Question 2

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

(a) K__I___{3}

(b) H_{2}__S___{4}O_{6}

(c) __Fe___{3}O_{4 }

(d) __C__H_{3}__C__H_{2}OH

(e) __C__H_{3}__C__OOH

Answer

(a) K__I___{3 }

Let assume oxidation number of l is x.

In KI_{3}, the oxidation number (O.N.) of K is +1.

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

⇒ +1 +3x = 0

⇒ 3x = -1

⇒ x = -1/3

Hence, the average oxidation number of I is - 1/3

However, O.N. cannot be fractional. Therefore, we will have to consider the structure of KI_{3} to find the oxidation states. In a KI_{3 }molecule, an atom of iodine forms a coordinate covalent bond with an iodine molecule.

Hence, in a KI_{3} molecule, the O.N. of the two I atoms forming the I_{2} molecule is 0, whereas the O.N. of the I atom forming the coordinate bond is –1.

(b) H_{2}__S___{4}O_{6}

Let assume oxidation number of S is x.

The oxidation number (O.N.) of H is +1.

The oxidation number (O.N.) of O is -2.

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

⇒ 2 + 4x - 12 = 0

⇒ 4x -10 = 0

⇒ 4x = +10

⇒ x = +10/4

However, O.N. cannot be fractional. Hence, S must be present in different oxidation states in the molecule.

The O.N. of two of the four S atoms is +5 and the O.N. of the other two S atoms is 0.

**(c)** Fe_{3}O_{4 }

Let assume oxidation number of Fe is x.

The oxidation number (O.N.) of O is -2.

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

⇒ 3x - 8 = 0

⇒ 3x = 8

⇒ x = 8/3

However, O.N. cannot be fractional.

Here, one of the three Fe atoms exhibits the O.N. of +2 and the other two Fe atoms exhibit the O.N. of +3.

(d) __C__H_{3}__C__H_{2}OH

Let assume oxidation number of C is x.

The oxidation number (O.N.) of O is -2.

The oxidation number (O.N.) of H is +1.

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

⇒ x +3 + x +2 - 2 + 1 = 0

⇒ 2x + 4 = 0

⇒ 2x = -4

⇒ x = -2

Hence, the oxidation number of C is -2.

(e) __C__H_{3}__C__OOH

Let assume oxidation number of C is x.

The oxidation number (O.N.) of O is -2.

The oxidation number (O.N.) of H is +1.

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

⇒ 2x + 3 - 2 - 2 + 1 = 0

⇒ 2x + 0 = 0

⇒ x = 0

However, 0 is average O.N. of C.

The two carbon atoms present in this molecule are present in different environments. Hence, they cannot have the same oxidation number. Thus, C exhibits the oxidation states of +2 and –2 in CH_{3}COOH.

- Q:-
Calculate the amount of carbon dioxide that could be produced when

(i) 1 mole of carbon is burnt in air.

(ii) 1 mole of carbon is burnt in 16 g of dioxygen.

(iii) 2 moles of carbon are burnt in 16 g of dioxygen.

- Q:-
The mass of an electron is 9.1 × 10

^{–31}kg. If its K.E. is 3.0 × 10^{–25}J, calculate its wavelength. - Q:-
Calculate the wavelength of an electron moving with a velocity of 2.05 × 10

^{7}ms^{–1}. - Q:-
Balance the following redox reactions by ion – electron method :

(a) MnO

_{4 }^{– }(aq) + I^{ – }(aq) → MnO_{2 (s)}+ I_{2(s)}(in basic medium)(b) MnO

_{4}^{–}(aq) + SO_{2 (g) }→ Mn^{2+}(aq) + HSO_{4}^{–}(aq) (in acidic solution)(c) H

_{2}O_{2 (aq)}+ Fe^{2+ }(aq) → Fe^{3+}(aq) + H_{2}O (l) (in acidic solution)(d) Cr

_{2}O_{7}^{2–}+ SO_{2(g)}→ Cr^{3+}_{(aq)}+ SO_{4}^{2– }(aq) (in acidic solution) - Q:-
In a process, 701 J of heat is absorbed by a system and 394 J ofwork is done by the system. What is the change in internal energy for the process?

- Q:-
What will be the minimum pressure required to compress 500 dm

^{3}of air at 1 bar to 200 dm^{3}at 30°C? - Q:-
In a reaction A + B2 → AB2 Identify the limiting reagent, if any, in the following reaction mixtures.

(i) 300 atoms of A + 200 molecules of B

(ii) 2 mol A + 3 mol B

(iii) 100 atoms of A + 100 molecules of B

(iv) 5 mol A + 2.5 mol B

(v) 2.5 mol A + 5 mol B

- Q:-
At 0°C, the density of a certain oxide of a gas at 2 bar is same as that of dinitrogen at 5 bar. What is the molecular mass of the oxide?

- Q:-
Which one of the following will have largest number of atoms?

(i) 1 g Au (s)

(ii) 1 g Na (s)

(iii) 1 g Li (s)

(iv) 1 g of Cl2(g)

- Q:-
Density of a gas is found to be 5.46 g/dm

^{3}at 27 °C at 2 bar pressure. What will be its density at STP?

- Q:-
Define environmental chemistry.

- Q:-
Draw the cis and trans structures of hex-2-ene. Which isomer will have higher b.p. and why?

- Q:-
Starting with sodium chloride how would you proceed to prepare

(i) sodium metal (ii) sodium hydroxide (iii) sodium peroxide (iv) sodium carbonate?

- Q:-
How will you convert benzene into

(i) p - nitrobromobenzene

(ii) m - nitrochlorobenzene

(iii) p - nitrotoluene

(iv) acetophenone?

- Q:-
Explain the formation of a chemical bond.

- Q:-
For the reaction

2 A(g) + B(g) → 2D(g)

ΔU

^{0}= –10.5 kJ and ΔS^{0}= –44.1 JK^{–1}.Calculate ΔG

^{0}for the reaction, and predict whether the reaction may occur spontaneously. - Q:-
The ionization constant of acetic acid is 1.74 x 10

^{-5}. Calculate the degree of dissociation of acetic acid in its 0.05 M solution. Calculate the concentration of acetate ion in the solution and its pH. - Q:-
What happens when

(i) magnesium is burnt in air

(ii) quick lime is heated with silica

(iii) chlorine reacts with slaked lime

(iv) calcium nitrate is heated ?

- Q:-
Thermodynamically the most stable form of carbon is

(a) diamond (b) graphite (c) fullerenes (d) coal

- Q:-
On the basis of quantum numbers, justify that the sixth period of the periodic table should have 32 elements.

Deepak rao
2019-05-19 14:46:36

Thanks for your NCERT solution

- NCERT Chapter