Group 7- the halogens

Trend in electronegativity

Decreases down the group, as the distance between the nucleus and outer shell increases is shielding increases and the ability of an atom to attract a shared pair of electrons to the nucleus decreases

Trend in boiling point

Increases down the group, as the size of the molecule increases so there are stronger van de wall forces between the molecules, which need more energy to overcome

Trend in electronegativity

Decreases down the group, as the distance between the nucleus and outer shell increases is shielding increases and the ability of an atom to attract a shared pair of electrons to the nucleus decreases

Trend in boiling point

Increases down the group, as the size of the molecule increases so there are stronger van de wall forces between the molecules, which need more energy to overcome

Trend in reducing ability of halide ions

Reducing power increases as you go down the group, the ability ti lose an electron depends on shielding and atomic radius: atomic radius increases and the electrons are further away from the nucleus so it is easier for the ion to lose an electron and reduce another substance

How do sodium fluoride and chloride react with sulphuric acid?

• HF or HCl gas forms- misty white fumes

• Not strong enough to reduce H₂SO₄

How does sodium bromide react with sulphuric acid?

• Redox reaction

• Steamy brown fumes of hydrogen bromide

• Sulphur dioxide: colourless gas

• Bromide ions reduce sulphuric acid to sulphur dioxide

• Bromine ions are themselves oxidises to bromine gas

• H₂SO₄  +  2H⁺  +  2Br^-  

  

   Br₂  +   SO₂  +  2H₂O

How does sodium iodide react with sulphuric acid?

• Iodide ions are a very strong reducing agent, and will reduce the sulphuric acid to sulphur dioxide, then sulphur and then hydrogen sulfide

• Iodide ions are oxidised to iodine

• Steamy fumes of hydrogen iodide

• Hydrogen sulphide is toxic and has a bad smell

How can acidified silver nitrate solution be used to identify halide ions?

Precipitate of silver halide will form, and the colour helps us distinguish the halide ion present

Why must the silver nitrate solution be acidified?

Removes any excess ions which mask the results e.g. also form a coloured precipitate

What colour precipitate does silver fluoride form?

no ppt

What colour precipitate does silver chloride form?

white ppt

What colour precipitate does silver bromide form?

cream ppt

What colour precipitate does silver iodide form?

yellow ppt

why is ammonia solution added?

each silver halide has a different solubility in the ammonia solution, so can help with identification

trend in solubility of the silver halides in ammonia

Decreases down group

what does silver chloride do in ammonia?

white ppt dissolves in dilute ammonia

what does silver bromide do in ammonia?

cream ppt dissolves in concentrated ammonia

what does silver iodide do in ammonia?

insoluble in concentrated ammonia

What is the reaction of chlorine with water?

Cl₂ + H2O turns to 2H + Cl^- + ClO^- (chloride and chlorate ions form)

What happens when chlorine reacts with water in sunlight?

2Cl₂ + 2H2O turns to 4H + 4Cl^- + O₂

Chlorine in water treatment

Chlorate ions kill bacteria/ pathogens in water so it is swimable and drinkable

Advantage of chlorine use in water

• Kills bacteria and pathogens

• Inhibits algal growth and removes smells/ tastes

• Remains in water so no reinfection

Disadvantages of chlorine in water

• toxic and harmful: can cause respiratory problems if inhales

• can cause chemical burns

• chlorinated hydrocarbons cause cancer

Reaction of chlorine with cold, dilute, aqueous Na

Cl₂ + 2NaOH turns to NaClO + NaCl + H₂O

Used to kill bacteria in houses and water