Reactions of the Halogens

Specification Point 6.9:
  • Describe the reactions of the halogens, chlorine, bromine and iodine, with metals to form metal halides, and use this pattern to predict the reactions of other halogens.
  • The halogens react with some metals to form ionic compounds which are metal halide salts.
  • The halide ion carries a -1 charge so the ionic compound formed will have different numbers of halogen atoms, depending on the valency of the metal.
  • E.g. sodium is a Group 1 metal:

2Na + Cl2 → 2NaCl

  • Calcium is a Group 2 metal:

Ca + Br2 → CaBr2

  • The halogens decrease in reactivity moving down the group but they still form halide salts with some metals including iron.
  • The rate of reaction is slower for halogens which are further down the group such as bromine and iodine.

Formation Halide Metal, Edexcel GCSE Chemistry

Sodium donates its outer electron to chlorine forming the metal halide salt NaCl

Hydrogen Halides

Specification Point 6.10:
  • Recall that the halogens, chlorine, bromine and iodine, form hydrogen halides which dissolve in water to form acidic solutions, and use this pattern to predict the reactions of other halogens.
  • The halogens react with hydrogen to form hydrogen halides.
  • The reactions are less vigorous as we move down the group due to the decrease in reactivity of the halogens.
  • Chlorine which is higher up the group forms a hydrogen halide in the presence of sunlight:

CH4 + Cl2 → CH3Cl + HCl

  • Bromine requires a higher temperature for a hydrogen bromide to form.
  • Halogens below bromine would require a much higher temperature to form their respective hydrogen halides.
  • Those above chlorine would react more vigorously.

Trend in Group VII Activity

Specification Point 6.11:
  • Describe the relative reactivity of the halogens chlorine, bromine and iodine, as shown by their displacement reactions with halide ions in aqueous solution, and use this pattern to predict the reactions of astatine.
  • The reactivity of Group 7 non-metals increases as you go up the Group.
  • A halogen displacement reaction occurs when a more reactive halogen displaces a less reactive halogen from an aqueous solution of its halide.

Chlorine and Bromine

  • If you add chlorine solution to colourless potassium bromide solution, the solution becomes orange as bromine is formed and released into the reaction mixture.
  • Chlorine is above bromine in Group 7 and will therefore displace bromine from an aqueous solution of metal bromide:

2KBr (aq) + Cl2 (aq) → 2KCl (aq) + Br2(aq)

Bromine and Iodine

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

Br2 (l) + 2MgI (aq) → 2MgBr (aq) + I2 (aq or s)

  • Out of the 3 halogens, chlorine, bromine and iodine, chlorine is the most reactive and iodine is the least reactive.
  • Displacement reactions between these three halogens allows us to construct a halogen reactivity series:

More reactive Cl → B → I Less reactive

  • Based on this series we can predict that astatine is the least reactive and therefore could be displaced by any other halogen.

Halogen Displacement Colours, Edexcel GCSE Chemistry

Chlorine displaces bromine and iodine but no reaction occurs in a solution of its own salt

Reactivity & Electronic Configuration

Specification Point 6.13:
  • Explain the relative reactivity of the halogens in terms of electronic configurations.
  • In the halogens each outer shell contains seven electrons and when they react, they need to gain one outer electron to get a full outer shell of electrons.
  • As you go up Group 7, the number of shells of electrons decreases (Period number decreases moving up the Periodic Table).
  • This means that the outer electrons are closer to the nucleus so there are stronger electrostatic forces of attraction and less shielding.
  • This allows an electron to be attracted more readily, so the higher up the element is in Group 7 then the more reactive it is.

Electronic Configuration Halogens, Edexcel GCSE Chemistry

Electron shell diagrams of the first three elements in Group 7

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.