The Halogens

Trends in group 7: Electronegativity

• Electronegativity decreases

  • The number of principle energy levels increases

  • There is a weaker attraction between the nucleus and outer electron in a covalent bond

Trends in group 7: Boiling point

• Boiling point increases

  • The size of the atom increases, so there are more electrons

  • There are more London forces between the molecules that need to be broken

Fluorine at Room Temperature: F₂

Pale yellow gas

Chlorine at Room Temperature: Cl₂

Pale green gas

Bromine at room temperature: Br₂

Dark red/orange liquid

Iodine at Room Temperature: I₂

Dark purple/black solid

What happens when a halogen acts as an oxidising agent?

• It gains an electron and is reduced

• ½ X₂ + e^- → X^-

Why does the ability of a halogen to act as an oxidising agent decrease as you go down the group?

• Atomic radius increases

• Shielding increases

• Gains an electron less easily

Observation when a chlorine ion displaces a bromine gas

No visible change

Observation when a chlorine ion displaces a iodine gas

No visible change

Observation when a bromine ion displaces a chlorine gas

Yellow solution

Observation when a bromine ion displaces a iodine gas

No visible change

Observation when a iodine ion displaces a chlorine gas

• Aqueous solution - Brown

• Organic solution - Purple

Observation when a iodine ion displaces a bromine gas

• Aqueous solution - Brown

• Organic solution - Purple

Ionic equations for displacement reactions

• 2Br^- + Cl₂ → 2Cl^-+ Br₂

• 2I^- + Cl₂ → 2Cl^- + I₂

• 2I^- + Br₂ → 2Br^- + I₂

Reagents and conditions to identify halide ions in solution

• Reagents - Silver Nitrate

• Conditions - Acidified

Observation of silver halides

• AgF - No visible change

• AgCl - White ppt

• AgBr - Cream ppt

• AgI - Yellow ppt

How to distinguish between AgCl and AgBr

• Add dilute Ammonia as AgCl dissolves, AgBr does not

How to distinguish between AgBr and AgI

• Add concentrated Ammonia as AgBr dissolves, AgI does not

Ionic equations of silver halide ions

• Ag⁺(aq) + Cl^-(aq) → AgCl(s)

• Ag+(aq) + Br^-(aq) → AgBr(s)

• Ag⁺(aq) + I^-(aq) → AgI(s)

Disproportionation

Oxidation and Reduction of the same element

What is the correct sequence to test for inorganic ions?

• Carbonate

• Sulfate

• Halide

Carbonate test

• React CO₃^2- with Nitric Acid

• 2H⁺ + CO₃^2- → H₂O + CO₂

• If present we would get on observation of Effervescence

Sulfate test

• Add Barium nitrate

• Ba²⁺(aq) + SO₄^2-(aq) → BaSO₄

• If Sulfate ions are present we would observe a white ppt

Test for cations

• Ammonium salt is heated with aqueous sodium hydroxide to produce ammonia gas

• NH₄⁺(aq) + OH^-(aq) → NH₃(g) + H₂O

• Observation - Turns moist litmus paper blue