AQA A-Level Chemistry (new spec) 2.3 Group 7 - The Halogens

What are the 5 halogens?

Fluorine, Chlorine, Bromine, Iodine & Astatine

Why isn't astatine studied?

Because it's very radioactive and can't exist for more than a few seconds before decaying.

What oxidation state are the G7 elements usually found in?

-1, found as X-1 ions (known as halide ions)

Appearance of G7 elements: Fluorine

Very pale yellow gas, highly reactive

Appearance of G7 elements: Chlorine

Pale green gas, poisonous in high concentrations

Appearance of G7 elements: Bromine

Red liquid, gives off dense brown or orange poisonous fumes

Appearance of G7 elements: Iodine

Shiny grey solid that sublimes to a purple gas

Appearance of G7 elements: Astatine

Black solid, is radioactive

Trends of G7: Boiling point as you go down the group

Increases

-Number of VDW forces increase between molecules as they're larger

-More energy is needed to break the bonds

Trends of G7: Electronegativity

Decreases

-Atomic radius increases

-Shielding increases

-Less attraction between the outer electrons and the protons in the nucleus

Are the halogens oxidising or reducing agents?

Oxidising

-They can accept electrons and are reduced

F2(g) + 2e- --> 2F-(aq)

Cl2(g) + 2e- --> 2Cl-(aq)

Br2(l) + 2e- --> 2Br-(aq)

I2(aq) + 2e- --> 2I-(aq)

Trends of G7: Oxidising power

Decrease

-Atoms become larger

-Shielding increases

-Outer e-'s are less attracted to the nucleus

Are halides oxidising or reducing agents?

Reducing

-They can lose electrons and can get oxidised

2F-(aq) --> F2(g) + 2e-

2Cl-(aq) --> Cl2(g) + 2e-

2Br-(aq) --> Br2(l) + 2e-

2I-(aq) --> I2(aq) + 2e-

Trends of G7: Reducing power

Increase

-Atoms larger

-Shielding increases

-Electrons aren't attracted to the nucleus as strongly so they're more easily lost

Trends of G7: Displacement reactions

The more reactive halogens (strongest oxidising agents) will displace the more reactive halides (strongest reducing agents) from solutions of their ions

What happens when: potassium chloride reacts with chlorine?

No reaction

What happens when: potassium chloride reacts with bromine?

No reaction

What happens when: potassium chloride reacts with iodine?

No reaction

What happens when: potassium bromide reacts with chlorine?

A yellow solution is formed, Cl has displaced Br.

Cl2(aq) + 2Br-(aq) --> 2Cl-(aq) + Br2(aq)

What happens when: potassium bromide reacts with bromine?

No reaction

What happens when: potassium bromide reacts with iodine?

No reaction

What happens when: potassium iodide reacts with chlorine?

A brown solution is formed, Cl has displaced I

Cl2(aq) + 2I-(aq) --> 2Cl-(aq) + I2(aq)

What happens when: potassium iodide reacts with bromine?

A brown solution is formed, Br has displaced I

Br2(aq) + 2I-(aq) --> 2Br-(aq) + I2(aq)

What happens when: potassium iodide reacts with iodine?

No reaction

Sulfuric acid can act as..?

An acid or an oxidising agent

Sulfuric acid acting as an acid

Can convert the sodium salts of the halides into the hydrogen halides

H2SO4(l) + NaX(s) --> NaHSO4(s) + HX(g)

Sulfuric acid acting as an oxidising agent

Can be reduced to either SO2, S or H2S.

H2SO4 + 2H+ + 2e- --> SO2 + 2H2O

H2SO4 + 6H+ + 6e- --> S + 4H2O

H2SO4 + 8H+ + 8e- --> H2S + 4H2O

Sulfuric Acid Reactions: Fluorine & Chlorine

No redox reactions, only acid-base reactions.

NaF(s) + H2SO4(l) --> NaHSO4(s) + HF(g)

White steamy fumes of HF are produced.

NaCl(s) + H2SO4(l) --> NaHSO4(s) + HCl(g)

White steamy fumes of HCl are produced.

Sulfuric Acid Reactions: Bromide

Br- ions are stronger reducing agents and after the acid-base reaction, they reduce the sulfur

NaBr(s) + H2SO(l) --> NaHSO4(s) + HBr(g)

White steamy fumes of HBr are produced

2HBr(g) + H2SO4(l) --> Br2(g) + SO2(g) + 2H2O

Red fumes of bromine are evolved and SO2 (a colourless acidic gas)

Sulfuric Acid Reactions: Iodide

The strongest reducing agent of the halides

NaI(s) + H2SO4(l) -->NaHS(s) + HI(g)

White steamy fumes of HI evolved

2HI(g) + H2SO4(l) --> I2(s) + SO2(g) + 2H2O(l)

Black solid and purple fumes of iodine are produced, as well as SO2

6HI(g) + H2SO4(l) --> 3I2(s) + S(s) + 4H2O

A yellow solid of Sulfur

8HI(g) + H2SO4(l) --> 4I2(s) + H2S(g) + 4H2O(l)

Hydrogen Sulphide gas is produced (gas with a pungent egg smell)

Silver Nitrate Reactions: Process

Make a halide solution and acidify with dilute nitric acid. Add silver nitrate solution and treat any excess with dilute ammonia solution. If there is still a precipitate, treat with concentrated ammonia solution

Silver Nitrate Reactions: Chloride

White precipitate formed, soluble in both concentrations

Dilute: AgCl(s) + 2NH3(aq) --> [Ag(NH3)2]+(aq) + Cl-(aq)

Silver Nitrate Reactions: Bromide

Cream precipitate formed, insoluble in dilute ammonia but soluble in conc. ammonia

Conc.: AgBr(s) + 2NH3(aq) --> [Ag(NH3)2]+(aq) + Br-(aq)

Silver Nitrate Reactions: Iodide

Yellow precipitate formed, insoluble in both concentrations

Why acidify with nitric acid?

Reacts with any carbonates present to prevent formation of the precipitate Ag2CO3, which would mask the desired observation

2HNO3 + Na2CO3 --> 2NaNO3 + H2O + CO2

What is disproportionation?

A reaction where an element oxidises and reduces simultaneously

Disproportionation of: Chlorine with water

CL2(aq) + H2O(l) --> HOCl(aq) + HCl(aq)

Disproportionation of: Chlorine with water in sunlight

2Cl2(aq) + 2H2O(l) --> 4H+ + 4Cl- + O2

What will happen when U.I (Universal Indicator) is added to chlorine and water solution?

It will turn red due to the acidity of both products, it will then turn colourless as the HClO bleaches the colour

Uses of chlorine

Water treatment to kill bacteria, it can treat drinking water and swimming pool water.

Reactions of Chlorine, Bromine and Iodine with cold, dilute NaOH solution

The colour will fade to colourless

Cl2(aq) + 2NaOH(aq) --> NaCl(aq) + NaOCl(aq) + H2O(l)

NaCl and NaClO uses

Bleach