3.2.3 Group 7 the halogens - Chemistry Alevel

What is the charge of a halide ion when bonded to a metal?

- 1

Room temperature appearance as element - F₂

Colourless/very pale yellow gas

Room temperature appearance as element - Cl₂

Pale green gas

Room temperature appearance as element - Br₂

Orange brown liquid

Room temperature appearance as element - I₂

Grey solid (sublimes to purple vapour)

Room temperature appearance - water - F₂

Colourless solution

Room temperature appearance in water - Cl₂

Colourless solution

Room temperature appearance in water - Br₂

Yellow solution

Room temperature appearance in water - I₂

Brown solution

Trend - boiling point - G7

The boiling points increase going down the group.

Trend - electronegativity - group

The electronegativity decreases going down the group.

Explanation - boiling point G7

An increase in the size of the molecules, an increase in the number of e⁻s, more chance of a temporary dipole forming, greater Van Der Waals forces between molecules and so the boiling point increases as you go down the group.

Explanation - electronegativity

The number of shells, atomic radius and amount of shielding increases as you go down the group and so there is a decrease in attraction between the nucleus and the electron density in a covalent bond.

Red or ox agents - halogens

Halogen act as oxidising agents.

Trend - oxidising agent strength

The halogens become weaker oxidising agents as you go down the group.

Explanation - Oxidising agent strength - G7

There is an increase in number of electron shells, atomic radius, amount of shielding. Hence there is a decrease in the ability of the nucleus to attract an additional electron.

Halogen displacement reactions - Br₂(aq - from their elements) & Cl⁻(aq - from colourless solution e.g. KX/NaX)

Stays yellow solution (no reaction)

Halogen displacement reactions - I₂(aq) & Cl⁻(aq)

Stays brown solution (no reaction)

Halogen displacement reactions - Cl₂(aq) & Br⁻(aq)

Yellow solution forms (Br₂ is produced). Cl₂ is the oxidising agent.

Halogen displacement reactions - Cl₂(aq) & Br⁻(aq) equation

Cl₂(aq) + 2Br⁻(aq) → 2Cl⁻(aq) + Br₂(aq)

Halogen displacement reactions - Cl₂(aq) & I⁻

Brown solution forms (I₂ is produced). Cl₂ is oxidising agent.

Halogen displacement reactions - Cl₂(aq) & I⁻ equation

Cl₂(aq) + 2I⁻(aq) → I₂(aq) + 2Cl⁻(aq)

Halogen displacement reactions - Br₂(aq) & I⁻(aq)

Brown solution forms (I₂ is produced). Br₂ is oxidising agent.

Halogen displacement reactions - Br₂(aq) & I⁻(aq) half equation

Br₂(aq) + 2I⁻(aq) → I₂(aq) + 2Br⁻(aq)

Halogen displacement reactions - I₂(aq) & Br⁻(aq)

Stays brown solution (no reaction)

Red or ox agents - halide ions

Halide ions act as reducing agents since they donate electrons.

Trend - halide- reducing agents strength

Halide ions become stronger reducing agents going down the group.

Explanation - reducing agents strength

There is an increase in number of electron shells, atomic radius, amount of shielding. There is a decrease in the attraction of the nucleus for outer electrons.Hence there is a increase in the ability of the nucleus to loose an electron.

Why can conc. sulphuric acid be used to illustrate the reducing agent strength?

It is a strong acid and an oxidising agent.

Reactions of halide ions with concentrated sulphuric acid - Chloride ion - observation

White Mistry fumes

Reactions of halide ions with concentrated sulphuric acid - Chloride ion - Product causing the observation

HCl

Reactions of halide ions with concentrated sulphuric acid - Chloride ion - Equation

NaCl + H₂SO₄ → NaHSO₄ + HCl

White MISTY fumes

Reactions of halide ions with concentrated sulphuric acid - Chloride ion - Type of reaction

Acid-base reaction (oxidation state of sulfur doesn't change). H₂SO₄ = proton donor, HCl = proton acceptor (acts as a base).

Reactions of halide ions with concentrated sulphuric acid - Bromide ion 1 - observation

White Mistry fumes

Reactions of halide ions with concentrated sulphuric acid - Bromide ion 1 - Product causing the observation

HBr

Reactions of halide ions with concentrated sulphuric acid - Bromide ion 1 - Equation

NaBr + H₂SO₄ → NaHSO₄ + HBr

Reactions of halide ions with concentrated sulphuric acid - Bromide ion 1 - Type of reaction

Acid-base reaction (oxidation state of sulfur doesn't change). H₂SO₄ = proton donor, HBr = proton acceptor (acts as a base).

Reactions of halide ions with concentrated sulphuric acid - Bromide ion 2 - observation

Brown vapour

Reactions of halide ions with concentrated sulphuric acid - Bromide ion 2 - Product causing the observation

Br₂

Reactions of halide ions with concentrated sulphuric acid - Bromide ion 2 - Equation

2H⁺ + H₂SO₄ + 2e⁻ →SO₂ + 2H₂O

2Br⁻ → Br₂ + 2e⁻

2HBr + H₂SO₄ → Br₂ + SO₂ + 2H₂O

Reactions of halide ions with concentrated sulphuric acid - Bromide ion 2 - Type of reaction

Redox

Br -1 → 0

S +6 → +4

Reactions of halide ions with concentrated sulphuric acid - Iodide ion 2 - observation

Purple vapour

Reactions of halide ions with concentrated sulphuric acid - Iodide ion 2 - Product causing the observation

I₂

Reactions of halide ions with concentrated sulphuric acid - Iodide ion 2 - Equation

2H⁺ + H₂SO₄ + 2e⁻ →SO₂ + 2H₂O

2I⁻ → I₂ + 2e⁻

2HI + H₂SO₄ → I₂ + SO₂ + 2H₂O

Reactions of halide ions with concentrated sulphuric acid - Iodide ion 2 - Type of reaction

Redox

I -1 → 0

S +6 → +4

Reactions of halide ions with concentrated sulphuric acid - Iodide ion 1 - observation

White Mistry fumes

Reactions of halide ions with concentrated sulphuric acid - Iodide ion 1 - Product causing the observation

HI

Reactions of halide ions with concentrated sulphuric acid - Iodide ion 1 - Equation

NaI + H₂SO₄ → NaHSO₄ + HI

Reactions of halide ions with concentrated sulphuric acid - Iodide ion 1 - Type of reaction

Acid-base reaction (oxidation state of sulfur doesn't change). H₂SO₄ = proton donor, HI = proton acceptor (acts as a base).

Reactions of halide ions with concentrated sulphuric acid - Iodide ion 3 - observation

Yellow solid

Reactions of halide ions with concentrated sulphuric acid - Iodide ion 3 - Product causing the observation

S

Reactions of halide ions with concentrated sulphuric acid - Iodide ion 3 - Equation

6H⁺ + H₂SO₄ + 6e⁻ → S + 4H₂O

6I⁻ → 3I₂ + 6e⁻

6HI + H₂SO₄ → 3I₂ + S + 4H₂O

Reactions of halide ions with concentrated sulphuric acid - Iodide ion 3 - Type of reaction

Redox

I -1 → 0

S +6 → 0

Reactions of halide ions with concentrated sulphuric acid - Iodide ion 4 - observation

Gas produced that smells of rotten eggs

Reactions of halide ions with concentrated sulphuric acid - Iodide ion 4 - Product causing the observation

H₂S

Reactions of halide ions with concentrated sulphuric acid - Iodide ion 4 - Equation

8H⁺ + H₂SO₄ + 8e⁻ → H₂S + 4H₂O

8I⁻ → 4I₂ + 8e⁻

8HI + H₂SO₄ → 4I₂ + H₂S + 4H₂O

Reactions of halide ions with concentrated sulphuric acid - Iodide ion 4 - Type of reaction

Redox

I -1 → 0

S +6 → -2

Why do you have to do tests to identify halide ions?

They have similar visual appearances of white solids and colourless solutions.

Test for identifying halides - steps

Add silver nitrate solution (AgNO₃ (aq)) acified with nitric acid and then test the solubility in NH₃ conc/dilute.

Test for identifying halides - ppt - fluoride ion - observation

Colourless solution

Test for identifying halides - ppt - fluoride ion - equation

No reaction.

Test for identifying halides - ppt - Chloride ion - observation

White ppt

Test for identifying halides - ppt - Chloride ion - equation

Ag⁺(aq) + Cl⁻(aq) → AgCl(s)

Test for identifying halides - ppt - Bromide ion - observation

Cream ppt

Test for identifying halides - ppt - Bromide ion - equation

Ag⁺(aq) + Br⁻(aq) → AgBr(s)

Test for identifying halides - ppt - Iodide ion - observation

Yellow ppt

Test for identifying halides - ppt - Iodide ion - equation

Ag⁺(aq) + I⁻(aq) → AgI(s)

Dilute NH₃ solubility - AgCl

Soluble

Dilute NH₃ solubility - AgBr

Insoluble

Dilute NH₃ solubility - AgI

Insoluble

Concentrated NH₃ solubility - AgCl

Soluble

Concentrated NH₃ solubility - AgBr

Soluble

Concentrated NH₃ solubility - AgI

Insoluble

Why is dilute nitric acid added - testing for halides?

Some anions, other than halides, also produce ppts with silver nitrate (e.g carbonate ions which makes Ag₂CO₃(s)). Nitric acid reacts away there carbonate ions (you can't use HCl as it contains Cl ions).

Dilute nitric acid reaction with carbonate ions

CO₃²⁻(aq) + 2H⁺ → CO₂(g) + H₂O(l)

Reaction of chlorine with water cold & hot - equation

Cl₂(aq) + H₂O(l) → HCl(aq) + HClO(aq)

&

2Cl₂(aq) + H₂O(l) → O₂(aq) + 4HCl(aq)

Chloric (1) acid

HClO(aq)

Reaction of chlorine with water - indicator

Full range indicator starts red (from H⁺ ions) and looses its colour (from ClO⁻ ions acting as bleach)

Reaction of chlorine with water - reaction type

Disproportionation reaction.

Why is chlorine added to drinking water?

As it sterilises the water by killing bacteria. However it can only be added in small amounts because Cl is toxic but the health benefits of sterilised water far outweigh the risk of toxic effects.

Reaction of chlorine with sodium hydroxide - equation

Cl₂(aq) + 2NaOH(aq) → NaCl(aq) + NaClO(aq) + H₂O(l)

Sodium chlorate (1)

NaClO(aq)

Reaction of chlorine with sodium hydroxide - reaction type

Disproportionation reaction.

Reaction of chlorine with sodium hydroxide - uses of product

NaClO(aq) produced is used as the active ingredient in bleach.