3.1.3 The halogens

Question 1

describe the trend in reactivity down g17 (7)

Answer 1

• reactivity decreases
  > atomic radius + electron shielding inc
  > nuclear attraction to outer shell e- dec
  > outer electron gained less easily so less reactive

Question 2

describe the trend in bp down g17(7)

Answer 2

• increases
  > more electrons + stronger London forces
  > more energy required to break them
• also elements change states from gas to solid

Question 3

describe the trend in electronegativity down g17(7)

Answer 3

• decreases
  > more electron shells + electron shielding as greater atomic radius
  > nuclear attraction to a shared pair of electrons decrease

Question 4

describe what happens when halogens undergo redox reactions

Answer 4

• halogen is reduced (gains e- and forms a 1- halide)
• another species is oxidised (loses e-)

Question 5

why are halogens oxidising agents

Answer 5

• they cause another species to be oxidised but is reduced itself

Question 6

describe the halogen displacement reactions

Answer 6

• a more reactive halogen displaces a less reactive one from its compound

Question 7

how can you tell solutions of iodine and bromine apart

Answer 7

• cyclohexane (organic non-polar solvent) added and mixture shaken
  > non-polar halogens dissolve more readily than in water
• br = orange colour
• iodine = deep violet

Question 8

what is the colour of chlorine in water + cyclohexane

Answer 8

• both are pale green

Question 9

what is the colour of bromine in water + cyclohexane

Answer 9

• both are orange

Question 10

what is the colour of iodine in water + cyclohexane

Answer 10

• brown in water
• violet in cyclohexane

Question 11

when chlorine displaces bromine what colour solution forms

Answer 11

• orange colour from Br2 formation
  Cl2(aq) + 2Br-(aq) —> 2Cl-(aq) + Br2(aq)

Question 12

when chlorine displaces iodine what colour solution forms

Answer 12

• violet from I2 formation
  Cl2(aq) + 2I-(aq) —> 2Cl-(aq) + I2(aq)

Question 13

when bromine displaces iodine what colour solution forms

Answer 13

• violet from I2 formation
  Br2(aq) + 2I-(aq) —> 2Br-(aq) + 12(aq)

Question 14

what is disproportionation

Answer 14

• a redox reaction in which the same element is both oxidised + reduced

Question 15

the disproportionate reaction of chlorine with water is useful because

Answer 15

• it can be used as a disinfectant for drinking water
• the chloric(l) acid and chlorate(l) ions kills bacteria + acts as a weak bleach

Question 16

give the formula for chlorine reacting with water

Answer 16

Cl2(aq) + H2O(l) —> HClO(aq) + HCl(aq)

Question 17

when universal indicator is added to HClO what colour does it turn

Answer 17

• first red due to acid
  > but then colour disappears as bleaching action takes place

Question 18

reaction of chlorine + water is limited by low solubility of chlorine in water, to combat this what is used instead

Answer 18

• if water contains dissolved sodium hydroxide, much more chlorine dissolves

Question 19

to make household bleach, what do you need to react + give full formula

Answer 19

• chlorine + cold dilute aqueous sodium hydroxide = soidum chlorate + sodium chloride + water
  Cl2(aq) + 2NaOH(aq) —> NaClO(aq) + NaCl(aq) + H2O(l)

Question 20

what are some benefits of chlorine use

Answer 20

• ensures drinking water is safe + bacteria is killed

Question 21

what are some risks of chlorine use

Answer 21

• chlorine is toxic gas
• chlorine is respiratory irritant in small concs + can be fatal in large concs
• if chlorine reacts with hydrocarbons, chlorinated hydrocarbons form which are suspected of forming cancer