Potassium Dichromate: Preparation & Reactions

CHEImportant Compounds

Potassium dichromate is prepared from chromite ore and is a key acidified oxidising agent. Exam focus: the preparation steps, the chromate–dichromate equilibrium, and balanced ionic oxidations in acidic medium.

Acidic-medium reduction half-reaction

Cr_2O_7^{2-} + 14H^+ + 6e^- \rightarrow 2Cr^{3+} + 7H_2O \qquad E^\circ = +1.33\ \text{V}

Cr goes

+6 \rightarrow +3

, gaining 6 electrons per dichromate ion.

Chromate–dichromate equilibrium

2CrO_4^{2-} + 2H^+ \rightleftharpoons Cr_2O_7^{2-} + H_2O

acid shifts right (orange dichromate); alkali shifts left (yellow chromate).

Oxidation of Fe(II) by dichromate

Cr_2O_7^{2-} + 14H^+ + 6Fe^{2+} \rightarrow 2Cr^{3+} + 6Fe^{3+} + 7H_2O

each

Fe^{2+}\rightarrow Fe^{3+}

supplies one of the 6 electrons.

Preparation from chromite $FeO\cdot Cr_2O_3$ : roast with $Na_2CO_3$ in air to yellow $Na_2CrO_4$ , acidify to orange $Na_2Cr_2O_7$ , then treat with $KCl$ to crystallise $K_2Cr_2O_7$ .
Chromate–dichromate equilibrium: $2CrO_4^{2-} + 2H^+ \rightleftharpoons Cr_2O_7^{2-} + H_2O$ ; adding acid gives orange dichromate, adding alkali shifts it back to yellow chromate.
In acidic medium the half-reaction is $Cr_2O_7^{2-} + 14H^+ + 6e^- \rightarrow 2Cr^{3+} + 7H_2O$ ( $E^\circ = +1.33$ V).
Oxidation of iron(II): $Cr_2O_7^{2-} + 14H^+ + 6Fe^{2+} \rightarrow 2Cr^{3+} + 6Fe^{3+} + 7H_2O$ .
Oxidation of iodide: $Cr_2O_7^{2-} + 14H^+ + 6I^- \rightarrow 2Cr^{3+} + 3I_2 + 7H_2O$ ; oxidation of $H_2S$ : $Cr_2O_7^{2-} + 8H^+ + 3H_2S \rightarrow 2Cr^{3+} + 3S + 7H_2O$ .
$Cr_2O_7^{2-}$ oxidises strongly in acidic medium because the $14H^+$ in the half-reaction drives it forward; in neutral or alkaline medium it converts to chromate, which is a poor oxidiser.
$K_2Cr_2O_7$ is a bright-orange crystalline solid, only moderately soluble in cold water, and is used as a primary standard in volumetric analysis because it is not deliquescent and can be obtained pure.
Structure: dichromate has two $CrO_4$ tetrahedra sharing one corner oxygen (a Cr–O–Cr bridge); both Cr are in the $+6$ state and tetrahedrally coordinated.
Oxidation of $SO_3^{2-}$ (or $SO_2$ ): $Cr_2O_7^{2-} + 8H^+ + 3SO_3^{2-} \rightarrow 2Cr^{3+} + 3SO_4^{2-} + 4H_2O$ ; the orange colour fades to green ( $Cr^{3+}$ ).
On heating, ammonium dichromate decomposes vigorously: $(NH_4)_2Cr_2O_7 \rightarrow N_2 + Cr_2O_3 + 4H_2O$ (the 'volcano' reaction).
The colour change orange ( $Cr_2O_7^{2-}$ ) to green ( $Cr^{3+}$ ) is the visual signal that dichromate has acted as an oxidant in acid.

Where the marks go

Balancing the oxidation in acidic medium with $OH^-$ on the wrong side; the acidic half-reaction uses $14H^+$ and produces $7H_2O$ with $Cr^{3+}$ .
Writing $Cr_2O_7^{2-}$ reduced to $Cr$ metal or leaving Cr as $+6$ ; it is reduced to $Cr^{3+}$ (a 6-electron change for two Cr).
Saying acid shifts the equilibrium toward chromate; acid gives the orange dichromate, alkali gives yellow chromate — students often reverse this.
Forgetting to multiply the reducing agent so electrons balance: 6 $Fe^{2+}$ , 6 $I^-$ , or 3 $H_2S$ are needed per dichromate, not arbitrary coefficients.
Claiming dichromate is a strong oxidiser in alkaline medium; in base it becomes chromate, which is a weak oxidant.

How the board asks it

Conversionacidic-medium oxidation half-reactions
Write the balanced ionic equation for the reaction of acidified $K_2Cr_2O_7$ with (i) $Fe^{2+}$ ions, (ii) iodide ions $I^-$ , and (iii) hydrogen sulphide $H_2S$ .
Applicationpreparation from chromite ore
Describe the preparation of $K_2Cr_2O_7$ from chromite ore $FeO\cdot Cr_2O_3$ , giving balanced equations for the roasting with $Na_2CO_3$ , the acidification, and the conversion to $K_2Cr_2O_7$ with $KCl$ .
Give reasonsmedium-dependence of oxidising power
Account for the fact that $K_2Cr_2O_7$ acts as a strong oxidising agent in acidic medium but is a poor oxidiser in alkaline medium.
Conversionchromate–dichromate equilibrium
How will you convert a solution of potassium chromate into potassium dichromate, and then back into chromate? Give the ionic equations involved.
Structure / namingcorner-shared tetrahedra
Draw the structure of the dichromate ion $Cr_2O_7^{2-}$ , indicating the oxidation state of chromium and the nature of the $Cr-O-Cr$ linkage.
Give reasonscolour change and primary standard
Give reasons: (i) the orange colour of acidified $K_2Cr_2O_7$ turns green when it oxidises $SO_3^{2-}$ to $SO_4^{2-}$ ; (ii) $K_2Cr_2O_7$ is used as a primary standard in volumetric analysis.

Practise this topic

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Written for Sublevo. Question text quoted anywhere in these notes is the Council’s and carries its year and paper; the board’s own diagrams are not reproduced.