3.1.2 Group 2 & 3.1.3 Halogens & 3.1.4 Qualitative analysis Flashcards
are group 2 elements oxidised or reduced
oxidised
why is group 2 element a reducing agent
as it reduces another species as the 2 electrons it loses another species gains to become reduced
what sub-shell are the 2 outer electrons in
S sub-shell
what do group 2 elements form when they react with oxygen
metal oxide with general formula MO due to 1:1 ratio of the -2 and +2
group 2 reactants react with water to form …..
alkaline hydroxide with general formula M(OH)2 and hydrogen gas
what happens to reactivity as you go down group 2
reactivity increases as you go down the group
why does reactivity increase as you go down group 2
as the atoms of group 2 elements react by losing electrons to form 2+ ions which requires input of 2 ionisation energies.
the ionisation energies decrease as you go down the group due to decrease in attraction between nucleus and outer electrons so less energy is needed to be given in to lose the electron and therefore react. this also causes them to become stronger reducing agents
what happens to ionisation energies as you go down group 2
decreases as the attraction between the nucleus and the outer electrons decreases as a result of increasing atomic radius and increasing shielding
oxide of group 2 + water=
releases hydroxide ions and then form alkaline solution of metal hydroxide
what happens to solubility of hydroxides in water as you go down group 2
increases so solutions contain more OH- and are more alkaline
what happens to pH as the solubility of hydroxides increases as you go down group 2
increases so therefore alkalinity increases
why do farmers add group 2 compounds to there fields
to increase the pH of the acidic soils to form water
why are group 2 compounds used in medicines
used as antacids to help neutralise stomach acid to produce water treating indegestion
Mg(OH)2 + 2HCl -> MgCl2 + 2H2O
what are the halogens at RTP
exist as diatomic molecules
trends of halogens as you go down the group
more electrons
stronger London forces
more energy required to break IMF
boiling point increases
appearance and state of fluorine at RTP
pale yellow gas
appearance and state of chlorine at RTP
pale green gas
appearance and state of bromine at RTP
red-brown liquid
appearance and state of iodine at RTP
shiny grey-black solid
how many electrons in the outer P sub-shell and outer S sub-shell of halogens
S= 2
P= 5
Is a halogen oxidised or reduced
reduced
gains 1 electron
is a halogen and oxidising or reducing agent
oxidising agent
what happens to reactivity as you go down the halogens
decreases
what reaction do you carry out to show reactivity of the halogens
displacement reaction
what other halogens does Cl- react with
Br- and I-
what other halogens does Br- react with
I- only
why does reactivity decrease as you go down the halogens
atomic radius increases
more inner shell so increase in shielding
less nuclear attraction
Disproportionation definition
a redox reaction in which the same element is both oxidised and reduced
example of disproportionation reaction
reaction of chlorine with water and with cold dilute sodium hydroxide= 2 examples
benefits of chlorine used in water treatments
kills bacteria
Cl2 + H2O
HClO + HCl
reaction to form bleach
Cl2 + 2NaOH -> NaClO + NaCl + H2O
risks of using chlorine
extremely toxic gas
respiratory irritant in small concs and fatal in large concs
chlorinated hydrocarbons are formed from drinking water reacting with methane formed by decaying veg is suspected to cause cancer
Test for halide ions
add silver nitrate to aqueous halide a white, cream and yellow precipitate will form
then add ammonium
qualitative analysis
simple observations such as gas bubbles, precipitates, colour change and identification of gases
carbonate test - CO3 2-
add dilute nitric acid to solution
if bubbles its a carbonate
how to prove gas is CO2
bubble gas through lime water
and the calcium hydroxide react with CO2 and will turn lime water cloudy
sulphate test
add barium nitrate and a white precipitate form as barium sulphate is formed
colour of chlorine ions in silver nitrate and solubility in NH3
white
soluble in dilute NH3
colour of bromine ions in silver nitrate and solubility in NH3
cream
soluble in concentrated NH3
colour of iodide ions in silver nitrate and solubility in NH3
yellow
insoluble in all NH3
what order do the anion tests go in
carbonate
sulphate
halides
why do sulphate test after carbonate test
if you carry out sulphate test on a carbonate you get a white precipitate aswell so must carry out carbonate first
why do halide test last
silver carbonate and nitrate are both insoluble in water and will form a precipitate in the test so carry out the carbonates and sulphur test before to rule out these possibilities
test for ammonium NH4 +
add NaOH (aq)
warm and ammonia gas will be produced
test with moist litmus paper and if present it will turn blue
