Chemistry · Chemistry of the elements · Grade 9

Chromium and manganese: coloured ions

Why can chromium compounds be yellow, orange, green and blue?

The situation

The tap in the kitchen shines — it is chromium-plated; spoons and pans are made of “stainless steel”, which contains ≈ 18 % chromium. In the laboratory, chromium salts are colourful: potassium chromate is yellow, dichromate orange, chromium(III) salts green, chromium(II) sky blue. The “permanganate” of the neighbouring element manganese is violet, decolourises in acid when a reducing agent is added, and turns green in alkali. What does colour depend on, and how can a single element take on so many colours?

Topic

Chromium and manganese: coloured valencies. Chrome-plated trim and stainless steel: potassium dichromate from orange to green, permanganate from purple to colourless

Key ideas

Oxidation state and colour
Chromate ⇌ dichromate
KMnO₄ in three media

Formulas

2 CrO_{4}^{2 -} + 2 H^{+} \overset{}{⇌} Cr_{2} O_{7}^{2 -} + H_{2} O

Chromate ⇌ dichromate: add acid and the yellow solution turns orange, add alkali and the opposite happens; the oxidation state +6 does not change.

Cr_{2} O_{7}^{2 -} + 14 H^{+} + 6 e^{-} \overset{}{\to} 2 Cr^{3 +} + 7 H_{2} O

Dichromate is a strong oxidising agent: in acidic medium each Cr⁺⁶ takes three electrons and becomes green Cr³⁺.

Cr (OH)_{3} + 3 HCl \overset{}{\to} CrCl_{3} + 3 H_{2} O Cr (OH)_{3} + 3 NaOH \overset{}{\to} Na_{3} [Cr (OH)_{6}]

Cr(OH)₃ is an amphoteric hydroxide (lab work 14).

MnO_{4}^{-} \overset{\overset{}{H^{+}}}{\to} Mn^{2 +} MnO_{4}^{-} \overset{\overset{}{H_{2} O}}{\to} MnO_{2} MnO_{4}^{-} \overset{\overset{}{OH^{-}}}{\to} MnO_{4}^{2 -}

The reduction of KMnO₄ depends on the medium: colourless Mn²⁺ (5 e⁻), brown MnO₂ (3 e⁻), green manganate (1 e⁻).

Cr_{2} O_{3} + 2 Al \overset{}{\to} 2 Cr + Al_{2} O_{3}

Chromium is obtained by aluminothermy.

Theory

Chromium: position and properties (§31)

Chromium, Cr, is element 24, period 4, group 6 (VIB): [Ar] 3d⁵ 4s¹ — one electron has moved from 4s to 3d (a half-filled d-shell is stable). Oxidation states +2, +3, +6. Chromium is silvery blue-white and very hard (8.5 on the Mohs scale — the hardest of the pure metals), ρ = 7.19 g/cm³, melting at 1907 °C. In air it becomes covered with a thin Cr₂O₃ film and is passivated — which is why it does not rust. Its main ore is chromite FeCr₂O₄; the metal is obtained by aluminothermy ( $Cr_{2} O_{3} + 2 Al \overset{}{\to} 2 Cr + Al_{2} O_{3}$ ), while for steel ferrochrome is smelted. If more than ≈ 11 % chromium is added to steel, a self-healing oxide film forms on its surface — stainless steel (for kitchenware often 18 % Cr and 8–10 % Ni, “18/8”). Chromium plating is the deposition of a shiny layer of chromium thinner than 1 µm onto an object (usually over nickel); Cr(VI) compounds are used in the baths.

Chromium compounds in the +2, +3 and +6 states (§32, lab work 14)

Cr(II): CrO is a basic oxide; Cr²⁺ solutions are sky blue and strongly reducing — in air they are quickly oxidised to Cr³⁺: $4 Cr^{2 +} + O_{2} + 4 H^{+} \overset{}{\to} 4 Cr^{3 +} + 2 H_{2} O$ . Cr(III): Cr₂O₃ is green and amphoteric (a paint pigment); Cr(OH)₃ is a grey-green amphoteric precipitate: $Cr^{3 +} + 3 OH^{-} \overset{}{\to} Cr (OH)_{3}$ , it dissolves in acid ( $Cr (OH)_{3} + 3 HCl \overset{}{\to} CrCl_{3} + 3 H_{2} O$ ), and in excess alkali it forms emerald-green hexahydroxochromate ( $Cr (OH)_{3} + 3 OH^{-} \overset{}{\to} [Cr (OH)_{6}]^{3 -}$ ). Solutions of Cr³⁺ salts are green or violet-green. Cr(VI): CrO₃ is a dark red acidic oxide; chromates (CrO₄²⁻) are yellow, dichromates (Cr₂O₇²⁻) orange — strong oxidising agents in acidic medium. In lab work 14: dichromate is reduced by zinc in hydrochloric acid first to green Cr³⁺, and in a closed test tube to sky-blue Cr²⁺ ( $Cr_{2} O_{7}^{2 -} + 3 Zn + 14 H^{+} \overset{}{\to} 2 Cr^{3 +} + 3 Zn^{2 +} + 7 H_{2} O$ , $2 Cr^{3 +} + Zn \overset{}{\to} 2 Cr^{2 +} + Zn^{2 +}$ ); in alkaline medium H₂O₂ oxidises Cr(III) back to yellow chromate: $2 [Cr (OH)_{6}]^{3 -} + 3 H_{2} O_{2} \overset{}{\to} 2 CrO_{4}^{2 -} + 2 OH^{-} + 8 H_{2} O$ ; in acid, sulfite reduces dichromate to green Cr³⁺: $Cr_{2} O_{7}^{2 -} + 3 SO_{3}^{2 -} + 8 H^{+} \overset{}{\to} 2 Cr^{3 +} + 3 SO_{4}^{2 -} + 4 H_{2} O$ .

Medium and colour: chromate ⇌ dichromate

Chromate and dichromate convert into each other depending on the medium: $2 CrO_{4}^{2 -} + 2 H^{+} \overset{}{⇌} Cr_{2} O_{7}^{2 -} + H_{2} O$ . This is not a redox reaction — chromium stays at +6, only the composition of the ion changes. By Le Chatelier's principle, adding H⁺ shifts the equilibrium to the right (orange), adding OH⁻ shifts it to the left (yellow): $Cr_{2} O_{7}^{2 -} + 2 OH^{-} \overset{}{\to} 2 CrO_{4}^{2 -} + H_{2} O$ . More precisely, there are three species in solution: CrO₄²⁻, HCrO₄⁻ and Cr₂O₇²⁻ (25 °C: $\lg K$ (2HCrO₄⁻ ⇌ Cr₂O₇²⁻ + H₂O) = 1.76, p $K_{a}$ of HCrO₄⁻ ≈ 6.1). The share of dichromate also depends on concentration: in a 0.04 mol/L solution at pH < 5, ≈ 63 % of the chromium is present as dichromate and the rest as HCrO₄⁻; at pH > 8 nearly all of it is chromate. The graph calculates this distribution exactly from the charge balance.

Manganese (§33)

Manganese, Mn, is element 25, group 7 (VIIB): [Ar] 3d⁵ 4s². Oxidation states +2, +4, +6, +7, each with its own colour: Mn²⁺ is very pale pink (almost colourless), MnO₂ is a brown-black solid, manganate MnO₄²⁻ is green, permanganate MnO₄⁻ is violet. KMnO₄ (“permanganate”) is a strong oxidising agent, and the product of its reduction depends on the medium: in acid $2 MnO_{4}^{-} + 5 SO_{3}^{2 -} + 6 H^{+} \overset{}{\to} 2 Mn^{2 +} + 5 SO_{4}^{2 -} + 3 H_{2} O$ , in neutral $2 MnO_{4}^{-} + 3 SO_{3}^{2 -} + H_{2} O \overset{}{\to} 2 MnO_{2} + 3 SO_{4}^{2 -} + 2 OH^{-}$ , in alkali $2 MnO_{4}^{-} + SO_{3}^{2 -} + 2 OH^{-} \overset{}{\to} 2 MnO_{4}^{2 -} + SO_{4}^{2 -} + H_{2} O$ . When heated it releases oxygen: $2 KMnO_{4} \overset{}{\to} K_{2} MnO_{4} + MnO_{2} + O_{2}$ . MnO₂ is a catalyst for the decomposition of hydrogen peroxide: $2 H_{2} O_{2} \overset{}{\to} 2 H_{2} O + O_{2}$ . In steelmaking, manganese binds oxygen and sulfur; manganese (Hadfield) steel (≈ 11–14 % Mn, ≈ 1–1.3 % C) work-hardens on its surface under impact — excavator buckets and the crossing parts of rails are made from it.

Oxidation state and properties

In d-block elements, as the oxidation state increases, the character of oxides and hydroxides changes from basic → amphoteric → acidic: CrO (basic) → Cr₂O₃ (amphoteric) → CrO₃ (acidic, corresponding to H₂CrO₄ and H₂Cr₂O₇); MnO (basic) → MnO₂ (amphoteric) → Mn₂O₇ (acidic, HMnO₄). The lowest state is reducing (Cr²⁺ is oxidised in air), the highest is oxidising (Cr₂O₇²⁻, MnO₄⁻). Colours arise from the absorption of light by electrons in a partly filled d-shell (Cr³⁺, Cr²⁺, Mn²⁺) or from charge transfer between oxygen and the metal (CrO₄²⁻, MnO₄⁻ — these ions have no d-electrons, yet they are intensely coloured).

Safety and the model

Safety: Cr(VI) compounds — chromates, dichromates, CrO₃ — are toxic and carcinogenic (International Agency for Research on Cancer, IARC, Group 1): if their dust or mist is inhaled they raise the risk of lung cancer, they irritate the skin and mucous membranes, and they cause allergies. Work with them only in gloves and safety goggles, in a fume cupboard, and in small amounts; the waste is not poured down the sink — it is collected and reduced to Cr(III). Metallic chromium and Cr(III) compounds are much less hazardous. KMnO₄ crystals and concentrated solution burn the skin; the crystals can start a fire with organic substances such as glycerol. Pour acid into water, never the other way round. Model: the Cr(VI) distribution is exact (Cruywagen et al., 1998, 25 °C, ionic strength 0.1 M); the pH in the test tubes is approximate, reactions follow stoichiometry, and slow processes with zinc and air run in model time (1 s ≈ 1 min). Colours are approximate, by the Beer–Lambert law. Steel corrosion is only a qualitative model (1 s ≈ 1 day).

Did you know?

The name chromium comes from the Greek “chroma” — “colour”: its compounds are red, orange, yellow, green, violet and sky blue. The red of a ruby comes from small amounts of Cr³⁺ ions in an Al₂O₃ crystal, and the green of an emerald is also mainly from Cr³⁺ (sometimes vanadium).
Chromium has a hardness of 8.5 on the Mohs scale — the hardest of the pure metals: among the elements only diamond (carbon) and boron are harder.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

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