Halogen Displacement Reactions — Concept Studio

What is a displacement reaction?

A more reactive halogen will displace a less reactive halide ion from solution — taking its place and pushing the less reactive halogen out as a free element. This is a redox reaction: the incoming halogen is reduced (gains electrons), and the halide ion already in solution is oxidised (loses electrons).

X₂(aq) + 2Y⁻(aq) → 2X⁻(aq) + Y₂(aq) (only proceeds if X is more reactive than Y)

Half-equation (reduction): X₂ + 2e⁻ → 2X⁻
Half-equation (oxidation): 2Y⁻ − 2e⁻ → Y₂ (usually written 2Y⁻ → Y₂ + 2e⁻)

Reactivity trend down Group 7

Oxidising power — the ability of a halogen molecule to pull an electron away from something else — decreases going down the group: F₂ > Cl₂ > Br₂ > I₂.

Atomic radius increases down the group, so the incoming electron ends up further from the nucleus.
Increased electron shielding from inner shells further weakens the pull the nucleus has on that incoming electron.
Both effects reduce the effective nuclear attraction felt by an electron joining the outer shell, so each halogen further down the group is a progressively weaker oxidising agent.

Exam phrasing to use: "atomic radius increases and shielding increases down the group, so the attraction between the nucleus and an incoming electron decreases — oxidising power decreases."

Colours — verified reference table

Colours below are cross-checked against exam-board revision sources. The organic-solvent column is the more reliable diagnostic: aqueous iodine solution can look deceptively similar to bromine water, but cyclohexane makes the three unmistakable.

Species	Colour in water	Colour in cyclohexane (upper layer)
Chlorine, Cl₂	very pale green / near colourless	pale yellow-green
Bromine, Br₂	orange	orange (slightly paler)
Iodine, I₂	brown	violet / purple
Chloride, bromide, iodide ions in water (no reaction)	colourless

⚠ Why this is a Concept Studio, not a Core Practical

Chlorine water and bromine water are genuinely hazardous at the concentrations needed to see a clear colour change: CLEAPSS safety data classifies bromine solutions above roughly 0.06 M as an eye/skin irritant and above 0.2 M as corrosive, and chlorine gas dissolved in water carries similar handling risks. Real school practice usually keeps this as a small-scale teacher demonstration in a fume cupboard, or skips direct student handling of chlorine/bromine water altogether — so most students only ever see it once, from a distance, if at all.

This simulation lets every student run all nine halogen/halide combinations themselves, as many times as they like, with zero real hazard — which is exactly the gap it exists to fill.

The real-world method this simulates

Add ~1 cm depth of potassium halide solution

KCl(aq), KBr(aq) or KI(aq) — all colourless — into a clean test tube.

Add a few drops of halogen water

Chlorine water, bromine water or iodine solution, added dropwise down the side of the tube.

Observe any colour change

A colour appearing means the halogen you added has displaced the halide already in solution.

Confirm with cyclohexane (optional but recommended)

Add ~1 cm of cyclohexane, stopper the tube, shake gently, then let it stand. The organic layer floats on top (it's less dense than water) and gives an unambiguous colour, especially useful for telling iodine apart from bromine.

⚠ Real-lab hazards this removes

Chlorine and bromine vapour are toxic and corrosive — real handling needs a fume cupboard, gloves and eye protection.
Cyclohexane is highly flammable — no naked flames allowed near it.
In the simulation, none of this applies. You can be as exploratory and repetitive as you like.

Your observation log

Every combination you run in the Simulation tab is recorded here automatically, in the order you tried it.

#	Halide	Halogen added	Predicted	Actual result	Correct?	Ionic equation

No attempts yet — head to the Simulation tab.

Progress saves automatically on this browser.

Rank the halogens — from your own data

Based on which of your attempts actually reacted, put the three halogens in order of decreasing oxidising power (most reactive first). Use the formulas: Cl₂, Br₂, I₂.

> >

Cl₂ > Br₂ > I₂. Chlorine displaced both bromide and iodide in your data; bromine displaced iodide but not chloride; iodine displaced neither. That ordering — able to displace everything below it, nothing above it — is exactly what "oxidising power decreases down the group" predicts.

Q1 — Write the ionic equation

Chlorine water is added to potassium iodide solution. Fill in the missing formulas for the ionic equation (state symbols not required).

+ →

+

Cl₂ + 2I⁻ → 2Cl⁻ + I₂. Chlorine is reduced (0 → −1), iodide is oxidised (−1 → 0). Chlorine is the oxidising agent; iodide is the reducing agent.

Q2 — Oxidising and reducing agents

In the reaction Br₂(aq) + 2I⁻(aq) → 2Br⁻(aq) + I₂(aq), identify each role.

Oxidising agent (the species reduced)

Reducing agent (the species oxidised)

Q3 — Explain the trend

Before the model answer unlocks, complete this sentence using the word bank: increases, decreases, radius, shielding, nucleus, weaker.

"Going down Group 7, atomic and electron both , so the attraction between the and an incoming electron gets ."

Going down Group 7, atomic radius and electron shielding both increase, so the attraction between the nucleus and an incoming electron gets weaker — each halogen further down is a progressively weaker oxidising agent, which is why reactivity/oxidising power decreases down the group.

Q4 — Predict and justify

Iodine solution is added to potassium chloride solution. Will a reaction occur?

Why?

Q5 — Reflection

Why might a school choose to run this as a teacher demonstration (or skip it) rather than let every student handle chlorine and bromine water themselves — and what does a simulation like this let you do that the real demonstration can't?

Chlorine and bromine water are toxic/corrosive enough that CLEAPSS guidance pushes schools toward small-scale fume-cupboard use or teacher-only demonstration, which usually means each student sees the reaction happen once, from a distance, rather than running it themselves. A simulation removes the hazard entirely, so every student can test all nine combinations, repeat any of them, and build their own evidence for the reactivity trend — at the cost of never seeing the real colours, smells and physical behaviour of the actual chemicals.

Q6 — Wider context

Chlorine is added to drinking water and swimming pools in small, carefully controlled amounts. Using what you now know about the reactivity of halogens, suggest one reason chlorine — rather than iodine — is used for this.

Chlorine's stronger oxidising power makes it more effective at destroying bacteria and other pathogens at low concentration than iodine would be — plus it's cheaper and easier to dose accurately at large scale. (Iodine is sometimes used instead in situations like emergency water purification tablets, where its slower action is an acceptable trade-off for portability.)