Group 7- The Halogens

What is the trend in boiling point down the group?

• Increases

• More electrons

• Therefore stronger van der Waals forces

What is the trend in reactivity down the group?

• Decreases

• Atomic radius increases *more energy levels

• Weaker attraction between positive nucleus and outer energy level due to increased distance

• Therefore harder to attractive force pulling an electron in

What does trend in reactivity tell us about the oxidising ability (as oxidising agents) of each halogen?

• Oxidation is the loss of electrons

• Oxidising agents cause another species to be oxidised and are themselves reduced

• This means that fluorine and chlorine will be the strongest oxidising agents because halogens want to gain an electron upon reaction, causing the species they react with to be oxidised

Describe and explain halogen displacement reactions, including displacement equations

• Chlorine, bromine and iodine as diatomic molecules in solution are pale green, orange and brown respectively

• When a more reactive halogen reacts with a less reactive halogen in solution, a reaction takes place

• Chlorine is more reactive than bromine and iodine so it displaces them and cannot be displaced by them

  • When added to bromide ions in solution, the colour changes from pale green to brown- the colour change is caused by the diatomic molecule in solution

• Bromine is more reactive that iodine but less reactive then chlorine so it cannot be displaced by iodine but can be displaced by chlorine

• Iodine is the least reactive so it cannot displace either of them and will be displaced by both of them

What is the ionic and overall equation for the reaction of chlorine with water?

Ionic equation: Cl₂ + H₂O ⇌ 2H⁺ + Cl^- + ClO^-

Overall equation: Cl₂ + H₂O ⇌ HCl + HClO

How is the reaction of chlorine with water used?

• Used in swimming pools and drinking water- halogens are good antimicrobial agents

• Chlorine is used in water supplies as it is toxic to bacteria/microbes

• Adding it to water supplies is beneficial for the population

Why is chlorine which is toxic used in drinking water?

• Despite the risks, the benefits of adding chlorine to our drinking water outweighs the risks

• It provides safe drinking water and the chlorine is in too low concentrations to be harmful to humans, but is enough for bacteria

What is the reaction of chlorine with water in direct sunlight?

2Cl₂ + 2H₂O → 4HCl + O₂

Give the symbol equation for the reaction of chlorine with cold, aqueous sodium hydroxide

2NaOH _(aq) + Cl_{2 (g)} → NaClO _(aq) + NaCl _(aq) + H₂O _(l)

What is the reaction of chlorine with cold, aqueous sodium hydroxide used for?

• Household bleach commonly contains the chlorate (I) ion in the form of sodium chlorate (I), formed when chlorine reacts with cold, aqueous sodium hydroxide

• Bleach acts as an oxidising agent against organic compounds in food stains, bacteria and dyes

What are the reactions of chlorine with water and cold, aqueous sodium hydroxide examples of?

Disproportionation reactions

What are reducing agents?

• The cause other species to be reduced and are themselves oxidised

• Electron donors

What is the trend in reducing ability of the halogens?

• Reducing ability increases as you go down the group

• Chlorine is the weakest reducing agent and iodine is the strongest

• This is because Iodine’s ionic radius is the greatest and there is the greatest shielding between outer electrons and the positive nucleus due to more energy levels

• This means it is easier to lose and electron, making iodine the strongest reducing agent

What test can we do to see the difference in reducing ability between the halogens?

Reactions of X^- ions with concentrated sulfuric acid

Give the equation for the reaction between chloride ions in potassium chloride and concentrated sulfuric acid and any other resulting reactions. Then state what tests can be done to test the reducing ability of chloride ions.

KCl _(s) + H₂SO_{4 (aq)} → KHSO_{4 (aq)} + HCl _(aq)

• Fizzing- gas formed

• Universal indicator turns red when hovering over solution- acidic gas given off

• Mistry white fumes when ammonia is held at the mouth of test tube- HX gas is produced (HCl)

• Oxidation states tell us this is not a redox reaction

Give the equation for the reaction between bromide ions in potassium bromide and concentrated sulfuric acid and any other resulting reactions. Then state what tests can be done to test the reducing ability of bromide ions.

KBr _(s) + H₂SO_{4 (aq)} → KHSO_{4 (aq)} + HI _(aq)

2HBr (g) + H₂SO₄ (aq)→ SO₂ (g)+ Br₂ (g) + 2H₂O (l)

• Fizzing- gas formed

• Universal indicator turns red when hovering over solution- acidic gas given off

• Mistry white fumes when ammonia is held at the mouth of test tube- HX gas has been produced (HBr)

• Reacts with K₂Cr₂O₇/H⁺ paper (potassium dichromate paper) and its turns from orange to green - SO₂ gas produced:

• Orange gas produced (Br₂)

• This is a redox reaction

Why is SO₂ hazardous?

It’s a choking gas

Give the equation for the reaction between iodide ions in potassium iodide and concentrated sulfuric acid and any other resulting reactions. Then state what tests can be done to test the reducing ability of iodide ions.

KI _(s) + H₂SO_{4 (aq)} → KHSO_{4 (aq)} + HI _(aq)

HI (g) + H₂SO₄ (aq) → SO₂ (g) + HI (g)

6HI (g) + H₂SO_{4 (aq)} → S (s) + 3I₂ (g) + 4H₂O (l)

8HI (g) + H₂SO_{4 (aq)} → H₂S (g) + 4H₂O (l) + 4I_{2 (g)}

• Fizzing- gas formed

• Universal indicator turns red when hovering over solution- acidic gas given off

• Mistry white fumes when ammonia is held at the mouth of test tube- HX gas has been produced (HBr)

• Reacts with K₂Cr₂O₇/H⁺ paper (potassium dichromate paper) and it turns from orange to green- SO₂ gas produced

• Purple gas produced (I₂)

• Yellow solid produced (sulfur)

• Lead acetate paper turns black- H₂S gas produced

• This is a redox reaction

How can we test for halide ions?

• Add acidified silver nitrate

• F^- stays colourless

• Cl^- forms a white precipitate

• Br^- forms a cream precipitate

• I^- forms a yellow precipitate

Why is silver nitrate acidified when testing for halide ions?

• Add acidified silver nitrate- acidified allows us to get rid of ions causing false positives like CO₃^- and OH^- ions

  • AgOH masks other results

  • Ag₂CO₃ forms a cream precipitate that could be mistaken for bromine

How can we differentiate between bromide and iodide ions when tested for with acidified silver nitrate?

• Add dilute ammonia

  • Silver chloride ions turn colourless, but silver bromide and iodide ions have no visible change

• Add concentrated ammonia

  • Silver bromide ions turn colourless but silver iodide shows no visible change

What is the ionic equation for the reaction between halide ions and silver nitrate?

Ag⁺ (aq)+ X^- (aq)→ AgX (s)

What is the reaction for the addition of ammonia to silver halides?

AgX (s) + 2NH₃ (aq) → [Ag(NH₃)]X^- (aq)

How can we test for ammonium (NH₄⁺) ions and what is the ionic equation for this?

• Add NaOH and warm (in a water bath)

• Hold damp red litmus paper over mouth of test tube

• Red litmus paper turns blue

• NH₄⁺ + OH^- → NH₃ + H₂O

• NH₃ is an alkaline gas