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Better Equipped Practical Teaching Guides
A complete A-Level Chemistry practical guide to investigating how initial iodide concentration affects reaction rate using an iodine clock, including collision theory, fixed-endpoint timing, relative rate using 1/t, initial-concentration calculations, reaction-order analysis, uncertainty, troubleshooting, advanced evaluation and technician support.
Teacher note: This resource supports practical teaching and equipment planning. Teachers should adapt the method and risk assessment to their examination-board specification, exact reagent concentrations and products, current supplier safety data, CLEAPSS or SSERC guidance where applicable, local laboratory procedures and departmental policies.
The rate of a chemical reaction describes how quickly reactants are converted into products. In this practical, students investigate how changing the initial concentration of iodide ions affects reaction rate while the other reagent concentrations, total reaction volume, temperature, mixing method and clock endpoint are kept constant.
The reaction is monitored using an iodine clock. Iodine is produced after the reactants are mixed but is initially removed by a fixed amount of thiosulfate. Once the thiosulfate has been consumed, iodine or triiodide can accumulate and form a sudden blue-black complex with starch. The time taken to reach this fixed visible endpoint provides a relative measure of the initial rate.
Core enquiry question: How does changing the initial iodide concentration affect the relative initial rate of the iodine clock reaction, and what does the data suggest about the order with respect to iodide ions?
Reaction rate is a measure of how quickly reactants are converted into products. It can be expressed using a change in concentration, amount or another measurable quantity per unit time. At constant temperature, increasing concentration places more reacting particles in each unit volume. This increases collision frequency. The approximate fraction of collisions with sufficient energy is unchanged, but more successful collisions can occur each second.
Rate
rate = change in concentration ÷ time
Typical concentration-rate units: mol dm−3 s−1
Hydrogen peroxide oxidises iodide ions in acidic solution:
H2O2(aq) + 2I−(aq) + 2H+(aq) → I2(aq) + 2H2O(l)
The exact dependence of rate on reactant concentration must be determined experimentally and is represented by the reaction order.
Iodine is removed rapidly by thiosulfate:
I2(aq) + 2S2O32−(aq) → 2I−(aq) + S4O62−(aq)
When all thiosulfate has been consumed, iodine can accumulate and starch gives the sudden blue-black endpoint. In excess iodide, dissolved iodine may also be represented as triiodide; the iodine-starch description here is a simplified school-level model.
One mole of iodine reacts with two moles of thiosulfate. Keeping the thiosulfate amount and concentration constant therefore fixes the amount of iodine that must be produced before the endpoint is reached.
With 2.0 cm3 of 0.0100 mol dm−3 thiosulfate, approximately 1.00 × 10−5 mol of I2 is consumed before iodine can begin to accumulate; the visible endpoint then requires a small excess above this fixed threshold.
If the same fixed amount of iodine is produced in less time, the reaction is faster.
As a reaction proceeds, reactants are consumed and concentrations decrease. A clock method therefore uses a short interval in which only a small amount of rate-forming reactant is consumed. Under these conditions, the average rate over the clock interval can approximate the initial rate.
Relative rate
relative rate ∝ 1/t
Important distinction
1/t has units of s−1 and is a relative rate. It is not the same as an actual concentration-change rate in mol dm−3 s−1.
Suggested filename: relative_rate_and_one_over_t_explained.jpg 
Technique: use clean, dry, clearly labelled measuring equipment for each solution to reduce cross-contamination and unintended dilution.
Mixture A: KI + water = 50.0 cm3, plus 2.0 cm3 thiosulfate and 2.0 cm3 starch.
Mixture B: 10.0 cm3 sulfuric acid + 2.0 cm3 hydrogen peroxide.
Final volume in every trial: 66.0 cm3.
| Reagent | Working concentration | Volume per trial | Variable or fixed? |
|---|---|---|---|
| Potassium iodide, KI(aq) | 0.100 mol dm−3 | 10.0-50.0 cm3 | Variable |
| Distilled or deionised water | - | 40.0-0.0 cm3 | Adjusted to compensate |
| Sodium thiosulfate, Na2S2O3(aq) | 0.0100 mol dm−3 | 2.0 cm3 | Fixed |
| Soluble starch indicator | Approximately 0.40% (w/v) | 2.0 cm3 | Fixed |
| Hydrogen peroxide, H2O2(aq) | 0.250 mol dm−3, approximately 3 volume or 0.8-0.85% (w/v) | 2.0 cm3 | Fixed |
| Sulfuric acid, H2SO4(aq) | 0.250 mol dm−3 | 10.0 cm3 | Fixed |
| Experiment | 0.100 mol dm−3 KI / cm3 | Water / cm3 | Initial [I−]0 after final mixing / mol dm−3 |
|---|---|---|---|
| 1 | 10.0 | 40.0 | 0.0152 |
| 2 | 20.0 | 30.0 | 0.0303 |
| 3 | 30.0 | 20.0 | 0.0455 |
| 4 | 40.0 | 10.0 | 0.0606 |
| 5 | 50.0 | 0.0 | 0.0758 |
Calculating the initial iodide concentration after mixing
[I−]0 = 0.100 × (VKI / 66.0)
Critical control: only the initial iodide concentration should change between trials. Thiosulfate, starch, hydrogen peroxide and sulfuric acid amounts, final volume, temperature, timing, mixing and endpoint judgement must remain consistent.
The table below reflects the small-scale recipe in this guide and is a model only. Complete and approve a local risk assessment using the exact products, concentrations and quantities in your laboratory, current supplier safety data sheets and relevant local guidance.
| Material or activity | Main practical risk | Control measures / PPE | Response, disposal and notes |
|---|---|---|---|
| 0.250 mol dm−3 sulfuric acid, 10.0 cm3 per trial | Acidic solution; splashes may irritate skin and eyes. | Wear splash-resistant eye protection. Use labelled measuring apparatus. Avoid skin and eye contact and keep containers closed. | Rinse splashes immediately with plenty of water and report exposure. Clean spills promptly using the local procedure. |
| 0.250 mol dm−3 hydrogen peroxide, 2.0 cm3 per trial | Splashes may irritate eyes and skin. Contamination may affect reagent stability. | Wear eye protection. Use clean, dedicated apparatus. Avoid contact with skin, eyes and clothing. Keep the bottle closed and protected from contamination. | Rinse splashes immediately with water and report them. Remake contaminated solutions if necessary. |
| 0.100 mol dm−3 potassium iodide, up to 50.0 cm3 per trial | Low practical hazard at this concentration, but avoid ingestion and eye contact. | Wear eye protection. Never pipette by mouth. Use labelled apparatus and wash hands after the practical. | Clean splashes promptly and follow the supplier SDS and local aqueous-waste procedure. |
| 0.0100 mol dm−3 sodium thiosulfate, 2.0 cm3 per trial | Low practical hazard; spilled liquid may cause contamination or slipping. | Keep containers labelled and capped. Avoid unnecessary contact and wipe small spills promptly. | Wash hands after use and dispose of completed mixtures according to the local approved route. |
| Soluble starch indicator, approximately 0.40% (w/v), 2.0 cm3 per trial | Low practical hazard; splashes and spills may contaminate surfaces or create a slip risk. | Use labelled apparatus and keep the working area clear and dry. | Wipe spills promptly. Dispose of with the completed reaction mixture where locally approved. |
| Iodine / triiodide formed in situ; endpoint threshold approximately 1.00 × 10−5 mol I2 per trial | May stain skin, clothing and surfaces; avoid direct contact and uncontrolled splashing. | Use only the specified small-scale recipe. Keep completed mixtures in the designated area and avoid splashing. | Transfer reaction mixtures to the labelled iodine-containing aqueous waste stream or follow the approved local disposal route. |
| Glassware and liquid spillages | Broken glass may cause cuts; spills may cause slips and chemical contact. | Inspect glassware before use, keep the workspace uncluttered and wear eye protection throughout. | Report breakages immediately. Use a brush and pan, not hands, to collect broken glass. Follow local spill procedures. |
Minimum controls: splash-resistant eye protection, suitable laboratory clothing and closed footwear, one student pouring while another starts the stopwatch where possible, fixed 66.0 cm3 total reaction volume, separate labelled apparatus for each reagent and prompt reporting of spills, breakages or exposure incidents.
Put on safety goggles and place the 250 cm3 conical flask on a white tile. Check that all reagents are at the same temperature and record the temperature before beginning.
Measure the required volume of 0.100 mol dm−3 potassium iodide into the conical flask. Add enough distilled water to make the combined KI + water volume 50.0 cm3. Then add 2.0 cm3 sodium thiosulfate solution and 2.0 cm3 starch indicator. Swirl gently to mix.
Measure 10.0 cm3 of 0.250 mol dm−3 sulfuric acid and 2.0 cm3 of 0.250 mol dm−3 hydrogen peroxide into a separate labelled vessel. Prepare one experiment at a time and do not combine mixtures A and B until the stopwatch is ready.
Pour mixture B rapidly into mixture A. Start the stopwatch as the two solutions first make contact.
Swirl the flask through three complete rotations over approximately two seconds. Return the flask to the white tile and observe it under consistent lighting.
Stop the stopwatch at the first uniform blue-black colour that persists. Record the time to the nearest 0.1 s, or to the resolution of the instrument being used.
Dispose of the completed mixture as instructed and prepare a fresh reaction mixture. Obtain at least three time measurements for each initial iodide concentration. Investigate anomalous results rather than simply ignoring them.
Calculate the mean time for each iodide concentration and then calculate the relative rate using 1 divided by the mean time. Plot relative rate against the initial iodide concentration after all solutions have been mixed.
Technique for reliable timing: where students work in pairs, one student should pour mixture B while the other starts the stopwatch as the liquids first make contact. Keep the same roles, swirling method, endpoint judgement and lighting for all trials.
For each concentration, calculate a mean time from repeat measurements and then calculate the reciprocal. The reciprocal is used because the clock endpoint corresponds to the production of the same fixed amount of iodine in every trial.
| Initial [I−]0 / mol dm−3 | Trial 1 / s | Trial 2 / s | Trial 3 / s | Mean time, t / s | Relative rate, 1/t / s−1 |
|---|---|---|---|---|---|
| 0.0152 | |||||
| 0.0303 | |||||
| 0.0455 | |||||
| 0.0606 | |||||
| 0.0758 |
Mean time
t̄ = sum of valid repeat times ÷ number of valid repeats
Relative rate
relative rate = 1/t̄
If the mean time is 25.0 s:
1 / 25.0 = 0.0400 s−1
x-axis: initial iodide concentration, [I−]0 / mol dm−3
y-axis: relative rate, 1/t̄ / s−1
Expected trend: as initial iodide concentration increases, the blue-black endpoint should appear sooner, the measured time should decrease and 1/t should increase. A straight-line relationship through or close to the origin would be consistent with first-order behaviour with respect to iodide under the fixed conditions of this investigation.
| Initial iodide concentration | Expected time to blue-black endpoint | Expected relative rate, 1/t |
|---|---|---|
| Low | Long | Low |
| Medium | Moderate | Intermediate |
| High | Short | High |
| Mistake | Consequence |
|---|---|
| Starting the timer late | Measured time is too short, so 1/t is too large and the relative rate is overestimated. |
| Stopping the timer late | Measured time is too long, so 1/t is too small and the relative rate is underestimated. |
| Judging the endpoint too early | The measured time is too short and the relative rate is overestimated. |
| Judging the endpoint too late | The measured time is too long and the relative rate is underestimated. |
| Inaccurate volume measurements | The intended initial concentrations are incorrect. |
| Poor or inconsistent mixing | Results become less consistent and the timing no longer reflects the same mixing conditions. |
| Using only one trial | Reliability is reduced and anomalous values are harder to identify. |
Quick examiner tip: describing the trend is not enough for a high-mark explanation. Link increased concentration to more particles per unit volume, greater collision frequency, more successful collisions per second and therefore a faster reaction.
Reaction order is determined experimentally. When comparing two trials, all other initial concentrations and the temperature must be held constant.
If concentration doubles and the relative rate approximately doubles, the reaction is first order with respect to the reactant being varied.
If concentration doubles and the relative rate approximately quadruples, the reaction is second order with respect to that reactant.
If concentration doubles and the relative rate remains approximately unchanged, the reaction is zero order with respect to that reactant.
Rate-ratio relationship
rate2 / rate1 = ([I−]2 / [I−]1)n
Calculate the order, n
n = log(rate2 / rate1) / log([I−]2 / [I−]1)
| Concentration | Relative rate |
|---|---|
| 0.10 | 0.020 |
| 0.20 | 0.040 |
| 0.30 | 0.060 |
When concentration doubles from 0.10 to 0.20, the rate doubles from 0.020 to 0.040. This is consistent with first-order behaviour with respect to that reactant.
Higher-level point: varying only iodide allows students to determine the order only with respect to iodide. A full rate equation requires separate concentration series in which the other rate-forming reactants are varied independently.
Initial iodide concentration after all solutions have been mixed.
Time to the first persistent uniform blue-black endpoint, processed as the relative rate 1/t.
Hydrogen peroxide, sulfuric acid, thiosulfate and starch amounts; final volume; temperature; flask; mixing; timing; lighting and endpoint judgement.
| Source | Effect | Improvement |
|---|---|---|
| Human start/stop time | Timing uncertainty | One student pours while another starts the timer; use automated timing where available. |
| Subjective blue-black endpoint | Endpoint variation | Use fixed lighting or a colorimeter threshold. |
| Volume measurement | Incorrect initial concentrations | Use suitable volumetric pipettes for fixed volumes and a graduated pipette or burette for variable KI volumes. |
| Temperature variation | Changed reaction rate | Use a thermostatically controlled water bath and allow solutions to reach the target temperature before mixing. |
| Inconsistent swirling | Variable mixing and timing | Standardise the number, duration and style of swirls in every trial. |
| Single measurements | Poor reliability and difficulty identifying anomalies | Repeat each condition several times and calculate a mean. |
When the endpoint is reached very quickly, mixing and human reaction time form a larger fraction of the measured value. This can dominate the relative uncertainty.
A long clock interval allows greater reactant depletion, so the average rate over the interval becomes less representative of the true instantaneous rate at t = 0.
Advanced improvement: using a colorimeter and data logger can make endpoint detection more objective. Continuous monitoring of iodine concentration would be a different method and could provide more detailed kinetic information than a single clock endpoint.
| Problem | Possible cause | Solution |
|---|---|---|
| No blue-black colour appears | Starch omitted or ineffective; hydrogen peroxide, iodide or acid omitted; degraded hydrogen peroxide; too much thiosulfate. | Check the recipe and reagent identity, confirm measured volumes, and prepare a fresh mixture. |
| Colour change happens immediately | Thiosulfate omitted or too little added; iodine contamination; reaction began before timing; wrong concentration or volume. | Check thiosulfate addition, reagent concentrations and the order of mixing, then repeat with a fresh mixture. |
| Colour change takes too long | Too much thiosulfate; iodide or peroxide concentration too low; mixture too cold; degraded hydrogen peroxide. | Check concentrations, temperature and measured volumes, then repeat using fresh reagents if needed. |
| Results vary widely | Inconsistent timing, mixing, temperature or endpoint judgement. | Standardise the method, keep the same operator roles where possible and repeat the trials. |
| Unexpected trend | Incorrect KI or water volume, cross-contamination, inconsistent total volume or temperature drift. | Recheck measurements, dedicated apparatus, total-volume control and solution temperature before repeating. |
Quick check before repeating: correct reagent concentrations, accurate volume measurements, timer started immediately, consistent mixing, clean dry glassware and a clearly defined endpoint.
Incorrect. At constant temperature, increasing concentration means more particles per unit volume. Collision frequency increases; particle speed is not increased by concentration itself.
Incorrect. The fixed thiosulfate amount means the same fixed amount of iodine must be produced before the endpoint in every trial.
Incorrect. Reactants are consumed, so their concentrations and the rate usually change as the reaction proceeds.
Incorrect. Iodine is initially removed by thiosulfate. The visible colour appears only after the thiosulfate has been consumed.
A fixed amount of thiosulfate requires the same fixed amount of iodine to be produced before the endpoint. The average iodine-production rate is therefore proportional to 1/t, provided the total volume and endpoint amount remain constant.
Keeping the total reaction volume constant means changing the KI volume changes the initial iodide concentration without unintentionally changing the concentrations of the fixed-amount reagents.
There are more reacting particles per unit volume, so collisions occur more frequently. This leads to more successful collisions per second.
Repeats improve reliability and precision, allow a mean to be calculated and help identify anomalous measurements.
Human reaction time when starting or stopping the stopwatch, subjective judgement of the first persistent blue-black endpoint, volume measurement uncertainty or temperature variation.
Use a colorimeter with data logging to detect a defined optical endpoint automatically, alongside improved temperature control and more precise volumetric apparatus.
Examiner tip: do not stop at "more collisions". For higher-mark explanations, link the chain explicitly: more particles per unit volume → greater collision frequency → more successful collisions per second → faster reaction.
| Activity | Suggested time |
|---|---|
| Introduction and theory | 15 min |
| Apparatus setup and safety briefing | 10 min |
| Preparation and pilot trial | 10-15 min |
| Full investigation, repeats and clean-up | 30-45 min |
| Data processing | 15 min |
| Evaluation and discussion | 10 min |
Examiner advice summary:
Explore chemistry glassware, measuring equipment, timers, laboratory consumables and other practical resources for teaching rates of reaction and physical chemistry.
Browse chemistry equipmentAbout this guide: This web page is designed to support the accompanying Better Equipped A-Level Chemistry practical resource. It is intended as teaching and equipment-planning support and should be used alongside the relevant examination-board specification, approved local risk assessment and current laboratory safety guidance.
This guide was written and reviewed by Better Equipped's technical team and further reviewed by former A Level Science Teachers. Our technical team draw on experience supplying practical science equipment to schools, colleges, laboratories and science departments throughout the UK. They include ex-school laboratory technicians and are here to support schools, colleges and laboratories. If you have feedback on this guide, we'd love to here it so please contact us. Don't forget we will be regularly updating our guides and resources on our Better-Resources Hub. If you would like us to cover a particular subject matter in these guides or have some top tips you'd like to share then again we'd love to hear from you.
Last reviewed and updated: August 2026
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