🧪 2019 P5 — Redox roles of hydrogen peroxide(Version B: written answers, auto-marked)
📖 What's the chemistry?
The three solutions (printed in the paper!)
W = hydrogen peroxide (colourless), X =
aqueous potassium iodide (colourless), Y =
aqueous potassium manganate(VII) (purple). The deductions are about the
ROLE the hydrogen peroxide plays in each test.
The four tests
(a) Y dropwise into W: the purple loses its colour, then a brown-black solid (MnO₂) forms and
settles; effervescence of oxygen; the test-tube becomes hotter.
(b) acidified W + four 5-drop portions of Y: the first portion turns the solution pink, which fades
to colourless with tiny bubbles; each later portion is decolourised faster (Mn²⁺ autocatalysis)
→ H₂O₂ = reducing agent.
(c) X + acid + W: colourless → yellow → brown solution, a black solid settles,
NO gas → H₂O₂ = oxidising agent.
(d) X + W: effervescence of oxygen; the colourless solution turns yellow; the test-tube
becomes warm.
The NEW gas test — a GLOWING splint
Oxygen relights a glowing splint: light the splint, click it to blow the flame out, then
hold the glowing tip at the tube mouth. A lighted splint is the wrong test.
How the auto-marking works
Your written answers are checked offline against the mark scheme's required points using keyword and
synonym patterns, so you need not match its wording exactly (but write words like "precipitate" in full,
as in the exam). Wrong-test wording is rejected — a "pop", a
"lighted splint", "exothermic" instead of warm, "clear" instead of colourless, and effervescence in (c)
all lose their points, with feedback telling you why.
Examiner tips
The test-tube becomes hotter (observation), not "exothermic" (conclusion).
No effervescence in (c) — no gas is evolved there.
(e)(i) = reducing agent (test (b)); (e)(ii) = oxidising agent (test (c)) — do not swap.
The manganate(VII) ion is MnO₄⁻ (the TYS misprints MnO₄²⁻).