3.3 the halogens

Question 1

fluorine

Answer 1

pale yellow gas

Question 2

chlorine

Answer 2

pale green gas

Question 3

bromine

Answer 3

brown-orange liquid

Question 4

iodine

Answer 4

grey solid

Question 5

boiling point trend down group 7

Answer 5

boiling point increases down group 7
London forces increase due to increasing size and relative mass

Question 6

electronegativity trend down group 7

Answer 6

electronegativity decreases down group 7
atoms get larger
distance between nucleus and bonding electrons increases
also more shielding

Question 7

what is electronegativity?

Answer 7

the ability for an atom to attract electrons towards itself in a covalent bond

Question 8

what is the use of organic solvent?

Answer 8

to see colour changes easily
halogen present dissolves readily in organic solvent which forms a layer above aqueous layer
e.g. hexane

Question 9

what happens in group 7 displacement reactions?

Answer 9

more reactive halogens displace less reactive halide ions

Question 10

reactivity trends down group 7

Answer 10

reactivity decreases down group 7
smaller atoms attract electrons better than larger atoms
halogens are also less oxidising as we move down group 7

Question 11

organic solvent colours

Answer 11

Br2 = orange
I2 = purple

Question 12

test for halides

Answer 12

add dilute nitric acid (HNO3) then silver nitrate solution (AgNO3)
*confirm with ammonia solution

Question 13

why do we add nitric acid first for halide testing?

Answer 13

nitric acid reacts with any other anions other than halides (e.g. carbonates) to avoid false results

Question 14

results for further halide test using ammonia (NH3)

Answer 14

Cl- white ppt dissolves in dilute NH3
Br- cream ppt dissolves in conc NH3
I- yellow ppt is insoluble in conc NH3

Question 15

what reaction is bleach made via?

Answer 15

disproportionation reaction
chlorine has been simultaneously reduced and oxidised

Question 16

bleach formation

Answer 16

mixing chlorine and sodium hydroxide will form sodium chlorate(I) solution - also know as bleach

Question 17

bleach equation

Answer 17

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

Question 18

uses of sodium chlorate(I)
NaClO (bleach)

Answer 18

• treating water
• bleaching paper and fabrics
• cleaning agents (bleach)

Question 19

water sterilisation

Answer 19

adding chlorine to water can kill bacteria in the water
produces chlorate ions (ClO-) which kill bacteria
useful in drinking water and pools

Question 20

equation for adding Cl2 to water

Answer 20

H2O(l) + Cl2(g) <–> HCl(aq) + HClO(aq)
DISPROPORTIONATION reaction

Question 21

Chloric(I) acid ionises to make chlorate(I) ions (hypochlorite ions)

Answer 21

HClO(aq) + H2O(l) <–> ClO-(aq) + H3O+ (aq)

Question 22

advantages of chlorinating drinking water

Answer 22

• destroys microorganisms that cause disease
• long lasting so reduces bacteria build up
• reduces growth of algae that discolours water and can give it a bad smell/taste

Question 23

disadvantages of chlorinating drinking water

Answer 23

• chloring gas is toxic and irritates the respiratory system
• liquid chloring causes severe chemical burns to the skin
• chloring can react with organic compounds present in water to make chloroalkanes which have been linked to causing cancer

Question 24

risk of not chlorinating water

Answer 24

lead to cholera epidemic

Question 25

ethical consideration of chlorinating drinking water

Answer 25

chlorination of water is mandatory across UK, some object to this claiming it is forced medication of a whole population

Question 26

alternatives to chlorination of water

Answer 26

1. ozone - powerful oxidising agent that kills microorganisms but has a short half life so treatment is not permanent and is expensive to produce 2. UV light - damages DNA in microorganisms however ineffective in cloudy water and does not prevent water becoming contaminated further down the process

Question 27

test for ammonium

Answer 27

- add warm sodium hydroxide (NaOH), if ammonium compound is present, ammonia gas will be produced - used damp red litmus paper, turns blue when ammonia dissolves in water

Question 28

test for hydroxides

Answer 28

turn red litmus paper blue hydroxides are alkaline (NOT A PURE TEST)

Question 29

test for carbonates

Answer 29

add acid (HCl) which reacts with carbonate to produce CO2 gas when CO2 gas is bubbled through limewater, it turns cloudy

Question 30

test for sulfates

Answer 30

add HCl to remove carbonates add barium chloride (BaCl2) white precipitate if sulfates are present Ba 2+(aq) + SO4 2-(aq) -> BaSO4(s)

Question 31

order of ion tests

Answer 31

1. carbonates (carbon dioxide gas produced) 2. sulfates (barium sulfate precipitate) 3. halides (white, cream, yellow precipitate)