Dichromate as an Oxidizing Agent
Unlike the chromate-dichromate equilibrium itself, dichromate genuinely acts as a strong oxidizing agent in three classic reactions: with hydrogen peroxide, with hydrogen sulfide, and — used the other way round, as the oxidant that reveals chloride — in the chromyl chloride test.
Chromyl Chloride Test for Chloride Ion is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Dichromate is reduced, not just discoloured, by H2O2
When acidic dichromate reacts with hydrogen peroxide, chromium is genuinely reduced from +6 down toward +3 (forming blue peroxochromate intermediates first, then ultimately Cr3+ species), so the orange colour of the solution changes as the oxidation state of chromium ACTUALLY DECREASES — this is real redox, unlike the acid-base chromate/dichromate interconversion covered in the companion cluster on this topic. Do not describe this reaction as the oxidation state of Cr staying constant; H2O2 here is acting as a reducing agent toward Cr(VI).
Acidic dichromate oxidizes H2S to elemental sulfur
Acidified potassium dichromate is a strong enough oxidizing agent to oxidize hydrogen sulfide (S at -2) all the way to elemental sulfur (S at 0), a visible yellow/white precipitate, while chromium is reduced from +6 to +3, appearing in solution as green Cr2(SO4)3 (chromium(III) sulfate, not any chromium(II) species — Cr2+ is not a product of this reaction). The potassium and remaining sulfate combine as K2SO4. Reading this reaction correctly means tracking BOTH half-reactions: sulfur is oxidized (loses electrons, -2 to 0) while chromium is reduced (gains electrons, +6 to +3) — a chromium(II) product would require chromium to be reduced even further than dichromate oxidizing power allows in this reaction.
The chromyl chloride test uses dichromate as an oxidant to confirm chloride
Warming a solid chloride salt with solid potassium dichromate and concentrated sulfuric acid produces deep red-orange chromyl chloride vapours (CrO2Cl2, volatile), a classic qualitative test that confirms the presence of chloride ion specifically (bromide and iodide salts do NOT give this test, since dichromate would oxidize them to the free halogen instead of forming an analogous volatile chromyl halide). Passing the red chromyl chloride vapours into sodium hydroxide solution hydrolyses them back to a yellow solution of sodium chromate. Critically, no chlorine gas is evolved in this test — the chloride is incorporated into the chromyl chloride molecule itself (as Cl in CrO2Cl2), not oxidized to free Cl2, so any statement claiming chlorine gas evolution during this specific test is the incorrect one to flag.
OlympIQ exam lens
- ✓H2O2 genuinely reduces Cr(VI) toward Cr(III) — the oxidation state of Cr decreases, it does not stay constant
- ✓Acidic dichromate oxidizes H2S to elemental sulfur (S precipitate), while Cr is reduced to Cr3+ (as Cr2(SO4)3), not to Cr2+
- ✓The chromyl chloride test (NaCl + K2Cr2O7 + conc. H2SO4, warmed) confirms chloride specifically — bromide/iodide do not give this test
- ✓Chromyl chloride vapours (CrO2Cl2, deep red) hydrolyse in NaOH to give yellow sodium chromate solution
- ✓No chlorine gas is evolved in the chromyl chloride test — the chloride ends up bound in CrO2Cl2, not oxidized to Cl2
Do not assume every dichromate-plus-chloride reaction releases chlorine gas just because dichromate is a strong oxidant. In the specific chromyl chloride test, the chloride is incorporated into the volatile CrO2Cl2 molecule rather than oxidized to free Cl2 — evolving Cl2 gas is the one statement in this classic test that is actually false.
Ready to try the pattern?
100% coverage across the 3 linked questions.
Practise 3 linked questions