Halogens

Question 1

Explanation for trends in bp down group

Answer 1

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.

Question 2

Explanation for trend in e- negativity

Answer 2

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.

Question 3

Trend in reactivity down group

Answer 3

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.

Question 4

How to discern relative reactivity of halogens

Answer 4

More reactive halogens are able to oxidise less reactive halides. (oxidising power decreases down group).

Question 5

Why is cyclohexane added to see which halogen is present?

Answer 5

Non-polar halogen able to dissolve into organic solution.

Question 6

What colour does iodine appear in solution

Answer 6

Purple

Question 7

What colour does bromine appear in solution

Answer 7

Orange

Question 8

Oxidation reaction with metals

Answer 8

Reacts with metal to form metal halide. Halogen reduced. Metal oxidised.

Question 9

Disproportionation of chlorine in water

Answer 9

Cl2 (g) + H2O (l) -> HCl (aq) + HOCl (aq)

Chlorine both -1 and +1.

Question 10

Disproportionation of chlorine with cold, dilute, aqueous NaOH to form bleach

Answer 10

Cl2 (g) + 2 NaOH (aq) -> NaCl (aq) + NaOCl (aq) + H2O

Bleach = OCl- ion

Question 11

Disproportionation reaction of chlorine with hot alkali

Answer 11

3 Cl2 (g)  +  6 NaOH (aq)  ->  5 NaCl (aq)  +  NaClO­3 (aq) +  3 H2O (l)
NaOCl disproportionates to NaClO3 and NaCl at 80C or above.

Question 12

How does bromine and iodine react with alkali

Answer 12

Bromine always disproportionates fully as chlorine does while hot above 0C. Iodine does all the time.

Question 13

Potassium chloride + conc. H2SO4 (also reducing power of HCl)

Answer 13

KCl + H2SO4 -> HCl + KHSO4

There is no further redox reaction after this non-redox reaction because chloride is not a good reducing agent.

Question 14

Potassium bromide + conc. H2SO4 (also reducing power of HBr)

Answer 14

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

Question 15

Potassium iodide + conc. H2SO4 (also oxidising power of HI)

Answer 15

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

Question 16

Observation for hydrogen halide

Answer 16

Dense white smoke on contact with NH3.

Question 17

Observation for bromine gas

Answer 17

Orange gas

Question 18

Observation for SO2

Answer 18

K2Cr2O7 oxidises this gas and the dichromate turns from orange (+6) to green (+3) as it is reduced on a piece of paper.

Question 19

Observation for iodine gas/solid

Answer 19

Purple gas/grey solid

Question 20

Observation for H2S

Answer 20

Bad egg smell, turns lead ethanoate paper black (reduces lead).

Question 21

Observation for S

Answer 21

Yellow solid

Question 22

Test for chloride ion

Answer 22

White precipitate formed (AgCl) when reacted with silver nitrate.
Ppt dissolves in dilute NH3 (aq).

Question 23

Test for bromide ion

Answer 23

Cream precipitate formed (AgBr) when reacted with silver nitrate.
Ppt dissolves in concentrated NH3 (aq) but not dilute.

Question 24

Test for iodide ion

Answer 24

Yellow ppt formed (AgI) when reacted with silver nitrate.

Ppt does not dissolve in conc. NH3.

Question 25

Hydrogen halides with water

Answer 25

React to produce acid, HCl + H2O -> H3O+ + Cl-

Question 26

Hydrogen halides with ammonia

Answer 26

React to form NH4Cl (s) and will appear as a dense white smoke.

Question 27

Silver halides in light

Answer 27

Decompose to form Ag and X2 and therefore appear grey in light.