Group 7: trends and displacement

Physical trends

Explaining trends

• Melting and boiling points increase down the group: more electrons, stronger London forces.
• Electronegativity decreases down the group: the bonding pair is further from the nucleus and more shielded.
• Reactivity (as oxidising agents) decreases down the group: the larger atom attracts an extra electron less strongly.

Displacement reactions

A more reactive halogen displaces a less reactive halide from solution: Cl2 + 2Br- -> 2Cl- + Br2.

Adding a hydrocarbon solvent such as cyclohexane shows the halogen clearly: Br2 is orange-red and I2 is purple in the organic layer (I2 is brown in water).

Testing for halide ions

Add dilute nitric acid (to remove carbonate ions, which would also give a precipitate) then silver nitrate solution:

• Cl-: white AgCl, dissolves in dilute ammonia
• Br-: cream AgBr, dissolves in concentrated ammonia only
• I-: yellow AgI, insoluble even in concentrated ammonia

Ag+(aq) + X-(aq) -> AgX(s)

Worked example

Chlorine water is added to potassium iodide solution, then cyclohexane is added and shaken. Describe and explain what is seen.

1. Chlorine is more reactive than iodine: Cl2 + 2I- -> 2Cl- + I2
2. Iodine turns the aqueous layer brown.
3. Iodine dissolves in cyclohexane and is purple there.

Answer: Brown solution, then a purple upper organic layer.

Check yourself

1. Which mixture reacts?
   • Iodine solution and potassium bromide solution
   • Bromine water and potassium chloride solution
   • Bromine water and potassium iodide solution
   • Iodine solution and potassium chloride solution
   Show answer

   Bromine water and potassium iodide solution. Br2 is more reactive than I2, so it displaces iodide.

2. Silver nitrate gives a cream precipitate that dissolves in concentrated, but not dilute, ammonia. Which ion is present?
   • Bromide
   • Chloride
   • Iodide
   • Fluoride
   Show answer

   Bromide. Cream AgBr dissolves only in concentrated ammonia.

3. Why is dilute nitric acid added before silver nitrate in the halide test?
   • To make the precipitate dissolve
   • To oxidise the halide
   • To remove carbonate ions that would also form a precipitate
   • To colour the precipitate
   Show answer

   To remove carbonate ions that would also form a precipitate. Ag2CO3 would form a precipitate and give a false result.

4. Why does the boiling point increase down Group 7?
   • The covalent bonds get stronger
   • The molecules have more electrons, so London forces are stronger
   • Hydrogen bonding increases
   • The atoms become ionic
   Show answer

   The molecules have more electrons, so London forces are stronger. Boiling separates molecules, overcoming London forces.

5. What colour is iodine dissolved in cyclohexane?
   • Brown
   • Orange
   • Colourless
   • Purple
   Show answer

   Purple. I2 is purple in hydrocarbon solvents, brown in water.

6. Why is chlorine a stronger oxidising agent than bromine?
   • Chlorine has more electrons
   • A chlorine atom is smaller, so it attracts an extra electron more strongly
   • Chlorine is a liquid
   • Chlorine has a lower electronegativity
   Show answer

   A chlorine atom is smaller, so it attracts an extra electron more strongly. The incoming electron is closer to the nucleus and less shielded.

7. What is the ionic equation for chlorine displacing bromide?
   • Cl- + Br2 -> Cl2 + Br-
   • Cl2 + Br- -> Cl- + Br
   • 2Cl + Br2 -> 2Br + Cl2
   • Cl2 + 2Br- -> 2Cl- + Br2
   Show answer

   Cl2 + 2Br- -> 2Cl- + Br2. Chlorine is reduced to chloride; bromide is oxidised to bromine.

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Sources

• Pearson Edexcel IAL Chemistry (2018) scheme of work
• Pearson Edexcel International Advanced Level Chemistry (2018) qualification page

Last updated 7 November 2026. Spotted a mistake? Tell us.