M atom loses two electrons, so it is oxidized.

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How can you justify that a reaction between magnesium and oxygen is a redox reaction, while the reaction shows only addition of oxygen (oxidation)

reaction between magnesium and oxygen atoms involves lose of electrons magnesium and gain of electrons (reduction) by oxygen. Hence, it is a redox

A reaction between carbon and oxygen involved only addition of oxygen (oxidation), but, it is called a redox reaction. Comment on this.

arbon is being oxidized as its oxidation number increases from 0 to +4 and oxygen is being reduced as its oxidation number decreases from 0 to –2. Both oxidation and reduction reactions are taking place.That is why it is called redox reaction.

Oxidation and reduction proceed simultaneously. Explain, with an examplewhere there is oxidation there is reduction. Both reactions proceed simultaneously.

In this reaction magnesium is oxidized and oxygen is reduced.

Identify which of the following is oxidation or reduction reaction?

K involves lose of electron, so it is an oxidation reaction.Br involves gain of electron, it is a reduction reaction.

involves lose of electron, it is an oxidation reaction.ion loses an electron, it is an oxidation reaction.

loses an electron, it is an oxidation reaction.

An element M reacts with another element X to form MX2 In terms of loss or gain of electrons, identify the element which is oxidized and which is reduced

M M+2 + 2e—

loses two electrons, so it is oxidized.

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How can you justify that a reaction between magnesium and oxygen is a redox reaction, while the reaction shows only addition of oxygen (oxidation)?

involves lose of electrons magnesium and gain of electrons (reduction) by oxygen. Hence, it is a redox

A reaction between carbon and oxygen involved only addition of oxygen (oxidation),

its oxidation number increases from 0 to +4 and oxygen is 2. Both oxidation and reduction

aneously. Explain, with an example?where there is oxidation there is reduction. Both reactions proceed simultaneously. for

In terms of loss or gain of electrons, identify the element which is oxidized and which is reduced?

Chapter 7


X atom gains two electrons

6. How can you justify that the following reaction is not only an oxidation reaction but also a complete redox reaction?

Ans. Removal of an oxygen atomremoved from FeO, so it is reduced. CO gains oxygen and is converted to CO

7. Explain the term oxidation on the basis of electronic concept withAns. Oxidation is loss of electron

1. Find out the oxidation numbers of the following elements marked in bold in the formulae:Ba3(PO4)2, CaSO4, Cu(NOAns.

(a) To find oxidation number of P in

3(oxidation number of Ba) + 2(oxidation number of P3(+2) + 2(oxidation number of P) + 8(6 + 2(O.N. of P) –16 = 02(O.N. of P) – 10 = 02(O.N. of P) = 10O.N. of P = 10

2Oxidation number of P = 5

(b)To calculate oxidation number of S in

(O.N. of Ca) + (O.N. of S(+2) + (oxidation number of S2 + (O.N. of S) – 8 = 0O.N. of S – 6 = 0Oxidation number of S = 6

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X + 2e— X—2

gains two electrons, so it is reduced.

M + X MX2

How can you justify that the following reaction is not only an oxidation reaction but also

Removal of an oxygen atom from a molecule is called reduction. , so it is reduced. CO gains oxygen and is converted to CO

Explain the term oxidation on the basis of electronic concept with an exampleis loss of electron by an atom or an ion. e.g.

Find out the oxidation numbers of the following elements marked in bold in the

O3)2, Al2(SO4)3.

To find oxidation number of P in Ba3(PO4)2

3(oxidation number of Ba) + 2(oxidation number of P) + 8(oxidation number of O2) + 2(oxidation number of P) + 8(– 2) = 0

number of P = 5

o calculate oxidation number of S in CaSO4

) + 4(O.N. of O) = 0oxidation number of S) + 4(– 2) = 0

number of S = 6

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How can you justify that the following reaction is not only an oxidation reaction but also

reduction. Oxygen atom has , so it is reduced. CO gains oxygen and is converted to CO2. It is oxidized.

an example?

Find out the oxidation numbers of the following elements marked in bold in the

) + 8(oxidation number of O) = 0

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(c)To calculate oxidation number of N in Cu(NO3)2

(O.N. of Cu) + 2(O.N. of N) + 6(O.N. of O) = 0(+2) + 2(O.N. of N) + 6(– 2) = 02 + 2(O.N. of N) – 12 = 02(O.N. of N) – 10 = 02(O.N. of N) = 10O.N. of N = 10/2Oxidation number of N = 5

(d)To calculate oxidation number of S in Al2(SO4)3

2(O.N.of Al) + 3(O.N.of S) + 12(O.N.of O) = 02(+3) + 3(oxidation number of S) + 12(–2) = 06 + 3(oxidation number of S) –24 = 03(oxidation number of S) –18 = 03(oxidation number of S) = 18O.N. of of S = 18/3O.N. of S = 6

2. In a compound MX3, find out the oxidation number of M and X?Ans.

Applying formula for MX3

(O.No. of M) + 3(O.No. of X) = 0(O.No. of M) +3(–1) = 0O.No. of M –3 = 0O.No. of M = 3

Applying formula for MX3

(O.No. of M) + 3(O.No. of X) = 03 + 3(O.No. of X) = 03(O.No. of X) = –3 O.No. of X = –3/3O.No. of X = –1

3. Why the oxidation number of oxygen in OF2 is +2?Ans. In OF2 molecule both oxygen and fluorine atoms are electronegative. Fluorine beingmost electronegative atom gets negative ( –4) oxidation state and oxygen atom being less electron negative gets positive (+2) oxidation state.

4. In H2S, SO2 and H2SO4 the sulphur atom has different oxidation number. Find out the oxidation number of sulphur in each compound.Ans.

(a) H2S2(O.N of H ) + O.N of S = 02(+1) + O.N of S = 02 + O.N of S = 0O.N of S = –2

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(b) SO2

O.N of S + 2(O.N of OO.N of S +2(–2) O.N of S –4 = 0O.N of S = 4

(c) H2SO4

2(O.N of H ) + (O.N of S2(+1) + O.N of S + 4(2 + O.N of S –8 = 0O.N of S –6 = 0O.N of S = 6

5. An element X has oxidation state 0. What will be its oxidation state when it gains three electrons?Ans. when X element gains three electrons its oxidation state changes from 0 toIt can be represented as:

X + 3e

6. An element in oxidation state +7 gains electrons to be reduced to oxidation state +2. How many electrons did it accept?Ans. when an element in oxidation state +7 gains five (5) electrons its oxidation state reduced to +2.It can be represented as:

7. If the oxidation state of an element changes from +5 to oxidized? How many electrons are involved in this process?Ans. If the oxidation state of an element changes from +5 to gained eight (8) electrons.

1. In the following reaction, how can you justify that H

Ans. Chemical equation with oxidation number

S+4O2—4 + 2H

From above equation we can 0. So, H2S is oxidized. In SOreduced.

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O.N of S + 2(O.N of O ) = 0 = 0

O.N of S) + 4(O.N of O) = 02(+1) + O.N of S + 4(–2) = 0

8 = 06 = 0

An element X has oxidation state 0. What will be its oxidation state when it gains three

when X element gains three electrons its oxidation state changes from 0 toIt can be represented as:

X + 3e-- X3--

An element in oxidation state +7 gains electrons to be reduced to oxidation state +2. How many electrons did it accept?

an element in oxidation state +7 gains five (5) electrons its oxidation state

It can be represented as:X7+ + 5e-- X2+

If the oxidation state of an element changes from +5 to -3. Has it been reduced or electrons are involved in this process?

If the oxidation state of an element changes from +5 to -3, it has been reduced. It has electrons.

X 5+ + 8e-- X3--

In the following reaction, how can you justify that H2S is oxidized and

Ans. Chemical equation with oxidation numbers can be written as:

+ 2H2+2S—2 2H2

+2O—2 + 3S0

equation we can see that oxidation number of S in H2S has changed from S is oxidized. In SO2, oxidation number of S has changed from +4 to 0. So, SO

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An element X has oxidation state 0. What will be its oxidation state when it gains three

when X element gains three electrons its oxidation state changes from 0 to –3.

An element in oxidation state +7 gains electrons to be reduced to oxidation state +2.

an element in oxidation state +7 gains five (5) electrons its oxidation state

3. Has it been reduced or

, it has been reduced. It has

S is oxidized and SO2 is reduced.

can be written as:

S has changed from –2 to number of S has changed from +4 to 0. So, SO2 is

Chapter 7


2. The reaction between MnOequation.

Find out:

a. The substance oxidized.

b. The substance reduced.

c. The substance which acts

d. The substance which acts as a reducing agent.

Ans. chemical equation

Mn+4O2-4 + 4H+1Cl

The above reaction shows that(a) HCl is oxidized.(b) MnO2 is reduced.(c) MnO2 acts an oxidizing agent.(d) HCl acts as a reducing agent.

3. The following reactions are redox reactions.Find out the element which has been reduced and the element which has been oxidized.

Ans.

(a) Chemical equation with oxidation numbers

Zn0 + Cu

Above reaction shows that

i. Zinc (Zn0) is oxidized.ii. Copper (Cu2+)

(b) Chemical equation with oxidation numbers can be written as:

Cu0 + 2Ag


i. Copper (Cu0)ii. Silver (Ag+1) is reduced.

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The reaction between MnO2 and HCl is a redox reaction written as balance chemical

a. The substance oxidized.

b. The substance reduced.

c. The substance which acts as an oxidizing agent.

d. The substance which acts as a reducing agent.

chemical equation with oxidation numbers can be written as:

Cl-1 Mn+2Cl2-2 + 2H2

+2O-2 + Cl

The above reaction shows that

acts an oxidizing agent.HCl acts as a reducing agent.

The following reactions are redox reactions.Find out the element which has been reduced and the element which has been

with oxidation numbers can be written as:

+ Cu+2SO4-2 Zn+2SO4

-2 + Cu0


is oxidized.) is reduced.

Chemical equation with oxidation numbers can be written as:

+ 2Ag+1N+5O3-6 Cu+2(N+10O3

-12)2 + 2Ag

reaction shows that

is oxidized.is reduced.

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and HCl is a redox reaction written as balance chemical

+ Cl20

Find out the element which has been reduced and the element which has been

+ 2Ag0

Chapter 7


(c) Chemical equation with oxidation numbers can be written as:

H2+2S + Cl


i. Sulphur (S) is oxidized.ii. Chlorine (Cl) is reduced.

4. Why the following reaction is not a redox reaction. Explain with reasons?

Ans. Chemical equation with oxidation numbers can be written as:

Na+1O-2H+1

It is clear from above reaction, there isTherefore, no oxidation-reduction reaction taking place. So, it is not a redox reaction.

1. Why are the strong electrolytes termed as good conductors?Ans. Strong electrolytes are termed as good conductors because they ionize almost completely in their solutions. The ions of electrolytes are responsible for the conduct of electricity.

2. Does non-electrolytes forms ions in solution?Ans. Non-electrolytes do not

3. What is difference between a strong electrolyte and a weak electrolyte?Ans. Strong electrolyte: The electrolytes which ionize almost completely in their aqueous solutions and produce more ions, are called strong electrolytes. electrolytes are aqueous solutions of NaCl, NaOH and HWeak electrolyte: The electrolytes which ionizeand could not produce more ions are called weak electrolytes. Acetic acid (CHCa(OH)2 when dissolved in water, ionize to a small extent and are good examples of weak electrolytes.

4. Identify a strong or CuSO4, H2CO3, Ca(OH)2

Ans. Strong electrolyteweak electrolyte

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S + Cl20 2H+1Cl-1 + S0

is oxidized.is reduced.

Why the following reaction is not a redox reaction. Explain with reasons?


+1 + H+1Cl-1 Na+1Cl-1 + H2+2O-2

ar from above reaction, there is no change in oxidation number of any element. reduction reaction taking place. So, it is not a redox reaction.

Why are the strong electrolytes termed as good conductors?trong electrolytes are termed as good conductors because they ionize almost

completely in their solutions. The ions of electrolytes are responsible for the conduct of

electrolytes forms ions in solution?do not ionize in their aqueous solutions.

What is difference between a strong electrolyte and a weak electrolyte?

The electrolytes which ionize almost completely in their aqueous solutions and produce more ions, are called strong electrolytes. Examples of strong

aqueous solutions of NaCl, NaOH and H2SO4.The electrolytes which ionize to a small extent when dissolved in water

and could not produce more ions are called weak electrolytes. Acetic acid (CHwhen dissolved in water, ionize to a small extent and are good examples of weak

Identify a strong or weak electrolyte among the following compounds:2 , HCl, AgNO3

Strong electrolyte CuSO4, HCl, AgNO3

H2CO3, Ca(OH)2

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Why the following reaction is not a redox reaction. Explain with reasons?

oxidation number of any element. reduction reaction taking place. So, it is not a redox reaction.

trong electrolytes are termed as good conductors because they ionize almost completely in their solutions. The ions of electrolytes are responsible for the conduct of

What is difference between a strong electrolyte and a weak electrolyte?

The electrolytes which ionize almost completely in their aqueous Examples of strong

to a small extent when dissolved in water and could not produce more ions are called weak electrolytes. Acetic acid (CH3COOH) and

when dissolved in water, ionize to a small extent and are good examples of weak

weak electrolyte among the following compounds:

Chapter 7


5. Which force drives the nonAns. An electric force drives

6. Which type of chemical reaction takes place in electrolytic cell?Ans. A Non-spontaneous redox reaction takes place in

7. What type of reaction takes place at anode in Ans. An oxidation reaction takes place at anode in electrolytic cell

8. Why the positively charged electrode is called anode in electrolytic cell?Ans. In electrolytic cell, positively charged electrode isbattery, that is why it is called an anode.

9. In the electrolysis of water, towards which terminal HAns. In the electrolysis of water, H

10. In the electrolysis of water, where is the oxygen produced?Ans. In the electrolysis of water,

11. Towards which electrode of the electrolytic cell moves the cations and what does they do there ?Ans. In electrolytic cell cations move toward cathode. electrons.

12. How the half-cells of a galvanic cell are connected? What is function of saltAns. A galvanic cell consists of two cells.connected electrically by a salt bridge. In each of the halfsolution of its own salt and connected through a wire to an external circuit. the salt bridge is to keep the solutions of two halfmigration of ions.

1. Anode of Downs cell is made of a nonthis anode?

Ans. Graphite acts as anode in The function of graphite anode is to oxidize Cl

2. Where does the sodium metal is collected in Downs cell?Ans. Sodium ions (Na+) aredenser molten salt mixture from where it is collected in a side tube.

3. What is the name of the byAns. Chlorine gas (Cl2) is produced

4. Are anodes of Down’s cell name ?Ans. Yes, anodes of both of element is graphite.

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Which force drives the non-spontaneous reaction to take place?electric force drives the non-spontaneous reaction to take place

Which type of chemical reaction takes place in electrolytic cell?spontaneous redox reaction takes place in an electrolytic cell

What type of reaction takes place at anode in electrolytic cell?reaction takes place at anode in electrolytic cell.

Why the positively charged electrode is called anode in electrolytic cell?positively charged electrode is connected to po

that is why it is called an anode.

In the electrolysis of water, towards which terminal H+ ions move?In the electrolysis of water, H+ ions move towards cathode.

In the electrolysis of water, where is the oxygen produced?the electrolysis of water, oxygen is produced at anode.

Towards which electrode of the electrolytic cell moves the cations and what does they

cations move toward cathode. They get reduced there by gaining

cells of a galvanic cell are connected? What is function of saltanic cell consists of two cells. Each cell is called as half

connected electrically by a salt bridge. In each of the half-cell, an electrode is dipped in 1 M solution of its own salt and connected through a wire to an external circuit. the salt bridge is to keep the solutions of two half-cells neutral by providing a pathway for

Anode of Downs cell is made of a non-metal, what is its name? What is the function of

as anode in Down’s cell. The function of graphite anode is to oxidize Cl-- ions to Cl2 by taking electrons from it.

2Cl-- Cl2 + 2e--

Where does the sodium metal is collected in Downs cell?) are reduced at cathode and molten sodium (Na

denser molten salt mixture from where it is collected in a side tube.

What is the name of the by-product produced in the Down’s cell?is produced as a by-product in the Down’s cell.

s cell and Nelson cell made of same element? If yes, what is its

both Down’s cell and Nelson cell are made of same element

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spontaneous reaction to take place.

electrolytic cell.

Why the positively charged electrode is called anode in electrolytic cell?connected to positive terminal of the

Towards which electrode of the electrolytic cell moves the cations and what does they

They get reduced there by gaining

cells of a galvanic cell are connected? What is function of salt-bridge?cell is called as half-cell. These are

cell, an electrode is dipped in 1 M solution of its own salt and connected through a wire to an external circuit. The function of

al by providing a pathway for

metal, what is its name? What is the function of

by taking electrons from it.

Na) metal floats on the

and Nelson cell made of same element? If yes, what is its

made of same element. The name

Chapter 7


5. What is the shape of cathode in Nelson’s cell? Why is it perforated?Ans. U-shaped iron cathodesodium hydroxide solution slowly into a catch basin

6. Which ions are discharged at cathode in Nelson’s cell and what is produced at cathode?Ans. The H+ ions are discharged at cathode and H

2H

1. What is the difference between corrosion and rusting?Ans.

Corrosion is slow and continuous eating away of a metal by the surrounding medium.

Corrosion is a general term but corrosion of iron is called

2. What happens to iron in the rusting process?Ans. By rusting process the whole piece of iron is eaten away

3. Rusting completes in how many redox reactions?Ans. The rusting of iron

i.

ii.

iii.

iv.

4. Explain the role of O2 in rusting?Ans. Oxygen plays an important role in rusting.A region of relatively high Oas cathode and electrons reduce the oxygen molecule in the presence of H

This chemical reaction starts rusting of iron.

5. State the best method for proAns. The best method for protection against the corrosion of metals exposed to acidic conditions is coating the metal with other metal. Corrosion resistant metals like (Sn) and chromium (Cr) are coated on the surface of iron to protect it from corrosion.

6. What do you mean by galvanizingAns. The process of coating a thin layer of zinc on iron is called galvanizing. This process is carried out by dipping a clean iron sheet in a zinc iron sheet is removed, rolled into molten zinc metal bath and finally air

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What is the shape of cathode in Nelson’s cell? Why is it perforated?iron cathode is used in Nelson’s cell. It is to be perforated for

sodium hydroxide solution slowly into a catch basin.

Which ions are discharged at cathode in Nelson’s cell and what is produced at

ions are discharged at cathode and H2 gas is produced at cathode.

2H2O + 2e-- H2 + 2OH—

What is the difference between corrosion and rusting?

is slow and continuous eating away of a metal by the surrounding

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

What happens to iron in the rusting process?the whole piece of iron is eaten away.

Rusting completes in how many redox reactions?of iron completes is these four steps:

in rusting?n important role in rusting.

region of relatively high O2 concentration near the surface surrounded by water layer acts as cathode and electrons reduce the oxygen molecule in the presence of H

This chemical reaction starts rusting of iron.

State the best method for protection of metal from corrosion?The best method for protection against the corrosion of metals exposed to acidic

conditions is coating the metal with other metal. Corrosion resistant metals like are coated on the surface of iron to protect it from corrosion.

galvanizing?The process of coating a thin layer of zinc on iron is called galvanizing. This process is

carried out by dipping a clean iron sheet in a zinc chloride bath and then heating it. After this iron sheet is removed, rolled into molten zinc metal bath and finally air-cooled.

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What is the shape of cathode in Nelson’s cell? Why is it perforated?. It is to be perforated for percolation of

Which ions are discharged at cathode in Nelson’s cell and what is produced at

gas is produced at cathode.

is slow and continuous eating away of a metal by the surrounding

rusting.

concentration near the surface surrounded by water layer acts as cathode and electrons reduce the oxygen molecule in the presence of H+ ions.

The best method for protection against the corrosion of metals exposed to acidic conditions is coating the metal with other metal. Corrosion resistant metals like zinc (Zn), tin

are coated on the surface of iron to protect it from corrosion.

The process of coating a thin layer of zinc on iron is called galvanizing. This process is chloride bath and then heating it. After this

cooled.

Chapter 7


7. What is the advantage of galvanizing?Ans. Advantage of galvanizing is that zinc protects the iron against corrosion even coating surface is broken.

8. Why tin plated iron is rusted rapidly when tin layer is broken?Ans. The tin protects the iron only as long as its protective layer remains intact. Once it is broken and the iron is exposed to the air and water, a rusts rapidly.

9. Name the metal which is used for galvanizing iron?Ans. Zinc metal is used for

1. Define electroplating?Ans. An electroplating is depositing of one metal over the other by

2. How electroplating of zinc is carried out?Ans. The target metal is cleaned in alkaline detergent solutions and it is treated with acid, in order to remove any rust or surface scales. Then, the zinc is deposited on the metal by immersing it in a chemical bath containing electrolyte zinc sulphate. A current is applied, which results in depositing of zinc on the target metal i.e. cathode.

3. Which material is used to make cathode in electroplating?Ans. The cathode is made up of the up of iron.

4. Why is the anode made up of a metal to be deposited during electrolysis?Ans. The anode is made of the metal, which is to be deposited like Cr, Ni.is passed, the metal from anode dissolves in the solution and metallic ions migrate to the cathode and discharge or deposit on the cathode (object).

SHORT ANSWER QUESTIONS

1. Define oxidation in termAns. Oxidation is loss of electrons by an atom

2. Define reduction in terms of loss or gain of oxygAns. Reduction is defined as addition of hydrogen or removal of oxygen during a chemical reaction.Let us have an example

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What is the advantage of galvanizing?Advantage of galvanizing is that zinc protects the iron against corrosion even

coating surface is broken.

Why tin plated iron is rusted rapidly when tin layer is broken?The tin protects the iron only as long as its protective layer remains intact. Once it is

broken and the iron is exposed to the air and water, a galvanic cell is established and iron

Name the metal which is used for galvanizing iron?Zinc metal is used for the galvanizing of iron.

lectroplating is depositing of one metal over the other by means of electrolysis

How electroplating of zinc is carried out?The target metal is cleaned in alkaline detergent solutions and it is treated with acid, in

order to remove any rust or surface scales. Then, the zinc is deposited on the metal by immersing it in a chemical bath containing electrolyte zinc sulphate. A current is applied, which results in depositing of zinc on the target metal i.e. cathode.

Which material is used to make cathode in electroplating?The cathode is made up of the object that is to be electroplated like some sheet made

Why is the anode made up of a metal to be deposited during electrolysis?The anode is made of the metal, which is to be deposited like Cr, Ni.

rom anode dissolves in the solution and metallic ions migrate to the cathode and discharge or deposit on the cathode (object).

SHORT ANSWER QUESTIONS

Define oxidation in terms of electrons. Give an example?Oxidation is loss of electrons by an atom or an ion. e.g.

Define reduction in terms of loss or gain of oxygen or hydrogen. Give an example?Reduction is defined as addition of hydrogen or removal of oxygen during a chemical

based upon removal or addition of oxygen.

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Advantage of galvanizing is that zinc protects the iron against corrosion even after the

The tin protects the iron only as long as its protective layer remains intact. Once it is galvanic cell is established and iron

means of electrolysis.

The target metal is cleaned in alkaline detergent solutions and it is treated with acid, in order to remove any rust or surface scales. Then, the zinc is deposited on the metal by immersing it in a chemical bath containing electrolyte zinc sulphate. A current is applied,

object that is to be electroplated like some sheet made

Why is the anode made up of a metal to be deposited during electrolysis?The anode is made of the metal, which is to be deposited like Cr, Ni. When the current

rom anode dissolves in the solution and metallic ions migrate to the

en or hydrogen. Give an example?Reduction is defined as addition of hydrogen or removal of oxygen during a chemical

Chapter 7


Let us have another example

3. What is difference between valency and oxidation state?Ans.

Valency It is the combining capacity of an

atom to other atom. It is a number only. It has no plus or

minus sign with it. It can not be zero. It is always a whole number.

4. Differentiate between oxidizing and reducing agentsAns.

Oxidizing agent An Oxidizing agent is the specie that

oxidise a substance by taking electrons from it.

Non-metals are oxidizing agents because they accept electrons being more electronegative elements.

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another example based upon removal or addition of hydrogen.

What is difference between valency and oxidation state?

Valency Oxidation StateIt is the combining capacity of an

to other atom.It is a number only. It has no plus or minus sign with it.It can not be zero.It is always a whole number.

It is the apparent charge assigned to an atom of an element in a molecule or in an ion.

It may be positive or negative number.

It may be zero. It may be whole number or in

fraction.

Differentiate between oxidizing and reducing agents?

Oxidizing agent Reducing agentAn Oxidizing agent is the specie that oxidise a substance by taking

oxidizing agents because they accept electrons being more electronegative elements.

Reducing agent is the specie that reduces a substance by donating electron to it.

Almost all metals are good reducing agents because they have the tendency to lose

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based upon removal or addition of hydrogen.

Oxidation Statehe apparent charge assigned to

an atom of an element in a molecule

It may be positive or negative

It may be whole number or in

Reducing agentReducing agent is the specie that reduces a substance by donating

Almost all metals are good reducing agents because they have the tendency to lose electrons.

Chapter 7


5. Differentiate between strong and weak electrolytes?Ans.

Strong Electrolytes

The electrolytes which ionize almost completely in their aqueous solutions and produce more ions, are called strong electrolytes.

Example of strong electrolytes are aqueous solutions of NaCl, NaOH and H2SO4, etc.

6. How electroplating of tin on steel is carried out?Ans. Tin is usually electroplated on steel by placing the steel into a container containing a solution of tin salt. The steel is connected to an electrical circuit, acting as cathode. While the other electrode made of tin metal acts as anode. When an electrical current passes through the circuit, tin metal ions present in the solution deposit on steel.

7. Why steel is plated with nickel beforeAns. The steel is usually plated first with nickel or copper and then by chromium because it does not adhere well on the steel surface. Moreover, it allows moisture to pass through it and metal is stripped off. The nickel or copper provides adhesion and then chromium deposited over the adhesive layer of copper lasts longer.

8. How can you explain, that following reaction is oxidation in terms of increase of oxidation number?

Ans. Oxidation is loss of electrons by an atom or an ionthree electrons and its oxidation state has increased from 0 to +3

9. How can you prove with an example that conversion of an ion to an atom is an oxidation process?Ans. Oxidation is loss of electrons by an atom or an ionoxidation of iron Fe+2 ion.

10. Why does the anode carries negative charge in galvanic cell but positive charge in electrolytic cell? Justify with comments.Ans. In electrolytic cell, anode carries positive charge because it is connected to positive terminal of the battery.In galvanic cell, anode carried negative charged due to deposition of electrons which are lost by metal acting as anode.

11. Where do the electrons flow from Zn electrode in Daniel’s cell?Ans. The electrons flow from Znelectrode. These electrons are gained by the copper ions of the solution deposit at electrode.

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en strong and weak electrolytes?

Strong Electrolytes Weak Electrolytes

The electrolytes which ionize almost completely in their aqueous solutions and produce more ions, are called strong electrolytes. Example of strong electrolytes are aqueous solutions of NaCl, NaOH

The electrolytes which ionize to a small extent when dissolved in water and could not produce more ions are called weak electrolytes.

Acetic acid (CHCa(OH)2 when dissolved in water, ionize to a small extent and are good examples of weak electrolytes.

How electroplating of tin on steel is carried out?Tin is usually electroplated on steel by placing the steel into a container containing a

of tin salt. The steel is connected to an electrical circuit, acting as cathode. While the other electrode made of tin metal acts as anode. When an electrical current passes through the circuit, tin metal ions present in the solution deposit on steel.

steel is plated with nickel before the electroplating of chromium?he steel is usually plated first with nickel or copper and then by chromium because it

does not adhere well on the steel surface. Moreover, it allows moisture to pass through it metal is stripped off. The nickel or copper provides adhesion and then chromium

deposited over the adhesive layer of copper lasts longer.

How can you explain, that following reaction is oxidation in terms of increase of

is loss of electrons by an atom or an ion. In above reaction, Aluminum loses three electrons and its oxidation state has increased from 0 to +3.

How can you prove with an example that conversion of an ion to an atom is an

is loss of electrons by an atom or an ion. e.g. a reaction given below is an ion.

Why does the anode carries negative charge in galvanic cell but positive charge in electrolytic cell? Justify with comments.

In electrolytic cell, anode carries positive charge because it is connected to positive

In galvanic cell, anode carried negative charged due to deposition of electrons which are lost by metal acting as anode.

ns flow from Zn electrode in Daniel’s cell?The electrons flow from Zn-electrode through the external wire in a circuit to copper

electrode. These electrons are gained by the copper ions of the solution

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Weak Electrolytes

The electrolytes which ionize to a small extent when dissolved in water and could not produce more ions are called weak electrolytes.Acetic acid (CH3COOH) and

when dissolved in water, ionize to a small extent and are good examples of weak electrolytes.

Tin is usually electroplated on steel by placing the steel into a container containing a of tin salt. The steel is connected to an electrical circuit, acting as cathode. While the

other electrode made of tin metal acts as anode. When an electrical current passes through

the electroplating of chromium?he steel is usually plated first with nickel or copper and then by chromium because it

does not adhere well on the steel surface. Moreover, it allows moisture to pass through it metal is stripped off. The nickel or copper provides adhesion and then chromium

How can you explain, that following reaction is oxidation in terms of increase of

In above reaction, Aluminum loses

How can you prove with an example that conversion of an ion to an atom is an

a reaction given below is an

Why does the anode carries negative charge in galvanic cell but positive charge in

In electrolytic cell, anode carries positive charge because it is connected to positive

In galvanic cell, anode carried negative charged due to deposition of electrons which are lost

electrode through the external wire in a circuit to copper electrode. These electrons are gained by the copper ions of the solution and copper atoms

Chapter 7


12. Why do electrodes get their names ‘anode’ and cathode in galvanic cell?Ans. In galvanic cell, the electrode from which electrons flow out of the cell is called The electrode from which electrons flow into the cell is called

13. What happens at the cathode in a galvanic cell?Ans. Reduction reaction takes place at cathode in a galvanic cell.

14. Which solution is used as an electrolyte in Nelson’s cell?Ans. Aqueous solution of NaCl (brine) is used as an electrolyte in

15. Name the by-products produced in Nelson’s cell?Ans. Hydrogen (H2) and Chlorine (Cl

16. Why is galvanizing done?Ans. Galvanizing (coating a thin layer of zinc on iron) is done to protect iron from rusting.

17. Why an iron grill is painted frequently?Ans. An iron grill is painted frequently

18. Why is O2 necessary for rusting?Ans. Oxygen plays an important role in rusting. A region of relatively high Oas cathode and electrons reduce the oxygen molecule in the presence of H

In the absence of oxygen (O

19. In electroplating of chromium, which Ans. Chromium sulphate

20. Write the redox reaction taking place during the electroplating of chromium?Ans. Following reactions takes place at electrodes during the

21. In electroplating of silver, from where AgAns. When the current is passed through the cellproduce Ag+ ions, that migrate towardsdeposited on the object.The chemical reaction can be represented as:

At Anode:

At Cathode:

What is the nature of electrode used in electrolyting of Ans. The object to be electroplated acts as cathode while anode is made of antimonial lead

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Why do electrodes get their names ‘anode’ and cathode in galvanic cell?In galvanic cell, the electrode from which electrons flow out of the cell is called

The electrode from which electrons flow into the cell is called cathode.

at the cathode in a galvanic cell?Reduction reaction takes place at cathode in a galvanic cell.

Which solution is used as an electrolyte in Nelson’s cell?Aqueous solution of NaCl (brine) is used as an electrolyte in Nelson’s cell

products produced in Nelson’s cell?) and Chlorine (Cl2) gases are produced as a by-product

Why is galvanizing done?alvanizing (coating a thin layer of zinc on iron) is done to protect iron from rusting.

an iron grill is painted frequently?n iron grill is painted frequently to protect iron surface from corrosion.

necessary for rusting?n important role in rusting.

A region of relatively high O2 concentration near the surface surrounded by water layer acts as cathode and electrons reduce the oxygen molecule in the presence of H

In the absence of oxygen (O2) this chemical reaction cannot takes place.

In electroplating of chromium, which salt is used as an electrolyte?hromium sulphate, Cr2(SO4)3 is used as an electrolyte in electroplatin

Write the redox reaction taking place during the electroplating of chromium?ollowing reactions takes place at electrodes during the electroplating of chromium

In electroplating of silver, from where Ag+ ions come and where they deposit?When the current is passed through the cell, anode (piece of silver strip)

that migrate towards cathode. At cathode they are discharged and

The chemical reaction can be represented as:

What is the nature of electrode used in electrolyting of chromium?The object to be electroplated acts as cathode while anode is made of antimonial lead

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Why do electrodes get their names ‘anode’ and cathode in galvanic cell?In galvanic cell, the electrode from which electrons flow out of the cell is called anode.

Nelson’s cell.

product in Nelson’s cell.

alvanizing (coating a thin layer of zinc on iron) is done to protect iron from rusting.

to protect iron surface from corrosion.

concentration near the surface surrounded by water layer acts as cathode and electrons reduce the oxygen molecule in the presence of H+ ions.

) this chemical reaction cannot takes place.

salt is used as an electrolyte?n electroplating of chromium.

Write the redox reaction taking place during the electroplating of chromium?electroplating of chromium.

ions come and where they deposit?(piece of silver strip) dissolves to

cathode. At cathode they are discharged and

The object to be electroplated acts as cathode while anode is made of antimonial lead.

Chapter 7 Electrochemistry

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Find out the oxidation numbers of the underlined elements in the following compounds.(a) Na2SO4 (b) AgNO3 (c) KMnO4

(d) K2Cr2O7 (e) HNO2

Ans.

(a) To find out oxidation number of S in Na2SO4

2(O.N. of Na) + (O.N. of S) + 4((O.N. of O) = 02(+1) + (O.N. of S) + 4(– 2) = 02 + (O.N. of S) – 8 = 0O.N. of S – 6 = 0O.N. of S = 6

(b) To find out oxidation number of N in AgNO3

O.N. of Ag + O.N. of N + 3(O.N. of O) = 0(+1) + O.N. of N + 3(– 2) = 0+1 + O.N. of N – 6 = 0O.N. of N – 5 = 0O.N. of N = 5

(c) To find out oxidation number of Mn in KMnO4

O.N. of K + O.N. of Mn + 4(O.N. of O) = 0(+1) + O.N. of Mn + 4(– 2) = 0+1 + O.N. of Mn – 8 = 0O.N. of Mn – 7 = 0O.N. of Mn = 7

(d) To find out oxidation number of Cr in K2Cr2O7

2(O.N. of K) + 2(O.N. of Cr) + 7(O.N. of O) = 02(+1) + 2(O.N. of Cr) + 7(--2) = 02 + 2(O.N. of Cr) – 14 = 02(O.N. of Cr) – 12 = 02(O.N. of Cr) = 12O.N. of Cr = 12

2O.N. of Cr = 6

(e) To find out oxidation number of N in HNO2

O.N. of H + O.N. of N + 2(O.N. of O) = 0(+1) + O.N. of N + 2(– 2) = 0+1 + O.N. of N – 4 = 0O.N. of N – 3 = 0O.N. of N = 3

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