8. Halogens

Question 1

What is the valence shell structure of the halogens?

Answer 1

ns^2 np^5

Question 2

What block do halogens belong to?

Answer 2

p block

Question 3

What state and colour are the halogens at rtp?

Answer 3

Fluorine - pale yellow gas
Chlorine - green gas
Bromine - red-brown liquid
Iodine - grey-black solid

Question 4

What colour are the halogens in water?

Answer 4

Chlorine - pale green/colourless
Bromine - orange
Iodine - yellow-brown

Question 5

What colour are the halogens in non-aqueous solvents?

Answer 5

Chlorine - pale green
Bromine - orange
Iodine - violet/purple

Question 6

What colour is iodine vapour?

Answer 6

Purple - sublimes from solid state

Question 7

Describe the solubility of halogens

Answer 7

Although they will dissolve slightly in water, they will dissolve more readily in non-polar solvents eg hexane as they exist as non-polar diatomic molecules

Question 8

Give the equation of chlorine with cold dilute sodium hydroxide solution (~15°C)

Answer 8

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

Question 9

What can be observed when chlorine reacts with cold dilute sodium hydroxide solution?

Answer 9

Faint smell of bleach (NaOCl)

Question 10

Give the chemical equation for the reaction of hot concentrated sodium hydroxide solution (>70°C) with chlorine

Answer 10

3Cl2(g) + 6NaOH(aq) > 5NaCl(aq) + NaClO3(aq) + 3H20(l)

Question 11

Give the equation of chlorine with water

Answer 11

Cl2(aq) + H2O(l) [<>] HClO(aq) + HCl(aq)

Question 12

Give the equation for bromine’s solubility in water

Answer 12

Br2(aq) + H2O [«>] HBrO(aq) +HBr(aq)

Question 13

Describe iodine’s solubility in water

Answer 13

Doesn’t dissolve well in water and therefore doesn’t go on to react with water. It will dissolve in water containing a high conc of iodide ions eg potassium iodide
I2(s) + KI(aq) [<>] KI3(aq)

Question 14

What is the trend for displacement reactions in the halogens?

Answer 14

Halogens which are higher up the group will displace halide lower down the group from their salts

Question 15

Give the chemical equation for the reaction of chlorine with potassium bromide and give the expected colour change

Answer 15

Cl2 + 2KBr > 2KCl + Br2

Pale green to yellow/orange

Question 16

Give the chemical equation for the reaction of chlorine with conc sulphuric acid

Answer 16

NaCl(S) + H2SO4(l) > HCl(g) + NaHSO4(aq)

Question 17

What can be observed in a reaction with sodium chloride and conc sulphuric acid?

Answer 17

Steamy/misty fumes of HCl

Question 18

How can HCl be tested for?

Answer 18

• damp blue litmus paper turns red
• glass rod dipped in ammonia will yield white fumes (NH4Cl(s))
• acidified silver nitrate solution will yield white fumes

Question 19

Give the equations for the reaction of sodium bromide with conc sulfuric acid

Answer 19

NaBr + H2SO4 > HBr + NaHSO4

2HBr + H2SO4 > Br2 + SO2 + 2H2O

Question 20

Why do two reactions occur when sodium bromide reacts with conc sulphuric acid?

Answer 20

Bromine is a reducing agent and sulphuric acid is an oxidising agent

Question 21

What can be observed from the reaction of sodium bromide with conc sulphuric acid?

Answer 21

White fumes HBr
Pungent smell SO2
Red-brown vapour (Br2)
Heat released, gas produced, solid disappears

Question 22

Give the reactions of sodium iodide with conc sulphuric acid

Answer 22

NaI + H2SO4 > HI + NaHSO4
2HI + H2SO4 > I2 + SO2 + 2H2O
6HI + H2SO4 > 3I2 + S + 4H2O
8HI + H2SO4 > 4I2 + H2S + 4H2O

Question 23

What can be observed in the reaction between sodium iodide and conc sulphuric acid?

Answer 23

Misty fumes HI
Purple vapour I2
Rotten egg smell H2S
Yellow solid S
Grey-black solid on sides of test tube I2
Heat released, gas produced, solid disappears

Question 24

Describe the preparation of the hydrogen halides

Answer 24

• conc H2SO4 can be used to prepare HF and HCl but not HBr or HI as it oxidises immediately to Br2 and I2.
• HBr and HI can be prepared using phosphoric acid (H3PO4), which doesn’t act as an oxidant

Question 25

Give the chemical equation of the reaction of sodium iodide and conc phosphoric acid

Answer 25

NaI(s) + H3PO4 > HI(g) + NaH2PO4(aq)

Question 26

What observations will the reactions of halides with phosphoric acid yield?

Answer 26

Misty fumes of the respective hydrogen halide

Question 27

What is the trend of acid strength in the halogens?

Answer 27

Acid strength increases down the group (of hydrogen halides)

Question 28

Why does respective acid strength increase down the group of the halogens?

Answer 28

Longer bonds, weaker bonds and lower bond enthalpy causes the compound to more readily form H+ and X-

Question 29

What are the advantages of adding chlorine to drinking water?

Answer 29

Cost effective, more soluble, provides residual protection

Question 30

What are the disadvantages of adding chlorine to drinking water?

Answer 30

Gives unpleasant taste and smell, doesn’t kill all microorganisms, toxic in large doses

Question 31

What are the advantages of adding ozone to drinking water?

Answer 31

More effective in killing bacteria, mo residual chemicals to smell or taste (breaks down to O2)

Question 32

What are the disadvantages of adding ozone to drinking water?

Answer 32

Higher costs, no residual protection, less soluble than Cl2 so requires special mixing techniques