3.1.3 Halogens Flashcards
outline the basics of halogens
- group 17
- most reactive non-metallic group
- halogen = element VS halide = ion (how they are naturally found, or in compounds)
- at RTP, exist as diatomic molecules
- as solids, form lattices with simple molecular structure
how does F2 appear
pale yellow gas
how does Cl2 appear
pale green gas
how does Br2 appear
- red-brown liquid
- orange vapour
how does I2 appear
- shiney grey-black solid
- purple vapour
how does At2 appear
never been seen
what is the trend in boiling points down the halogens
INCREASES down group
(why F2 is a gas and I2 is a solid at room temperature)
explain the trend in boiling points in halogens as you go down the group
1) more electrons
2) stronger London dispersion forces
3) more energy required to break the IMFs
4) boiling point INCREASES
what is the most common reaction of group 17
redox reactions
how do halogens undergo redox reactions
- have 7 electrons in outer shell (2 in outer s sub-shell and 5 in p sub-shell, s2p5)
- the halogen is REDUCED:
- gains 1 e-
- forms 1- ion
- Cl2 + 2e- ===> 2Cl-
why are halogens referred to as oxidising agents
- cause other species to lose electrons to the halogen atoms
what displacement reaction can be used to show the relative reactivities of group 7 elements
halogen-halide displacement reactions
- can happen on a test-tube scale
explain the displacement reactions of halogen-halides
1) add a solution of halogen
2) to an aqueous solutions of other halide ions
3) if halogen is more reactive than halide present
- reaction takes place and halogen displaces halide out of solution
- the solution changes colour to least reactive halide
what colour are the solutions of halogens in water
Cl2 = pale green
Br2 = orange
I2 = brown
what is the problem of using halogen solutions in water to test for displacement
solutions of iodine and bromine in water can appear a similar orange-brown shade, so not good for noticing a colour change
which solutions of halogens do you actually use when carrying out displacement reactions
- add a organic, non-polar solvent to mixture and shake to tell apart
- in CYCLOHEXANE:
- Cl2 = pale green
- Br2 = orange
- I2 = VIOLET
write the equation and colour change for chlorine added to bromide ions
Cl2(aq) + 2Br-(aq) ===> Br2(aq) + 2CL- (aq)
- from pale green to orange (Br2)
write the equation and colour change for adding chlorine to iodide ions
Cl2(aq) + 2I- ===> 2Cl-(aq) + I2(aq)
- from pale green to violet (I2)
write the equation and colour change for adding bromine to iodide ions
Br2(aq) + 2I-(aq) ===> 2Br-(aq) + I2(aq)
- from orange to violet (I2)
explain the redox reaction taking place in displacement of halogens and halides, including oxidation numbers
HALOGEN : 0 to -1 (reduced)
HALIDE : -1 to 0 (oxidised)
- total increase and decrease by 2 (as 2 atoms present)
explain why F2 and At2 aren’t included in the halide-halogen displacement reactions
F2: pale yellow gas, which is VERY REACTIVE and reacts with almost everything
At2: very rare as radioactive so decays very fast, never actually seen but assumed to be the least reactive
what is the trend of reactivity down group 7
DECREASES:
- redox occurs by halogens GAINING an e-, and down the group, the tendency for them to do this decreases
- become weaker oxidising agents down the group
explain the trend of reactivity down group 7
1) atomic radius increases
2) more inner shells, so shielding increases
3) increase in nuclear charge, but this is outweighed by other factors
4) less nuclear attraction to capture e- from other species
5) so reactivity DECREASES
what is disproportionation
a redox reaction where the same element is both oxidised and reduced
what are 2 examples of disproportionation
chlorine + water
chlorine + cold, dilute sodium hydroxide
why is chlorine used in water
used to kill bacteria, as a disinfectant to keep water sterile for swimming and drinking
- the hydrogen chlorate (I) / chloric acid produced, along with the chlorate ions ClO-, are the component which kills the bacteria
- can also act as a weak bleach (if you add indicator, it will turn red, due to presence of two acids, but the colour will soon disappear as the bleaching properties appear)
write the redox reaction for chlorine + water
Cl2(aq) + H2O(l) ===> HClO(aq) + HCl(aq)
CL2= 0
HClO = +1 (oxidised)
HCl = -1 (reduced)
explain the reaction of chlorine and cold, dilute sodium hydroxide
- much more chlorine will dissolve, as was limited in water
- used in household bleach, which contains large concentrations of ClO- ions
write down the equation of chlorine and cold, dilute sodium hydroxide
Cl2(aq) + 2NaOH(aq) ===> NaClO + NaCl + H2O(l)
Cl2= 0
NaClO = +1 (oxidised)
NaCl = -1 (reduced)
what are the benefits of using chlorine in water treatment
kills bacteria so water is sterile and safe to drink
what are the risks of using chlorine in water treatment
- chlorine is toxic
- if mixed with hydrocarbons (e.g. methane produced by decaying vegetation), can form chlorinated hydrocarbons which are potential carcinogens
why is chlorine still used in water treatment despite the drawbacks
the benefits outweigh the risks (may cause breakout of disease is water is unsanitized)
how do you test for presence of halides
react with aqueous silver ions, forming silver halide precipitates
Ag+(aq) + X-(aq) ===> AgX(s)
