Lecture Notes: Halogens and Their Reactions

Introduction

• Halogens are elements in Group 7 of the periodic table.
• Elements include: Fluorine, Chlorine, Bromine, Iodine, Astatine.
• Halogens are toxic in their elemental form but have useful compounds.
• They have seven electrons in their valence shell and form ions with -1 charge by accepting one electron.
• General Trend: As you move down the group:
  • Atomic radii, melting points, boiling points, and density increase.
  • Reactivity decreases.

Physical States and Properties

• Fluorine: Pale green gas.
• Chlorine: Greenish-yellow gas.
• Bromine: Reddish-brown liquid.
• Iodine: Gray solid that sublimes to purple vapor.
• Astatine: Black solid, radioactive.

Reactivity and Displacement Reactions

• A more reactive halogen displaces a less reactive halogen in a compound.
• Example Reactions:
  • Chlorine Water with Potassium Bromide: Chlorine displaces bromine. Reddish-brown solution forms.
  • Chlorine Water with Potassium Iodide: Chlorine displaces iodine. Dark brown solution forms.
  • Bromine Water:
    • With Potassium Chloride: Solution turns reddish-brown, no displacement.
    • With Potassium Bromide: No reaction.
    • With Potassium Iodide: Bromine displaces iodine. Dark brown solution forms.
  • Iodine: No displacement reactions with these solutions, as iodine is less reactive.

Reactions with Metals

• Halogens react with certain metals to form metal halides.
• Example: Sodium chloride (table salt), formed from sodium and chlorine.
  • Reaction is highly exothermic and should not be done in the lab.

Reactions with Hydrogen

• Halogens react with hydrogen gas to form hydrogen halides.
• Reactions are highly exothermic.
• Example: Hydrogen chloride from chlorine gas and hydrogen gas.

Summary

• Halogens are Group 7 elements with distinct physical states and properties.
• They undergo displacement reactions where a more reactive halogen displaces a less reactive one.
• They react with metals to form metal halides and with hydrogen to form hydrogen halides.