Iodine Clock Reaction | mysimulator.uk

Reactant Concentrations

Iodide [I⁻]₀ 0.020 M

Persulfate [S₂O₈²⁻]₀ 0.020 M

Thiosulfate scavenger [S₂O₃²⁻]₀ 0.60 mM

Temperature 25 °C

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Elapsed time

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Rate constant k

— M⁻¹s⁻¹

Instant. rate r

— M/s

Predicted t_clock

— s

How it works

This is the classic persulfate–iodide clock reaction (a Landolt-type clock). Two reactions run at once in the same well-mixed flask:

(1) S₂O₈²⁻ + 2 I⁻ → 2 SO₄²⁻ + I₂        (slow, rate-determining)
(2) I₂ + 2 S₂O₃²⁻ → 2 I⁻ + S₄O₆²⁻       (fast, "invisible" scavenger)

Reaction (1) is second order overall — first order in each reactant — with an Arrhenius-activated rate constant:

rate  r = k [I⁻][S₂O₈²⁻]
k(T)  = A · exp(−Eₐ / R T)

Every I₂ molecule made by (1) is instantly consumed by reaction (2) as long as thiosulfate remains — so no colour appears. Because (1) is slow relative to (2), [I⁻] and [S₂O₈²⁻] barely change before the scavenger runs out, so r stays ≈ constant and the scavenger's stock depletes linearly. The clock time is simply how long that stock lasts:

t_clock ≈ [S₂O₃²⁻]₀ / (2 r) = [S₂O₃²⁻]₀ / (2 k [I⁻]₀[S₂O₈²⁻]₀)

The instant the scavenger hits zero, newly formed I₂ starts to accumulate and immediately pairs with starch indicator to form the deep blue-black starch–I₂ complex — everywhere in the flask at once, since the solution is uniformly mixed. That sudden, simultaneous colour flip is why it is called a "clock": timing it at several concentrations is the standard lab method for measuring reaction order and, via runs at different temperatures, the activation energy Eₐ.

[I⁻]₀ / [S₂O₈²⁻]₀ sliders — raise the reaction rate, so the clock trips sooner (t_clock ∝ 1/[I⁻][S₂O₈²⁻]).
Thiosulfate slider — more scavenger means a longer delay before the colour appears (t_clock ∝ [S₂O₃²⁻]₀).
Temperature slider — raises k through the Arrhenius equation (Eₐ ≈ 45 kJ/mol used here), shortening the clock time roughly the way a real kinetics lab observes.

I⁻ / S₂O₈²⁻ solution Starch–I₂ complex

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