PAG1: Reactivity Trends and Halogen Displacement
A more reactive halogen can displace a less reactive halogen from a suitable halide solution. Distinguish chlorine molecules, for example, from chloride ions. Naming both as “chlorine” loses the chemical distinction needed to interpret the reaction.
Before you start
- Use your teacher’s tier checklist: this overview can include Higher-only material, and is not a tier-specific revision list.
- OCR Gateway A separate GCSE Chemistry J248. This is a suitable practical-group example; centres can choose other activities covering the required apparatus and techniques.
- Recall halogens, halide ions and electron transfer.
- Use the centre’s approved supervised method, apparatus, heating and disposal arrangements. Written preparation does not replace performing and recording the practical activities.
Watch, pause, try
Video by Royal Society of Chemistry. Use the demonstration alongside this lesson’s specification reference and written explanations.
Viewing focus: Use controlled comparisons to infer a halogen reactivity order
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Which species is a halide ion?
Show answer and reasoning
Br⁻. The negative ion and diatomic element are different species.
After watching 2: Why state whether a colour is aqueous or in another phase?
Show answer and reasoning
The appearance can depend on the phase. Interpret the observation under the actual conditions.
What you'll be able to do
- Use controlled comparisons to infer a halogen reactivity order
- Distinguish a halogen from its halide ion
- Explain a displacement using a balanced ionic equation
Compare a defined halogen with a defined halide
A more reactive halogen can displace a less reactive halogen from a suitable halide solution. Distinguish chlorine molecules, for example, from chloride ions. Naming both as “chlorine” loses the chemical distinction needed to interpret the reaction.
Use the approved small-scale solutions and handling method. Compare matched volumes, concentrations and observation conditions, with appropriate starting-solution references. Adding two coloured substances can change the observed colour without establishing displacement, so the control observations and chemical model matter.
State the phase and interpretation of colour evidence
Record the actual observations and the phase in which they occur. A halogen can have a different appearance in an aqueous and an appropriate organic phase, so a colour rule copied from one phase may misidentify another. Use only the centre’s chosen solvent and viewing method.
A set of suitable positive and negative comparisons can support the order chlorine, bromine, iodine for the stated Group 7 examples. Do not infer that every absence of an obvious colour change proves zero reaction: concentrations, mixing and detection can limit observation. Explain why the combined evidence supports the proposed order.
Connect displacement to electron transfer
For chlorine reacting with bromide, Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂ conserves both atoms and charge. Chlorine gains electrons and bromide loses electrons. The product halogen is different from the starting halide ion; the accompanying cation can be a spectator in this ionic model.
Use the reactivity relationship to predict a new comparison, then distinguish that prediction from an observation actually made. The lesson’s halogen example develops an OCR practical group; suitable alternative activities can cover the required apparatus and techniques when selected by the centre.
Worked applications
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Example 1Predict the displacement result when chlorine is added to a suitable bromide solution, and state the ionic equation.
Example 2Should iodine displace chlorine from chloride in this model?
Common mistakes to avoid
Key takeaways
- You can use controlled comparisons to infer a halogen reactivity order.
- You can distinguish a halogen from its halide ion.
- You can explain a displacement using a balanced ionic equation.
Apply it in an exam
Question 1: Should iodine displace chlorine from chloride in this model?
Show answer and reasoning
No displacement is predicted by this reactivity model; actual observations still need appropriate controls.
- Iodine is less reactive than chlorine in the stated order.
- The displacement direction is not supported.
Explore further
- OCR J248 official specification (opens in a new tab)
Check the assessed scope and practical or mathematical requirements.
- Read or explore this topic (opens in a new tab)
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Return to this tomorrow
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
- Why state whether a colour is aqueous or in another phase?
- What happens to chlorine in the stated bromide reaction?
Test yourself
Answer a few questions, then review anything you missed.
Keep practising
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.