Topic 2.3 - Group 7

What is the trend in bpt

down group 7? Why?

Increases down the group

Because: size of atom increases as more occupied electron

shells → stronger van der Waals forces of attraction between

molecules, take more energy to break

What is the trend in

electronegativity down

group 7? Why?

Decreases

Because: more occupied electron shells → greater atomic

radius and outer electrons are further from the positive

charge of the nucleus → lower force of attraction between the

nucleus and electron pair in the covalent bond

What do you use to test for

halide ions?

Acidified AgNO

Why do you add HNO 3 ?

Why not HCl?

To remove CO₃2-

Adding HCl would add Cl- ions, giving a false

positive result

Result and equation for Cl-

test?

white ppt

Ag + + Cl - → AgCl (s)

What is the result and

equation for the test for Br -

?

Cream ppt

Ag + + Br - → AgBr (s)

What is the result and

equation for the test for I -

?

Yellow ppt

Ag + + I - → AgI (s)

What happens (+ equations)

to each of the silver halide

precipitates when

dilute/conc NH 3 are added?

AgCl- dissolves in both dilute and conc

AgCl (s) + 2NH₃ (aq) → [Ag(NH₃)₂] + (aq) + Cl -

AgBr- only dissolves in conc

AgBr (s) + 2NH₃ (aq) → [Ag(NH₃)₂] + (aq) + Br -

AgI- will not dissolve in either

What is the trend in

oxidising ability down the

group? Why?

Decreases down group (Cl best, I worst)

Because: Cl has fewest occupied electron shells, greatest

force of attraction between outer electrons and nucleus,

easiest to gain electrons and be reduced → best oxidising

agent

Write the equation for Cl 2

oxidising 2I -

Cl₂ + 2I - → 2Cl - + I₂

What is the trend in

reducing ability of the

halides down the group?

Why?

Increases down the group (Cl - worst, I - best)

Because: I - has the most occupied electron shells, so outer

electrons are further from the nucleus, weakest force of

attraction between outer electrons and positive charge of

nucleus → easiest to be oxidised and lose electrons → best

reducing agent

What products are formed

when I - reduces H 2 SO 4 ? Do

equations for all 4

H₂SO₄ + 2I - → SO₄2- + 2HI

H₂SO₄ + 2H + + 2I - → SO₂ + I₂ + 2H₂O (SO 2 is a choking gas with a pungent odour)

H₂SO₄ + 6H + + 6I - → S + 3I₂ + 4H₂O (S is a yellow solid)

H₂SO₄ + 8H + + 8I - → H₂S + 4I₂ + 4H₂O (H 2 S smells of bad/rotten eggs)

What are the products of Br -

+ H 2 SO 4 ?

HBr and SO₂

Does Cl - reduce H 2 SO 4 ?

No, not a powerful enough reducing agent; only

HCl is formed

Why is chlorine added to

drinking water? Why is it

safe?

Forms ClO - ions which oxidise (kill) all

microorganisms in water

Once it has done its job, little remains, and the

health benefits outweigh the risks of using it

What are potential risks of

adding chlorine to drinking

water?

Chlorine is toxic and damages the respiratory

system in large enough quantities; can form

carcinogens with hydrocarbons

Why is ozone not used to

purify water in the UK?

More expensive than chlorine, evaporates from

water more quickly

What is the equation for the

reaction of Cl 2 with water?

Cl₂ (g) + H₂O (l) → HClO (aq) + HCl (aq)

What type of reaction is the

reaction of chlorine with

water?

Disproportionation; chlorine is both oxidised and

reduced

What are the two forms of

the chlorate ion?

ClO - is chlorate (I)

ClO₃- is chlorate (V

What is the equation for

making bleach?

Cl₂ + 2NaOH → NaCl + NaClO + H₂O

NaClO is bleach

Give the equation for the

reaction of chlorine and

water in the presence of

sunlight

2Cl₂ + 2H₂O → 4HCl + O₂ (g)

What is desalination?

Converts saltwater into clean, potable water

Either by reverse osmosis (using a smart

membrane) or by vacuum distillation at low

pressure and low temperature

What are the advantages

and disadvantages of

desalination?

Advantages - safe, clean, drinkable water produced in places

where it might not otherwise be available

Disadvantages - uses lots of energy, reverse osmosis has

low efficiency, can disturb marine ecosystems