I- Group 7 Halogens Flashcards
(34 cards)
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)
