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QUESTION: 1

Which substance serves as reducing agent in the following reaction ? [1994]

Solution:

The element undergo oxidation itself and reduces others is known as reducing agent.

In this reaction O. N. of Ni Changes from 0 to + 2 and hence Ni acts as a reducing agent.

QUESTION: 2

Phosphorus has the oxidation state of + 3 in

Solution:

O.N. of P in H_{3}PO_{3} (phosphorous acid) 3 × 1 + x + 3 × (– 2) = 0 or x = + 3 In orthophosphoric acid (H_{3}PO_{4}) O.N. of P is + 5, in hypophosphorous acid (H_{3}PO_{2}) it is + 1 while in metaphosphoric acid (HPO_{3}), it is + 5,

QUESTION: 3

In which of the following reactions, there is no change in valency ? [1994]

Solution:

In this reaction, none of the elements undergoes a change in oxidation number or valency.

QUESTION: 4

The oxidation number of chromium in potassium dichromate is [1995]

Solution:

Let x = oxidation no. of Cr in K_{2}Cr_{2}O_{7}.

∴ (2 × 1) + (2 × x) + 7 (– 2) = 0

or 2 + 2x – 14 = 0 or x = + 6.

QUESTION: 5

The loss of electron is termed as [1995]

Solution:

Losing of electron is called oxidation,

QUESTION: 6

The oxide, which cannot act as a reducing agent, is[1995]

Solution:

Carbon has the maximum oxidation state of + 4, therefore carbon dioxide (CO_{2}) cannot act as a reducing agent.

QUESTION: 7

Which of the following involves a redox reaction?

Solution:

(not redox reaction)

here oxidation of N_{2} & reduction of O_{2} is taking place

(not redox reaction)

QUESTION: 8

The oxidation number of phosphorus in pyrophosphoric acid is [1999]

Solution:

Pyrophosphoric acid H_{4}P_{2}O_{7}

Let oxidation state of phosphorus is x(4 × 1 + (– 2) × 7 + 2 x) = 0

∴ 2x = 10 or x = +5

QUESTION: 9

The following redox reaction is balanced by which set of coefficients ? [1999]

Solution:

operate eq. (1) × 4 + eq. (2) × 1

QUESTION: 10

A compound contains atoms of three elements A, B and C. If the oxidation number of A is +2, B is +5, and that of C is –2, the possible formula of the compound is : [2000]

Solution:

Oxidation number of a compound must be 0.

Using the values for A, B and C in the four options we find that A3(BC_{4})_{2} is the answer.

Check : (+2)3 + [(+5)+4(–2)]2 = 6 + (5–8)2 = 0

QUESTION: 11

Zn gives H_{2} gas with H_{2}SO_{4} and HCl but not with HNO_{3} because [2002]

Solution:

Zinc gives H_{2 }gas with dil H_{2}SO_{4}/HCl but not with HNO_{3} because in HNO_{3}, NO_{3}^{–} ion is reduced and give NH_{4}NO_{3}, N_{2}O, NO and NO_{2} (based upon the concentration of HNO_{3})

Zn is on the top position of hydrogen in electrochemical series. So Zn displaces H_{2} from dilute H_{2}SO_{4} and HCl with liberation of H_{2}.

QUESTION: 12

The oxidation states of sulphur in the anions SO_{3}^{2–}, S_{2}O_{4}^{2– }and S_{2}O_{6}^{2–} follow the order [2003]

Solution:

The correct answer is option B

Oxidation state of S_{2}O_{4}^{2−}

2(x) + 4(−2) = −2

2x = 8 − 2

2x = 6

x = 3

Oxidation state of SO_{3}^{2−}

x + 3(−2) = −2

x = 6 − 2

x = 4

Oxidation state of S_{2}O_{6}^{2−}

2(x) + 6(−2) = −2

2x = 12 − 2

2x = 10

x = 5

So the oxidation state of sulphur in the anions S_{2}O_{4}^{2−}, S_{2}O_{4}^{2−} and S2O62− follows the order.S_{2}O_{4}^{2−} < SO_{3}^{2−} < S_{2}O_{6}^{2−}.

QUESTION: 13

Oxidation numbers of P in PO_{4}^{3}^{–} , of S in SO_{4}^{2–} and that of Cr in Cr_{2}O_{7}^{2– } are respectively [2009]

Solution:

(i) Sum of oxidation states of all atoms = charge of ion.

(ii) oxidation number of oxygen = -2

Let the oxidation state of P in PO_{4}^{3-} is x.

PO_{4}^{3-}

x + 4 (-2) = - 3

x-8 = - 3

x = +5

Let the oxidation state of S in SO_{4}^{2-} is y

y + 4(-2) = -2

y-8 = - 2

y = +6

Let the oxidation state of Cr in Cr_{2}O_{7}^{2-} is z.

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

2z-14 = - 2

z=+6

Hence, oxidation state of P, S and Cr are +5, +6 and +6

QUESTION: 14

When Cl_{2} gas reacts with hot and concentrated sodium hydroxide solution, the oxidation number of chlorine changes from : [2012]

Solution:

The correct answer is Option B.

The reaction of chlorine gas with hot and concentrated sodium hydroxide solution is

3Cl2 + 6NaOH ⟶ NaClO3 + 5NaCl + 3H2O

Oxidation number of Cl is 0 in Cl2, −1 in NaCl and +5 in NaClO3

So, the oxidation number of chlorine changes from Zero to -1 and Zero to +5.

QUESTION: 15

A mixture of potassium chlorate, oxalic acid and sulphuric acid is heated. During the reaction which element undergoes maximum change in the oxidation number ? [2012]

Solution:

i.e. maximum change in oxidation number is observed in Cl (+5 to –1).

QUESTION: 16

Standard reduction potentials of the half reactions are given below :

The strongest oxidising and reducing agents respectively are : [2012 M]

Solution:

Higher the value of reduction potential higher will be the oxidising power whereas the lower the value of reduction potential higher will be the reducing power.

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