Chemistry IA · Reactivity 2 · How much, how fast, how far? · topic 17 of 25

The vitamin C clock: finding activation energy · IB Chemistry IA idea: guide, sample graph and step-by-step plan

A colour change at a time you choose, then an Arrhenius plot from it. The clock does the timing, you do the thinking.

• ✓ you like a reaction that changes colour on cue
• ✓ you can keep temperatures steady
• ✓ you enjoy a straight-line Arrhenius plot

The big idea

In the vitamin C clock, hydrogen peroxide oxidises iodide to iodine, and ascorbic acid turns the iodine straight back. When the ascorbic acid runs out, iodine builds up and starch turns blue-black. The time tells you the average rate, which is proportional to 1/t. Warmer solutions are faster, and the Arrhenius equation links rate to temperature. A plot of ln(1/t) against 1/T gives a straight line whose gradient is −Ea/R. Everything depends on holding the other variables still and on timing short reactions reliably.

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.

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Your Chemistry IA research question: run the check

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

• Names the independent variable (temperature, with range in °C or K) and the dependent variable (time for the blue-black colour), and how the activation energy follows.
• Names every control: concentrations, volumes, starch, light and stirring, and how each is kept constant.
• States how each solution is brought to temperature before mixing, and how the mixture's temperature is checked.
• Says how the endpoint is judged, and what is the shortest time you can time reliably.
• Has at least 5 temperatures spread over a wide range, with 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 real volumes and temperatures, for the Method section.

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.

What your Chemistry IA graph could look like

Made-up data to show the form of a strong graph, not the answer. An Arrhenius plot with error bars on both axes. example only

• Temperature is in kelvin on the x axis. The factor 1000 keeps the numbers readable, so remember it for Ea.
• The horizontal error bars come from the thermometer, the vertical ones from timing.
• Draw max and min lines through the error bars to find the uncertainty of the gradient.
• Check the units: the gradient × R × 1000 gives Ea in J mol⁻¹, so convert to kJ mol⁻¹.
• Another graph to consider: time against temperature, to show how fast the reaction speeds up before you linearise.

Which uncertainty matters most?

Rank by the effect on your result, the gradient and so Ea, not on the raw readings. Short times at the hot end are the usual weak spot, but check what your own data say.

Your game plan

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

0 of 10 done

1. Read and predictFind the reactions behind the clock and the Arrhenius equation. Predict how time changes with temperature and how a plot of ln(1/t) against 1/T should look.
2. Trial run (1 lesson)Run it at two temperatures that are far apart. Is the colour change sharp? Is the warm run shorter than about 10 seconds? Adjust the concentrations until all runs can be timed.
3. Decide temperatures and repeatsChoose at least 5 temperatures from about 5 to 45 °C and the number of repeats. Write down why.
4. Plan temperature controlDecide how you bring both solutions to temperature, how long that takes, and how you check the mixture's temperature.
5. Collect the dataRecord bath temperature before and after, and the time to the first blue-black colour. Randomise the order of temperatures.
6. ProcessFind the mean time and spread at each temperature, then 1/t, ln(1/t) and 1/T in kelvin, with uncertainties.
7. Graph and fitPlot ln(1/t) against 1/T with error bars. Fit a line and get the gradient and its uncertainty from max and min lines.
8. Compare and concludeConvert the gradient to Ea, compare with a cited value (with conditions) and say what your uncertainty allows.
9. EvaluateWhich uncertainty or systematic error mattered most? 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

• IB Chemistry data booklet →
• Uncertainties and error bars (physics examples, same rules) →
• IB command terms explained →

Traps to avoid in this Chemistry IA

• Mixing before both solutions reach the bath temperature: the reaction runs at a temperature you did not record.
• Using °C in the Arrhenius plot. It has to be kelvin.
• Times under about 10 seconds at the warm end, where your reaction time is a large part of the reading.
• Treating 1/t as the rate without saying it assumes the same amount of reaction at the end of every run.
• A straight line forced through the data when the points bend, which hides a change in mechanism or a control that drifted.
• Forgetting the factor of 1000 (or R in the wrong units) when turning the gradient into Ea.

More Chemistry IA ideas like this

• Does the rate really double every 10 °C?●●○
• Iodine and propanone: which reactant sets the rate?●●○
• Iron thiocyanate equilibrium: how does K change with T?●●○