Initial rate: iodine clock template

Stopwatch, starch, hydrogen peroxide in a measuring cylinder and a beaker of iodide, acid and thiosulfate on a white tile. A-level / AP / IB.

What the template shows

• Time how long a fixed amount of iodine takes to form; rate ∝ 1/t
• H₂O₂(aq), added last
• Pour it all in at once and start the clock
• Blue-black once all the thiosulfate is used up and free I₂ remains
• Stop at the first blue-black colour; repeat with other concentrations
• Reaction mixture
• KI, dilute H₂SO₄, a small fixed amount of Na₂S₂O₃, starch and water to the same total volume each run
• H₂O₂ + 2H⁺ + 2I⁻ → I₂ + 2H₂O, then I₂ + 2S₂O₃²⁻ → 2I⁻ + S₄O₆²⁻. Persulfate version: S₂O₈²⁻ + 2I⁻ → 2SO₄²⁻ + I₂
• White tile
• Swirl once to mix; judge the colour change against the tile the same way each run
• Reaction setup

Drawn with 13 shapes – every one editable in MolDraw.

Sources

• Atkins, de Paula & Keeler, Atkins' Physical Chemistry, 11th ed., OUP 2018 Textbook