8.2 The Halogens Flashcards
What do halogens exist as at RTP?
Diatomic molecules
Describe the appearance of all the halogens
F2 = pale yellow gas Cl2 = pale green gas Br2 = red - brown liquid I2 = shiny grey - black solid At2 = never been seen ( probably dark/ black solid )
Describe and explain the trend in boiling point down the group
Boiling point increases because :
More electrons
Stronger London forces
More energy required to break the intermolecular forces
What is the outer shell configuration of halogens?
s2p5
Describe halogens in redox reactions
Reduced, gaining one electron to form a 1- halide ion.
Another species loses electrons to halogen atoms ( oxidised ).
Therefore, halogens are oxidising agents.
Describe reactivity down the group
Decreases because :
Atomic radius increases
More inner shells so shielding increases
Less nuclear attraction to capture an electron from another species
How do you carry out displacement reactions?
Solution of halogen added to aqueous solution of other halide. If halogen added is more reactive :
- reaction takes place, displacing the halide from solution
- solution changes colour
Describe the colours of chlorine, bromine and iodine in water and in cyclohexane
In water
Cl2 = pale green Br2 = orange I2 = Brown
After cyclohexane ( top layer )
Cl2 = pale green Br2 = orange I2 = violet
What will displace what?
Chlorine will displace both Br - and I -
Bromine will displace I - only
Iodine displaces nothing
Define disproportionation. State the two examples
A redox reaction in which the same element is both oxidised and reduced
Eg chlorine with water
Eg chlorine with cold, dilute sodium hydroxide
List the uses of chlorine
Water treatment
Bleach
Describe the disproportionation reaction of chlorine with water. How does this help purify water?
When chlorine is added to water, for each chlorine molecule, one chlorine atom is oxidised and the other chlorine atom is reduced.
Cl2 + H2O -> HClO + HCl
The two products are both acids, chloric acid ( HClO ) and hydrochloric acid ( HCl ).
The bacteria are killed by chlorate ions, ClO - .
Describe the disproportionation reaction of chlorine with cold, dilute sodium hydroxide. What is it’s use?
Chlorine dissolves a lot better here than in water.
Cl2 + 2 NaOH -> NaClO + NaCl + H2O
Resulting solution contains a lot of chlorate ions , ClO -, from the sodium chlorate ( NaClO) that is formed.
This solution is used as bleach.
Give the benefits and risks of using chlorine
+ve
Purifies water and kills bacteria
-ve
Extremely toxic gas -> respiratory irritant
In water, can react with organic hydrocarbons, forming chlorinated hydrocarbons ( potentially cancerous )
However, more risky to drink untreated water than dangers of chlorine. ( typhoid , cholera )
How do you test for halide ions?
Aqueous halide ions react with aqueous silver ions to form precipitates of silver halides.
