3.3 the halogens Flashcards
fluorine
pale yellow gas
chlorine
pale green gas
bromine
brown-orange liquid
iodine
grey solid
boiling point trend down group 7
boiling point increases down group 7
London forces increase due to increasing size and relative mass
electronegativity trend down group 7
electronegativity decreases down group 7
atoms get larger
distance between nucleus and bonding electrons increases
also more shielding
what is electronegativity?
the ability for an atom to attract electrons towards itself in a covalent bond
what is the use of organic solvent?
to see colour changes easily
halogen present dissolves readily in organic solvent which forms a layer above aqueous layer
e.g. hexane
what happens in group 7 displacement reactions?
more reactive halogens displace less reactive halide ions
reactivity trends down group 7
reactivity decreases down group 7
smaller atoms attract electrons better than larger atoms
halogens are also less oxidising as we move down group 7
organic solvent colours
Br2 = orange
I2 = purple
test for halides
add dilute nitric acid (HNO3) then silver nitrate solution (AgNO3)
*confirm with ammonia solution
why do we add nitric acid first for halide testing?
nitric acid reacts with any other anions other than halides (e.g. carbonates) to avoid false results
results for further halide test using ammonia (NH3)
Cl- white ppt dissolves in dilute NH3
Br- cream ppt dissolves in conc NH3
I- yellow ppt is insoluble in conc NH3
what reaction is bleach made via?
disproportionation reaction
chlorine has been simultaneously reduced and oxidised
bleach formation
mixing chlorine and sodium hydroxide will form sodium chlorate(I) solution - also know as bleach
bleach equation
2NaOH(aq) + Cl2(g) -> NaClO(aq) + NaCl(aq) + H2O(l)
uses of sodium chlorate(I)
NaClO (bleach)
- treating water
- bleaching paper and fabrics
- cleaning agents (bleach)
water sterilisation
adding chlorine to water can kill bacteria in the water
produces chlorate ions (ClO-) which kill bacteria
useful in drinking water and pools
equation for adding Cl2 to water
H2O(l) + Cl2(g) <–> HCl(aq) + HClO(aq)
DISPROPORTIONATION reaction
Chloric(I) acid ionises to make chlorate(I) ions (hypochlorite ions)
HClO(aq) + H2O(l) <–> ClO-(aq) + H3O+ (aq)
advantages of chlorinating drinking water
- destroys microorganisms that cause disease
- long lasting so reduces bacteria build up
- reduces growth of algae that discolours water and can give it a bad smell/taste
disadvantages of chlorinating drinking water
- chloring gas is toxic and irritates the respiratory system
- liquid chloring causes severe chemical burns to the skin
- chloring can react with organic compounds present in water to make chloroalkanes which have been linked to causing cancer
risk of not chlorinating water
lead to cholera epidemic
ethical consideration of chlorinating drinking water
chlorination of water is mandatory across UK, some object to this claiming it is forced medication of a whole population
alternatives to chlorination of water
- ozone - powerful oxidising agent that kills microorganisms but has a short half life so treatment is not permanent and is expensive to produce
- UV light - damages DNA in microorganisms however ineffective in cloudy water and does not prevent water becoming contaminated further down the process
test for ammonium
- add warm sodium hydroxide (NaOH), if ammonium compound is present, ammonia gas will be produced
- used damp red litmus paper, turns blue when ammonia dissolves in water
test for hydroxides
turn red litmus paper blue
hydroxides are alkaline
(NOT A PURE TEST)
test for carbonates
add acid (HCl) which reacts with carbonate to produce CO2 gas
when CO2 gas is bubbled through limewater, it turns cloudy
test for sulfates
add HCl to remove carbonates
add barium chloride (BaCl2)
white precipitate if sulfates are present
Ba 2+(aq) + SO4 2-(aq) -> BaSO4(s)
order of ion tests
- carbonates (carbon dioxide gas produced)
- sulfates (barium sulfate precipitate)
- halides (white, cream, yellow precipitate)
