Chemistry IA · Reactivity 2 · How much, how fast, how far? · topic 19 of 25

Iodine and propanone: which reactant sets the rate? · IB Chemistry IA idea: guide, sample graph and step-by-step plan

a neat chance to catch a surprise

One of the reactants has no effect on the rate, and the data show which one. Can you prove it with a graph?

Difficulty●●○ Medium
KitColorimeter with blue filter, or stopwatch for timed colour loss
TimeAbout 10 hours
FitsHL (rate equations are AHL); SL: rate against [H⁺] only
  • ✓ you like rate laws that surprise you
  • ✓ you can mix and start a clock quickly
  • ✓ you enjoy testing an assumption with data

The big idea

← this is your RQ hook!

Propanone reacts with iodine, but the slow step involves the acid-catalysed formation of the enol, so iodine does not appear in the rate equation. The rate depends on propanone and on hydrogen ions. If you keep propanone and iodine fixed and vary the acid, the initial rate tells you the order with respect to H⁺. The usual way to measure it is by the loss of iodine colour. Check that absorbance falls on a straight line, then use initial rates from the first stage of the run, and test every order you claim.

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 →

Find all three orders

Run one series where only propanone changes, one where only iodine changes, and one where only the acid changes.

→ A full rate equation from three graphs. The iodine series is the surprising one, so it needs the best data.
Find the activation energy

Repeat one set of concentrations at several temperatures and plot ln k against 1/T.

→ Adds an Arrhenius analysis (HL), which needs good temperature control and kelvin on the axis.
Link to a mechanism

Write a mechanism with the enol as an intermediate and test whether your orders fit it.

→ Turns the results into an argument for the rate-determining step, not just a list of orders (HL).
Use a different catalyst

Compare hydrochloric acid with another acid at the same hydrogen ion concentration.

→ Tests whether the rate depends only on H⁺ or also on the anion, a stronger conclusion.
Switch from colorimeter to timing

Time how long a fixed amount of iodine colour takes to disappear.

→ A cheaper method with a bigger timing uncertainty. You compare how the two methods agree.

Your Chemistry IA research question: run the check

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

  • Names the independent variable (the reactant or catalyst whose concentration changes, with range) and the dependent variable (initial rate, with unit).
  • Names what stays fixed: the other concentrations, temperature, volumes, the cuvette and the wavelength or filter.
  • Says how the initial rate is found: which part of the absorbance-time graph, and how a calibration turns absorbance into concentration.
  • Gives the evidence you will use for an order: a straight line, a log-log gradient, or a ratio of rates.
  • Has at least 5 concentrations and repeats, and is narrow enough for about 10 hours.

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The set-up

One way to set it up. Draw your own version, with your real volumes and the wavelength or filter you used, for the Method section.

blue filter: iodineabsorbs blue lightsame cuvette, sameway round each timeabsorbance falls:is it a straight line?calibrate first:A against [I₂]mix, start the clock; usethe first 10 to 20 %

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; why the order matters (it tells you about the mechanism); the order you predict for each species and why.
Background chemistry~450
Rate equation and order (HL; SL students introduce them); the reaction and its acid catalysis; why absorbance tracks iodine (Beer–Lambert is not in the course, so cite it); what each graph should look like. Cite sources.
Variablestable
Independent, dependent and controlled variables, with how each control is held and checked.
Method~400
Apparatus with resolutions; stock solutions and dilution; calibration of the colorimeter; how you mix and start; recording; repeats; enough detail to copy.
Safety, ethics, environment~80
Specific hazards: propanone is highly flammable and irritates eyes (no flames, ventilate); iodine in potassium iodide solution stains and irritates; hydrochloric acid at your stock concentration (an irritant at 2 mol dm⁻³). Goggles; waste collected, iodine reduced with sodium thiosulfate first if your school does that.
Raw datatable
Absorbance against time for each run, units and resolutions in the headers; qualitative notes.
Processing~300
Calibration line; gradient of absorbance against time converted to a rate; uncertainty; one worked example.
Graphs and analysis~350
Absorbance against time (straight?); initial rate against concentration, with error bars and fit; how you decide the order.
Conclusion~450
Answer with the order and its uncertainty; say whether iodine really is zero order; link to the mechanism; compare with a cited rate law.
Evaluation~600, the biggest part!
Weaknesses ranked by effect on the order: temperature control, mixing and start time, fit range; improvements linked to each.
Referencesnot counted
Every source for the method and the accepted rate law.

What your Chemistry IA graph could look like

draw both models on the graph

Made-up data to show the form of a strong graph, not the answer. Initial rate against acid concentration, with a second-order curve to compare. example only

0.00.10.20.30.40.50.60.70.80.91.00246810121416Concentration of HCl in the mixture / mol dm⁻³ (± 0.005)Initial rate / 10⁻⁶ mol dm⁻³ s⁻¹ (± 0.6)first order: rate ∝ [H⁺]second order: rate ∝ [H⁺]²your initial ratesthe curves meet at 1.0 mol dm⁻³:which points tell them apart?ILLUSTRATIVE · MADE-UP DATA
  • The two curves are drawn so the reader can see which model the data fit. A straight line alone is not enough.
  • Error bars on every point; the horizontal ones are small, the vertical ones carry the fitting uncertainty.
  • A line through the origin is physically sensible here: no acid, no catalysed rate. Check the intercept anyway.
  • Plot a log-log version too if you want the order as a gradient with an uncertainty.
  • Another graph to consider: absorbance against time for one run, to show the zero-order straight line for iodine.

Which uncertainty matters most?

Rank by the effect on the initial rate. Temperature changes this rate a lot, so a water bath is the single biggest improvement for most set-ups.

Temperature drift (± 0.5 K)5.8%Slope fit from noisy absorbance (est.)3%Propanone by cylinder (± 0.5 cm³ on 25)2%Calibration of absorbance (est.)1.5%Acid volume by pipette (± 0.05 cm³)0.6%biggest effect: discuss this first!% uncertainty in the initial rate (made-up)ILLUSTRATIVE · MADE-UP NUMBERS

Your game plan

the trial run fixes your concentrations

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

0 of 10 done

  1. Read and predictFind the reaction, its rate equation and the idea of initial rates. Predict the order for each species and what each graph will look like.
  2. Trial run (1 lesson)Calibrate the colorimeter with a few iodine concentrations, then run one mixture. Does absorbance fall on a straight line? How long does a run take? Fix your concentrations and mixing.
  3. Decide concentrationsChoose at least 5 acid concentrations and fixed values for propanone and iodine. Make sure iodine is limiting and the runs are neither too fast nor too slow.
  4. Plan temperature controlDecide how you hold the temperature, and how you check it before and after each run.
  5. Collect the dataMix quickly, start the record, and log absorbance at set times. Repeat each concentration and randomise the order.
  6. ProcessConvert absorbance to concentration with your calibration. Take the gradient over the early, linear part to get the initial rate, with uncertainty.
  7. Find the orderPlot rate against concentration, and a log-log graph if useful. Compare models and decide the order with its uncertainty.
  8. Compare and concludeCheck the orders against a cited rate law and say what mechanism fits them.
  9. EvaluateWhich uncertainty or systematic error mattered most for the order? Rank them and make each improvement specific.
  10. Write, then checkWrite in the order of the layout, then run the checklist below before your teacher's single read.

Chemistry you'll need

Traps to avoid in this Chemistry IA

  • Assuming iodine is zero order without checking: look at your own absorbance-time graph and see whether it is straight.
  • Using the whole run to find the rate, so the late curved part pulls the gradient.
  • Slow mixing or starting the clock late, so the first seconds are not reliable.
  • Letting the temperature drift between runs when the rate is this sensitive to it.
  • Using absorbance directly as concentration without a calibration at your wavelength.
  • Claiming an order from two points, when a graph with at least five concentrations is needed.

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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