Chemistry IA · Reactivity 3 · Mechanisms of chemical change · topic 25 of 25

How fast do halogenoalkanes hydrolyse? · IB Chemistry IA idea: guide, sample graph and step-by-step plan

mechanism + a clever twist: good choice

Drop a tertiary halogenoalkane into silver nitrate and a cream precipitate appears. How fast depends on the solvent, and the reason says something about the mechanism.

Difficulty●●● Stretch
KitStandard: test tubes, water bath, light sensor or phone
TimeAbout 10 to 12 hours
FitsHL (SN1 and SN2); SL: compare 1°, 2°, 3°
  • ✓ you like mechanisms
  • ✓ you can design a fair timing method
  • ✓ you enjoy explaining *why* a rate changes

The big idea

← your hook: rate tells you the mechanism

A halogenoalkane reacts with water and the bromide it releases forms a cream precipitate with silver nitrate. The time to reach a fixed turbidity gives a rate. A tertiary compound like 2-bromo-2-methylpropane is thought to react by an SN1 mechanism, in which a charged intermediate forms first. If so, a more polar solvent should help. Change the share of water in an ethanol–water mixture and see. Your rate method is the weak point: judging cloudiness by eye varies from run to run, so a sensor will carry your marks.

Make it yours: pick one twist

Copying a classic won't show your choices. Pick one of these (or invent your own) and your IA becomes yours.

Swipe for more twists →

Compare primary, secondary and tertiary

Hold the solvent fixed and compare 1-bromobutane, 2-bromobutane and 2-bromo-2-methylpropane.

→ Your independent variable becomes the structure. The mechanism explanation is different: it links to SN1 and SN2.
Compare the halogen

Use chloro-, bromo- and iodo- versions of the same alkyl group.

→ Tests the effect of bond strength on rate. Your graph compares three halogens, each at a single solvent.
Change the temperature too

Repeat the solvent series at two temperatures and find the effect of temperature on each rate.

→ Lets you estimate an activation energy for each solvent mix, which links to Reactivity 2.
Use a light sensor or camera

Record brightness over time instead of watching by eye.

→ You get a curve, not one time per run. You choose a fixed threshold and justify it, and the rate method becomes much more objective.
Use conductivity instead

Follow the production of ions with a conductivity probe in the absence of silver nitrate.

→ A different method for the rate. It lets you compare your two methods in the evaluation.
Change the context

Link to halogenoalkanes in solvents, pesticides, or drug synthesis.

→ Gives your introduction a purpose; the method and graph stay the same.

Your Chemistry IA research question: run the check

write it FIRST, then check it

Write your own question first. Then make sure it ticks every box:

  • Names the independent variable and its range (which solvent compositions, and how many).
  • Names the dependent variable (time to a fixed turbidity) and says how the endpoint is judged, not by eye alone.
  • Says what stays the same: temperature, total volume, amount of halogenoalkane, silver nitrate concentration.
  • Says which halogenoalkane and why it was chosen, and what mechanism you are testing.
  • Is narrow enough for about 10 to 12 hours, with repeats at each solvent composition.

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The set-up

One way to set it up. Draw your own with your real volumes and the position of your sensor for the Method section.

thalogenoalkane last: start the clockbath held at constant Tsame lamp, same distance, dark roomsensor: time to a fixed reading

How your Chemistry IA could be laid out

The usual order for a topic like this, what each part needs to show, and roughly how much of your 3,000 words it takes. The words are all yours.

Research question and context~250
Your question; where halogenoalkane reactions matter; the mechanism you think applies and why.
Background chemistry~450
SN1 and SN2 mechanisms, with the charged intermediate and the rate-determining step; why a polar solvent might matter; the silver bromide test; how rate relates to time.
Variablestable
Independent, dependent and controlled variables, each with how you set, measure or keep it constant.
Method~450
How you made each solvent mix; how you held the temperature; how you added the halogenoalkane; how you detected the endpoint; repeats. Repeatable by someone else.
Safety, ethics, environment~80
2-bromo-2-methylpropane and the other halogenoalkanes are highly flammable and irritant: a few drops only, no flames, good ventilation or a fume cupboard. Ethanol is highly flammable. Silver nitrate (about 0.05 mol dm⁻³) irritates and stains skin: gloves and goggles. Silver and organic waste collected separately, never down the sink. Iodo- compounds only with your teacher's approval.
Raw datatable
Each timing for each composition, with units and uncertainties in the headers, and the endpoint you used.
Processing~250
Mean time, the rate you derive from it, and the uncertainty; why 1/time is an acceptable measure of rate here; one worked example.
Graphs and analysis~350
Rate (or its logarithm) against the percentage of water, with error bars, and what the shape says.
Conclusion~400
Answer with numbers: how the rate changed across your range, and how that supports or does not support your proposed mechanism.
Evaluation~650, the biggest part!
Strengths, weaknesses ranked by their effect on the rate, and improvements linked to each.
Referencesnot counted
Sources for the mechanism and any rate or solvent data you compare with.

What your Chemistry IA graph could look like

log scale, and say why

Made-up data, to show you the form of a strong graph, not the answer. Yours will look different. example only

10203040506070-3.0-2.8-2.5-2.2-2.0-1.8-1.5-1.2-1.0-0.8-0.5-0.20.0Water in the solvent / % by volume (± 0.5)log₁₀((1/t) / s⁻¹) (t ± 0.3 s)steady trend (made up)your data, with error barseach 10% more water:by what factor does the rate change?ILLUSTRATIVE · MADE-UP DATA
  • Plot the logarithm of the rate: a change of 100 times would squash a plain graph into the corner.
  • Error bars on every point. The vertical bars should come from the repeat timings, not a guess.
  • A straight line on a log scale means a constant factor for each step in water content. Say whether yours is.
  • State the endpoint you used. Rates from different endpoints cannot be compared.
  • Another graph to consider: 1/t against % water on ordinary axes, to show how fast the rate climbs.

Which uncertainty matters most?

Rank by the effect on the rate at the fastest composition, where the time is only a few seconds. Notice how a small timing error is large when the reaction is quick.

Judging the cloudiness by eye (± 2 s)32%Mixing and starting the clock (± 1 s)16%Temperature (± 0.5 K)6.1%Solvent composition (± 0.5% water)5.9%Light sensor threshold (± 0.3 s)4.8%biggest effect: discuss this first!% uncertainty in the rate, t about 6 s (made-up)ILLUSTRATIVE · MADE-UP NUMBERS

Your game plan

Tick steps off as you go. We'll remember on this device.

0 of 11 done

  1. Read and predictWrite the SN1 and SN2 mechanisms and decide which one a tertiary compound should follow. Predict how a more polar solvent should change the rate and why.
  2. Plan the rate methodDecide how you will detect the endpoint objectively and what fixed turbidity or brightness you will use. Check that the time is neither a few seconds nor an hour.
  3. Trial run (1 lesson)Try the extremes of your solvent range at one temperature. How long does each take? Is the mixture clear before the halogenoalkane is added, or is it already cloudy?
  4. Decide range, volumes and repeatsChoose at least five compositions, keep the total volume and the amount of halogenoalkane fixed, and decide on repeats. Write the reason for each choice.
  5. Control the temperatureDecide how you will hold the tubes at temperature and how you will check it, including after mixing.
  6. Collect the dataAlways add the halogenoalkane last and start timing at the same point. Record the time and note any cloudiness before the reaction started.
  7. ProcessTurn each mean time into a rate and decide how you will show the uncertainty. Say what 1/time assumes about the extent of reaction.
  8. Graph and interpretPlot the rate or its logarithm against solvent composition with error bars. What does the trend say about the mechanism? What else could explain it?
  9. EvaluateWhich uncertainty or systematic error mattered most? Rank them and make every improvement specific.
  10. Write, then cutWrite in the order of the layout above. Keep within 3,000 words: tables, graphs, equations and references are not counted, so let them carry detail.
  11. Check before your teacher's draftRun the checklist below, then get your RQ or draft checked. Your teacher reads one draft only, so make it count.

Chemistry you'll need

Traps to avoid in this Chemistry IA

by eye is the big one
  • Judging when it goes cloudy by eye: the endpoint changes from run to run. Use a sensor or a fixed phone reading.
  • Changing the total volume or the amount of halogenoalkane when you change the solvent mix, so two things change at once.
  • A halogenoalkane that does not dissolve in a water-rich mixture, so the cloudiness is the compound itself, not silver bromide.
  • Starting the timing at different points, such as adding the silver nitrate last in some runs and the halogenoalkane last in others.
  • Forgetting that the temperature of the mixture changes when the liquids are combined.
  • Claiming the mechanism is proved by one trend. Say what the data support, and what else could explain them.
  • Forgetting that silver ions can help pull the bromide off, so the silver nitrate concentration is a control you must hold and mention.

More Chemistry IA ideas like this

Make sure it's yours

This guide shows how a topic like this is usually approached. It is not an IA. Your question, choices, data, analysis and words must be your own: change something, tell your teacher where the idea came from, and follow your school's rules on outside help.

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