Test Session Chapter 13 Halogens (1st Year Chemistry)

Class: 1st Year

Subject: Chemistry (SNC)

Paper Date: To be announced

Time: 2 hours

Chapter 13: Halogens

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

Which halogen molecule has the strongest bond?

(A) F₂

(B) Br₂

(D) Cl₂

The volatility of the halogens (Group 17) generally …… as you move down the group (from Fluorine to Iodine).

(A) Increases

(B) Decreases

(D) Fluctuates unpredictably

As the size of halogen atoms increases:

(A) Volatility increases

(B) Boiling point decreases

(D) Volatility decreases

What trend is observed in volatility as we go down group 17?

(A) Increases

(B) Decreases

(D) Remains constant

Bond strength in halogen molecules is measured in:

(A) KJ/mol

(B) atm

(D) K

Fluorine displaces which of the following from its compound?

(A) Iodine

(B) Bromine

(D) All of the above

Which halogen reacts with hydrogen explosively in the dark?

(A) Bromine

(B) Iodine

(D) Chlorine

What type of agent is hypochlorous acid (HClO)?

(A) Reducing

(B) Bleaching

(D) Precipitation

Why is chlorine effective even in small concentrations?

(A) It reacts slowly

(B) It is highly reactive and oxidizing

(D) It makes water acidic

What is the role of ClO⁻ ion in water treatment?

(A) Neutralizes acid

(B) Acts as disinfectant

(D) Dissolves gases

Q2. Write down short answers of following questions. (15×2=30)

[1] Which halogen is the least reactive, which is the most? Why?

Fluorine

Most Reactive

Chlorine

Bromine

Iodine

Least Reactive

[ii] How does the reactivity of halogens with hydrogen vary?

Reactivity Trend

Reactivity with hydrogen decreases down the group:

F₂ > Cl₂ > Br₂ > I₂

[iii] What is the colour change when chlorine displaces bromine?

[iv] How the halogen acids are ionized in water?

[v] Why HF is weaker acid than HCl?

[vi] Describe a simple chemical test that could be used to distinguish between aqueous solutions of potassium bromide and potassium iodide. Include the reagents and expected observations.

Test Procedure

Reagent: Chlorine water or chlorine gas

Observation for KBr: Colorless to yellow/orange (Br₂ forms)

Observation for KI: Colorless to brown (I₂ forms)

[vii] Which halogens elements are radioactive? (Quick Check 13.1)

[viii] Is the reaction between NaCl(aq) and F₂ gas possible?

Displacement Reaction

2NaCl(aq) + F₂(g) → 2NaF(aq) + Cl₂(g)

Fluorine can displace chlorine from NaCl because fluorine is more reactive (higher oxidizing power) than chlorine.

[ix] What is the relationship between the oxidizing power of halogens and their standard reduction potential values? (Quick Check 13.2)

[x] What are the primary active species in the chlorination of water? Give equation that shows their production. (Quick Check 13.6)

Water Chlorination

Cl₂(g) + H₂O(l) ⇌ HClO(aq) + H⁺(aq) + Cl⁻(aq)

HClO(aq) ⇌ H⁺(aq) + ClO⁻(aq)

Active species: HClO (hypochlorous acid) and ClO⁻ (hypochlorite ion)

[xi] Which halogen is the most volatile among Cl₂, Br₂, and I₂?

[xii] Why does volatility of halogens decrease from chlorine to iodine?

[xiii] Explain how intermolecular forces affect the physical states of halogens at room temperature?

[xiv] What is meant by halogen displacement?

[xv] What is observed when chlorine reacts with hydrogen under UV light?

Photochemical Reaction

H₂(g) + Cl₂(g) → 2HCl(g)

Reaction is explosive under UV light or sunlight

Q3. Write detailed answers of the following questions. (Answer any 2) (2×5=10)

1. Describe the displacement reactions of halogens with halide ions and relate them to oxidizing strength.

Halogen	Displaces	Example Reaction
F₂ (Strongest oxidizer)	Cl⁻, Br⁻, I⁻	F₂ + 2NaCl → 2NaF + Cl₂
Cl₂	Br⁻, I⁻	Cl₂ + 2NaBr → 2NaCl + Br₂
Br₂	I⁻	Br₂ + 2NaI → 2NaBr + I₂
I₂ (Weakest oxidizer)	None	Does not displace any halide

2. Describe the reactions of halogens with hydrogen. Compare their reactivity trend and write balanced chemical equations.

Reactivity with Hydrogen

Fluorine: Reacts explosively even in dark and cold conditions

F₂(g) + H₂(g) → 2HF(g) (Explosive)

Chlorine: Reacts explosively in sunlight/UV light

Cl₂(g) + H₂(g) → 2HCl(g) (Explosive in light)

Bromine: Reacts on heating

Br₂(g) + H₂(g) → 2HBr(g) (Requires heating)

Iodine: Reversible reaction, requires continuous heating

I₂(g) + H₂(g) ⇌ 2HI(g) (Reversible, needs heating)

3. Describe how chlorine purifies drinking water. Include chemical reactions and the role of HClO and ClO⁻.

Water Purification Process

Step 1: Chlorine dissolves in water

Cl₂(g) + H₂O(l) ⇌ HClO(aq) + HCl(aq)

Step 2: Hypochlorous acid dissociates

HClO(aq) ⇌ H⁺(aq) + ClO⁻(aq)

Active disinfectants: HClO and ClO⁻ ions

Mechanism: These species oxidize and destroy microorganisms (bacteria, viruses) and organic matter in water

Advantages: Effective even at low concentrations, residual protection against recontamination

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