Chapter 12 Halogens FBISE Chemistry 1st Year Keynotes with Solved Exercise

Introduction to Halogens

⚛️ What are Halogens?

Group 17 (VII-A) elements
Name from Greek: “halous” (salt) + “gen” (former)
Highly reactive non-metals
Exist as diatomic molecules (X₂)
Include: F, Cl, Br, I, At, Og

📝 Key Terms: “Halogen = Salt former”

🎨 Physical States & Colors

Fluorine (F₂): Pale yellow gas

Chlorine (Cl₂): Greenish yellow gas

Bromine (Br₂): Reddish brown liquid

Iodine (I₂): Shiny greyish black solid

📉 Trends Down the Group

Volatility decreases: Gas → Liquid → Solid
Color darkens: Light → Dark
Reason: Increasing molecular size → stronger London forces
Br₂ is liquid, I₂ is solid at room temperature

Bond Strength of Halogens

🔗 Bond Energies (kJ/mol)

Halogen	Bond	Bond Energy (kJ/mol)
Fluorine	F-F	156
Chlorine	Cl-Cl	243
Bromine	Br-Br	193
Iodine	I-I	151

💡 Important Note: F-F bond weaker than Cl-Cl due to small size causing inter-electronic repulsions

📊 Trend Explanation

Bond energy decreases from Cl to I
F-F bond exceptionally weak
Factors: Atomic size, electron repulsion
Result: F₂ less stable than Cl₂

Reactivity of Halogens

⚡ Reactivity Trend

Based on ability to gain electrons
Forms halide ions: X⁻ (F⁻, Cl⁻, Br⁻, I⁻)
Fluorine most reactive (highest electronegativity)
Reactivity decreases down group

🔄 Oxidizing Power

F₂ > Cl₂ > Br₂ > I₂

Displacement Reactions:

F₂(g) + 2KCl(aq) → 2KF(aq) + Cl₂(g)

Cl₂(g) + 2KBr(aq) → 2KCl(aq) + Br₂(l)

I₂ cannot oxidize any halide ion

🔥 Reaction with Hydrogen

F₂ + H₂
Explosive, even in cold

→

Cl₂ + H₂
Explodes in sunlight

→

Br₂ + H₂
Heated, moderate reaction

→

I₂ + H₂
Partial, reversible

Hydrogen Halides (HX)

🌡️ Thermal Stability

HF > HCl > HBr > HI

Depends on H-X bond strength
Bond strength decreases from HF to HI
Reason: Decreasing electronegativity difference
HI least stable (easiest to decompose)

🧪 Acidic Strength

HI > HBr > HCl > HF

Opposite trend to thermal stability!
Weaker bond = stronger acid
HI strongest acid (bond breaks easily)
HF weakest acid (strong bond, low dissociation)

🎯 Memory Tip: Thermal stability & Acidic strength have opposite trends

12.2.4 Reactivity of Halide Ions

🔄 Reducing Power

I⁻ > Br⁻ > Cl⁻ > F⁻

Increasing down the group
I⁻ strongest reducing agent
F⁻ weakest reducing agent
Based on ease of losing electrons

🧪 Reaction with H₂SO₄

F⁻/Cl⁻: Weak reducing agents
NaX + H₂SO₄ → NaHSO₄ + HX Br⁻: Reduces H₂SO₄ to SO₂
2HBr + H₂SO₄ → Br₂ + SO₂ + H₂O I⁻: Strongest, reduces to H₂S
8HI + 4H₂SO₄ → 4I₂ + 4H₂S + 4H₂O

🔍 Silver Nitrate Test

Halide	Precipitate	NH₃(aq) Test
Cl⁻	White (AgCl)	Dissolves
Br⁻	Cream (AgBr)	Partially dissolves
I⁻	Yellow (AgI)	Does not dissolve

Special Reactions of Chlorine

🌡️ With NaOH

Cold, dilute:
Cl₂ + 2NaOH → NaOCl + NaCl + H₂O
(Sodium hypochlorite) Hot, concentrated:
3Cl₂ + 6NaOH → NaClO₃ + 5NaCl + 3H₂O
(Sodium chlorate)

Type: Disproportionation reactions

💧 With Water

Step 1:
Cl₂ + H₂O → HCl + HOCl

→

Step 2:
HOCl → HCl + [O]

→

Step 3:
[O] kills bacteria

Application: Water purification (chlorination)

🏥 Medical Use: Chlorine compounds used as disinfectants and bleaches

Quick Quiz & Concept Check

Q1: Why does fluorine have lower bond energy than chlorine?

Answer: Due to its small atomic size causing inter-electronic repulsions between lone pairs in the F-F bond.

Q2: Arrange in order of increasing volatility: I₂, F₂, Br₂, Cl₂

Answer: I₂ < Br₂ < Cl₂ < F₂

Explanation: Volatility decreases down the group as molecular size increases.

Q3: Which halogen cannot displace any other halogen from its salt solution?

Answer: Iodine (I₂)

Reason: It has the lowest oxidizing power among halogens.

Q4: Why is HI a stronger acid than HF?

Answer: Because H-I bond is weaker than H-F bond, making it easier for HI to dissociate and donate protons.

Q5: What is observed when AgNO₃ is added to NaCl, NaBr, and NaI solutions?

Answer:

NaCl: White precipitate (AgCl), dissolves in NH₃(aq)
NaBr: Cream precipitate (AgBr), partially dissolves
NaI: Yellow precipitate (AgI), does not dissolve

Q6: Write the disproportionation reaction of chlorine with cold NaOH.

Answer: Cl₂ + 2NaOH → NaOCl + NaCl + H₂O

Note: Chlorine is both oxidized (to +1 in OCl⁻) and reduced (to -1 in Cl⁻).

Q7: Why is chlorine used for water purification?

Answer: Because it produces hypochlorous acid (HOCl) which decomposes to give atomic oxygen [O] that kills bacteria.

Reaction: Cl₂ + H₂O → HCl + HOCl → HCl + [O]

Q8: Which halide ion can reduce concentrated H₂SO₄ to H₂S?

Answer: Iodide ion (I⁻)

Reaction: 8HI + 4H₂SO₄ → 4I₂ + 4H₂S + 4H₂O

Q9: Explain the trend in reducing power of halide ions.

Answer: Reducing power increases down the group: I⁻ > Br⁻ > Cl⁻ > F⁻

Reason: As atomic size increases, the attraction between nucleus and valence electrons decreases, making it easier for the halide ion to lose electrons.

Q10: Compare thermal stability and acidic strength trends for hydrogen halides.

Answer:

Thermal Stability:
HF > HCl > HBr > HI
(Decreases down group) Acidic Strength:
HI > HBr > HCl > HF
(Increases down group)

Explanation: Stronger H-X bond = more thermally stable but weaker acid. Weaker H-X bond = less stable but stronger acid.