Test Session Chapter 10 Electrochemistry (1st Year Chemistry)

Q1. Choose the correct option. (10×1=10)

In NaCl, the oxidation number of Na is:

(A) +2

(B) -1

(D) 0

Oxidation number of Cl in NaClO₃ is:

(A) -1

(B) 0

(D) +5

The oxidation number of chromium in K₂Cr₂O₇ is:

(A) +3

(B) +6

(D) +2

When Na → Na⁺ + e⁻, Na acts as:

(A) Oxidizing agent

(B) Reducing agent

(D) Inter metal

What is the first step in balancing a redox equation?

(A) Balance water

(B) Balance hydrogen

(D) Add spectator ions

In electrolysis of CuBr₂, what is formed at cathode?

(A) Br₂ gas

(B) Cu metal

(D) H₂ gas

What happens at the anode in redox electrolysis?

(A) Electrons are gained

(B) Electrons are lost

(D) Metals are refined

Electrode potential is measured under:

(A) High temperature

(B) Standard conditions

(D) Vacuum

If Zn has E° = -0.76 V and Cu has E° = +0.34 V, which one is oxidized in a cell?

(A) Zn

(B) Cu

(D) Neither

The device used to measure electrode potential is:

(A) Galvanometer

(B) Ammeter

(D) Thermometer

Q2. Write down short answers of following questions.

[1] Define oxidation and reduction in terms of electron transfer with examples. (Quick Check 10.1)

Oxidation Example:

Mg → Mg²⁺ + 2e⁻

Reduction Example:

Cl₂ + 2e⁻ → 2Cl⁻

[ii] Determine Ox. No. of:

Oxidation Number Calculations

(i) Oxygen in Na₂O

(ii) Iodine in ICl₆

(iii) Nitrogen in NO₃⁻

(iv) Chromium in K₂Cr₂O₇

[iii] Define oxidation with an example.

[iv] Why is cellular respiration considered a redox process?

[v] Define oxidation number.

[vi] Explain why an oxidizing agent is itself reduced.

[vii] Balance K₂Cr₂O₇ + FeSO₄ + H₂SO₄ → Cr₂(SO₄)₃ + Fe₂(SO₄)₃ + K₂SO₄ + H₂O.

K₂Cr₂O₇ + 6FeSO₄ + 7H₂SO₄ → Cr₂(SO₄)₃ + 3Fe₂(SO₄)₃ + K₂SO₄ + 7H₂O

Balancing Steps

[viii] Why is Cl⁻ converted to Cl₂ at the anode during electrolysis?

Electrolysis Setup

Anode (+)

→

2Cl⁻ → Cl₂ + 2e⁻

[ix] Name any one factor affecting electrode potential.

[x] Why is SHE used as the standard reference electrode?

[xi] Describe the structure and working of SHE.

Standard Hydrogen Electrode

Pt electrode | H₂(g) | H⁺(aq, 1M)

2H⁺(aq) + 2e⁻ ⇌ H₂(g)

[xii] What is the E° value of SHE?

[xiii] Name the instrument used to measure electrode potential.

[xiv] What is a salt bridge?

[xv] How does organic matter affect BOD values?

Q3. Write detailed answers of the following questions. (Answer any 2)

1. Find the oxidation No. of S in Sulfur dioxide, SO₂. (Sample Problem 10.1)

Calculation Steps

Let oxidation number of S = x

Oxygen has oxidation number = -2

x + 2(-2) = 0

x – 4 = 0

x = +4

Therefore, oxidation number of S in SO₂ is +4

2. With the help of equations, describe oxidation and reduction during the electrolysis of molten salts and aqueous solutions.

Electrolysis of Molten NaCl

Cathode (-)

→

Na⁺ + e⁻ → Na

Anode (+)

→

2Cl⁻ → Cl₂ + 2e⁻

3. Explain the concepts of BOD and DO. Discuss their significance in environmental chemistry and water pollution monitoring.

Key Definitions

BOD (Biochemical Oxygen Demand): Amount of oxygen required by microorganisms to decompose organic matter in water.

DO (Dissolved Oxygen): Amount of oxygen dissolved in water, essential for aquatic life.

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