Chromium Chemistry

Variable Oxidation States: Chromium commonly exists in the $+2$, $+3$, and $+6$ oxidation states in aqueous environments. The $+3$ state is the most stable, while the $+6$ state is a powerful oxidizing agent often found in oxyanions.

Aqueous Colors: The hexaaquachromium(III) ion, $[Cr(H_2O)_6]^{3+}$, typically appears violet in solution, though it can appear green if certain ligands like chloride are present. The chromium(II) ion, $Cr^{2+}$, is a bright blue but is easily oxidized by atmospheric oxygen.

Chromium(VI) Species: In the $+6$ state, chromium does not exist as a simple cation but as the chromate ion ($CrO_4^{2-}$, yellow) or the dichromate ion ($Cr_2O_7^{2-}$, orange). These species are highly sensitive to the pH of the solution.

pH Dependency: The yellow chromate ion and orange dichromate ion exist in a dynamic equilibrium governed by the concentration of hydrogen ions. This is an acid-base reaction, not a redox reaction, as the oxidation state of chromium remains $+6$ in both species.

Equilibrium Shift: Adding acid ($H^+$) increases the concentration of reactants on the left, shifting the equilibrium to the right to produce orange dichromate. Conversely, adding an alkali ($OH^-$) removes $H^+$ ions, shifting the equilibrium back to the left to produce yellow chromate.

Chemical Equation: The relationship is represented by the reversible reaction: $2CrO_4^{2-}(aq) + 2H^+(aq) \rightleftharpoons Cr_2O_7^{2-}(aq) + H_2O(l)$

Definition of Amphoterism: Chromium(III) hydroxide, $Cr(OH)_3(H_2O)_3$, is an amphoteric compound, meaning it can react as both an acid and a base. This property allows it to dissolve in both strong acids and strong bases.

Reaction with Acid: When a strong acid is added to the green $Cr(OH)_3$ precipitate, it acts as a base, accepting protons to reform the violet hexaaquachromium(III) ion: $Cr(OH)_3(H_2O)_3(s) + 3H^+(aq) \rightarrow [Cr(H_2O)_6]^{3+}(aq)$

Reaction with Excess Base: When excess sodium hydroxide is added, the precipitate acts as an acid, losing further protons to form the dark green hexahydroxochromate(III) complex ion: $Cr(OH)_3(H_2O)_3(s) + 3OH^-(aq) \rightarrow [Cr(OH)_6]^{3-}(aq) + 3H_2O(l)$

Oxidation to Chromium(VI): Chromium(III) can be oxidized to the $+6$ state using hydrogen peroxide ($H_2O_2$) in alkaline conditions. The initial green precipitate of $Cr(OH)_3$ dissolves to form a bright yellow solution of chromate ($CrO_4^{2-}$).

Reduction of Dichromate: Orange dichromate ions can be reduced back to Chromium(III) using a reducing agent like Zinc in acidic conditions. The solution changes from orange to green as $Cr^{3+}$ ions are formed.

Further Reduction: If Zinc is used in excess, the reduction can continue from the $+3$ state to the $+2$ state. This results in a color change from green to a distinct blue, though this $Cr^{2+}$ state is unstable in the presence of air.

Identify Non-Redox Steps: Always check if an oxidation state change occurs. The chromate-dichromate transition is a frequent 'trick' question; remember that both species are $+6$ and the change is purely an equilibrium shift driven by pH.

Precipitate vs. Complex: Distinguish between the neutral green precipitate $Cr(OH)_3$ and the charged green complex $[Cr(OH)_6]^{3-}$. The latter only forms in excess alkali.

Color Sequences: Memorize the sequence for reduction: Orange ($+6$) $\rightarrow$ Green ($+3$) $\rightarrow$ Blue ($+2$). For oxidation: Green ($+3$) $\rightarrow$ Yellow ($+6$).