Halogen displacement: use the reactivity order
Predict which halogen can replace another from a halide salt, then justify the reaction or absence of displacement.
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall chlorine > bromine > iodine in reactivity.
- Distinguish a free halogen from its halide salt.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which is more reactive in the required halogen order?
What you will learn
- Describe displacement reactions of halogens with other halide ions.
- Explain displacement predictions through the chlorine–bromine–iodine reactivity order.
The more reactive halogen displaces the less reactive one
A more reactive halogen displaces a less reactive halogen from its halide ions in the stated aqueous comparisons. Chlorine displaces bromine from bromide and iodine from iodide; bromine displaces iodine from iodide. Iodine does not displace bromine from bromide or chlorine from chloride. Name both the incoming element and the halide starting species before comparing their parent halogens.
For sodium bromide, Cl₂+2NaBr→2NaCl+Br₂ represents chlorine displacing bromine. Chlorine becomes part of the chloride salt while bromine appears as the element. The two sodium ions remain associated with dissolved salts in the full equation; they do not become sodium metal. Keep formulas fixed while choosing coefficients so the two-atom halogen molecules balance.
Interpret observations with the medium specified
In a supplied aqueous comparison, formation of bromine can give a red-brown/orange-brown colour and formation of iodine a brown colour, depending on concentration and conditions. These are solution observations, not bromine's r.t.p. liquid state or iodine's solid description. Use the stated colour key if another solvent or extraction layer is present; a different medium can give a different observation.
A no-displacement prediction follows when the added halogen is less reactive than the halogen belonging to the existing halide. It does not imply that every possible chemistry in an unspecified mixture is absent. Here the controlled question is the simple halogen/halide displacement pattern. Extended redox lessons can describe the electron transfer, while the required Core explanation uses the relative reactivity ordering.
Sources for this lesson
Pause and explain
Which pair supports displacement in the stated model?
Worked example
Predict bromine with potassium iodide and iodine with potassium bromide in the stated displacement comparison.
Show the worked solution
- Bromine is more reactive than iodine, so Br₂+2KI→2KBr+I₂ occurs in the model.
- Iodine is less reactive than bromine, so I₂ does not displace bromine from KBr.
- The opposite outcomes follow from one consistent reactivity order rather than from salt name or colour alone.
Answer Bromine displaces iodine from KI; iodine does not displace bromine from KBr.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Predict chlorine with sodium iodide and write the balanced full equation.
Give me a hint
Chlorine is more reactive than iodine; both elements are diatomic.
Compare my reasoning
- Chlorine displaces iodine from the iodide salt.
- Write Cl₂+NaI→NaCl+I₂ with correct formulas.
- Balance Cl₂+2NaI→2NaCl+I₂.
Cl₂+2NaI→2NaCl+I₂; iodine is displaced.
Look for these in your work
- Reactivity justifies displacement.
- Halogen atom counts balance.
Predict bromine with sodium chloride and with sodium iodide, explaining each result.
Give me a hint
Compare bromine separately with chlorine and iodine.
Compare my reasoning
- Bromine is less reactive than chlorine, so no chlorine displacement from NaCl is predicted.
- Bromine is more reactive than iodine, so it displaces iodine from NaI.
- The reacting equation is Br₂+2NaI→2NaBr+I₂.
NaCl: no displacement. NaI: Br₂+2NaI→2NaBr+I₂.
Look for these in your work
- Both comparisons use the same order.
- The reacting equation is balanced.
In a supplied comparison, unknown halogen Q displaces iodine from iodide but does not displace bromine from bromide. Is that enough to prove Q is chlorine? Explain.
Give me a hint
Q is a question label here; chlorine would also displace bromine.
Compare my reasoning
- Displacing iodine places the labelled halogen above iodine in reactivity.
- Failure to displace bromine does not support chlorine, which is above bromine in the required order.
- The evidence does not prove chlorine; it could fit bromine for these two simple tests, and the label is not an element formula.
The evidence does not prove chlorine; chlorine would displace bromine as well. The stated pattern could fit bromine.
Look for these in your work
- Both observations constrain the inference.
- A question label is not mistaken for a real chemical formula.
Common mistakes
- Letting the less reactive halogen displace the more reactive one.
- Comparing the alkali-metal names instead of the parent halogens.
- Writing halide ions as neutral diatomic elements before reaction.
- Using one solution colour as a universal appearance in every solvent.
What can you explain now?
Will iodine displace chlorine from sodium chloride in this model? Explain.
Compare with the explanation
No; iodine is less reactive than chlorine.
The added element must be more reactive than the parent halogen of the halide for this displacement pattern.
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Make it stick
Build a displacement prediction for each halogen with the other two halide salts. Explain both positive and negative predictions, then balance one reacting equation.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.