Chemistry 6.11-6.13 - Halogen displacement and reactivity

Displacement reactions as evidence

A halogen can be added to an aqueous solution containing halide ions. A reaction occurs only when the added halogen is more reactive than the halogen represented by the halide ion.

The evidence gives this order:

chlorine > bromine > iodine

Reactivity therefore decreases down Group 7. The complete pattern for the three halogens is:

Worked example: a reaction occurs

Will chlorine water react with potassium bromide solution?

1. Compare the halogens: chlorine is above bromine in Group 7, so chlorine is more reactive.
2. Apply the rule: chlorine displaces bromine from bromide ions.
3. Name the products: potassium chloride and bromine form.
4. Write and balance the equation:

$\ce{Cl2(aq) + 2KBr(aq) -> 2KCl(aq) + Br2(aq)}$

1. Connect equation to evidence: the solution becomes orange because bromine has formed. The colour change supports the claim that chlorine is more reactive than bromine.

Worked example: no reaction occurs

Will iodine solution react with potassium chloride solution? Iodine is below chlorine, so iodine is less reactive. It cannot displace chlorine from chloride ions, so there is no displacement reaction. Do not invent products and do not write a reaction equation.

Astatine lies below iodine. Extending the pattern predicts that astatine is less reactive than iodine: chlorine, bromine and iodine should each displace astatine from astatide ions, while astatine should not displace chloride, bromide or iodide ions.

Investigating the reactivity order

The purpose of this investigation is to use displacement results to determine the relative reactivity of chlorine, bromine and iodine. It is a useful suggested investigation for this topic, but it is not a named core practical.

Apparatus and variables

Use a white spotting tile, labelled dropping pipettes, dilute teacher-prepared chlorine water, bromine water and iodine solution, dilute potassium chloride, potassium bromide and potassium iodide solutions, distilled water and eye protection.

Two categorical factors are changed systematically: the identity of the halogen solution and the identity of the halide solution. The dependent result is whether displacement occurs, judged by a persistent product-colour change compared with a dilution control. Keep the number of drops, solution concentrations, observation time, temperature, lighting and white background the same.

Workable microscale method

1. Label a $3\times3$ grid on the spotting tile with halogen solutions as rows and halide solutions as columns.
2. Place two drops of the correct halogen solution into each dimple in its row. Use a separate labelled pipette for every solution.
3. Add two drops of potassium chloride to the first column, potassium bromide to the second and potassium iodide to the third.
4. Set up three reference dimples containing two drops of each halogen solution plus two drops of distilled water. These show the colour change caused by dilution alone.
5. Observe every mixture against the white background after the same time. Record the final colour and whether it differs from the matching water control.
6. Repeat the complete grid with fresh drops. If duplicates disagree, repeat that combination again rather than selecting the preferred result.

Record results in a table with the two solution identities, final colour and a displacement/no displacement conclusion. The halogen that displaces two others is most reactive; the one that displaces one other is next; the one that displaces none is least reactive. The expected order is therefore chlorine > bromine > iodine.

Safety and disposal

Chlorine can escape from chlorine water as a toxic gas, and bromine water is harmful and can irritate eyes, skin and the respiratory system. Use only small drops, wear eye protection, keep bottles stoppered except while dispensing, avoid skin contact and never smell the solutions. Carry out the work with the ventilation or fume control required by the school's risk assessment. Rinse a splash immediately with plenty of water and report it. Put all mixtures and contaminated rinsings into the labelled waste container provided; do not pour them into the sink unless the local disposal procedure specifically permits it.