11. Group 17

Question 1

Colour of chlorine at RTP, in aqueous solution and in organic solvent

Answer 1

RTP: pale green gas

aq: very pale green / colourless solution
organic: yellow solution

Question 2

Colour of bromine at RTP, in aqueous solution and in organic solvent

Answer 2

RTP: red-brown liquid

aq: yellow solution
organic: brown solution

Question 3

Colour of iodine at RTP, in aqueous solution and in organic solvent

Answer 3

RTP: black solid which sublimes to a violet vapour on warming

aq: brown solution
organic: violet solution

Question 4

Explain the trend in volatility of the halogens

Answer 4

• going down the group, the volatility decreases and the boiling point increases
• b/c going down the group, the number of electrons per molecule increases and there are increasing induced dipoles, so the strength of van der Waals’ forces increases

Question 5

Relative reactivity of the halogens as oxidising agents

Answer 5

going down the group, the reactivity of the halogens decreases and they become weaker oxidising agents

Question 6

Describe and explain the reactions of the elements with hydrogen (to form hydrogen halides/halogen hydrides)

Answer 6

• reactivity decreases down the group
• the H-X bond formed becomes longer and weaker, bond energy decreases

chlorine: explosive in sunlight but slow in the dark
bromine: requires heat and catalyst
iodine: slow even on heating; forms an equilibrium mixture

Question 7

Describe and explain the relative thermal stabilities of the hydrides in terms of bond energies

Answer 7

• thermal stability decreases down the group
• H-Hal BOND ENERGY decreases
• H-X bond is WEAKER and is more readily broken
• due to GREATER DISTANCE / SHIELDING of nucleus from bond pair
• ATTRACTION of nucleus for SHARED pair of electrons is weaker

HCl: no reaction on heating
HBr: partially decomposes to H2 and Br2 on heating; red-brown bromine fumes
HI: completely decomposes to H2 and I2 on heating; violet iodine vapour

Question 8

Reaction of chloride ions with aqueous Ag+ ions followed by aqueous ammonia (dilute and concentrated)

Answer 8

with aqueous Ag+ : white precipitate
ppt and dilute aqueous NH3 : soluble
ppt and conc. aqueous NH3 : soluble

Question 9

Reaction of bromide ions with aqueous Ag+ ions followed by aqueous ammonia (dilute and concentrated)

Answer 9

with aqueous Ag+ : cream precipitate
ppt and dilute aqueous NH3 : sparingly soluble / insoluble
ppt and conc. aqueous NH3 : soluble

Question 10

Reaction of iodide ions with aqueous Ag+ ions followed by aqueous ammonia (dilute and concentrated)

Answer 10

with aqueous Ag+ : yellow precipitate
ppt and dilute aqueous NH3 : insoluble
ppt and conc. aqueous NH3 : insoluble

Question 11

Reaction of chloride ions with concentrated sulfuric acid

Answer 11

2NaCl + H2SO4 –> Na2SO4 + 2HCl

- steamy white fumes of HCl gas are produced

Question 12

Reaction of bromide ions with concentrated sulfuric acid

Answer 12

2NaBr + H2SO4 –> Na2SO4 + 2HBr
- steamy white fumes of HBr gas are produced

Concentrated H2SO4 oxidises HBr to Br2
2HBr + H2SO4 –> Br2 + SO2 + 2H2O
- red-brown fumes of Br2 gas are produced

Question 13

Reaction of iodide ions with concentrated sulfuric acid

Answer 13

2NaI + H2SO4 –> Na2SO4 + 2HI
- steamy white fumes of HI gas are produced

Concentrated H2SO4 is reduced to several sulfur products (HI + H2SO4 –> I2 + sulfur product + H2O)
- iodine released as violet vapour / black solid
- sulfur products are:
+4, SO2 (choking pungent smell)
0, S (yellow solid)
-2, H2S (gas with rotten egg smell)

Question 14

Disproportionation reaction of chlorine with cold aqueous NaOH (15°C)

Answer 14

Cl2(aq) + 2NaOH(aq) –> NaCl(aq) + NaClO(aq) + H2O(l)

- chlorate(I) ClO-

Question 15

Disproportionation reaction of chlorine with hot aqueous NaOH (70°C)

Answer 15

3Cl2(aq) + 6NaOH(aq) –> 5NaCl(aq) + NaClO3(aq) + 3H2O(l)

- chlorate(V) ClO3-

Question 16

Explain the use of chlorine in water purification

Answer 16

• chlorine KILLS BACTERIA, so it is used to purify drinking water and sterilise water in swimming pools
• chlorine forms chloric(I) acid / hypochlorous acid
• HClO decomposes slowly in solution to produce reactive oxygen atoms that can kill bacteria in water
• the bleaching agent is the chlorate(I) ion, ClO-
  *chlorine undergoes disproportionation reaction in water (moderately soluble)
  Cl2(aq) + H2O(l) –> HCl(aq) + HClO(aq)

Question 17

Uses of halogens

Answer 17

1. bleaches
2. plastic PVC
3. CHCl3 (trichloromethane/chloroform) as solvents
4. CFCs as refrigerants and in aerosols; cause ozone depletion
5. KClO3 as match heads

Question 18

Explain the trend in oxidising power of halogens. [3m]

Answer 18

• oxidising power decreases down the group
• ability to accept electrons reduces down the group
• b/c outer shell experiences more shielding; increased distance from nucleus to outer shell