Reactions of Halogens (WJEC GCSE Chemistry)

Reactions of Halogens

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

Relative Reactivity of Chlorine, Bromine & Iodine

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

• 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 

Properties & Uses of Chlorine & Iodine

• 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