Reactions of Halogens (WJEC GCSE Chemistry: Combined Science)

The Reaction of Halogens with Alkali Metals

• The halogens react with Group 1 metals (the alkali metals) to form ionic compounds which are metal halide salts
• Some of these reactions are: 
  • Lithium reacts with bromine to form sodium bromide: 

lithium  +  bromide →   lithium bromide

 2Li + Br₂ → 2LiBr

• • Chlorine will react with sodium to form sodium chloride: 

sodium  +  chlorine  →   sodium chloride 

2Na + Cl₂ → 2NaCl

• • Potassium reacts with iodine to form potassium iodide 

potassium  +  iodine  →   potassium iodide

2K + I₂ → 2KI

• The word and symbol equations all have the same format, you just need to change the name or the symbol in each case.
• All of the reactions between lithium and halogens produce a crimson flame
• All of the reactions between sodium and halogens produce an orange flame 
• All of the reactions between potassium and halogens produce a lilac flame  
• The reactions must be carried out in a fume cupboard due to the toxic nature of the halogens 
• The halogens decrease in reactivity moving down the group so the reactions become less vigorous going down the group

The Reaction of Halogens with Iron

• Group 7 elements react with iron to form an iron halide
• For example, chlorine will react with iron to form iron(III) chloride 

iron  +  chlorine  →   iron(III) chloride 

2Fe  +   3Cl₂   →   2FeCl₃

• Each reaction should be carried out in a fume cupboard
• Going down the group, the reaction between the halogen and iron becomes less vigorous 

The Reaction of Iron with the Halogens

Halogen	Reaction with iron wool
Fluorine	Cold iron wool burns to produce white iron(III) fluoride
Chlorine	Hot iron wool burns vigorously to form an orange-brown solid of iron(III) chloride
Bromine	Hot iron wool burns vigorously to form an orange-brown solid of iron(III) bromide
Iodine	Hot iron wool reacts slowly with iodine vapour to form a grey solid of iron(III) iodide

Higher Tier

• Displacement reactions can show the relative reactivity of the Group 7 elements
• A halogen displacement reaction occurs when a more reactive halogen displaces a less reactive halogen from an aqueous solution of its halide
• Aqueous halide solutions are colourless
• The reactivity of Group 7 elements decreases as you move down the group
• You only need to learn the displacement reactions with chlorine, bromine and iodine
  • Chlorine is the most reactive and iodine is the least reactive

Chlorine with Bromides & Iodides

• If you add chlorine solution to colourless potassium bromide or potassium iodide solution a displacement reaction occurs:
  • The solution becomes orange as bromine is formed or
  • The solution becomes brown as iodine is formed

• Chlorine is above bromine and iodine in Group 7 so it is more reactive
• Chlorine will displace bromine or iodine from an aqueous solution of the metal halide:

chlorine + potassium bromide →  potassium chloride + bromine

Cl₂ + 2KBr → 2KCl + Br₂

chlorine + potassium iodide →  potassium chloride + iodine

Cl₂ + 2KI → 2KCl + I₂

Bromine with Iodides

• Bromine is above iodine in Group 7 so it is more reactive
• Bromine will displace iodine from an aqueous solution of the metal iodide

bromine + potassium iodide →  potassium bromide + iodine

Br₂ + 2KI → 2KBr + I₂

Table to show the summary of displacement reactions

	Chlorine (Cl2)	Bromine (Br2)	Iodine (I2)
Potassium chloride (KCl)	x	No reaction	No reaction
Potassium bromide (KBr)	Chlorine displaces the bromide ions Yellow-orange colour of bromine seen	x	No reaction
Potassium Iodide  (KI)	Chlorine displaces the iodide ions Brown colour of iodine is seen	Bromine displaces the iodide ions Brown colour of iodine is seen	x

• From this pattern of reaction we can predict that:
  • Fluorine will displace all other halogens from their compounds
  • Astatine will be displaced by all the halogens from its compounds

• Having said that, astatine is the rarest naturally occurring element so there is not enough around to actually test!
• These displacement reactions provide stronger evidence for the decreasing reactivity down Group 7 than that gained from the elements reactions with iron 
  • The halogens having different states at room temperature can make it difficult to make a fair comparison when observing their reaction with iron whereas in displacement reactions they are competing directly against one another 

• Chlorine and iodine are both located in Group 7 of the Periodic Table 
• The both exist as diatomic molecules, so are made up of pairs of atoms 
• Although both in Group 7, their appearance and state are different: 
  • Chlorine is a green-yellow gas at room temperature which is toxic 
  • Iodine is a dark grey solid which can form a purple vapour when it is warmed 
• Chlorine is used to sterilise drinking water, making it safe for us to drink
• It is also used to kill the bacteria in swimming pools
  • The levels of chlorine in swimming pools have to be carefully monitored because chlorine is toxic
  • There needs to be enough chlorine present to kill bacteria and sterilise the water without causing any harm to us
• Iodine can be used as an antiseptic following hospital procedures, in plasters and sterilising sprays
  • Iodine is also toxic would levels are carefully monitored 

Photo by Jason Hawke on Unsplash 

Chlorine levels need to be carefully monitored in swimming pools