i) Action of heat : Potassium dichromate when heated strongly. Decomposes to give oxygen.
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4K2Cr2O7(s) ————→ 4K2CrO4(s) + 2Cr2O3(s) + 3O2
(ii) Action of acids
(a) In cold, with concentrated H2SO4, red crystals of chromium trioxide separate out.
K2Cr2O7(aq) + conc.H2SO4 → KHSO4(aq) + 2CrO3(s) + H2O
On heating a dichromate-sulphuric acid mixture, oxygen gas is given out.
2K2Cr2O7 + 8H2SO4 → 2K2SO4 + 2Cr2(SO4)3 + 8H2O + 3O2
(b) With HCl, on heating chromic chloride is formed and Cl2 is liberated.
K2Cr2O7(aq) + 14HCl(aq) → 2CrCl3(aq) + 2KCl(aq) + 7H2O + 3Cl2(g)
(iii) Action of alkalies: With alkalies, it gives chromates. For example, with KOH,
K2Cr2O4 + 2KOH → 2K2CrO4 + H2O
orange yellow
On acidifying, the color again changes to orange-red owing to the formation of dichromate.
2K2CrO4 + H2SO4 → K2Cr2O7 + K2SO4 + H2O
Actually, in dichromate solution, the
ions are in equilibrium with
ions.
Cr2O72– + H2O 2CrO42– + 2H+
(iv) Oxidizing nature : In neutral or in acidic solution, potassium dichromate acts as an excellent oxidizing agent, and Cr2O72–
gets reduced to Cr3+. The standard electrode potential for the reaction,
Cr2O72– + 14H+ + 6e– → 2Cr+3 + 7H2O is + 1.31V.
This indicates that dichromate ion is a fairly strong oxidizing agent, especially in strongly acidic solutions. That is why potassium dichromate is widely used as an oxidizing agent, for quantitative estimation of the reducing agents such as, Fe2+. It oxidizes,
(a) Ferrous salts to ferric salts
K2CrO7 + 4H2SO4 → K2SO4 + Cr2(SO4)3 + 4H2O + 3[O]
2FeSO4 + H2SO4 + [O] → Fe2[SO4]3 + H2O × 3
K2Cr2O7 + 6FeSO4 + 7H2SO4 → K2SO4 + Cr2(SO4)3 + 3Fe2(SO4)3 + 7H2O
Ionic equation
Cr2O72– + 14H+ + 6Fe2+ → 2Cr3+ + 6Fe3+ + 7H2O
(b) Sulphites to sulphates and arsenites to arsenates.
K2Cr2O7 + 4H2SO4 → K2SO4 + Cr2(SO4)3 + 4H2O + 3[O]
Na2SO3 + [O] → Na2SO4] × 3
K2Cr2O7 + 4H2SO4 + 3Na2SO3 → K2SO4 + Cr2(SO4)3 + 3Na2SO4 + 4H2O
Ionic equation
Cr2O72– + 8H+ + 3SO32– → 2Cr3+ + 3SO42– + 4H2O
Similarly, arsenites are oxidised to arsenates.
Cr2O72– + 8H+ + 3AsO33– → 2Cr3+ + 3AsO43– + 4H2O
(c) Hydrogen halides to halogens.
K2Cr2O7 + 4H2SO4 → K2SO4 + Cr2(SO4)3 + 4H2O + 3[O]
2HX + O → H2O + X2] × 3
K2Cr2O7 + 4H2SO4 + 6HX → K2SO4 + Cr2(SO4)3 + 7H2O + 3X2
where, X may be Cl, Br, I.
Cr2O72– + 8H+ + 6HX → 2Cr3+ + 3X2 + 7H2O
(d) Iodides to iodine
K2Cr2O7 + H2SO4 → K2SO4 + Cr2(SO4)3 + 4H2O + 3[O]
2KI + H2O + [O] → 2KOH + I2] × 3
K2Cr2O7 + 7H2SO4 + 6KI → 4K2SO4 + Cr2(SO4)3 + 3I2 + 7H2O
Ionic equation :
Cr2O72– + 14H+ + 6I– → 2Cr3+ + 7H2O + 3I2
Thus, when KI is added to an acidified solution of K2Cr2O7 iodine gets liberated.
(e) It oxidizes H2S to S.
K2Cr2O7 + 4H2SO4 → K2SO4 + Cr2(SO4)3 + 4H2O + 3[O]
H2S + [O] → H2O + S] × 3
K2Cr2O7 + 4H2SO4 + 3H2S → K2SO4 + Cr2(SO4)3 + 7H2O + 3S
Ionic equation
Cr2O72– + 8H+ + H2S → 2Cr3+ + 3S + 7H2O
(v) Formation of insoluble chromates : With soluble salts of lead, barium etc., potassium dichromate gives insoluble chromates. Lead chromate is an important yellow pigment.
2Pb(NO3)2
+ K2Cr2O7 + H2O → 2PbCrO4 + 2KNO3 + 2HNO3
(vi) Chromyl chloride test : When potassium dichromate is heated with conc. H2SO4 in the presence of a soluble chloride salt, the orange-red vapors of chromyl chloride (CrO2Cl2) are formed.
heat
K2Cr2O7 +4NaCl + 6H2SO4————→ 2KHSO4 + 4NaHSO3 + 2CrO2Cl2
chromyl chloride
(orange-red vapours)
Chromyl chloride vapors when passed through water give yellow-colored solution containing chromic acid.
CrO2Cl2 + 2H2O → 2HCl + H2CrO4
Chromic acid
(yellow solution)
Chromyl chloride test can be used for the detection of chloride ion is any mixture.
Uses : Potassium dichromate is used as,
(i) An oxidizing agent
(ii) In chrome tanning
(iii) The raw material for preparing large number of chromium compounds
(iv) Primary standard in the volumetric analysis.
Structures of Chromate and Dichromate Ions
Chromates and dichromate are the salts of chromic acid (H2CrO4). In solution, these ions exist in equilibrium with each other. Chromate ion has four oxygen atoms arranged tetrahedrally around Cr atom. (see Fig). Dichromate ion involves a Cr–O–Cr bond as shown in Fig.
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