CrO4 is commonly encountered in aqueous inorganic chemistry as the chromate oxyanion, written as CrO42−. Its charge and chromium(VI) oxidation state control formula writing, compound naming, and characteristic equilibria such as chromate–dichromate interconversion.
Charge and oxidation state
The oxide convention assigns oxygen an oxidation state of \(-2\) in typical oxyanions. With four oxygens and an overall ion charge of \(-2\), the oxidation state \(x\) of chromium satisfies:
\[ x + 4(-2) = -2 \;\;\Longrightarrow\;\; x - 8 = -2 \;\;\Longrightarrow\;\; x = +6 \]Structure and bonding description
Chromate is an oxyanion with four oxygen atoms arranged around a central chromium atom in an approximately tetrahedral geometry. Resonance descriptions distribute negative charge largely onto oxygen atoms, consistent with strong polarity in Cr–O bonds.
Naming salts that contain CrO4
The ion name “chromate” appears unchanged in ionic compound names. The cation name is stated first, followed by “chromate.” Charge balance determines the stoichiometric subscripts in the empirical formula.
| Formula | Compound name | Charge balance statement |
|---|---|---|
| K2CrO4 | potassium chromate | \(2(+1) + (-2) = 0\) |
| Na2CrO4 | sodium chromate | \(2(+1) + (-2) = 0\) |
| CaCrO4 | calcium chromate | \((+2) + (-2) = 0\) |
| BaCrO4 | barium chromate | \((+2) + (-2) = 0\) |
Chromate–dichromate interconversion in water
Acid–base conditions shift the distribution of chromium(VI) oxyanions in solution. In more acidic media, dichromate becomes more prominent; in more basic media, chromate becomes more prominent. A common net equilibrium representation is:
\[ 2\,\mathrm{CrO_4^{2-}} + 2\,\mathrm{H^+} \rightleftharpoons \mathrm{Cr_2O_7^{2-}} + \mathrm{H_2O} \]The same chromium oxidation state \(+6\) persists in both chromate and dichromate; the shift reflects condensation/protonation equilibria rather than a redox change.
Precipitation behavior and solubility expressions
Many chromate salts have limited solubility, enabling precipitation reactions used in qualitative analysis. A representative net ionic precipitation reaction is:
\[ \mathrm{Ba^{2+}(aq)} + \mathrm{CrO_4^{2-}(aq)} \rightarrow \mathrm{BaCrO_4(s)} \]The solubility product expression for barium chromate is:
\[ K_{sp} = [\mathrm{Ba^{2+}}]\,[\mathrm{CrO_4^{2-}}] \]Common pitfalls
- Charge omission: CrO4 in aqueous chemistry most often implies CrO42−, not a neutral molecule.
- Oxidation-state confusion: chromium in chromate is \(+6\), consistent with the label chromium(VI) and distinct from chromium(III) compounds.
- Redox misclassification: chromate–dichromate conversion reflects acid–base/condensation equilibria; oxidation state remains unchanged.