Halogens Flashcards
Explanation for trends in bp down group
As you go down group, size of atoms (and molecules) increase due to increase atomic number and shells. More electrons = stronger London = more energy to break IMF.
Explanation for trend in e- negativity
As you go down group, atom gets larger due to more shells. Increased shielding from bonding pair of e- and nucleus therefore less effective nuclear charge and weaker e-static forces of attraction.
Trend in reactivity down group
Halogens react by gaining electron in outer shell. Descend group = more shells. Gained electron in outer shell experiences less attraction (shielding distance, despite nuclear charge) therefore more energy to add electron.
How to discern relative reactivity of halogens
More reactive halogens are able to oxidise less reactive halides. (oxidising power decreases down group).
Why is cyclohexane added to see which halogen is present?
Non-polar halogen able to dissolve into organic solution.
What colour does iodine appear in solution
Purple
What colour does bromine appear in solution
Orange
Oxidation reaction with metals
Reacts with metal to form metal halide. Halogen reduced. Metal oxidised.
Disproportionation of chlorine in water
Cl2 (g) + H2O (l) -> HCl (aq) + HOCl (aq)
Chlorine both -1 and +1.
Disproportionation of chlorine with cold, dilute, aqueous NaOH to form bleach
Cl2 (g) + 2 NaOH (aq) -> NaCl (aq) + NaOCl (aq) + H2O
Bleach = OCl- ion
Disproportionation reaction of chlorine with hot alkali
3 Cl2 (g) + 6 NaOH (aq) -> 5 NaCl (aq) + NaClO3 (aq) + 3 H2O (l) NaOCl disproportionates to NaClO3 and NaCl at 80C or above.
How does bromine and iodine react with alkali
Bromine always disproportionates fully as chlorine does while hot above 0C. Iodine does all the time.
Potassium chloride + conc. H2SO4 (also reducing power of HCl)
KCl + H2SO4 -> HCl + KHSO4
There is no further redox reaction after this non-redox reaction because chloride is not a good reducing agent.
Potassium bromide + conc. H2SO4 (also reducing power of HBr)
KBr + H2SO4 -> HBr + KHSO4 (NOT REDOX)
Then, 2HBr + H2SO4 -> Br2 + SO2 + 2H2O (REDOX)
This happens as hydrogen bromide is a good reducing agent. S +6 -> S +4
Br -1 -> Br 0
Potassium iodide + conc. H2SO4 (also oxidising power of HI)
KI + H2SO4 -> HI + KHSO4 (NOT REDOX)
Then, 2HI + H2SO4 -> I2 + SO2 + 2H2O (REDOX)
Then, 4HI + SO2 -> I2 + S + 2H2O (REDOX)
Then, 2HI + S -> I2 + H2S (REDOX)
This is because I is the best reducing agent out of the three. S +6 -> S -2
I -1 -> I 0
Observation for hydrogen halide
Dense white smoke on contact with NH3.
Observation for bromine gas
Orange gas
Observation for SO2
K2Cr2O7 oxidises this gas and the dichromate turns from orange (+6) to green (+3) as it is reduced on a piece of paper.
Observation for iodine gas/solid
Purple gas/grey solid
Observation for H2S
Bad egg smell, turns lead ethanoate paper black (reduces lead).
Observation for S
Yellow solid
Test for chloride ion
White precipitate formed (AgCl) when reacted with silver nitrate.
Ppt dissolves in dilute NH3 (aq).
Test for bromide ion
Cream precipitate formed (AgBr) when reacted with silver nitrate.
Ppt dissolves in concentrated NH3 (aq) but not dilute.
Test for iodide ion
Yellow ppt formed (AgI) when reacted with silver nitrate.
Ppt does not dissolve in conc. NH3.
Hydrogen halides with water
React to produce acid, HCl + H2O -> H3O+ + Cl-
Hydrogen halides with ammonia
React to form NH4Cl (s) and will appear as a dense white smoke.
Silver halides in light
Decompose to form Ag and X2 and therefore appear grey in light.
