# How Fast a Reaction Goes

Laplace’s Mini World · Plate 93 · Matter, Light & Sound · Matter

> Two clear solutions poured together, a wait, then all at once blue-⁠black. More iodide, shorter wait.

- Interactive model: https://world.lapla.si/en/reactionrate/
- Embed: https://world.lapla.si/embed/en/reactionrate/
- 中文: https://world.lapla.si/zh/reactionrate/
- Updated: 2026-11-04

## 01 Five Beakers, Mixed at Once

Five 250 mL beakers stand on five magnetic stirrers. Into each go 100 mL of solution A and 100 mL of solution B at the same moment. Both are clear, and so is the mixture.

A holds potassium iodide, sodium thiosulfate and starch; B is dilute hydrogen peroxide with acetic acid. The beakers differ in one thing only: the iodide in A, from 50 g down to 10 g a litre.

In this page’s model at 20 °C, the beaker with the most iodide turns blue-⁠black after 19.6 seconds, the next ones at 24.5, 32.7 and 49.1 seconds, and the one with the least waits 98.2 seconds. The Royal Society of Chemistry gives this recipe as about 20 seconds at room temperature.

*50, 40, 30, 20, 10 g of KI a litre · blue at 19.6, 24.5, 32.7, 49.1, 98.2 s (20 °C, this page’s model)*

## 02 While Nothing Seems to Happen

Nothing stops during those 20 seconds. All along, the peroxide turns iodide into iodine: H₂O₂ + 2I⁻ + 2H⁺ → I₂ + 2H₂O. In this page’s model it makes about 0.49 millimoles of iodine per litre each second.

Each bit of iodine is turned straight back into iodide by thiosulfate: I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻. That step is much faster, so the liquid stays clear and the iodide never runs low.

At 19.6 seconds the thiosulfate is gone. The iodine now stays, mostly joined to iodide as I₃⁻, which slips inside the coils of the starch and shows blue-⁠black. Enough of it to show builds up within a fraction of a second, so the change looks sudden. By then only about 2% of the peroxide has been used.

*About 0.49 mmol of iodine per litre each second · thiosulfate gone at 19.6 s · only about 2% of the peroxide used*

## 03 Half the Iodide

This step goes in proportion to the iodide, and in proportion to the peroxide: rate = k\[H₂O₂\]\[I⁻\]. Halve the iodide and iodine is made half as fast, so eating through the same thiosulfate takes twice as long. Among the five beakers, 40 g a litre takes 24.5 seconds and 20 g takes 49.1.

In one of the Royal Society of Chemistry’s practical videos the peroxide is changed instead: five flasks with 30, 25, 20, 15 and 10 mL turned blue at 15, 20, 25, 32 and 46 seconds. From 30 mL to 15 mL, the wait went from 15 seconds to 32.

Not every reaction behaves like this. If the rate does not depend on a substance at all (zero order), halving it changes nothing; if it goes with the square (second order), the rate drops to a quarter. Orders are found by experiment; they cannot be read off the balanced equation.

*Rate = k\[H₂O₂\]\[I⁻\] · half the iodide, about twice the wait · measured: 30 mL of peroxide 15 s, 15 mL 32 s*

## 04 Collisions and the Barrier

To react, molecules first have to meet. In water they jostle without end (see Brownian Motion). Estimated from how fast they diffuse, an iodide ion in this beaker meets a peroxide molecule about 3 billion times a second, yet reacts only about once in 5 minutes. That is an order of magnitude, no more.

Most meetings come to nothing because they are not hard enough. The energy has to clear a barrier before the old bonds break, the activation energy. For this step it is 56 kJ⁠/⁠mol, and at 20 °C only about one molecule in ten billion has that much.

Enough energy is not all: the molecules must also meet the right way round. Hit at the wrong place, they bounce apart.

*Each iodide meets peroxide about 3 × 10⁹ times a second · reacts about once in 5 minutes · about 1 × 10⁻¹⁰ over 56 kJ⁠/⁠mol (20 °C)*

## 05 Ten Degrees Warmer

The same recipe waits for different times at different temperatures. In this page’s model it takes 44 seconds at 10 °C, 19.6 seconds at 20 °C, 9.2 at 30 °C and only 4.5 at 40 °C.

Warmer molecules meet harder, so the small share that clears the barrier grows: from 20 °C to 30 °C it becomes about 2.1 times as large, and the reaction goes about 2.1 times as fast. That comes from the Arrhenius equation, k = A·exp(−Ea/RT), with Ea at 56 kJ⁠/⁠mol.

The familiar “10 °C warmer, twice as fast” is only a rough rule for many reactions near room temperature. The higher the barrier, the more warming helps; an exact doubling from 20 °C to 30 °C goes with an activation energy of about 51 kJ⁠/⁠mol.

*10 °C 44 s · 20 °C 19.6 s · 30 °C 9.2 s · 40 °C 4.5 s (this page’s model, Ea = 56 kJ⁠/⁠mol)*

## 06 Lumps and Powder

The two flasks on the shelf hold the same dilute hydrochloric acid, one with lumps of marble, one with marble powder. Both give off carbon dioxide: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. Over the powder the bubbles come thick and fast.

The acid can only reach the surface of the solid. Take the same marble from 1 cm cubes down to 0.1 mm grains, and its surface becomes 100 times as large.

Faster is not more. In the Royal Society of Chemistry’s example, 1 g of marble in 100 mL of 1 M hydrochloric acid gives the same gas in the end as lumps or as powder, about 240 mL by this page’s count; the powder only gets there sooner.

Surface area matters only where a solid takes part. The beakers in the row mix two solutions, so there is no surface to speak of. Fine burnable dust in the air can explode: in 2008 a sugar dust explosion at an American refinery killed 14 people.

*1 cm lumps to 0.1 mm powder: surface × 100 · 1 g of marble ends at about 240 mL of CO₂ either way (20 °C)*

## 07 A Black Powder That Stays

The conical flask on the shelf holds hydrogen peroxide with a pinch of black manganese dioxide at the bottom. Peroxide breaks down into water and oxygen slowly on its own; with the manganese dioxide, oxygen pours out at once: 2H₂O₂ → 2H₂O + O₂. A little washing-⁠up liquid in the flask turns it into foam.

When it is over, the manganese dioxide is still there; it can be filtered off, dried and weighed back. A catalyst gives the reaction another path with a lower barrier. Manganese dioxide does it on its surface; we found no reliable figure for how far it lowers the barrier. The measured case is another catalyst, iodide: about 75 kJ⁠/⁠mol for peroxide breaking down alone, about 56 kJ⁠/⁠mol with iodide ions. That 56 is the clock’s own slow step: both reactions begin with iodide meeting peroxide.

Living things have catalysts too. The catalase in puréed potato breaks down peroxide as well; discs of potato are much slower. In 16 catalases measured, one molecule broke down 54 000 to 833 000 peroxide molecules a second.

A catalyst speeds the reverse reaction just as much, so it does not change how far a reaction goes in the end (see Pale in Ice, Dark in Heat); for iron and vanadium oxide at work in industry, see Nitrogen from the Air and Sulfuric Acid from Sulfur.

*2H₂O₂ → 2H₂O + O₂ · the manganese dioxide is not used up · measured with iodide: about 75 kJ⁠/⁠mol alone, about 56 kJ⁠/⁠mol with iodide*

## 08 A Clock Beating Seconds

In the 1860s, Harcourt and Esson measured this very reaction: hydrogen peroxide with hydriodic acid. They let sodium thiosulfate take up the iodine, used starch to show it, and timed it against “a clock beating seconds”. Their finding: at each moment, the amount of change is proportional to the amount of substance still changing.

In 1884 van ’t Hoff published his Études de dynamique chimique; in 1901 he won the first Nobel Prize in Chemistry. In 1886 Landolt described the reaction of iodic and sulfurous acid: with starch in it, the liquid stays clear for seconds or minutes, then suddenly turns deep blue, and under the same conditions the delay is the same every time.

In 1889 Arrhenius, studying sugar breaking down in acid, proposed that only a small share of “active” molecules react, and that warmth makes the share larger; today’s Arrhenius equation comes from this. His Nobel Prize of 1903 was for something else, his theory of ions. In 1895 Harcourt and Esson measured temperature as well: their reaction went about 2.1 times as fast at 10 °C as at 0 °C.

*1866, 1867 Harcourt and Esson · 1884 van ’t Hoff · 1886 Landolt · 1889 Arrhenius · 1901 the first Nobel Prize in Chemistry*

## 09 Bench and Shelf

On the bench: five magnetic stirrers, each with a 250 mL low-⁠form beaker and a 25 mm stir bar; a stop clock with a 110 mm dial. On the shelf: a 250 mL conical flask of peroxide over manganese dioxide; a watch glass with manganese dioxide and a spatula; two 100 mL conical flasks of dilute acid, one with marble lumps, one with marble powder.

*Stirrers · beakers · stop clock · shelf · peroxide · manganese dioxide · marble*

## 10 Set the Recipe, Start the Clock

The middle beaker follows your recipe: 10 to 50 g of potassium iodide a litre, half to twice the recipe’s thiosulfate, 10 to 40 °C. The stop clock runs from the moment the two solutions meet; when the thiosulfate is gone, the beaker turns blue-⁠black.

This is the page’s model: rate = k\[H₂O₂\]\[I⁻\], with k at 0.0115 M⁻¹ s⁻¹ at 25 °C and an activation energy of 56 kJ⁠/⁠mol; the rest follows the Royal Society of Chemistry’s recipe. Twice the thiosulfate means twice the iodine to eat first, so the wait comes out close to twice as long.

*This page’s model · k = 0.0115 M⁻¹ s⁻¹ (25 °C) · Ea = 56 kJ⁠/⁠mol*

**Try this**: In the lab, halve the iodide: how long until it turns blue? Then warm it by 10 °C?

## What the model leaves out

The bench is this page’s model, not a copy of any one set. The beakers follow DURAN’s 250 mL low form (70 mm across, 95 mm high), the flasks DIN ISO 1773, the stop clock one with a 110 mm dial. The waits come from the one slow step alone: rate = k\[H₂O₂\]\[I⁻\], with k and the activation energy as measured by Liebhafsky and Mohammad in 1933 (quoted by Vitz, 2007), taken as constant from 10 to 40 °C. At this pH the acid-⁠driven term adds about 0.3% and is left out; so is the slow direct reaction of peroxide with thiosulfate, which one study suggests could turn the low-⁠iodide beakers somewhat early, so “half the iodide, twice the wait” is exact in the model and only roughly so in a real beaker. Volumes are taken as adding up and the solutions as ideal; the pH comes from the acetate buffer’s concentrations, without activities. Twenty-⁠volume peroxide is taken as 1.66 mol a litre, after CLEAPSS. The colour comes up over 0.7 seconds; its depth, the stirring swirls, the bubbles and the foam are drawn, not computed. The meeting counts are estimated from diffusion, an order of magnitude only.

## Easy to get wrong

The sudden colour is not the reaction suddenly starting: iodine is made from the first moment and eaten by thiosulfate as it forms. “10 °C warmer, twice as fast” is a rough rule; this reaction speeds up about 2.1 times, others more or less. A hard enough collision may still not react if the molecules meet the wrong way round. Surface area matters only where a solid takes part, and 100 times the surface is not 100 times the speed. A catalyst is not used up, but it speeds the reverse reaction too and leaves the final yield unchanged. The peroxide in these demonstrations is much stronger than the 3% sold for the home: 20-⁠volume (5⁠–⁠8%) irritates the eyes, and over 8% is corrosive; even dilute hydrochloric acid may harm the eyes or skin.

## Common questions

### Why does the iodine clock turn blue all at once?

At 19.6 seconds the thiosulfate is gone. The iodine now stays, mostly joined to iodide as I₃⁻, which slips inside the coils of the starch and shows blue-⁠black. Enough of it to show builds up within a fraction of a second, so the change looks sudden.

See: [02 While Nothing Seems to Happen](https://world.lapla.si/en/reactionrate/#wait)

### Why does halving the concentration double the time?

This step goes in proportion to the iodide, and in proportion to the peroxide: rate = k\[H₂O₂\]\[I⁻\]. Halve the iodide and iodine is made half as fast, so eating through the same thiosulfate takes twice as long.

See: [03 Half the Iodide](https://world.lapla.si/en/reactionrate/#half)

### Why do most collisions between molecules not react?

Most meetings come to nothing because they are not hard enough. The energy has to clear a barrier before the old bonds break, the activation energy.

See: [04 Collisions and the Barrier](https://world.lapla.si/en/reactionrate/#collide)

### Does a 10 °C rise always double the rate?

The familiar “10 °C warmer, twice as fast” is only a rough rule for many reactions near room temperature. The higher the barrier, the more warming helps; an exact doubling from 20 °C to 30 °C goes with an activation energy of about 51 kJ⁠/⁠mol.

See: [05 Ten Degrees Warmer](https://world.lapla.si/en/reactionrate/#warm)

### Is a catalyst used up?

When it is over, the manganese dioxide is still there; it can be filtered off, dried and weighed back. A catalyst gives the reaction another path with a lower barrier.

See: [07 A Black Powder That Stays](https://world.lapla.si/en/reactionrate/#catalyst)

## Sources

1. [RSC / Nuffield: Iodine clock reaction demonstration (the recipe; about 20 seconds at room temperature; first order in H₂O₂ and I⁻; thiosulfate sets the time)](https://edu.rsc.org/experiments/iodine-clock-reaction-demonstration-method/744.article)
2. [RSC: Rates of reaction practical videos 16⁠–⁠18 (five flasks of different peroxide: 15, 20, 25, 32, 46 s)](https://edu.rsc.org/practical/rates-of-reaction-practical-videos-16-18-students/4014325.article)
3. [E. Vitz (2007): J. Chem. Educ. 84, 1156 (quoting Liebhafsky and Mohammad, 1933: k = 0.69 M⁻¹ min⁻¹, Ea = 56 kJ⁠/⁠mol)](https://doi.org/10.1021/ed084p1156)
4. [H. MacKeown et al. (2022): Water Research 217, 118417 (H₂O₂ with I⁻: k = 0.011 M⁻¹ s⁻¹; the triiodide constant)](https://doi.org/10.1016/j.watres.2022.118417)
5. [OpenStax: Chemistry 2e, 12.2 (surface area matters between phases; many rates roughly double for 10 °C)](https://openstax.org/books/chemistry-2e/pages/12-2-factors-affecting-reaction-rates)
6. [OpenStax: Chemistry 2e, 12.3 (rate laws; orders found by experiment)](https://openstax.org/books/chemistry-2e/pages/12-3-rate-laws)
7. [OpenStax: Chemistry 2e, 12.5 (collision theory: orientation and energy; the Arrhenius equation)](https://openstax.org/books/chemistry-2e/pages/12-5-collision-theory)
8. [OpenStax: Chemistry 2e, 12.7 and 13.3 (a catalyst lowers the barrier, is not consumed, speeds the reverse too, leaves the equilibrium alone)](https://openstax.org/books/chemistry-2e/pages/12-7-catalysis)
9. [J. Gutow (ed.): Chem 370 Physical Chemistry (LibreTexts, 6.1: diffusion-⁠controlled rate constants in solution)](https://chem.libretexts.org/Courses/University_of_Wisconsin_Oshkosh/Chem_370%3A_Physical_Chemistry_1_-_Thermodynamics_(Gutow))
10. [S. Pesek and R. Silaghi-⁠Dumitrescu (2024): Molecules 29, 641 (the starch⁠–⁠iodine blue: polyiodide in the amylose helix)](https://doi.org/10.3390/molecules29030641)
11. [D. Cheung (2007): Education in Chemistry, Investigating activation energies (peroxide decomposition: about 75 kJ⁠/⁠mol alone, 56 with iodide)](https://edu.rsc.org/feature/investigating-activation-energies/2020172.article)
12. [D. Fleming (2018): Education in Chemistry, The genie in the bottle (manganese dioxide filtered, dried and weighed back)](https://edu.rsc.org/exhibition-chemistry/the-genie-in-the-bottle/3009156.article)
13. [RSC: Hydrogen peroxide decomposition using different catalysts (foam out of the cylinder; a glowing spill relights)](https://edu.rsc.org/experiments/hydrogen-peroxide-decomposition-using-different-catalysts/831.article)
14. [Practical Biology: catalase and hydrogen peroxide concentration (the catalase in puréed potato; discs too slow)](https://practicalbiology.org/bio-molecules/factors-affecting-enzyme-activity/investigating-an-enzyme-controlled-reaction-catalase-and-hydrogen-peroxide-concentration.html)
15. [J. Switala and P. C. Loewen (2002): Arch. Biochem. Biophys. 401, 145 (16 catalases, 54 000 to 833 000 a second)](https://pubmed.ncbi.nlm.nih.gov/12054464/)
16. [RSC: Interpreting rate of reaction graphs (1 g of marble in 100 mL of 1 M acid; lumps and powder end at the same gas)](https://edu.rsc.org/lesson-plans/interpreting-rate-of-reaction-graphs-14-16-years/95.article)
17. [OSHA: Combustible Dust (fine powders can explode; the 2008 sugar dust explosion killed 14)](https://www.osha.gov/combustible-dust)
18. [CLEAPSS: Student Safety Sheet 57 (hydrogen peroxide: 5⁠–⁠8% an eye irritant, over 8% corrosive; what “volume” strength means)](https://science.cleapss.org.uk/Resource/SSS057-Hydrogen-peroxide.pdf)
19. [CLEAPSS: Student Safety Sheet 20 (dilute hydrochloric acid may still harm the eyes or skin)](https://science.cleapss.org.uk/Resource/SSS020-Hydrochloric-acid.pdf)
20. [A. G. Vernon Harcourt and W. Esson (1866, 1867): Phil. Trans. 156, 193; 157, 117 (peroxide and hydriodic acid; thiosulfate and starch; “a clock beating seconds”)](https://doi.org/10.1098/rstl.1867.0007)
21. [A. G. Vernon Harcourt and W. Esson (1895): Phil. Trans. A 186, 817 (Bakerian Lecture: temperature and the rate of change)](https://doi.org/10.1098/rsta.1895.0022)
22. [H. Landolt (1886): Berichte 19, 1317 (the time of the reaction of iodic and sulfurous acid)](https://doi.org/10.1002/cber.188601901293)
23. [S. Arrhenius (1889): Z. phys. Chem. 4, 226 (the rate of sucrose inversion by acids and temperature; “active” sucrose)](https://archive.org/details/sim_zeitschrift_physikalische_chemie_1889_4)
24. [NobelPrize.org: the Nobel Prize in Chemistry 1901 (van ’t Hoff, the laws of chemical dynamics and osmotic pressure)](https://www.nobelprize.org/prizes/chemistry/1901/summary/)
25. [NobelPrize.org: the Nobel Prize in Chemistry 1903 (Arrhenius, the electrolytic theory of dissociation)](https://www.nobelprize.org/prizes/chemistry/1903/summary/)
26. [DWK: DURAN low-⁠form beakers (ISO 3819; 250 mL: 70 mm across, 95 mm high)](https://katalog.hermeslab.sk/en/Beakers-glass-DURAN-sup-R-sup-low-form-p)
27. [DWK: DURAN narrow-⁠neck conical flasks (DIN ISO 1773; 250 mL: 85 × 145 mm; 100 mL: 64 × 105 mm)](https://katalog.hermeslab.sk/en/Erlenmeyer-flasks-narrow-neck-DURAN-sup-R-sup-p)

## Data

The clock’s recipe, its chemistry and its timing after the Royal Society of Chemistry and the Nuffield Foundation (Practical Chemistry: Iodine clock reaction demonstration; Rates of reaction practical videos 16⁠–⁠18); the rate constant and activation energy of hydrogen peroxide with iodide after E. Vitz (J. Chem. Educ., 2007), quoting H. A. Liebhafsky and A. Mohammad (J. Am. Chem. Soc., 1933), and H. MacKeown and co-⁠workers (Water Research, 2022); triiodide after the same review; rate laws, collision theory, the Arrhenius equation and catalysis after OpenStax Chemistry 2e; encounter rates after the LibreTexts physical chemistry text (J. Gutow, after S. Lower); the starch⁠–⁠iodine complex after S. Pesek and R. Silaghi-⁠Dumitrescu (Molecules, 2024). The waits, the timeline, the shares over the barrier and the encounter counts are computed here. Catalysis after the Royal Society of Chemistry (Classic chemistry demonstrations; Education in Chemistry, D. Cheung 2007 and D. Fleming 2018), Practical Biology and J. Switala and P. C. Loewen (Arch. Biochem. Biophys., 2002); marble and acid after the RSC’s Interpreting rate of reaction graphs; dust after OSHA; safety after CLEAPSS Student Safety Sheets 20 and 57; history after A. G. Vernon Harcourt and W. Esson (Phil. Trans., 1866, 1867, 1895), H. Landolt (Berichte, 1886), S. Arrhenius (Z. phys. Chem., 1889) and NobelPrize.org. The bench is this page’s model: 250 mL low-⁠form beakers of 70 × 95 mm (after DWK DURAN, ISO 3819), conical flasks after DIN ISO 1773 (250 mL: 85 × 145 mm; 100 mL: 64 × 105 mm), 25 mm PTFE stir bars, a stop clock with a 110 mm dial (after Hanhart); not a copy of any one set. Material presets and the model kit as on the site’s other model plates (public/looks⁠/⁠CREDITS.md).

## Licence CC BY-NC 4.0

The text, pictures and models on this page are licensed CC BY-NC 4.0: free to use and adapt for teaching and other non-commercial purposes, with credit to world.lapla.si. Third-party material listed above keeps its own licence.

https://creativecommons.org/licenses/by-nc/4.0/deed.en

## Cite this plate

Laplace’s Mini World, Plate 93, “How Fast a Reaction Goes”, https://world.lapla.si/en/reactionrate/, CC BY-NC 4.0, updated 2026-11-04.