🧪 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 observable change → no sulfate (so not H₂SO₄).
• NaOH + aluminium foil, warmed → effervescence; the gas turns damp red litmus paper blue → ammonia → 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, which makes it much easier 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).
• To improve accuracy: use a burette or pipette in place of the measuring cylinders, and keep the temperature constant (e.g. a water bath). Do not suggest swirling the mixture — that is technique, and the method already tells you to stir with the glass rod.