Group 7 - Halogens

Question 1

what is the colour and physical state of fluorine?

Answer 1

pale yellow gas

Question 2

what is the colour and physical state of chlorine?

Answer 2

green gas

Question 3

what is the colour and physical state of bromine?

Answer 3

red-brown liquid

Question 4

what is the colour and physical state of iodine?

Answer 4

grey/black solid

Question 5

describe and explain the trend in atomic radius going down group 7

Answer 5

• increases going down the group
• additional electron shells

Question 6

describe and explain the trend in reactivity/oxidising power/electronegativity/ionisation energy going down group 7

Answer 6

• decreases going down the group
• increase in atomic radius due to additional shells = further distance between nucleus and outer electrons + increase in shielding
• weaker force of attraction between nucleus and outer electron so harder to accept donated electron
• so less reactive/electronegative

Question 7

describe and explain the trend in boiling points going down group 7

Answer 7

• increases going down the group
• strength of vdw forces increases due to increasing size and relative mass of the atoms
• therefore higher bp as takes more energy to overcome strength of vdw forces

Question 8

write an equation for the displacement reaction of chlorine and bromine

Answer 8

Cl_2(aq) +2Br^-_(aq) –> 2Cl^-_(aq) + Br_2(aq)

Question 9

write an equation for the displacement reaction of chlorine and iodine

Answer 9

Cl_2(aq) +2I^-_(aq) –> 2Cl^-_(aq) + I_2(aq)

Question 10

write a general equation for a reaction between a halogen and a less reactive halide ion

Answer 10

(more reactive halogen)_(aq) + (less reactive halide)_(aq) —> 2(halogen)^-_(aq) + (halide as a halogen (e.g Br^- to Br₂))_(aq)

Question 11

what reaction will take place when adding chlorine water (Cl_2(aq) (colourless) to KCl_(aq) (colourless)? why?

Answer 11

• no reaction
• chlorine won’t displace itself

Question 12

what reaction will take place when adding bromine water (Br_2(aq) (orange) to KCl_(aq) (colourless)? why?

Answer 12

• no reaction
• bromine not reactive enough to displace chlorine

Question 13

what reaction will take place when adding iodine water (I_2(aq) (brown) to KCl_(aq) (colourless)? why?

Answer 13

• no reaction
• iodine not reactive enough to displace chlorine

Question 14

what reaction will take place when adding chlorine water (Cl_2(aq) (colourless) to KBr_(aq) (colourless)? why?

Answer 14

• orange solution formed (bromine water - Br_2(aq))
• chlorine displaces the bromine as it is more reactive

Question 15

what reaction will take place when adding bromine water (Br_2(aq) (orange) to KBr_(aq) (colourless)? why?

Answer 15

• no reaction
• bromine won’t displace itself

Question 16

what reaction will take place when adding iodine water (I_2(aq) (brown) to KBr_(aq) (colourless)? why?

Answer 16

• no reaction
• iodine not reactive enough to displace bromine

Question 17

what reaction will take place when adding chlorine water (Cl_2(aq) (colourless) to KI_(aq) (colourless)? why?

Answer 17

• brown solution formed (iodine water - I_2(aq))
• chlorine displaces the iodine as it is more reactive

Question 18

what reaction will take place when adding bromine water (Br_2(aq) (orange) to KI_(aq) (colourless)? why?

Answer 18

• brown solution formed (iodine water - I_2(aq))
• bromine displaces the iodine as it is more reactive

Question 19

what reaction will take place when adding iodine water (I_2(aq) (brown) to KI_(aq) (colourless)? why?

Answer 19

• no reaction
• iodine won’t displace itself

Question 20

what is a disproportionation reaction?

Answer 20

a reaction in which an element is both reduced and oxidised in the same reaction

Question 21

what kind of reaction forms bleach? what are the reactants and what is the correct name for “bleach”? write an equation for this reaction.

Answer 21

• disproportionation reaction
• mixing chlorine and sodium hydroxide will form sodium chlorate (I) solution (bleach)
• 2NaOH_(aq) + Cl_2(g) –> NaClO_(aq) + NaCl_(aq) +H₂O_(l)

Question 22

list 3 uses of sodium chlorate (I)

Answer 22

• water sterilisation
• bleaching paper and fabrics
• cleaning agents (e.g. bleach)

Question 23

explain how chlorine can be used in water sterilisation. write an equation for any reactions that take place.

Answer 23

• adding water to chlorine will produce chlorate (I) ions (ClO^-) which kill bacteria
  H₂O_(l) + Cl_2(g) –> 2H⁺_(aq) + Cl^-_(aq) + ClO^-_(aq)

Question 24

why is it important that chemicals in chlorinated are regularly replaced? include any necessary equations to support answer.

Answer 24

• sunlight can decompose chlorinated water meaning no ClO^-_(aq) ions are produced (which are responsible for killing the bacteria)
• no bacteria will be killed meaning the water will be unsafe
• 2H₂O_(l) + Cl_2(g) –> 4H⁺_(aq) + 2Cl^-_(aq) + O_2(g)

Question 25

what are the advantages of chlorinating drinking water?

Answer 25

• destroys microorganisms that cause disease
• long-lasting so reduces bacteria build up further own the supply
• reduces the growth of algae that discolours water and can give it a bad taste and smell
• not chlorinating water could lead to a cholera epidemic which outweighs the disadvantages as is far more lethal

Question 26

what are the disadvantages of chlorinating drinking water?

Answer 26

• chlorine gas is toxic and irritates the respiratory system
• liquid chlorine causes severe chemical burns to skin
• chlorine can react with organic compounds present in water to make chloroalkanes which are carcinogens

Question 27

what makes halide ions good reducing agents?

Answer 27

• ionic radius increases down a group, so distance between nucleus + outer electrons increases
• also more shielding
• outer e more easily lost
• lose electrons in reactions = reducing agent
• halides at the bottom of the table better reducing agents

Question 28

what would you see when adding NaF or NaCl to sulfuric acid?

Answer 28

misty white fumes

Question 29

write an equation for the reaction of NaF with sulfuric acid. which other halides would also react this way?

Answer 29

• NaF_(s) + H₂SO_4(l) –> NaHSO_4(s) + HF_(g)
• all halides react this way with sulfuric acid

Question 30

write an equation for the reaction of NaBr with sulfuric acid. explain why this happens. which other halides would also react this way?

Answer 30

• NaBr_(s) + H₂SO_4(l) –> NaHSO_4(s) + HBr_(g)
• larger than chloride so better reducing agent, causing reaction to continue further
• HBr +H₂SO_4(l) –> Br_2(s) + SO₂ + 2H₂O
• 2NaBr + H₂SO₄ –> Na₂SO₄ + Br₂ + SO₂ + 2H₂O
• 2nd reaction is redox
• would see brown fumes
• only bromide and iodine react this way with sulfuric acid

Question 31

write an equation for the reaction of NaI with sulfuric acid. explain why this happens. which other halides would also react this way?

Answer 31

• 2NaI + 2H₂SO₄ –> Na₂SO₄ + 2H₂O + I₂ + SO₂
• NaI_(s) + H₂SO_4(l) –> NaHSO_4(s) + HI_(g)
• HI +H₂SO_4(l) –> I_2(s) + SO₂ + 2H₂O
• 2nd reaction is redox
  6HI + SO₂ –> H₂S + 3I_2(s) + 2H₂O
• black solid forms
• larger than bromide so better reducing agent, causing reaction to continue further
• only iodine reacts this way with sulfuric acid

Question 32

describe how to test for the presence of halide ions using silver nitrate

Answer 32

1. add dilute nitric acid (to react with and therefore remove anions other than halide ions, e.g. carbonates, which would give a false result)
2. add a few drops of silver nitrate solution (AgNO_3(aq)). a precipitate of the silver halide will form
   - chloride = white precipitate
   - bromide = cream precipitate
   - iodide = yellow precipitate
3. further test by testing the solubility in ammonia
   - Cl^- = white precipitate dissolves in dilute NH₃
   - Br^- = cream precipitate dissolves in concentrated NH₃
   - I^- = yellow precipitate insoluble in concentrated NH₃

Question 33

describe how to test for the presence of group 2 ions in a compound using flame tests

Answer 33

1. dip nichrome wire in concentrated HCl
2. dip loop in unknown compound sample
3. hold loop in blue bunsen burner flame and observe colour
   - calcium = brick red
   - strontium = red
   - barium = pale green

Question 34

describe how to test for the presence of group 2 ions using dilute NaOH solution

Answer 34

1. add NaOH dropwise to a test tube containing the metal ion solution and observe the precipitate that forms
2. keep adding NaOH until it is in excess and record any changes
   - Mg²⁺ = slight white precipitate -> white precipitate
   - Ca²⁺ = slight white precipitate -> slight white precipitate
   - Sr²⁺ = slight white precipitate -> slight white precipitate
   - Ba²⁺ = no change -> no change

Question 35

describe how to test for the presence of ammonia ions
write an equation for the reaction

Answer 35

1. add NaOH and gently heat - if ammonium compound present, ammonia gas will be produced
2. hold damp red litmus paper in the gas - if ammonia present the ammonia will dissolve in the water and the paper will turn blue
   (paper must be damp so ammonia can dissolve)

• NH₄⁺_(aq) +OH^-_(aq) –> NH_3(g) + H₂O_(l)

Question 36

describe how to test for the presence of hydroxide ions. why are further tests necessary?

Answer 36

1. dip a piece of red litmus paper into the solution - if hydroxide ions present paper will turn blue

• further tests required as test does not prove hydroxide present: red litmus paper turns blue for any alkali

Question 37

describe how to test for the presence of carbonates

Answer 37

1. add HCl to sample - if carbonate ions present, will react + produce CO₂ gas which will be evident through the effervescence
2. bubble gas produced through limewater - CO₂ will turn a cloudy white colour

Question 38

what order would be best to test for ions to avoid false positives?

Answer 38

carbonates -> sulfates -> halides