Topic 9: Halogens

Halogens - Group VII Elements

Overview of Halogens

• Positioned in Group VII on the right-hand side of the Periodic Table.

• Comprise diatomic non-metals:

  • Fluorine (F)

  • Chlorine (Cl)

  • Bromine (Br)

  • Iodine (I)

  • Astatine (At) (radioactive)

General Physical Properties

1. Physical States and Colors at Room Temperature (RTP):

   • Fluorine (F₂): Gas, Pale yellow-green

   • Chlorine (Cl₂): Gas, Yellowish green

   • Bromine (Br₂): Liquid, Red-brown

   • Iodine (I₂): Solid, Grey-black (also purple gas)

   • Astatine (At₂): Solid, Black

2. Conductivity:

   • Poor conductors of heat and electricity (lack free ions/electrons).

3. Solubility:

   • Low solubility in water.

4. Toxicity:

   • All halogens are toxic.

Trends in Halogens

• As you move down the group:

  • Density: Increases

  • Reactivity: Decreases

Testing for Chlorine Gas

• Procedure:

  • Approach damp litmus paper.

• Observations:

  • Blue litmus turns red then bleaches.

  • Red litmus bleaches.

Test for Halide Ions

• Procedure:

  • Add a few drops of silver nitrate (AgNO₃) acidified with nitric acid to the aqueous solution.

• Observations:

  • Chloride (Cl⁻): White precipitate of silver chloride (AgCl) formed, darkens with light.

  • Reaction:
    $Ag^+(aq) + Cl^-(aq) <br>ightarrow AgCl(s)$

  • Bromide (Br⁻): Creamy precipitate of silver bromide (AgBr) formed, darkens with light.

  • Reaction:
    $Ag^+(aq) + Br^-(aq) <br>ightarrow AgBr(s)$

  • Iodide (I⁻): Yellow precipitate of silver iodide (AgI) formed, darkens with light.

  • Reaction:
    $Ag^+(aq) + I^-(aq) <br>ightarrow AgI(s)$

Reactivity of Halogens

• Fluorine is the most reactive halogen and most reactive non-metal.

• Reactivity decreases moving down the group.

Displacement Reactions of Halogens

• General Equation:
  $X<em>2(aq) + 2Y^-(aq) ightarrow 2X^-(aq) + Y</em>2(aq)$

  • Where:

  • $X$: Halogen displacing another halogen

  • $Y$: Halogen being displaced

• Examples:

  • $Cl<em>2(g) + 2NaBr(aq) ightarrow 2NaCl(aq) + Br</em>2(aq)$ (Colorless to orange due to Br₂ formation)

  • $Cl<em>2(g) + 2NaI(aq) ightarrow 2NaCl(aq) + I</em>2(aq)$ (Colorless to dark reddish-brown due to I₂ formation)

  • $Br<em>2(l) + 2NaI(aq) ightarrow 2NaBr(aq) + I</em>2(aq)$ (Colorless to dark reddish-brown due to I₂ formation)

Predicting Reactivity

• Use Trends in Reactivity:

  • $Cl<em>2 + 2KI ightarrow I</em>2 + 2KCl$

  • $I<em>2 + 2KF ightarrow F</em>2 + 2KI$

  • $Br<em>2 + 2KCl ightarrow Cl</em>2 + 2KBr$

Colors in Cyclohexane

• Observe Colors of Solutions:

  • Bromine solution: [Color specific]

  • Iodine solution: [Color specific]