Group 7: The Halogens

Fluorine colour

Pale yellow

Chlorine colour

Green

Bromine colour

Brown

Iodine colour

Dark grey

Astatine colour

Black

Fluorine state at room temperature

Gas

Chlorine state at room temperature

Gas

Bromine state at room temperature

Liquid

Iodine state at room temperature

Crystals

Astatine state at room temperature

Solid

What is the trend in melting and boiling point as you go down group 7?

The points increase

What is the trend in atomic radius as you go down group 7?

Increase

What is the trend in electronegativity as you go down group 7?

Decrease

Using a solubility in water versus hydrocarbon test, how can you tell which solvent the halogens are more soluble in?

They are more soluble in the hydrocarbon as there is a more intense colour.

Why are halogens more soluble in hydrocarbon than in water?

In a hydrocarbon, the VdW forces are easier to break however the hydrogen bonding in water is stronger.

What are the halogen displacement reactions?

Chlorine displaces bromine and iodine

Bromine displaces iodine but not chlorine

Iodine doesn’t displace any halogen

Why can chlorine and bromine displace halogens?

They are more reactive and stronger oxidising agents.

Which halogen in a displacement reaction is oxidised?

The ion that is displaced loses an electron

Which halogen in a displacement reaction is reduced?

The atom that displaces gains an electron

Equation for reaction between chlorine and water

Cl₂ + H₂O → HCl + HCLO

Products of chlorine and water reaction

Hydrochloric acid, Chloric (I) acid

Oxidation number of chlorine at the start of the reaction with water

0

Oxidation number of chlorine in HCl

-1

Oxidation number of chlorine in HClO

+1

How can you tell the reaction between chlorine and water is a disproportionation reaction?

Chlorine’s oxidation number has both increased (chloric acid) and decreased (hydrochloric acid)

Reaction between chlorine and water in the presence of sunlight

2Cl₂ + 2H₂O → 4HCl + O₂

Why is the reaction between chlorine and water different in the presence of sunlight?

Chlorine oxidises the water so is rapidly lost from the pool water .

What is an alternative to direct chlorination of swimming pools?

Adding solid sodium chlorate (NaClO) which dissolves in water to form chloric acid.

Equation for reaction between water and sodium chlorate

NaClO + H₂O <-> Na⁺ + OH^- + HClO

Why is acidity of swimming pools necessary?

To prevent the equilibrium in the equation from moving to the left which leads to HClO being removed as ClO^- ions.

Why is the acidity of swimming pools monitored?

So the water never gets acidic enough to corrode metal or affect swimmers.

Why is chlorine added to water for drinking purposes?

It kills pathogens responsible for water-borne diseases (cholera).

Kills microrganisms

Chloric (I) acid is an oxidising agent that kills bacteria by oxidation

In the reaction between halogens and cold alkali, what happens to the halogen’s oxidation number.

It decreases and increases. Therefore, it is a disproportionation reaction.

Equation for reaction between halogen and cold alkali

X₂ + 2NaOH → NaX + NaXO + H₂O

Products of reaction between halogen and cold alkali

Sodium (chlor)ide

Sodium (chlor)ate

When the halogen is reduced in reaction with cold alkali, what is the product?

Sodium Halide (0 → -1)

When the halogen is oxidised in reaction with cold alkali, what is the product?

Sodium Halate (0 → +1)

Are halogens oxidised or reduced when they form halide ions?

They are reduced as their oxidation number decreases from 0 to -1.

How to test for halide ions

Add nitric acid, silver nitrate and dilute/concentrated ammonia solution.

Why is dilute nitric acid added before the other testing reagents?

To remove impurities like carbonates or sulfates

Observation when nitric acid/ silver nitrate solution is added to chloride ions

White precipitate

Observation when nitric acid/ silver nitrate solution is added to bromide ions

Cream precipitate

Observation when nitric acid/ silver nitrate solution is added to iodide ions

Yellow precipitate

Observation when dilute ammonia solution is added to chloride ions

Redissolves

Observation when dilute ammonia solution is added to bromide ions

Nothing, bromide is insoluble

Observation when dilute ammonia solution is added to iodide ions

Nothing, iodide is insoluble

Observation when concentrated ammonia solution is added to bromide ions

Redissolves

Observation when concentrated ammonia solution is added to iodide ions

Nothing, iodide is insoluble

What is (concentrated) sulfuric acid’s role when reacted with halogens?

Acid and oxidising agent

What happens to the sulfur in sulfuric acid when it’s reacted with halogens?

It is reduced

What are the possible products when concentrated sulfuric acid is reacted with halogens?

Hydrogen halide (HX)

Sulfur dioxide (SO₂)

Sulfur (S)

Hydrogen sulfide (H₂S)

Why are iodide ions stronger reducing agents than chloride or bromide ions?

They are larger structures, reducing the forces of attraction between the nucleus and outer electrons, making it easier to donate electrons.

Products of reaction between sodium chloride and concentrated sulfuric acid

Hydrogen chloride (HCl)

Sodium bisulfate (NaHSO₄)

Products of reaction between sodium bromide and concentrated sulfuric acid

Hydrogen Bromide (HBr)

Sodium bisulfate (NaHSO₄)

Sulfur dioxide (SO₂)

(Water (H₂O))

Products of reaction between sodium iodide and concentrated sulfuric acid

Hydrogen iodide (HI)

Sodium bisulfate (NaHSO₄)

Sulfur dioxide (SO₂)

Sulfur (S)

Hydrogen sulfide (H₂S)

(Water (H₂O))

Oxidation number of sulfur in concentrated sulfuric acid and sodium bisulfate

+6

Change in oxidation number of sulfur in concentrated sulfuric acid to form sulfur dioxide

+6 → +4

Change in oxidation number of sulfur in concentrated sulfuric acid to form sulfur

+6 → 0

Change in oxidation number of sulfur in concentrated sulfuric acid to form hydrogen sulfide

+6 → -2

Equation when sodium bisulfate is formed

NaX + H₂SO₄ → HX + NaHSO₄

Equation when sulfur dioxide is formed

2X⁺ + H₂SO₄ + 2e^- + 2H⁺ + → X₂ + SO₂ + H₂O

Equation when sulfur is formed

6I^- + H₂SO₄ + 6H⁺ + 6e^- → 3I₂ + S + 4H₂O + 6e^-

Equation when hydrogen sulfide is formed

8I^- + H₂SO₄ + 8H⁺ + 8e^- → 4I₂ + 8e^- + H₂S + 4H₂O

Observation when sodium bisulfate is produced

Steamy fumes (white, brown, purple)

Observation when sulfur dioxide is formed

Colourless gas

Observation when sulfur is produced

Yellow solid

Observation when hydrogen sulfide is produced

Pungent gas