est for Chromium ion | Cr Metal Reactions | Qualitative Analysis of Cr³⁺

Chromium (Cr) is a transition element in d block. Chromium has different oxidation states +2, +3 , +6. The oxidation state +6 has strong oxidizing ability. Chromium hydroxide shows amphoteric characteristics. Chromate (CrO₄^2-) and dichromate Cr₂O₇^2-) ions are strong oxidizing agents. Chromium is used to make alloys and in electroplating.

Electrons configuration of chromium

1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹

Major oxidation states of chromium

• +2 : [Cr(H₂O)₆]²⁺_(aq)
• +3 : Cr(OH)_3(s)
• +6 : CrO_3(s), Na₂Cr₂O₇

Properties of chromium metal

• A silvery blue metal.
• Much resistance to atmospheric corrosion and decay.
• Readily dissolve in acids to form +II oxidation state.
• Compounds of +III state occur with oxygen, sulfur and halogens.
• Density of Cr : 7.20 g cm^-3

Melting and boiling points

• Melting point : 1890⁰C
• Boiling point : 2480⁰C

Reactions of chromium and compounds

Chromium and dilute acids reaction

Chromium react with dilute HCl or dilute H₂SO₄ to form [Cr(H₂O)₆]²⁺_(aq) blue colour coordination complex.

Chromium and dilute HCl reaction

Chromium reacts with dilute HCl and form, hydrogen gas with chloride ion.

Chromium and dilute H₂SO₄

As with dilute HCl, chromium reacts with dilute H₂SO₄ and form hexaaquachromium(ii) ion, hydrogen gas with sulfate ion. Aqueous solution is blue colour.

[Cr(H₂O)₆]²⁺_(aq) complex oxidizing behavior

Hexaaquachromium(ii) ion [Cr(H₂O)₆]²⁺_(aq), easily oxidises to Cr³⁺ ions by oxygen under atmospheric conditions.

Chromium +3 oxidation state

• Most stable oxidation state of Chromium is +3.

Aqueous Chromium(III) ion with alkali reactions

[Cr(H₂O)₆]³⁺_(aq) changes its colour (blue-violet) to dark green by forming Cr(OH)_3(s) precipitate with alkali. With excess alkali, precipitate dissolves and green colour aqueous [Cr(OH)₆]^3-_(aq) complex is given.

Cr(OH)₃, this hydroxide is an amphoteric compound.

Chromium 3+ ion and zinc amalgam

Cr³⁺ can be reduced to Cr²⁺ by zinc amalgam. Zinc amalgam is used in clemmensen reduction to reduce aldehyde and ketone to alkanes.

Chromium +6 oxidation state

Aqueous Cr³⁺ ions can be oxidized to chromate(+6) oxidation state by H₂O₂ in the presence of alkali.

Equilibrium of chromate ions and dichromate ions

Chromate(+6) ions undergo following equilibrium in acidic medium to form dichromate ( Cr₂O₇^2- ).

Reactions summary of chromium and compounds

Test for chromium ion

Here we discuss some experiments to test Cr³⁺ ion presence in different compoundsunder qualitative analysis. Chromium forms colourful precipitates and solutions with other elements. Therefore we use those colours, precipitates to identify chromium ion.

Aqueous ammonium sulfide and Cr³⁺ solution reaction

Addition of aqueous ammonium sulfide ( (NH₄)₂S_(aq) ) to Cr³⁺_(aq) solution forms a Cr₂S₃ precipitate. It quickly hydrolyzes to Cr(OH)_3(s) which is a green precipitate.

Ammonia and salts of weak acids such as sodium carbonate ( Na₂CO₃ ) , ammonium sulfide ( (NH₄)₂S ) also precipitates Cr³⁺ ions as chromium hydroxide ( Cr(OH)₃ ). This precipitate somewhat soluble in excess aqueous ammonia solution.

NaOH and chromium ion reaction

Aqueous chromium ion reacts with NaOH and form chromium hydroxide, green precipitate which has amphoteric characteristics. Chromium hydroxide will dissolve in excess aqueous NaOH and give green solution by forming sodium hexahydroxychromium(III).

How to identify chromium hydroxide and nickel hydroxide? - video

NaOH, Na₂O₂ and Cr³⁺ solution reaction

Addition of NaOH, Na₂O₂ to Cr³⁺ solution forms Na₂CrO₄, a yellow solution. Further addition of acetic acid ( CH₃COOH ) and BaCl₂, a yellow precipitate BaCrO₄ is obtained. BaCrO₄ is insoluble in acetic acid, but soluble in HNO₃ acid.

Ammonium buffer mixture with Cr³⁺

Cr³⁺ is completely precipitated as Cr(OH)₃ by ammonium buffer solution ( NH₄OH / NH₄Cl ). Cr(OH)₃ is a green precipitate.

Formation of sparingly soluble salts of CrO₄^2-

The anion CrO₄^2- forms sparingly soluble precipitates with Pb²⁺ , Ag⁺ , Ba²⁺. This can be used to detect and separate chromates.

• PbCrO₄: yellow
• BaCrO₄: yellow
• Ag₂CrO₄: Brick red

Uses of chromium and its compounds

Chromium(IV) oxide ( CrO₂ )

In magnetic recording tapes for better resolution and high frequency response.

Chromium(III) oxide (Cr₂O₃)

In abrasives, refractory materials, and semiconductors, as a green pigment, especially for colouring glass when chemical and heat resistance are required

Chromium(VI) oxide (CrO₃)

Chromium plating, Copper stripping as a corrosion inhibitor in photography

Sodium dichromate (Na₂Cr₂O₇)

Leather tanning, textile manufacture, metal corrosion inhibitor

More to learn about Cr³⁺ ion

• CrPO₄ is a greenish precipitate.
• Cr³⁺ ions can be oxidized by KMnO₄, (NH₄)₂S₂O₈ in acidic solutions.

Questions

Reaction of chromium with sodium hydroxide

Chromium does not react with aqueous sodium hydroxide. But chromium oxide and chromium hydroxide react with aqueous sodium hydroxide because they have amphoteric characteristics.

What is the chromium and halogens reaction?

Chromium metal reacts with the halogens to form chromium(III) halides. As an example, chromium reacts with chlorine gas to give chromium(III) chloride.

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