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
