chapter 10: group 7 the halogens

physical properties

• swimming pool smell

• astatine is radioactive and rare

• exist as diatomic molecules

colors of the halogens

fluorine = pale yellow

chlorine = green

bromine = red-brown

iodine = black solid or purple gas

states of the halogens at room temperature

fluorine = gas

chlorine = gas

bromine = liquid

iodine = solid

astatine = solid

halogens react with metals

• form salts and their names change

• bromine → bromide

• + ide to the name

• halogen means salt former

why is fluorine an exception to the general trends going down group 17/7 ?

the F-F covalent bond is very weak

this leads to repulsion because of its very small size

this repulsion between the non bonding electrons is because they are very close to each other

compared to other halogens

general trends in group 17/7 going down

• electronegativity decreases

• atomic radius/size of atoms increases

• melting/boiling point increases

• bond energ/ energy required to break covalent bonds decreases going down

why does the atomic radius increase going down?

• there is one extra filled main shell

• so the shielding increases going down

why does the electronegativity decrease going down?

• the bonding outer shell electron get farther away

• increased shielding

• nuclear charge increases

  distance between outer bonding shell electron is more important than nuclear charge

why do melting/boiling points increase going down?

• atomic radius increases → gets larger

• more electrons → more/stronger van der waal’s forces

  intermolecular forces involved

trends in oxidising ability of halogens

going down the oxidising ability decreases

there fore F is th strongest oxidising agent

what does OILRIG stand for

Oxidation

Is

Loss

Reduction

Is

Gain

displacement

when a more reactive halide replaces a less reactive one

example of a displacement reaction

Cl2 + 2NaBr→ Br2 + NaCl

bromine is formed so the colourless salt turns yellow/brown

the more reactive halogen is the oxidising agent

why is bromine extracted from sea water

trace amounts of bromide ions irritate the body and bromine gas is highly toxic

half equation for extracting bromide ions from sea water

Cl2 + 2Br→ Br2 + 2Cl-

ionic equation for extracting iodide ions from kelp to produce iodine

2I- + MnO2 + 4H+ → Mn2+ + 2H2O + I2

general trend of halide ions as reducing agents

going down the halide ions are better reducing agents

why does the reducing ability of halide ions increase going down

• atomic radius increases

• weaker electrostatic force of attraction between nucleus and valence shell electron

sodium choride + sulfuric acid observations

• oxidation state stays the same

• weak oxidising agent

• steamy fumes

sodium chloride + sulfuric acid symbol equation

NaCl(s) + H2SO4(aq) → NaHSO4 (s)+ HCl(g)

sodium bromide + sulfuric acid obsevations

• steamy fumes

• brown fumes of bromine

• sulfur dioxide gas

• stronger than chlorine so reduces sulfur dioxide and changes the oxidation state

sodium bromide + sulfuric acide symbol equation

NaBr (s) + H2SO4(aq) → NaHSO4(s) + HBr (s)

sodium iodide + sulfuric acid observations

• very strong reducing agent

• steamy fumes

• black solide

• smells like bad eggs

sodium iodide + sulfuric acide symbol equation

NaI + H2SO4 → NaHSO4 + HI

8HI + H2SO4 → I2 + H2S + 4H2O

postive test results for halide ions

fluoride = no precipiate

chloride = white precipitate

bromide = cream precipitate

iodide = yellow precipitate

further test results for hallide ions

chloride = dissolves in dilute ammonia

bromide = dissolves in concentrated ammonia

iodide = dissolve in concentrated ammonia

why don’t fluoride ions produce preciptates with silver nitrate

silver fluoride is soluble in water

why is nitric acid added before silver nitrate to a metall hallide

to remove any soluble carbonate

reacts to give a false positive result such as silver carbonate

2Ag + CO3 → Ag2CO3

anion test for carbonates

test: dilute HCl

observation: bubble gas through lime water turns milky/white

anion test for sulfates

test: dilute HCl and barium chloride

observation: white precipitate

anion test for hydroxides

universal indicator

turns purple/blue

or

warm an ammonium salt

ammonia gas is formed

reaction of chlorine in the presence of sunlight

Cl2​+H2​O⇌HCl+HOCl

goes through disporportionation

reaction of chlorine in the absence of sunlight

2Cl2(g) + 2H2O(l) → 4HCl(aq) +O2(g)

disproportionation

• a type of redox reaction where the oxidation state of atoms of the same elements increase and others decrease

uses of chlorine in water

• purifying it

• kill bacteria

• kills diseases in swimming pools

• health benefits outweigh the risks even though its toxic

why should chlorine be constantly added to swimming pools

• chlorine gets lost from the sunlight

• so to maintain equilibrium swimming pools are slightly acidic

how does chlorine react with alkaline solutions

• forms bleach

  Cl2+ 2NaOH → NaClO + Nacl + H20