[8.2] the halogens

Question 1

what are the properties of fluorine (F₂)?

Answer 1

• pale yellow gas
• very reactive
• toxic

Question 2

what are the properties of chlorine (Cl₂)?

Answer 2

• pale green gas
• very reactive
• toxic

Question 3

what are the properties of bromine (Br₂)?

Answer 3

• red-brown liquid
• very reactive
• toxic
• often used in solution (bromine water) to test for alkenes

Question 4

what are the properties of iodine (I₂)?

Answer 4

• grey crystals
• reactive
• toxic
• easily turns into purple vapour
• used in solution as antiseptic

Question 5

what is the trend in reactivity going down group 7?

Answer 5

reactivity decreases (explain why)

Question 6

what is a halogen displacement reaction?

Answer 6

when a more reactive halogen will displace a less reactive one from its compound

• Cl₂ (aq) + NaBr (aq) -> NaCl (aq) + Br₂ (aq)

Question 7

what is the colour of the halogens in water?

Answer 7

• chlorine = pale green
• bromine = orange
• iodine = brown

Question 8

what is the colour of halogens in cyclohexane?

Answer 8

• chlorine = pale green
• bromine = orange
• iodine = violet

Question 9

full and ionic equation for halogen displacement reactions

Answer 9

• Cl₂ (aq) + 2KBr (aq) -> 2KCl (aq) + Br₂ (aq)
• Cl₂ (aq) + 2Br⁻ (aq) -> 2Cl⁻ (aq) + Br₂ (aq)

Question 10

what colour change is observed during the 3 reactions that take place when a halogen water is added to KCl/Br/I?

Answer 10

• chlorine water + KBr = from pale green to orange
• chlorine water + KI = from pale green to brown
• bromine water + KI = from orange to brown

Question 11

what colour change is observed during the 3 reactions that take place when cyclohexane is added after a halogen water is added to KCl/Br/I?

Answer 11

• chlorine water followed by cyclohexane + KBr = from pale green to orange
• chlorine water followed by cyclohexane + KI = from pale green to brown
• bromine water followed by cyclohexane + KI = from orange to violet

Question 12

halogens and oxidisation

Answer 12

• halogens can act as oxidising agents
• it gains electrons (taken from the oxidising species)
• going down the group, the halogens’ oxidising power gets weaker

Question 13

what is the test for chlorine gas?

Answer 13

damp litmus paper turns red then is decolourised by chlorine water

Question 14

what is the equation for the test for chlorine gas?

Answer 14

Cl₂ (g) + H₂O (l) ⇌ HCl (aq) + HClO (aq)

Question 15

what is a disproportionation reaction?

Answer 15

when a species is oxidised and reduced simultaneously. for example, in the test for chlorine gas

Question 16

chlorine reaction with sodium hydroxide

Answer 16

• sodium hydroxide needs to be cold, aqueous, and dilute
• 2NaOH (aq) + Cl₂ (aq) -> NaCl (aq) + NaClO (aq) + H₂O (l)
• sodium chlorate is used in bleach (NaClO)

Question 17

benefits of chlorine

Answer 17

• kills micro-organisms
• prevents growth of algae
• eliminates bad tastes and
  smells
• removes discolouration caused by organic compounds.

Question 18

risks of chlorine

Answer 18

• Cl₂ can irritate respiratory system if breathed in
• liquid chlorine can cause severe chemical burns
• chlorine can react with organic compounds (eg. decomposing plant material) to form chlorinated hydrocarbons which cause cancer