Redox Reactions - Online Test

# Redox Reactions # CBSE 11TH CHEMISTRY Prepare / Learn

Q1. The term oxidation was coined for the class of chemical reactions wherein one of the following options take place.

A. addition of oxygen to a substance

B. addition of hydrogen to a substance

C. removal of electronegative element from a substance

D. addition of electropositive element to a substance

Answer : Option A
Explaination / Solution:

Oxidation means addition of oxygen, removal of hydrogen, increase in oxidation number, addition of a electronegative element or removal of an electropositive element

The term oxidation was first used to describe chemical reactions in which oxygen was added to an element of a compound. Later on the term oxidation was extended to describe many more reactions which occurred without the use of even oxygen.

Addition of Oxygen: Oxidation is a chemical reaction in which oxygen is added to any chemical species (atom, ion or molecule). Example

2Mg + O2 → 2MgO

S + O2 → SO2

2CO + O2 → 2CO2

Na2SO3 + H2O2 → Na2SO4 + H2O

Removal of Hydrogen : Oxidation is a chemical reaction in which hydrogen is removed from any chemical species (atom, ion or molecule).

H2S + Cl2 → 2HCl + S

4HI + O2 → 2H2O + 2I2

4HI + MnO2 → MnCl2 + 2H2O + Cl2

Addition of an Electronegative Element: Oxidation is a chemical reaction in which an electronegative element is added into any chemical species (atom, ion or molecule).

Fe+ S → FeS (Oxidation of iron)

SnCl2 + Cl2→ SnCl4(Oxidation of stannous chloride)

2Fe + 3F2 → 2FeF3(Oxidation of iron)

Removal of an Electropositive Element: Oxidation is a chemical reaction in which an electropositive element is removed from any chemical species (atom, ion or molecule).

2KI + H2O2 → 2KOH + I2 (Oxidation of potassium iodide)

2K2MnO4 + Cl2 → 2KCl + 2KMnO4 (Oxidation of potassium manganate)

2KI + Cl2 → 2KCl + I2 (Oxidation of potassium iodide)

A substance which brings oxidation is known as oxidizing agentt

# Redox Reactions # CBSE 11TH CHEMISTRY Prepare / Learn

Q2. The term reduction has one of the following options taking place.

A. removal of hydrogen to a substance

B. removal of electropositive element from a substance

C. addition of electronegative element to a substance

D. removal of oxygen from a substance

Answer : Option D
Explaination / Solution:

Reduction is addition of hydrogen, removal of oxygen, removal of an electronegative element or addition of an electropositive element.

Removal of Oxygen: Reduction is a chemical reaction in which oxygen is removed from any chemical species (atom, ion or molecule).
CuO + C → Cu + CO
H2O + C → CO + H2
2CO + O2 → 2CO2
Fe3O4 + 4H2 → 3F2 + 4H2O
Addition of Hydrogen : Reduction is a chemical reaction in which hydrogen is added to any chemical species (atom, ion or molecule).
Cl2 + H2 → 2HCl
S + H2 → H2S
C2H4 + H2 → C2H6
Removal of an Electronegative Element: Reduction is a chemical reaction in which an electronegative element is removed from any chemical species (atom, ion or molecule).

2HgCl2 + SnCl2 → Hg2Cl2 + SnCl4 (Reduction of mercuric chloride)
2FeCl3 + H2 → 2FeCl2 + 2HCl (Reduction of ferric chloride)
2FeCl3 + H2S → 2FeCl2 + 2HCl + S (Reduction of ferric chloride)

Addition of an Electropositive Element: Reduction is a chemical reaction in which an electropositive element is added any chemical species (atom, ion or molecule).
HgCl2 + Hg → Hg2Cl2 (Reduction of mercuric chloride)
CuCl2 + Cu → Cu2Cl2 (Reduction of cupric chloride)
The substance which brings reduction is known as reducing agent.
A substance, which undergoes oxidation, acts as a reducing agent while a substance, which undergoes reduction, acts as an oxidizing agent.

Mg, S, Cu, Na2SO3, H2S, HI, H2, C, KI are reducing agents, while O2, Cl2, F2, H2O2, MnO2, FeCl3, CuCl2, Fe3O4, CuO, etc., are oxidizing agents in the above examples.

All oxidation and reduction reactions are complimentary of one another and occur simultaneously, one cannot take place without the other.
No single oxidation and no single reduction process is known. The simultaneously oxidation and reduction reactions are generally termed as redox reactions. e.g., 2FeCl3 + SnCl2 → 2FeCl2 + SnCl4.

In above example iron undergoes reduction from +3 to +2.

# Redox Reactions # CBSE 11TH CHEMISTRY Prepare / Learn

Q3.

Oxidation and reduction always occur simultaneously, hence, the word “redox” was coined for this class of chemical reactions. In the following redox reaction, identify the species undergoing oxidation and reduction: $3F e_{3} O_{4} (s) + 8 Al (s) \to 9 Fe (s) + 4A l_{2} O_{3} (s)$

3F e_{3} O_{4} (s) + 8 Al (s) \to 9 Fe (s) + 4A l_{2} O_{3} (s)

F e_{3} O_{4}

is oxidised, Fe is reduced

F e_{3} O_{4}

is oxidised, Aluminium is reduced

C. Aluminium (Al) is oxidised, ( Fe3O4 ) is reduced

F e_{3} O_{4}

A l_{2} O_{3}

is reduced,

F e_{3} O_{4}

is oxidised

Answer : Option C
Explaination / Solution:

In the above reaction oxygen is transferred from Fe3O4 to Al, so Fe3O4 is reduced while Al is oxidised.

As per definition of oxidation and reduction , loss of oxygen i.e reduction occurs in Fe3O4 and gain of oxygen i.e Oxidation occurs in Al.

# Redox Reactions # CBSE 11TH CHEMISTRY Prepare / Learn

Q4.

The formation of sodium chloride involves two half reaction, which clearly show involvement of electrons.
2 $Na (s) \to 2 Na + (g) + 2e$
$C l_{2} (g) + 2e \to 2 Cl (g)$
Sum of the half reactions gives
$2 Na (s) + C l_{2} (g) \to 2 Na + Cl (s) or 2 NaCl (s)$
In terms of electron-transfer change, name the oxidising agent(s) and reducing agent(s)

A. reducing agents:Chlorine, oxygen and oxidising agents:sodium and sulphur

B. oxidising agents:Chlorine, oxygen and reducing agents:sodium and sulphur

C. Oxidising agent: Chlorine and Reducing agent: Sodium

D. reducing agents:Chlorine, oxygen and sulphur and oxidising agent:sodium

Answer : Option C
Explaination / Solution:

According to the modern concept, loss of electrons is oxidation whereas gain of electrons is reduction.Redox reaction involves two half reactions, one involving loss of electron or electrons (oxidation) and the other involving gain of electrons or electrons (reduction)

Oxidizing agent is a species which can gain one or more electrons.
Reducing agent is a species which can lose one or more electrons.

In the given reaction Sodium(Na) is loosing 2 electrons, thus it is the reducing agent. Chlorine(Cl) is gaining 2 electrons , thus it is oxidising agent.

# Redox Reactions # CBSE 11TH CHEMISTRY Prepare / Learn

Q5. Copper nitrate is a blue coloured solution. Place a strip of metallic zinc in an aqueous solution of copper nitrate for about one hour. What happens?

A. The aqueous solution of copper nitrate turns green in colour and mettallic zinc strip turns darker in colour.

B. zinc strip becomes coated with blue colour

C. the blue colour of the solution turns to red

D. the blue colour of the solution becomes more intense blue.

Answer : Option A
Explaination / Solution:

On placing a strip of mettallic zinc in an aqueous solution of copper nitrate for about one hour, the copper nitrate solution turns green in colour and zinc strip turns darker.

Formula and Ionic equation for the reaction is as follows.

Formula Equation:	Zn (s) + Cu(NO3)2 (aq) → Cu (s) + Zn(NO3)2 (aq)
Ionic Equation:	Zn (s) + Cu2+ (aq) → Zn2+ (aq) + Cu (s)

As is seen in ionic equation Zn is loosing 2 electrons, thus oxidation occurs and Cu is gaining 2 electrons, thus reduction occurs. Since oxidation and reduction takes place simultaneously in this reaction, it is a redox reaction. Also since Zn is loosing electrons and becoming darker in colour, it is the reducing agent and Cu is gaining 2 electrons, it is the reducing agent. The resultant solution Zinc nitrate [Zn(NO3)2] is green in colour.

# Redox Reactions # CBSE 11TH CHEMISTRY Prepare / Learn

Q6. Redox reaction between copper and aqueous solution of silver nitrate happens when copper rod is dipped in silver nitrate solution. What happens in the final stage?

A. the solution turns intense blue; and silver is deposited on copper rod

silver deposits on the rod; Ag+(aq) is oxidised to Ag(s)

C. copper deposits in the solution; Cu(aq) is reduced to

{Cu}_{2}^{+}

(s)

D. silver deposits in the solution; Ag+(aq) is reduced to Ag(s).

Answer : Option A
Explaination / Solution:

This is due to the formation of Cu2+ ions in the solution illustrated in following ionic reaction:

Cu(S)+2Ag+(aq)--------------˃Cu2+(aq)+2Ag(s)

Here, Cu is oxidized to Cu2+ (means relasing of 2 electrons from Cu to Cu2+ ) and Ag+ is reduced to Ag(s) (means gaining of 2 electrons from 2Ag+ to 2Ag ) and thus, silver converted into solid state and deposited on copper rod.

# Redox Reactions # CBSE 11TH CHEMISTRY Prepare / Learn

Q7. The electron releasing tendency of the metals, zinc, copper and silver is in the order:

A. Cu>Ag>Zn

B. Zn>Ag>Cu

C. Zn>Cu>Ag

D. Ag>Cu>Zn

Answer : Option C
Explaination / Solution:

Zinc relases electrons to copper and copper releases electron to silver therefore the electron releasing tendency of the metals, zinc, copper and silver is in the order Zn>Cu>Ag.This is on the basis of electrochemical series or metal activity series.

# Redox Reactions # CBSE 11TH CHEMISTRY Prepare / Learn

Q8. In the reaction of metallic cobalt placed in nickel sulphate solution, there is a competition for release of electrons At equilibrium, chemical tests reveal that both Ni2+ (aq) and Co2+ (aq) are present at moderate concentrations. The result is that:

A. neither the reactants nor the products are greatly favoured.

Only one reactant and one product is greatly favoured.

Only

{Co}_{2}^{+}

(aq) and Ni (s)] are favoured

D. Only [Co(s) and

{Ni}_{2}^{+}

(aq)] are favoured

Answer : Option A
Explaination / Solution:

Reaction will not complete and It will remain at equilibrium.

# Redox Reactions # CBSE 11TH CHEMISTRY Prepare / Learn

Q9. Oxidation number denotes the oxidation state of an element in a compound ascertained on the basis that electron in a covalent bond belongs

A. entirely to more electronegative element

B. entirely to transition elements if present

C. entirely to more electronegative element

D. entirely to halogens in a compound

Answer : Option C
Explaination / Solution:

Oxidation number is a number assigned to an element in a compound according to some rules. This number enable us to describe oxidation-reduction reactions, and balancing redox chemical reactions. When a covalent bond forms between two atoms with different electronegativities the shared electrons in the bond lie closer to the more electronegative atom eg. HCl The oxidation number of an atom is the charge that results when the electrons in a covalent bond are assigned to the more electronegative atom. It is the charge on an atom would possess if the bonding were ionic. In HCl (above) the oxidation number for the hydrogen would be +1 and that of the Cl would be -1

# Redox Reactions # CBSE 11TH CHEMISTRY Prepare / Learn

Q10. The highest value of oxidation number changes from 1 to 7

A. the first three groups

B. In alkaline earth metals

C. in the atoms of transition elements

D. across the third period in the periodic table

Answer : Option D
Explaination / Solution:

The highest value of oxidation number exhibited by an atom of an element generally increases across the period in the periodic table.

REDO THIS TEST AGAIN : ( TEST 1 )

START NEXT : ( TEST 2 )