I- Group 7 Halogens Flashcards
How to distinguish between aq solutions of bromine and iodine?
- both red-brown in water
- add hexane (non-polar solvent)
- iodine turns purple
Why do halogens dissolve in water?
- charge in H2O
- attracts charge in temp dipole in halogen
Halogens still dissolve better in non-polar solvents
Explain why chlorine is the best oxidising agent among Cl2, Br2, I2.
- Cl2 best at gaining e-
- harder to gain e- down the group
- larger atoms
- ↑ shielding ∵ extra e- shell
- ↓ attraction
Result when sodium bromide added to chlorine.
- yellow solution
- Br2 forms
- + hexane –> red-brown
- Cl2 + 2Br- –> 2Cl- +Br2
you can tell Cl2 is the oxidising agent as its oxidation state changes from 0 to -1.
Result when sodium iodide added to chlorine/ bromine.
- brown solution
- I2 forms
- + hexane –> purple
Reaction when sodium chloride added to bromine
- x reaction- stays yellow
- + hexane–> red-brown (shows Br still present)
Reaction when sodium chloride/ bromide added to iodine.
- x reaction- stays brown
- + hexane–> purple (violet) (I2 still present)
Describe the steps for the test for halide ions.
- silver nitrate solution
- acidify w/ a few drops of nitric acid (to remove CO3 2- ions)
- halide solution –> record results
- dilute ammonia (ammonium hydroxide) –> record
- concentrated ammonia in fume cupboard + record
Testing for halide ions:
Observations for F-
3 steps
- AgNO3- x ppt
- dilute NH3- x further change
- conc. NH3- x further change
Testing for halide ions:
Observations for Cl-
3 steps
1 AgNO3- white ppt
- Ag+ (aq) + Cl- (aq) –> AgCl (s)
2 dilute NH3- soluble
- AgCl(s) + 2NH3(aq) –> [Ag(NH3)2]+ (aq) + Cl- (aq)
- diammine silver (I) [complex]
3 conc. NH3- x further change
Testing for halide ions:
Observations for Br-
3 steps
1 AgNO3- cream ppt.
- Ag+ (aq) + Br- (aq) –> AgBr (s)
2 dilute NH3- insoluble
3 conc. NH3- soluble
- AgBr(s) + 2NH3(aq) –> [Ag(NH3)2]+ (aq) + Br-
Testing for halide ions:
Observations for I-
1 AgNo3- yellow ppt
- Ag+ (aq) + I- (aq) –> AgI (s)
2 dilute NH3- insoluble
3 conc. NH3- insoluble
Physical appearances of halides (F2 - I2) at room temp
- F2- pale yellow gas
- Cl2- pale green gas
- Br2- red-brown/ orange liquid
- I2- grey solid –> sublimes–> violet vapour
Trends in physical properties of halides
down the group:
- darker
- denser
they all have ‘swimming bath’ smell
Predict the appearance of astatine
- black solid (darker)
- x contained in a container ∵ radioactive + short half life
Safety precautions with concentrated sulphuric acid (VI)
corrosive
- gloves + goggles
releases heat when reacted w. water –> run thru cold water for long time if in contct
Test for reducing ability of halide ions
- spatula of sodium halide–> boiling tube
- conc. sulphuric acid (VI)- powerful oxidising agent
Are fluorides and chlorides reducing agents?
No ∵ x reducing ability
- NaF + H2SO4 –> NaHSO4 + HF
- NaF= base (proton acceptor); H2SO4= proton donor
- Acid-base reaction (x redox!!!)
Are bromides good reducing agents?
Weak reducing agent- S: +6 –> +4
1. NaBr + H2SO4 –> NaHSO4 + HBr (acid- base)
2. HBr –> H+ + Br-
3. 2H+ +2Br- +H2SO4 –> Br2 + SO2 + 2H2O
Are iodides good reducing agents?
Strong reducing agents- S: +6 –> -2
1. NaI + H2SO4 –> NaHSO4 + HI
2. HI –> H+ + I-
3. 2H+ + 2I- + H2SO4 –> I2 +SO2 + 2H2O
4. 8H+ + 8I- + 2SO2 –> 4I2 + 2S + 4H2O
5. 6H+ + 6I- + SO2 –> H2S + 3I2 + 2H2O
Observations of Cl- added to concentrated sulphuric acid (VI)
- white steamy fumes of HCl gas
Observations of Br- added to concentrated sulphuric acid (VI)
- white steamy fumes of HBr
- red-brown vapour Br2 ; SO2 w/ pungent smell
Observations of I- added to concentrated sulphuric acid (VI)
- white steamy fumes of HI
- purple vapour, grey solid I2 ; SO2- pungent smell
- H2S- bad egg smell ; S- yellow solid
Trend of reducing power down group 7
↑ reducing power
- easier to lose e- down the group
- larger ions
- ↑ shielding (extra e- shell)
What is a disproportionation reaction?
- a species being both oxidised & reduced in the same reaction
- both the oxidising & reducing agent
Eg. of a disproportionation reaction-
Cl + H2O
Cl2 (g) + H2O (l) ⇌ HClO (aq) + HCl (aq)
0 –> +1 & -1
- reversible
- HCl + chloric (I) acid
What are the 2 uses of chloric (I) acid formed in the reaction of chlorine and water?
- bleaches
- kill bacteria
Why is chlorine added into drinking water if it is toxic?
benefits to health (eg. prevents cholera) outweighs its toxic effects
What is the disadvantage of using chlorinated water in outdoor swimming pools?
has to be replaced frequently
- ∵ lost easily in sunlight
- 2 Cl2 (g) + 2 H2O (l) –> 4HCl (aq) + O2 (g)
What is a safer alternative to direct chlorination of water?
Add sodium chlorate (I)
+ in powder form- safer than Cl2 gas
+ easier to handle
+ X make Cl2 but still makes chloric acid (I) to kill bacteria
– NaClO (s) + H2O (l) ⇌ Na+ (aq) + OH- (aq) + HClO (aq)
Eg. of disproportionation reaction- Chlorine + sodium hydroxide
- x reversible
- Cl2 (g) + 2NaOH (aq) –> NaClO (aq) + H2O (l)
- Cl2 (g) + 2OH- (aq) –> ClO- (aq) + Cl- (aq) + H2O (l)
– 0 –> +1 & -1
Explain why the pH of outdoor swimming pools must be constantly monitored.
- ↓pH = Cl reacting w/ water under sunlight –>HCl + O2
- ↓ chloric (I) acid to kill bacteria
Test for nitrates
- add a few drops of NaOH + warm
- damp red litmus held at end of test tube turns blue
- ammonia produced- alkaline
