It is important in this practical activity to ensure the safe use and careful handling of liquids, including careful mixing of reagents. In particular, solutions of the group 7 elements are toxic, irritating or harmful.
This outlines one way to carry out the practical. Eye protection must be worn.
A more reactive halogen can displace a less reactive halogen from solutions of its salts. For example, chlorine is more reactive than iodine. A solution of chlorine can displace iodine from potassium iodide solution:
chlorine + potassium iodide → potassium chloride + iodine
Cl2(aq) + 2KI(aq) → 2KCl(aq) + I2(aq)
The reaction mixture turns darker and iodine solution forms.
Observe and record what happens in each well at step 3 or 4 in a suitable table. This table shows some sample results.
|KCl(aq)||KBr(aq)||Kl(aq)||Number of reactions seen|
|Cl2(aq)||Not done||Solution darkens||Solution darkens||2|
|Br2(aq)||No visible reaction||Not done||Solution darkens||1|
|I2(aq)||No visible reaction||No visible reaction||Not done||0|
Count the number of reactions seen with each halogen solution.
Use the results in the table to deduce an order of reactivity, starting with the most reactive halogen.
The order of reactivity is: chlorine > bromine > iodine. This is because chlorine could displace bromine and iodine, bromine could only displace iodine, but iodine could not displace chlorine or bromine.
Suggest an explanation for why three combinations of halogen and salt solution were not done.
A halogen cannot displace itself from a solution of one of its salts, so these combinations were not done.
Astatine is placed below iodine in group 7. Predict whether astatine will displace iodine from potassium iodide solution.
Astatine is less reactive than iodine, so it will not displace iodine from potassium iodide solution.
Example - Higher
a) Write an ionic equation for the reaction between chlorine solution and bromide ions from potassium bromide solution.
Cl2(aq) + 2Br-(aq) → 2Cl-(aq) + Br2(aq)
b) Write two half equations for this reaction.
Cl2(aq) + 2e- → 2Cl-(aq)
2Br-(aq) → Br2(aq) + 2e-
c) Explain why this displacement reaction is also a redox reaction.
Evaluate the hazards and the precautions needed to reduce the risk of harm. For example:
|Hazard||Possible harm||Possible precaution|
|Chlorine solution||Chlorine gas is produced, which is toxic if inhaled.||Make sure the lab is well ventilated and avoid inhaling the gas.|
|Bromine solution||Toxic bromine vapour is produced. Solution irritates the skin.||Make sure the lab is well ventilated and avoid inhaling the vapour. Wear gloves.|
|Iodine solution||Harmful to the skin. Stains clothes.||Wear gloves. Avoid contact with clothing.|