GROUP 7

properties of halogens

fluorine gas

pale yellow

chlorine gas

green

bromine liquid

red-brown

iodine solid

grey

boiling points INCREASE down group 7

because of increasing strength of van der waals forces as the size and relative mass of molecules increase

electronegativity DECREASES down group 7

larger atoms attracts electrons less than smaller ones because their valent electron is further from the nucleus and there’s more shielding

when halogens react they gain electrons

making them oxidising agents

going down group 7 halogens become less oxidising

because atoms get larger n distance increases

fluoride displaces

all halide ions

chlorine displaces

bromine (forms orange solution ) and iodine (forms brown solution)

bromide displaces

iodine forms brown solution

halide ions are colourless in solutions

but when the halogen is displaced it shows a distinctive colour

MIXING CHLORINE GAS W COLD DILUTE SODIUM HYDROXIDE AT ROOM TEMP

produces sodium chlorate NaClO, aka bleach in a disproportionation reaction

chlorine is oxidised and reduced

sodium chlorate uses

• water treatment

• bleach paper and textiles

• good product

chlorine mixed w water undergoes disproportionation again

to ptoduce a chloride ion and chlorate ion

in sunlight chlorine can also decompose water to form chloride ions and oxygen

chlorate ions kill bacteria

adding chlorine to water can make it safe to drink or swim in, however chlorine is toxic

chlorine kills disease causing microorganisms and prevents reinfection

it also prevents the growth of algae eliminating bad tastes and smells and removes discolouration caused by organic compounds

chlorine has is very harmful if breathed in

it irritates the respritory system

liquid chlorine on the skin or eyes causes severe chemicla burns

accidents involving chlorine could be serious and even fatal

water contains a variety of organic compounds like decomposed plants

chlorine reacts w these compounds to form chlorinated hydrocarbons which are carcionogenic

however this increased cancer risk is small compared to the risks from untreated water and cholera epidemic

we weight the risk and benefits when making decisions ab what chemicals to add to drinking water supplies

in reactions halogens gain 1 electron

to form halide ions

halides in reactions get oxidised (lose their extra electron)

they are reducing agents

going down group 7

reducing power of halide increases

as you go down group 7 the attraction gets weaker between valent electron and nucleus

because the ions get bigger so electrons are further away from positive nucleus with extra shells in the way

All halide react w conc sulfuric acid to give a hydrogen halide

NaX + H2SO4 → NaHSO4 + HX

some halides are strong enough reducing agents to reduce sulfuric acid to water and sulfur dioxide

2HX + H2SO4 → X2 + SO2 + 2H2O

iodine is such a strong reducing agent is can reduce SO2 into H2S or S

6HI + SO2 → H2S + 3I + 2H20

halogens have very distinctive colours

halides are all colourless

use silver nitrate to test for halides

• add a few drops of dilute nitric acid to remove ions which might interfear w the test

• add a few drops of silver nitrate solution (AgNO3)

• a precipitate is formed or silver halide

white precipitate

chloride ion

cream precipitate

bromide ion

yellow precipitate

iodide ion

adding ammonia solution to each precipitate mixture

indicates the different solubilities of the silver halides formed

white precipitate + dilute ammonia

dissolves → chloride ion confirmed

cream precipitate + conc ammonia

dissolves → bromide ion confirmed

yellow precipitate + conc ammonia

DOESNT DISSOLVE → iodide ion confirmed

group 2 + flame

distinct colour

flame test:

1. dis cichrome wire loop in conc HCL to clean

2. then dip in unknown compound

3. hold loop in the clear blue part of a bunsen burner flame and observe colour change

Ca 2+ flame

brick red

Sr2+ flame

red

Ba2+ flame

pale green

NaOH identifies group 2 ions

observe the rpecipitate that formes from excess NaOH + metal ion solution

Mg 2+

• OH- = slight white ppt

• excess OH- = white ppt

Ca2+

• OH- =slight white ppt

• excess OH- = slight white ppt

Sr 2+

• OH- = slight white ppt

• excess OH- =slight white ppt

Ba 2+

• OH- = no change

• excess OH- = no change

NH3 is an alkaline

using a damp red litmus so ammonia gas can dissolve will turn it blue

OH- + NH4+ → ammonia gas and water

to test for ammonia in a substance w ammonium ions:

• add dilute NaOH to substance

• gently heat mixture

• hold a damp red litmus over test tube

• if it changes to blue ammonia gas has been given off

test for sulfate ions

• add dilute HCl + barium chloride (BaCl2) to substance

• if white precipitate forms = barium sulfate

• therefore origional compound contains sulfate

test for hydroxide ions:

• hydroxide ions make a solution alkaline

• add ph indicator (eg red litmus will turn blue)

test for halide ions:

• add dilute nitric acid (HNO3) then silver nitrate solution AgNO3

• chloride = white ppt

• bromide = cream ppt

• iodide = yellow ppt

to differenciate between the similar silver halide precipitate colours add ammonia solution

• silver chloride + dilute ammonia = dissolves

• silver bromide + conc ammonia = dissolves

• silver iodide + conc ammonia = no change

test for carbonate (CO₃ ^2-) ion

• add dilute hydrochloric acid

• solution will fizz, CO2 produced

• CO₃ ^2- + 2H⁺ → CO₂ + H₂O

• you can test for the CO2 using lime water, if it turns cloudy CO2 is present