Reactivity of the Halogens (Oxford AQA IGCSE Chemistry) : Revision Note

Alexandra Brennan

Written by: Alexandra Brennan

Reviewed by: Stewart Hird

Updated on

Displacement Reactions of the Halogens

  • Reactivity of Group 7 non-metals decreases as you go down the group

  • This trend in reactivity can be observed in displacement reactions

  • A halogen displacement reaction occurs when a more reactive halogen displaces a less reactive halogen from an aqueous solution of its salt

  • For example:

    sodium bromide  +  chlorine  →  sodium chloride  +  bromine

  • 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 Bromine & Iodine

  • Chlorine is more reactive than bromine and iodine

  • Chlorine will displace bromine or iodine from an aqueous solution of the metal halide

  • When chlorine solution is added to colourless potassium bromide solution :

    • The solution becomes orange as bromine is formed or

    • The solution becomes brown as iodine is formed

chlorine + potassium bromide  potassium chloride + bromine

Cl2 + 2KBr  2KCl + Br2

chlorine + potassium iodide  potassium chloride + iodine

Cl2 + 2KI 2KCl + I2

Bromine with Iodine

  • Bromine is more reactive than iodine

  • Bromine will displace iodine from an aqueous solution of the metal iodide

  • The solution becomes brown as iodine is formed

bromine + potassium iodide  potassium bromide + iodine

Br2 + 2KI 2KBr + I2

Observations during displacement reactions

Chlorine (Cl2)

Bromine (Br2)

Iodine (I2)

Potassium chloride

x

No reaction

No reaction

Potassium bromide

Yellow-orange colour of bromine

x

No reaction

Potassium iodide

Brown colour of iodine

Brown colour of iodine

x

Reactivity of the Halogens

  • Going down Group 7, reactivity decreases

  • Group 7 elements react in a similar way because they all have 7 electrons in their outer shell

    • However, some elements are more reactive than others

  • The reactivity of the halogens is determined by how easily it will gain one electron to achieve a full outer shell

  • Reactivity decreases because:

    • Going down the group, the number of shells increases

    • The outer shell is therefore further away from the nucleus

    • There is less attraction between an incoming electron and the nucleus

    • It is harder to gain an electron

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Alexandra Brennan

Author: Alexandra Brennan

Expertise: Chemistry Content Creator

Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.

Stewart Hird

Reviewer: Stewart Hird

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.

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