Chapter 8 Reactivity Trends Flashcards
How do halogens exist and boiling point trends
They exist as diatomic molecules at RTP
F2 - pale yellow gas
CL2 - Pale green gas
BR2 - Red-brown liquid
I2 - Shiny grey-black solid
AT2 - never been seen before
Their boiling points increase as you go down the group
Trends as you go down the group in general
more electrons
stronger london forces
more energy required to break the intermolecular forces
boiling point increases
Halogen - halide displacement reaction
How it is done
a solution of each halogen is added to aqueous solutions of the other halides.
If the halogen added is more reactiove than the halide present, a reaction takes place, the halogen displacing the halide from solution, and the solution changes colour.
Halogen - Halide displacement reactions results.
in water and cyclohexane
CL2 - pale green
BR2 - Orange
I2 - Brown
it is difficult to tell between the bromine and Iodine so you will have to add cyclohexane.
chlorine stays pale green
bromine goes orange
iodine goes violet
Trends in reactivity in halogens
Atomic radius increases down the group
more inner shells so shielding increases
less nuclear attraction to capture an electron from another species.
reactivity decreases down the groups
Disproportionation
is a redox reaction when the same element is oxidised and reduced.
This includes the reaction of chlorine with water and the reaction of chlorine with cold, dilute aqueous sodium hydroxide.
The reaction of chlorine with water
small amounts of chlorine added to water,
oxidation of chlorine into the HClO and reduction of chlorine into HCl.
Products are both acids.
HClO - chloric acid acts as a weak bleach. Proven by adding some indicator solution, it will turn red and then disappear because of bleaching.
The reaction of chlorine with cold, dilute sodium hydroxide
forms NaClO and NaCl and H2O.
Chlorine is oxidised into NaClO and reduced into NaCl
This is the main component of bleach
Benefits and risks of chlorine use
chlorine is a toxic gas
respiratory irritant and large concentrations can be fatal
chlorinated hydrocarbons are suspected of causing cancer.
Test for halide ions and their solubility
Precipitation reactions with aqueous silver ions.
Ag+ + X- -> AgX
Since the precipitates are all quite similar in colour, you add aqueous ammonia to differentiate between them. This will cause some to precipitate and some to stay the same.
Silver chloride is white soluble in dilute ammonia
Silver bromide is cream-coloured soluble in conc ammonia
silver iodide is yellow insoluble in conc ammonia
Test for carbonate
React the compound with nitric acid.
You will see bubbles which could be CO2.
To prove it is carbon dioxide, bubble it through lime water Ca(OH)2. CO2 will react to form calcium carbonate which is a white precipitate if carbonate ions are present.
Sulfate test
add aqueous barium ions to form barium sulfate.
if u are doing halide test afterwards use barium nitrate instead of barium chloride.
A dense white precipitate will form of BaSO4
Sequence of tests
If to find an unknown compound you will need to carry out the tests in a specific order.
1. carbonate
2. sulfate
3. halides
you will need to do the carbonate test first because it will produce CO2 bubbles after the white precipitate in order to differentiate from sulfate.
Test for cations - ammnoim ion
aqueous sodium hydroxide is added to a solution of ammonium ions.
ammonia gas is produced. When you warm the mixture it will release the ammonia gas.
you might be able to smell it but you will have to test it with pH indicator paper which it will turn blue.
