Halogen Displacement Reactions

Introduction

How can you prove, without handling dangerous pure halogens, that chlorine is more reactive than bromine? Chemists use displacement reactions . If a more reactive halogen meets the ions of a less reactive one, it "pushes" the less reactive halogen out of its compound. The evidence is a clear colour change in a test tube. These reactions confirm the order of reactivity in Group 7 and are a favourite topic in exams.

Core explanation

The idea. A more reactive halogen has a stronger tendency to form halide ions. If it is added to a solution containing the halide ions of a less reactive halogen, it takes their electrons: the more reactive halogen becomes ions and the less reactive halogen is released as the element.

Example. Adding chlorine water to potassium bromide solution:

chlorine + potassium bromide → potassium chloride + bromine

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

The colourless solution turns orange , the colour of dissolved bromine. Chlorine has displaced bromine.

The order of reactivity. Displacement shows that reactivity decreases down the group:

F₂ > Cl₂ > Br₂ > I₂

A halogen can displace any halogen below it from its salts, but none above it.

Results with chlorine, bromine and iodine solutions.

Halogen added Potassium chloride Potassium bromide Potassium iodide --- --- --- --- Chlorine water No reaction Turns orange (Br₂ formed) Turns brown (I₂ formed) Bromine water No reaction No reaction Turns brown (I₂ formed) Iodine solution No reaction No reaction No reaction

A halogen never "reacts" with its own halide, so those boxes show no change. Fluorine is not used in solution because it reacts with water itself.

Identifying the product. Because bromine and iodine solutions can look similar (orange and brown), a few drops of an organic solvent such as cyclohexane can be added. The halogen moves into the solvent layer, where bromine appears orange-red and iodine a distinctive violet .

Ionic equations. The metal ions (such as K⁺) do not change; they are spectator ions . Leaving them out gives the ionic equation:

Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂

Electron transfer (redox). Each bromide ion loses an electron (it is oxidised ) and each chlorine atom gains one (it is reduced ):

Cl₂ + 2e⁻ → 2Cl⁻ and 2Br⁻ → Br₂ + 2e⁻

So displacement is a redox reaction, and the more reactive halogen is the stronger oxidising agent .

Step-by-step reasoning

To predict and write a displacement reaction:

1. Identify the halogen added and the halide ion in the salt. 2. If the added halogen is higher in Group 7, a reaction occurs; if not, there is no reaction. 3. Swap the halogens: the added one becomes the halide ion, the other becomes the diatomic element. 4. Balance, then remove spectator ions for the ionic equation.

Visual explanation

Imagine three test tubes of colourless potassium iodide solution. A few drops of chlorine water turn the first brown; bromine water turns the second brown; iodine solution in the third simply adds its own colour with no reaction. Shaking each with cyclohexane shows a violet top layer wherever iodine was formed.

Real-world analogy

Think of a game of musical chairs where a stronger player can pull a weaker player off a chair, but never the other way round. The chair is the electron, and the stronger halogen always takes it from the weaker halogen's ion.

Real-world example

Bromine is extracted from seawater using exactly this chemistry. Seawater contains dissolved bromide ions. Chlorine is passed through it in an industrial plant, displacing bromine, which is then blown out as a vapour and collected. Much of the world's bromine is produced this way, including from the Dead Sea.

Why?

Why can chlorine displace bromine but bromine cannot displace chlorine? A chlorine atom attracts an extra electron more strongly than a bromine atom does, because its outer shell is closer to the nucleus. So electrons move from bromide ions to chlorine, but chloride ions will not give their electrons to bromine.

Common misconception

"The potassium takes part in the reaction." Potassium ions are present before and after unchanged. Only the halogen and the halide ions exchange electrons, which is why potassium is left out of the ionic equation.

Worked example

Question: Bromine water is added to sodium iodide solution. Predict what is seen, and write the balanced symbol and ionic equations.

Reasoning: Bromine is above iodine in Group 7, so it is more reactive and will displace iodine. Iodine forms in the solution, giving a brown colour.

Answer: The solution turns brown. Br₂ + 2NaI → 2NaBr + I₂; ionic: Br₂ + 2I⁻ → 2Br⁻ + I₂.

Quick check

1. Will iodine solution react with potassium bromide solution? Answer: No, because iodine is less reactive than bromine.

Exam focus

Examiners often give a results table to complete or interpret. State that a more reactive halogen displaces a less reactive halogen from its salt solution. Quote colours accurately (orange for bromine, brown for iodine in water) and be able to write ionic equations and identify which species is oxidised and which is reduced.

Advanced insight

Displacement is a practical demonstration of electrode potentials. The standard electrode potential for Cl₂/Cl⁻ is about +1.36 V, for Br₂/Br⁻ about +1.07 V and for I₂/I⁻ about +0.54 V. A more positive value means a stronger tendency to gain electrons, so any halogen can oxidise the halide ions of a halogen with a less positive potential.

Summary

In a halogen displacement reaction, a more reactive halogen takes electrons from the halide ions of a less reactive halogen, releasing it as the element. Reactivity order is F₂ > Cl₂ > Br₂ > I₂. Chlorine displaces bromine (orange) and iodine (brown); bromine displaces only iodine; iodine displaces neither. These are redox reactions, and the metal ions are spectators.

Practice questions

1. Complete the word equation: chlorine + sodium iodide → Answer: Sodium chloride + iodine. 2. Explain why bromine water does not react with potassium chloride solution. Answer: Bromine is less reactive than chlorine, so it cannot take electrons from chloride ions to displace chlorine. 3. Write the ionic equation for chlorine reacting with iodide ions. Answer: Cl₂ + 2I⁻ → 2Cl⁻ + I₂. 4. In the reaction Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂, which species is oxidised? Answer: The bromide ions, because they lose electrons to form bromine.