NCERT Solutions for Class 11 Chemistry: Chapter 8 (Redox Reactions)


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Then,

1(+1) + 2(+1) + 1(x) + 4(-2) = 0

1 + 2 + x -8 = 0

x = +5

Therefore, oxidation no. of P is +5.

(b) NaHSO4NaHunderline{S}O_{4}

Let x be the oxidation no. of S.

Oxidation no. of Na = +1

Oxidation no. of H = +1

Oxidation no. of O = -2

Then,

1(+1) + 1(+1) + 1(x) + 4(-2) = 0

1 + 1 + x -8 = 0

x = +6

Therefore, oxidation no. of S is +6.

(c) H4P2O7H_{4}underline{P}_{2}O_{7}


Let x be the oxidation no. of P.

Oxidation no. of H = +1

Oxidation no. of O = -2

Then,

4(+1) + 2(x) + 7(-2) = 0

4 + 2x – 14 = 0

2x = +10

x = +5

Therefore, oxidation no. of P is +5.

(d) K2MnO4K_{2}underline{Mn}O_{4}

Let x be the oxidation no. of Mn.

Oxidation no. of K = +1

Oxidation no. of O = -2

Then,

2(+1) + x + 4(-2) = 0

2 + x – 8 = 0

x = +6

Therefore, oxidation no. of Mn is +6.

(e) CaO2Caunderline{O}_{2}

Let x be the oxidation no. of O.

Oxidation no. of Ca = +2

Then,

(+2) + 2(x) = 0

2 + 2x = 0

2x = -2

x = -1

Therefore, oxidation no. of O is -1.

(f) NaBH4Naunderline{B}H_{4}

Let x be the oxidation no. of B.

Oxidation no. of Na = +1

Oxidation no. of H = -1

Then,

1(+1) + 1(x) + 4(-1) = 0

1 + x -4 = 0

x = +3

Therefore, oxidation no. of B is +3.

(g) H2S2O7H_{2}underline{S}_{2}O_{7}

Let x be the oxidation no. of S.

Oxidation no. of H = +1

Oxidation no. of O = -2

Then,

2(+1) + 2(x) + 7(-2) = 0

2 + 2x – 14 = 0

2x = +12

x = +6

Therefore, oxidation no. of S is +6.

(h) KAl(SO4)2.12H2OKAl(underline{S}O_{4})_{2}.12H_{2}O

Let x be the oxidation no. of S.

Oxidation no. of K = +1

Oxidation no. of Al = +3

Oxidation no. of O= -2

Oxidation no. of H = +1

Then,

1(+1) + 1(+3) + 2(x) + 8(-2) + 24(+1) + 12(-2) = 0

1 + 3 + 2x – 16 + 24 – 24 = 0

2x = +12

x = +6

Therefore, oxidation no. of S is +6.

OR

Ignore the water molecules because it is neutral. Then, the summation of the oxidation no. of all atoms of water molecules can be taken as 0. Hence, ignore the water molecule.

1(+1) + 1(+3) + 2(x) + 8(-2) = 0

1 + 3 + 2x -16 = 0

2x = 12

x = +6

Therefore, oxidation no. of S is +6.

 

2.What are the oxidation number of the underlined elements in each of the following and how do you rationalise your results?

(a) KI3Kunderline{I}_{3}

(b) H2S4O6H_{2}underline{S}_{4}O_{6}

(c) Fe3O4underline{Fe}_{3}O_{4}

(d) CH3CH2OHunderline{C}H_{3}underline{C}H_{2}OH

(e) CH3COOHunderline{C}H_{3}underline{C}OOH

 

Answer:

(a) KI3Kunderline{I}_{3}

Let x be the oxidation no. of I.

Oxidation no. of K = +1

Then,

1(+1) + 3(x) = 0

1 + 3x = 0

x = 13-frac{1}{3}

Oxidation no. cannot be fractional. Hence, consider the structure of KI3KI_{3}.

In KI3KI_{3} molecule, an iodine atom forms coordinate covalent bond with an iodine molecule.

Therefore, in KI3KI_{3} molecule, the oxidation no. of I atoms forming the molecule I2I_{2} is 0, while the oxidation no. of I atom which is forming coordinate bond is -1.

(b) H2S4O6H_{2}underline{S}_{4}O_{6}

Let x be the oxidation no. of S.

Oxidation no. of H = +1

Oxidation no. of O = -2

Then,

2(+1) + 4(x) + 6(-2) = 0

2 + 4x -12 = 0

4x = 10

x = +212+2frac{1}{2}

Oxidation no. cannot be fractional. Therefore, S would be present with different oxidation state in molecule.

The oxidation no. of two out of the four S atoms is +5 while that of other two atoms is 0.

(c) Fe3O4underline{Fe}_{3}O_{4}

Let x be the oxidation no. of Fe.

Oxidation no. of O = -2

Then,

3(x) + 4(-2) = 0

3x -8 = 0

x = 83frac{8}{3}

Oxidation no. cannot be fractional.

One of the three atoms of Fe has oxidation no. +2 and other two atoms of Fe has oxidation no. +3.

(d) CH3CH2OHunderline{C}H_{3}underline{C}H_{2}OH

Let x be the oxidation no. of C.

Oxidation no. of O= -2

Oxidation no. of H = +1

Then,

2(x) + 4(+1) + 1(-2) = 0

2x + 4 -2 = 0

x = -2

Therefore, oxidation no. of C is -2.

(e) CH3COOHunderline{C}H_{3}underline{C}OOH

Let x be the oxidation no. of C.

Oxidation no. of O= -2

Oxidation no. of H = +1

Then,

2(x) + 4(+1) + 2(-2) = 0

2x + 4 -4 = 0

x = 0

Therefore, average oxidation no. of C is 0. Both the carbon atoms are present in different environments so they cannot have same oxidation no. Therefore, carbon has oxidation no. of +2 and _2 in CH3COOHCH_{3}COOH.

 

3. Justify that the following reactions are redox reactions:

(a) CuO(s)  +  H2  (g)    Cu(s)  +  H2O(g)CuO_{(s)} ; + ; H_{2 ; (g)} ; ightarrow ; Cu_{(s)} ; + ; H_{2}O_{(g)}

(b) Fe2O3  (s)  +  3  CO(g)    2  Fe(s)  +  3  CO2  (g)Fe_{2}O_{3 ; (s)} ; + ; 3 ; CO_{(g)} ; ightarrow ; 2 ; Fe_{(s)} ; + ; 3 ; CO_{2 ; (g)}

(c)4  BCl3  (g)  +  3  LiAlH4  (s)    2  B2H6  (g)  +  3  LiCl(s)  +  3  AlCl3  (s)4 ; BCl_{3 ; (g)} ; + ; 3 ; LiAlH_{4 ; (s)} ; ightarrow ; 2 ; B_{2}H_{6 ; (g)} ; + ; 3 ; LiCl_{(s)} ; + ; 3 ; AlCl_{3 ; (s)}

(d) 2  K(s)  +  F2  (g)    2  K  +  F(s)2;K_{(s)};+;F_{2;(g)}; ightarrow;2;K;+;F_{(s)}

(e) 4  NH3  (g)  +  5  O2  (g)    4   NO(g)  +  6  H2O(g)4;NH_{3;(g)};+;5;O_{2;(g)}; ightarrow ; 4  ;NO_{(g)};+;6;H_{2}O_{(g)}

 

Answer:

(a) CuO(s)  +  H2  (g)    Cu(s)  +  H2O(g)CuO_{(s)} ; + ; H_{2 ; (g)} ; ightarrow ; Cu_{(s)} ; + ; H_{2}O_{(g)}

Oxidation no. of Cu and O in CuOCuO is +2 and -2 respectively.

Oxidation no. of H2H_{2} is 0.

Oxidation no. of Cu is 0.

Oxidation no. of H and O in H2OH_{2}O is +1 and -2 respectively.

The oxidation no. of Cu decreased from +2 in CuOCuO to 0 in Cu. That is CuOCuO is reduced to Cu.

The oxidation no. of H increased from 0 to +1 in H2H_{2}. That is H2H_{2} is oxidized to H2OH_{2}O.

Therefore, the reaction is redox reaction.

(b) Fe2O3  (s)  +  3  CO(g)    2  Fe(s)  +  3  CO2  (g)Fe_{2}O_{3 ; (s)} ; + ; 3 ; CO_{(g)} ; ightarrow ; 2 ; Fe_{(s)} ; + ; 3 ; CO_{2 ; (g)}

In the above reaction,

Oxidation no. of Fe and O in Fe2O3Fe_{2}O_{3} is +3 and -2 respectively.

Oxidation no. of C and O in CO is +2 and -2 respectively.

Oxidation no. of Fe is 0.

Oxidation no. of C and O in CO2CO_{2} is +4 and -2 respectively.

The oxidation no. of Fe decreased from +3 in Fe2O3Fe_{2}O_{3} to 0 in Fe. That is Fe2O3Fe_{2}O_{3} is reduced to Fe.

The oxidation no. of C increased from 0 to +2 in CO to +4 in CO2CO_{ 2 }. That is CO is oxidized to CO2CO_{ 2 }.

Therefore, the reaction is redox reaction.

(c) 4  BCl3  (g)  +  3  LiAlH4  (s)    2  B2H6  (g)  +  3  LiCl(s)  +  3  AlCl3  (s)4 ; BCl_{3 ; (g)} ; + ; 3 ; LiAlH_{4 ; (s)} ; ightarrow ; 2 ; B_{2}H_{6 ; (g)} ; + ; 3 ; LiCl_{(s)} ; + ; 3 ; AlCl_{3 ; (s)}

the above reaction,

Oxidation no. of B and Cl in BCl3BCl_{3} is +3 and -1 respectively.

Oxidation no. of Li, Al and H in LiAlH4LiAlH_{4} is +1, +3 and -1 respectively.

Oxidation no. of B and H in B2H6B_{2}H_{6} is -3 and +1 respectively.

Oxidation no. of Li and Cl in LiCl is +1 and -1 respectively.

Oxidation no. of Al and Cl in AlCl3AlCl_{3} is +3 and -1 respectively.

The oxidation no. of B decreased from +3 in BCl3BCl_{3} to -3 in B2H6B_{2}H_{6}. That is BCl3BCl_{3}is reduced to B2H6B_{2}H_{6}.

The oxidation no. of H increased from -1 in LiAlH4LiAlH_{4} to +1 in B2H6B_{2}H_{6}. That is LiAlH4LiAlH_{4} is oxidized to B2H6B_{2}H_{6}.

Therefore, the reaction is redox reaction.

(d) 2  K(s)  +  F2  (g)    2  K  +  F(s)2 ; K_{(s)} ; + ; F_{2 ; (g)} ; ightarrow ; 2 ; K ; + ; F_{(s)}

In the above reaction,

Oxidation no. of K is 0.

Oxidation no. of F is 0.

Oxidation no. of K and F in KF is +1 and -1 respectively.

The oxidation no. of K increased from 0 in K to +1 in KF. That is K is oxidized to KF.

The oxidation no. of F decreased from 0 in F2F_{2} to -1 in KF. That is F2F_{2} is reduced to KF.

Therefore, the reaction is a redox reaction.

(e) 4  NH3  (g)  +  5  O2  (g)    4  NO(g)  +  6  H2O(g)4 ; NH_{3 ; (g)} ; + ; 5 ; O_{2 ; (g)} ; ightarrow ; 4 ; NO_{(g)} ; + ; 6 ; H_{2}O_{(g)}

In the above reaction,

Oxidation no. of N and H in NH3NH_{3} is -3 and +1 respectively.

Oxidation no. of O2O_{2} is 0.

Oxidation no. of N and O in NO is +2 and -2 respectively.

Oxidation no. of H and O in H2OH_{2}O is +1 and -2 respectively.

The oxidation no. of N increased from -3 in NH3NH_{ 3 } to +2 in NO.

The oxidation no. of O2O_{2} decreased from 0 in O2O_{ 2 } to -2 in NO and H2OH_{ 2 }O. That is O2O_{ 2 } is reduced.

Therefore, the reaction is a redox reaction.

 

 

4. Fluorine reacts with ice and results in the change: 

H2O(s)  +  F2  (g)    HF(g)  +  HOF(g)H_{ 2 }O_{ (s) } ; + ; F_{ 2 ; (g) } ; ightarrow ; HF_{ (g) } ; + ; HOF_{ (g) }

Justify that this reaction is a redox reaction

 

Answer:

H2O(s)  +  F2  (g)    HF(g)  +  HOF(g)H_{ 2 }O_{ (s) } ; + ; F_{ 2 ; (g) } ; ightarrow ; HF_{ (g) } ; + ; HOF_{ (g) }

In the above reaction,

Oxidation no. of H and O in H2OH_{ 2 }O is +1 and -2 respectively.

Oxidation no. of F2F_{ 2 } is 0.

Oxidation no. of H and F in HF is +1 and -1 respectively.

Oxidation no. of H, O and F in HOF is +1, -2 and +1 respectively.

The oxidation no. of F increased from 0 in F2F_{ 2 } to +1 in HOF.

The oxidation no. of F decreased from 0 in O2O_{ 2 } to -1 in HF.

Therefore, F is both reduced as well as oxidized. So, it is redox reaction.

 

 

5. Calculate the oxidation no. of sulphur, chromium and nitrogen in H2SO5H_{ 2 }SO_{ 5 }, Cr2O72Cr_{ 2 }O_{ 7 }^{ 2- } and NO3NO_{ 3 }^{ – }. Suggest structure of these compounds. Count for the fallacy.

 

Answer:

For H2SO5H_{ 2 }SO_{ 5 }

Let x be the oxidation no. of S.

Oxidation no. of O= -2

Oxidation no. of H = +1

Then,

2(+1) + 1(x) + 5(-2) = 0

2 + x – 10 = 0

x = +8

But the oxidation no. of S cannot be +8 as S has 6 valence electrons. Therefore, the oxidation no. of S cannot be more than +6.

The structure of H2SO5H_{ 2 }SO_{ 5 } is as given below:

Now,

2(+1) + 1(x) + 3(-2) + 2(-1) = 0

2 + x – 6 – 2 = 0

x = +6

Therefore, the oxidation no. of S is +6.

 

For Cr2O72Cr_{ 2 }O_{ 7 }^{ 2- }

Let x be the oxidation no. of Cr.

Oxidation no. of O= -2

Then,

2(x) + 7(-2) = -2

2x -14 = -2

x = +6

There is no fallacy about the oxidation no. of Cr in Cr2O72Cr_{ 2 }O_{ 7 }^{ 2- }.

The structure of Cr2O72Cr_{ 2 }O_{ 7 }^{ 2- } is as given below.

Each of the two Cr atoms has the oxidation no. of +6.

For NO3NO_{ 3 }^{ – }

Let x be the oxidation no. of N.

Oxidation no. of O= -2

Then,

1(x) + 3(-2) = -1

x – 6 = -1

x = +5

There is no fallacy about the oxidation no. of N in NO3NO_{ 3 }^{ – }.

The structure of NO3NO_{ 3 }^{ – } is as given below.

Nitrogen atom has the oxidation no. of +5.

 

6. Write formulas for the following compounds:

(a) Mercury (II) chloride          (b) Nickel (II) sulphate

(c) Tin (IV) oxide                          (d) Thallium (I) sulphate

(e) Iron (III) sulphate                (f) Chromium (III) oxide

 

Answer:

(a) Mercury (II) chloride

HgCl2HgCl_{ 2 }

 

(b) Nickel (II) sulphate

NiSO4NiSO_{ 4 }

 

(c) Tin (IV) oxide

SnO2SnO_{ 2 }

 

(d) Thallium (I) sulphate

Tl2SO4Tl_{ 2 }SO_{ 4 }

 

(e) Iron (III) sulphate

Fe2(SO4)3Fe_{ 2 }(SO_{ 4 })_{ 3 }

 

(f) Chromium (III) oxide

Cr2O3Cr_{ 2 }O_{ 3 }

 

7. Suggest a list of the substances where carbon can exhibit oxidation states from –4 to +4 and nitrogen from –3 to +5.

 

Answer:

The compound where carbon has oxidation no. from -4 to +4 is as given below in the table:

CompoundsOxidation no. of carbon
CH2Cl2CH_{ 2 }Cl_{ 2 }0
HCCHHCequiv CH-1
ClCCClClCequiv CCl+1
CH3ClCH_{ 3 }Cl-2
CHCl3CHCl_{ 3 }, CO+2
H3CCH3H_{ 3 }C-CH_{ 3 }-3
Cl3CCCl3Cl_{ 3 }C-CCl_{ 3 }+3
CH4CH_{ 4 }-4
CCl4CCl_{ 4 }, CO2CO_{ 2 }+4

 

CompoundsOxidation no. of nitrogen
N2N_{ 2 }0
N2H2N_{ 2 }H_{ 2 }-1
N2ON_{ 2 }O+1
N2H4N_{ 2 }H_{ 4 }-2
NONO+2
NH3NH_{ 3 }-3
N2O3N_{ 2 }O_{ 3 }+3
NO2NO_{ 2 }+4
N2O5N_{ 2 }O_{ 5 }+5

 8. While sulphur dioxide and hydrogen peroxide can act as oxidising as well as reducing agents in their reactions, ozone and nitric acid act only as oxidants. Why?

 

Answer:

In sulphur dioxide (SO2SO_{ 2 }) the oxidation no. of S is +4 and the range of oxidation no. of sulphur is from +6 to -2.

Hence, SO2SO_{ 2 } can act as a reducing and oxidising agent.

 

In hydrogen peroxide (H2O2H_{ 2 }O_{ 2 }) the oxidation no. of O is -1 and the range of the oxidation no. of oxygen is from 0 to -2. Oxygen can sometimes attain the oxidation no. +1 and +2.

Therefore, H2O2H_{ 2 }O_{ 2 } can act as a reducing and oxidising agent.

 

In ozone (O3O_{ 3 }) the oxidation no. of O is 0 and the range of the oxidation no. of oxygen is from 0 to –2. Hence, the oxidation no. of oxygen only decreases in this case.

Therefore, O3O_{ 3 } acts only as an oxidant.

 

In nitric acid (HNO3HNO_{ 3 }) the oxidation no. of nitrogen is +5 and the range of the oxidation no. that nitrogen can have is from +5 to -3. Hence, the oxidation no. of nitrogen can only decrease in this case.

Therefore, HNO3HNO_{ 3 } acts only as an oxidant.

 

 

9.Consider the reactions:

(a) 6  CO2  (g)  +  6  H2O(l)    C6H12O6  (aq)  +  6  O2  (g)6 ; CO_{ 2 ; (g) } ; + ; 6 ; H_{ 2 }O_{ (l) } ; ightarrow ; C_{ 6 }H_{ 12 }O_{ 6 ; (aq) } ; + ; 6 ; O_{ 2 ; (g) }

(b) O3  (g)  +H2O2  (l)    H2O(l)  +  2  O2  (g)O_{ 3 ; (g) } ; + H_{ 2 }O_{ 2 ; (l) } ; ightarrow ; H_{ 2 }O_{ (l) } ; + ; 2 ; O_{ 2 ; (g) }

Why it is more appropriate to write these reactions as :

(a) 6  CO2  (g)  +  12  H2O(l)    C6H12O6  (aq)  +  6  H2O(l)  +  6  O2  (g)6 ; CO_{ 2 ; (g) } ; + ; 12 ; H_{ 2 }O_{ (l) } ; ightarrow ; C_{ 6 }H_{ 12 }O_{ 6 ; (aq) } ; + ; 6 ; H_{ 2 }O_{ (l) } ; + ; 6 ; O_{ 2 ; (g) }

(b) O3  (g)  +H2O2  (l)    H2O(l)  +  O2  (g)  +  O2  (g)O_{ 3 ; (g) } ; + H_{ 2 }O_{ 2 ; (l) } ; ightarrow ; H_{ 2 }O_{ (l) } ; + ; O_{ 2 ; (g) } ; + ; O_{ 2 ; (g) }

Also suggest a technique to investigate the path of the above (a) and (b) redox reactions

 

Answer:

(a)

Step 1 :

H2OH_{ 2 }O breaks to give H2H_{ 2 } and O2O_{ 2 }.

2  H2O(l)    2  H2  (g)  +  O2  (g)2 ; H_{ 2 }O_{ (l) } ; ightarrow ; 2 ; H_{ 2 ; (g) } ; + ; O_{ 2 ; (g) }

Step 2 :

The H2H_{ 2 } produced in earlier step reduces  CO2CO_{ 2 }, thus produce glucose and water.

6  CO2  (g)  +  12  H2  (g)    C6H12O6  (s)  +  6  H2O(l)6 ; CO_{ 2 ; (g) } ; + ; 12 ; H_{ 2 ; (g) } ; ightarrow ; C_{ 6 }H_{ 12 }O_{ 6 ; (s) } ; + ; 6 ; H_{ 2 }O_{ (l) }

The net reaction is as given below:

[2  H2O(l)    2  H2  (g)  +  O2  (g)2 ; H_{ 2 }O_{ (l) } ; ightarrow ; 2 ; H_{ 2 ; (g) } ; + ; O_{ 2 ; (g) }] × 6

6  CO2  (g)  +  12  H2  (g)    C6H12O6  (s)  +  6  H2O(l)6 ; CO_{ 2 ; (g) } ; + ; 12 ; H_{ 2 ; (g) } ; ightarrow ; C_{ 6 }H_{ 12 }O_{ 6 ; (s) } ; + ; 6 ; H_{ 2 }O_{ (l) }

————————————————————————————————————————–

6  CO2  (g)  +  12  H2O(l)    C6H12O6  (g)  +  6  H2O(l)  +  6  O2  (g)6 ; CO_{ 2 ; (g) } ; + ; 12 ; H_{ 2 }O_{ (l) } ; ightarrow ; C_{ 6 }H_{ 12 }O_{ 6 ; (g) } ; + ; 6 ; H_{ 2 }O_{ (l) } ; + ; 6 ; O_{ 2 ; (g) }

 

This is the suitable way to write the reaction as the reaction also produce water molecules in the photosynthesis process.

The path can be found with the help of radioactive H2O18H_{ 2 }O^{ 18 } instead of H2OH_{ 2 }O.

 

(b)

Step 1 :

O2O_{ 2 } is produced from each of the reactants O3O_{ 3 } and H2O2H_{ 2 }O_{ 2 }. That is the reason O2O_{ 2 } is written two times.

 

O3O_{ 3 } breaks to form O2O_{ 2 } and O.

Step 2 :

H2O2H_{ 2 }O_{ 2 } reacts with O produced in the earlier step, thus produce H2OH_{ 2 }O and O2O_{ 2 }.

 

O3  (g)    O2  (g)  +  O(g)  O_{ 3 ; (g) } ; ightarrow ; O_{ 2 ; (g) } ; + ; O_{ (g) };

 

  H2O2  (l)  +  O(g)    H2O(l)  +  O2  (g);H_{ 2 }O_{ 2 ; (l) } ; + ; O_{ (g) } ; ightarrow ; H_{ 2 }O_{ (l) } ; + ; O_{ 2 ; (g) }

———————————————————————————————-

H2O2