The Halogens and Qualitative Analysis

Trends in Group 7: Electronegativity

• Electronegativity decreases:

  • The number of principle energy levels increase as there is a weaker attraction between the nucleus and the outer electron

Trends in Group 7: Boiling Point

• Boiling Point increases:

  • The size of the atom increases so there are more electrons, this means there are more London forces between the molecules that need more energy to be broken

Fluorine at room temperature

Pale yellow gas

Chlorine at room temperature

Pale green gas

Bromine at room temperature

Dark red/orange liquid

Iodine at room temperature

Dark purple/black solid

Oxidising agent

Electron acceptor

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

• Halogen atom gains an electron less easily as there is a weaker attraction from the nucleus

Halogen displacement

Simple ionic equations for halogen displacement

• 2Br^- + Cl2 → 2Cl^- + Br₂

• 2I^- + Cl2 → 2Cl^- + I₂

• 2I^- + Br2 → 2Br^- + I₂

Reagents and Conditions to distinguish between halides

• Reagents - Silver Nitrate

• Conditions - Must be acidified

Aqueous ions reacted with silver halide

• AgF - No Visible Change

• AgCl - White precipitate

• AgBr - Cream precipitate

• AgI - Yellow precipitate

Why do we use nitric acid to to remove the hydroxide and carbonate ions?

They will interfere with the test

Overall and Ionic equation for nitric acid and carbonate

• 2HNO₃ + Na₂CO₃ → 2NaNO₃ + CO₂ + H₂O

• 2H⁺ + CO₃^2- → H₂0 + CO₂

How to distinguish between AgCl and AgBr

Add diluted Ammonia, AgCl dissolves and AgBr does not

How to distinguish between AgBr and AgI

Add concentrated ammonia, AgBr dissolves and AgI does not

Disproportionation

Oxidation and Reduction of the same element

What colour is present when chlorine is added to a universal indicator paper?

The paper first turns red, the red colour then disappears and the universal indicator paper turns white

Reaction of chlorine with cold dilute sodium hydroxide

• Cl₂ + 2NaOH → H₂0 + NaCl + NaClO

• Importance: NaClO is an active ingredient in household bleach

• Conditions: Dilute sodium hydroxide

Inorganic test for ions (CaSH)

Carbonate

Sulfate

Halide

Benefit of using chlorine in water treatment

Kills bacteria

• Disadvantage of treating water with chlorine

• Carcinogenic compounds are produced

• Toxic

Carbonate Test

• React Sodium Carbonate with Nitric Acid

• Ionic Equations: 2H⁺ + CO₃^2- → H₂0 + CO₂

• Observation: Effervescence

Sulfate Test

• React Sodium Sulfate with Barium Nitrate or Barium Chloride

• Ionic Equation: Ba₂⁺(aq) + SO₄^2-(aq) → BaSO₄

• Observation: White precipitate

Test for Cations

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

• Ionic equation: NH4⁺(aq) + OH^-(aq) → NH₃(g) + H₂0

• Observation: Turns moist litmus paper blue

How can a student modify their experiment when testing for haloalkanes

Heat test tubes in a water bath