Group 7: Halide Ions

Define reducing agent

An electron donor

What reaction can be used to measure the extent halide ions can act as reducing agents?

Reaction of halide ions with concentrated sulfuric acids

What are the 4 possible products if the reaction with halide ions and sulfuric acid. Give them in order of oxidation number (highest to lowest)

• H₂SO₄

• SO₂

• S

• H₂S

What is sulfurs oxidation number in H₂SO₄ (Sulfuric acid)?

+6

What is sulfurs oxidation number in SO₂ (Sulfur dioxide)? Describe its appearance

• +4

• Choking acidic gas

What is sulfurs oxidation number in S (Sulfur? Describe the appearance

• 0

• Yellow solid

What is sulfurs oxidation number in H₂S (Hydrogen sulfide)? Describe its appearance

• -2

• Gas with rotten egg smell

Describe the trend and explain the ability of halide ions acting as reducing agents down group 7

• Increases

• More shells and more shielding

• Attraction between nucleus and electron being donated is less

• Easier to lose electron

What are the 2 types of reactions that can occur when a solid halide salt reacts with concentrated sulfuric acid?

• Acid-base reaction

• Redox reaction

What does concentrated sulfuric acid act as in an acid-base reaction?

An acid

What does concentrated sulfuric acid act as in a redox reaction?

Oxidising agent

What is the observation for NaCl and concentrated H₂SO₄ in an acid-base reaction? What causes this observation?

• Steamy white fumes

• HCl

What is the observation for NaBr and concentrated H₂SO₄ in an acid-base reaction? What causes this observation?

• Steamy white fumes

• HBr

What is the observation for NaI and concentrated H₂SO₄ in an acid-base reaction? What causes this observation?

• Steamy white fumes

• HI

What is the observation for NaCl and concentrated H₂SO₄ in a redox reaction (Both oxidation and reduction product)? What causes this observation?

• No observation

• As Cl^- can not reduce sulfuric acid

• Chloride ions are weak reducing agents

What is the observation for NaBr and concentrated H₂SO₄ in a redox reaction (Both oxidation and reduction product)? What causes this observation?

• Oxidation product → Orange fumes (Br₂)

• Reduction product → Choking gas (SO₂)

What is the observation for NaI and concentrated H₂SO₄ in a redox reaction (Both oxidation and reduction products)? What causes this observation?

• Oxidation product → Black solid and purple vapours (I₂)

• Reduction products → Choking gas (SO₂), Yellow solid (S), Bad egg smell (H₂S)

What halogens can react with conc sulfuric acid to form SO₂?

• Bromine

• Iodine

What is the reaction of NaF or NaCl with H₂SO₄?

• NaF_(s) + H₂SO_4(l) → NaHSO_4(s) + HF_(g)

• NaCl _(s) + H₂SO_4(l) → NaHSO_4(s) + HCl _(g)

What are the 2 reactions of NaBr with H₂SO₄?

1. NaBr_(s) + H₂SO_4(l) → NaHSO_4(s) + HBr_(g)

2. 2HBr_(aq) + H₂SO_4(l) → Br_2(g) + SO_2(g) + 2H₂O_(l)

What are the 4 reactions of NaI with H₂SO₄?

1. NaI_(s) + H₂SO_4(l) → NaHSO_4(s) + HI_(g)

2. 2HI_(g) + H₂SO_4(l) → I_2(s) + SO_2(g) + 2H₂O_(l)

3. 6HI_(g) + H₂SO_4(l) → 3I_2(s) + S_(s) + 4H₂O_(l)

4. 8HI_(g) + H₂SO_4(l) → H₂S_(g) + 4I_2(s) + 4H₂O_(l)

Disproportionation

A redox reaction in which the same element is both oxidised and reduced

HCl formation observations

Damp blue litmus paper turns red

HClO formation observations

Damp litmus paper is bleached

Name a benefit and risk of using chlorine in water treatment

Benefit → Kills bacteria

Risk → Toxic in large quantities

Reaction of chlorine with cold dilute sodium hydroxide

Cl₂ + 2NaOH → NaCl + NaClO + H₂O

Main uses of this solution: Cl₂ + 2NaOH → NaCl + NaClO + H₂O

• As a bleach

• To kill bacteria

Describe test for halide ions

1. Add dilute nitric acid (To remove other ions which may interfere with the test)

2. Add silver nitrate solution

3. Observe precipitate formed

Why can you not test for F- ions with silver nitrate solution?

Silver Fluoride (AgF) is soluble in water

How do you distinguish between silver halides if colours are too similar?

Add dilute ammonia solution

• Silver chloride is soluble

• Silver bromide is insoluble

• Silver iodide is insoluble

Add concentrated ammonia solution

• Silver chloride is soluble

• Silver bromide is soluble

• Silver iodide is insoluble