Compare halogenoalkane rates with a fair silver nitrate test
Design a controlled comparison that uses time to a defined silver halide endpoint rather than precipitate color as the rate measure.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Use the named reagents, solvents, temperatures and pressure conditions; begin curly arrows at a bond or lone pair and distinguish typical mechanisms from absolute rules. Written practice is self-checked
Before you begin
- Use covalent formulas, nucleophiles, electrophiles, homolytic and heterolytic fission, and the alkene and alcohol pathways in Topics 13–14.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What should be timed when comparing halogenoalkane reactivity with silver nitrate?
What you will learn
- Choose a comparable halogenoalkane series
- Control conditions that affect reaction rate
- Separate precipitate identity from reaction speed
Change one structural feature at a time
To compare the C–X bond, use halogenoalkanes with the same carbon skeleton and the same halogen-bearing carbon class, changing only X where possible. Otherwise, structural differences can also affect the observed rate.
Use equal amounts and concentrations of each sample, the same volume and concentration of ethanolic silver nitrate, the same temperature, mixing and vessel. Keep the endpoint definition consistent.
Time a defined endpoint
Record the time until the first persistent precipitate or another agreed visible endpoint. Repeat trials and compare consistent results; a shorter time indicates faster halide release under those controlled conditions.
The precipitate color identifies AgX: AgCl is white, AgBr cream and AgI yellow. Color is not itself the rate measurement. The schematic curves are illustrative and do not represent measured data.
Pause and explain
Why use the same temperature in each silver nitrate trial?
Worked example
A student compares 1-chlorobutane, 1-bromobutane and 1-iodobutane. State one suitable endpoint, two controls and how the result tests the C–X bond trend.
Show the worked solution
- Define the endpoint as time to first persistent precipitate.
- Keep temperature and silver nitrate concentration the same, as well as sample amounts and volumes.
- Compare matched primary butyl compounds so the halogen is the main structural change.
Answer Measure time to a first persistent precipitate while keeping temperature, concentrations, amounts and volumes constant. Faster AgX appearance for iodoalkane than chloroalkane is consistent with the weaker C–I bond.
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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.
Propose a fair comparison of 1-chloro-, 1-bromo- and 1-iodobutane using ethanolic silver nitrate.
Give me a hint
Change the halogen while matching the rest of the test.
Compare my reasoning
- Use equal sample amounts and the same volume and concentration of ethanolic silver nitrate.
- Keep temperature, vessel and mixing consistent.
- Time to the same defined endpoint for each trial.
Run matched trials at the same temperature with equal sample amounts and reagent volumes/concentrations; measure time to first persistent precipitate.
Look for these in your work
- One variable selected
- At least two controls stated
- Endpoint defined
A student reports that the yellow tube reacted fastest because its precipitate is brightest. What is wrong with this conclusion?
Give me a hint
Separate the solid's identity from the time it formed.
Compare my reasoning
- Yellow identifies AgI and supports iodide release.
- Brightness or color does not tell how long formation took.
- Compare measured times to the same endpoint instead.
The yellow precipitate supports AgI formation but does not measure rate; compare the controlled time to a defined endpoint.
Look for these in your work
- Color interpretation bounded
- Rate measure corrected
- Fair comparison stated
The iodoalkane trial is performed at 35 °C and the chloroalkane trial at 20 °C. Can the times isolate the C–X effect? Explain.
Give me a hint
Temperature is an independent rate factor.
Compare my reasoning
- The warmer trial can react faster for a temperature reason.
- The conditions are not matched.
- Repeat both at the same controlled temperature.
No. Temperature could explain part of the difference, so repeat at the same temperature before attributing the rate difference to C–X bonding.
Look for these in your work
- Confounder identified
- Inference limited
- Control proposed
Common mistakes
- Using precipitate color as the rate measurement
- Changing the carbon skeleton and halogen at the same time
- Comparing trials at different temperatures or with different reagent concentrations
What can you explain now?
What should be timed when comparing halogenoalkane reactivity with silver nitrate?
Compare with the explanation
Time to a defined precipitate endpoint
A consistent time endpoint permits a rate comparison.
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Make it stick
How do you distinguish silver-halide identity from reaction rate in an ethanolic silver nitrate test?
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.