🧪 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: purple decolourised, then a brown-black solid (MnO₂) forms and
settles; effervescence of oxygen; the tube feels warm.
(b) acidified W + four 5-drop portions of Y: purple fades to colourless with tiny bubbles,
faster with each portion (Mn²⁺ autocatalysis) → H₂O₂ = reducing agent.
(c) X + acid + W: colourless → yellow → reddish-brown, black solids settle,
NO gas → H₂O₂ = oxidising agent.
(d) X + W: effervescence of oxygen; pale-yellow solution (TYS: remains colourless).
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 (e.g. "ppt" = "precipitate"). 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 tube feels warm (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⁻).