7.1.1 Reactivity Trends

Displacement Reactions

Practical Skills Development

• Safe use and careful handling of gases, liquids and solids, including:
  • Careful mixing of reagents under controlled conditions,
  • Using appropriate apparatus to explore chemical changes and/or products
• Presenting observations and other data using appropriate formats

Aim

• To use displacement reactions to identify the reactivity trend of Group 7 elements

Health & Safety Aspects

• The reagents used in these experiments are harmful so safety glasses should be worn
• The fumes are unpleasant so the reactions should be carried out in a well ventilated area or fume cupboard
• Small volumes should be used to minimise the risk of these hazardous substances
• Due to the hazards present this experiment is best carried out as a teacher demonstration

Materials

• spotting or dimple tile
• 0.1% solutions of Cl₂, Br₂ and I₂
• 0.1 mol / dm³ solutions of KCl, KBr and KI
• cyclohexane, C₆H₁₂
• Universal indicator paper

Method

1. Three drops of the halogens are added to separate wells in the dimple tile and the colours recorded
2. In a second set of wells, the same three halogen solutions are added separately, followed by three drops of potassium chloride
3. The procedure is repeated in a third and fourth set of wells using potassium bromide and potassium iodide , respectively
4. The colours of the resulting solutions are noted
5. The halogen colours are more easily seen if they are withdrawn in a pipette and added to a test tube containing 1-2 cm depth of cyclohexane and shaken
6. Finally, a piece of universal indicator is added, in turn, to each of the halogen solutions and the colour recorded
7. Record all observations in suitable tables

Results

The Colour of the Halogen Solutions

The Reactions of Halogens with Halide Ions

The Effect of Halogen Solutions on Indicator Paper

Conclusions

• Chlorine appears to displace bromine and iodine from solutions of their compounds
• Bromine displaces iodine from solutions of its compounds
• A more reactive halogen displaces a less reactive one
• The order of reactivity from this experiment is chlorine > bromine > iodine
• Chlorine is a strong bleaching agent, bromine less so and iodine not at all