🧪 2024 P5 — Acids in W + iodine clock (Version A: guided notebook)

📖 What's the chemistry?

(a) The acids in W

W is a mixture of hydrochloric acid and nitric acid.

• Silver nitrate → white precipitate (silver chloride) → chloride, so HCl.
• Barium nitrate → no change → no sulfate (so not H₂SO₄).
• NaOH + aluminium foil, warmed → ammonia (damp red litmus → blue) → nitrate, so HNO₃. (Aluminium reduces nitrate to ammonia in alkali.)

(b) The iodine clock

X (acidified iron(III) chloride) oxidises Y (potassium iodide) to make iodine; starch (S) turns it blue-black. Sodium thiosulfate (T) mops up the iodine first, so the blue-black appears suddenly after a delay — the easier it is to time.

More concentrated X reacts faster, so the blue-black comes sooner. Keeping X + water = 20.0 cm³ constant means only the concentration of X changes. Rate = 1000 / time. There is no graph — the rate is worked out by calculation.

Examiner tips

• Record the time to the nearest second, not to decimal places.
• The ratio is rate 2 ÷ rate 1 (not the reciprocal). For accuracy, use a burette/pipette and control temperature.