Chromate-Dichromate Interconversion, Structure and Colour
Yellow chromate and orange dichromate are the same oxidation state of chromium (+6) related by a simple, pH-driven, non-redox equilibrium. Almost every exam question on this pair comes down to remembering that this interconversion is acid-base chemistry, not electron transfer.
Chromate to Dichromate Conversion on Passing CO2 is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
The chromate-dichromate equilibrium is proton-driven, not a redox change
In basic or neutral solution, chromium(VI) exists as the yellow tetrahedral chromate ion, CrO4²-. Lowering the pH (adding acid) protonates chromate and drives two chromate ions to condense, releasing water and forming the orange dichromate ion, Cr2O7²-, in which two CrO4 tetrahedra share one bridging oxygen (2 CrO4²- + 2H+ <=> Cr2O7²- + H2O). Raising the pH again (adding base) reverses this, splitting dichromate back into two chromate ions. Because chromium stays at +6 throughout, this entire interconversion is NOT a disproportionation or any other redox process — it is exactly why chromate is the one common d-block oxoanion in this cluster that does NOT undergo disproportionation, unlike manganate (MnO4²-, which does disproportionate in acid) or other species where the central atom genuinely changes oxidation state. Any weak acid is enough to shift the equilibrium toward dichromate: dilute nitric acid protonates chromate directly, and even passing CO2 gas (which forms the weak acid H2CO3 in solution) is acidic enough to convert yellow Na2CrO4 into orange-red Na2Cr2O7, well short of any redox chemistry.
Why dichromate is orange and chromate is yellow
Both chromate and dichromate get their colour from charge-transfer transitions: in a d0 ion like Cr(VI) (chromium has lost all its d-electrons at the +6 oxidation state, so there is no d-d transition available), the colour instead comes from an electron being promoted from a filled oxygen-based orbital to an empty chromium-based orbital (a ligand-to-metal charge-transfer, LMCT). Because dichromate has a different, more extended bonding arrangement across its two bridged chromium centres than the single isolated tetrahedron of chromate, its charge-transfer transition occurs at a different energy, shifting the observed colour from the yellow of chromate to the orange of dichromate.
The bridging oxygen in dichromate makes six Cr-O bonds equivalent, not all of them
Dichromate, Cr2O7²-, consists of two CrO4 tetrahedra sharing exactly one oxygen atom (the bridging oxygen) between them. This bridging oxygen forms one Cr-O bond to each chromium (two bonds total, both slightly different from the rest by virtue of being a bridge), while each chromium additionally carries three TERMINAL oxygens that are not shared. The six terminal Cr-O bonds (three on each chromium) are equivalent to each other, but the two Cr-O(bridge) bonds are distinct from those six terminal bonds — so the correct description is that six Cr-O bonds are equivalent (the terminal ones), not that all seven Cr-O bonds in the ion are identical.
OlympIQ exam lens
- ✓Chromate (yellow, CrO4²-) and dichromate (orange, Cr2O7²-) both have Cr at +6 — their interconversion is a pH-driven condensation/hydrolysis equilibrium, not a redox reaction
- ✓Any acid (dilute HNO3, or even dissolved CO2 forming carbonic acid) shifts chromate toward dichromate; base reverses it
- ✓Because no oxidation state changes, chromate/dichromate is the example to reach for when asked which oxoanion does NOT disproportionate
- ✓The chromate/dichromate colour comes from a ligand-to-metal charge-transfer transition (Cr(VI) is d0, so no d-d transition is possible)
- ✓In dichromate, six Cr-O bonds (the terminal ones, three per chromium) are equivalent; the two Cr-O bonds to the single bridging oxygen are distinct from those six
Do not describe the chromate-to-dichromate conversion as a reduction or any kind of electron-transfer step just because the colour changes. Colour change alone does not imply redox — here it reflects a shift in the charge-transfer transition energy caused by a purely acid-base condensation reaction, with chromium remaining at +6 throughout.
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