Halogen displacement: predict before observing
| English | 中文 | Pinyin |
|---|---|---|
| displacement reaction/dɪˈspleɪsmənt rɪˈækʃn/ | 置换反应 | zhì huàn fǎn yìng |
| halide ion/ˈhælaɪd ˈaɪɒn/ | 卤离子 | lǔ lí zi |
What would explain this observation?
- Chlorine water added to potassium bromide can produce bromine. Reversing the materials does not reverse the reaction: bromine cannot displace the more reactive chlorine.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- A more reactive halogen displaces a less reactive halogen from an aqueous solution of its salt. Chlorine displaces bromine and iodine; bromine displaces iodine but not chlorine; iodine displaces neither chlorine nor bromine. For chlorine and potassium bromide, Cl₂ + 2KBr → 2KCl + Br₂. Potassium remains in the solution while the identity of the free halogen changes.
- displacement reaction 置换反应: A reaction in which a more reactive element replaces a less reactive element in a compound; halide ion 卤离子: A negatively charged ion formed when a halogen atom gains an electron.
Which pair produces a displacement reaction?
A displacement table should identify the added halogen, starting halide and expected product before recording colour. Bromine in water is orange/brown and iodine solutions can appear brown; observation must be compared with suitable blanks, because the added halogen also has colour. Same-halogen/halide combinations have no net displacement. A missing colour change alone may also reflect low concentration or observation limits.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- A displacement table should identify the added halogen, starting halide and expected product before recording colour. Bromine in water is orange/brown and iodine solutions can appear brown; observation must be compared with suitable blanks, because the added halogen also has colour. Same-halogen/halide combinations have no net displacement. A missing colour change alone may also reflect low concentration or observation limits.
- For AT6 school-supervised work, use approved dilute halogen solutions in small-scale containers under the teacher’s risk assessment. Keep solution volumes and concentrations comparable, label each well and include reference colours. Do not generate chlorine or use an unapproved extraction solvent. Record actual observations, then compare with the predicted ranking; this activity is not one of the separately numbered required practicals.
Which two habits make the investigation or model in this case more defensible?
For AT6 school-supervised work, use approved dilute halogen solutions in small-scale containers under the teacher’s risk assessment. Keep solution volumes and concentrations comparable, label each well and include reference colours. Do not generate chlorine or use an unapproved extraction solvent. Record actual observations, then compare with the predicted ranking; this activity is not one of the separately numbered required practicals.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: chlorine added to chloride, bromide and iodide solutions gives two predicted displacement reactions out of three tests. Bromine gives one, and iodine zero, under the stated matrix. For Cl₂ + 2KI → 2KCl + I₂, four KI formula units correspond to two I₂ molecules in the balanced particle model.
For Cl₂ + 2KI → 2KCl + I₂, how many I₂ molecules correspond to six KI formula units? Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
For Cl₂ + 2KI → 2KCl + I₂, how many I₂ molecules correspond to six KI formula units?
The result is 3 molecules. Known: chlorine added to chloride, bromide and iodide solutions gives two predicted displacement reactions out of three tests. Bromine gives one, and iodine zero, under the stated matrix. For Cl₂ + 2KI → 2KCl + I₂, four KI formula units correspond to two I₂ molecules in the balanced particle model.
Check the conclusion and its limits
- Displacement releases the less reactive halogen, not the more reactive one. A metal-ion spectator does not establish the reactivity order. Do not claim all orange solutions contain bromine without considering the starting reagents and reference observations. The separate HT ionic-equation lesson extends the both-tier prediction.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Iodine can displace chlorine because iodine has a higher relative atomic mass. This claim is false: Displacement releases the less reactive halogen, not the more reactive one. A metal-ion spectator does not establish the reactivity order. Do not claim all orange solutions contain bromine without considering the starting reagents and reference observations. The separate HT ionic-equation lesson extends the both-tier prediction.
Halogen displacement: predict before observing: A displacement table should identify the added halogen, starting halide and expected product before recording colour. Bromine in water is orange/brown and iodine solutions can appear brown; observation must be compared with suitable blanks, because the added halogen also has colour. Same-halogen/halide combinations have no net displacement. A missing colour change alone may also reflect low concentration or observation limits.
Iodine can displace chlorine because iodine has a higher relative atomic mass.
Displacement releases the less reactive halogen, not the more reactive one. A metal-ion spectator does not establish the reactivity order. Do not claim all orange solutions contain bromine without considering the starting reagents and reference observations. The separate HT ionic-equation lesson extends the both-tier prediction.
A reaction in which a more reactive element replaces a less reactive element in a compound: write the technical term.
displacement reaction means A reaction in which a more reactive element replaces a less reactive element in a compound.