Chemistry PREPARATION, PROPERTIES AND USES OF POTASSIUM DICHROMATE AND POTASSIUM PERMANGENATE

Potassium Dichromate(`K_2Cr_2O_7`) :

`(a)` `text(Preparation)` :

(i) It is prepared from Chromite ore `(FeCr_2O_4)`.

(ii) The preparation of `K_2Cr_2O_7` from chromite ore involve the following steps:

`(A)` `text(Conversion of Chromite ore to Sodium Chromate)` :

(i) The Chromite ore is fused with Sodium hydroxide or Sodium Carbonate in the presence of air.

`4FeCr_2O_4 + 16 NaOH + 7O_2 -> 8Na_2CrO_4 + 2Fe_2O_3 + 8 H_2O`

`4 FeCr_2O_4 + 8 Na_2CO_3 + 7O_2 -> 8 Na_2CrO_4 + 2Fe_2O_3 + 8 CO_2`

Sod. Chromite is extracted with water & ferric oxide is left behind.

`(B)` `text(Conversion of Sodium chromate to Sodium dichromate)` :

(i) The Sod. chromite is acidified with dilute `H_2SO_4` giving its dichromate.

`2 Na_2CrO_4 + H_2SO_4 -> Na_2Cr_2O_7 + Na_2SO_4 + H_2O`

(ii) On concentration, the less soluble sulphate crystallises and is filtered out. The resulting solution contains Sod. dichromate.

`(C)` `text(Conversion of Sod. dichromate to Pot. dichromate)` :

(i) Hot concentrated solution of `Na_2Cr_2O_7` with `KCl` in equimolar proportion

`Na_2Cr_2O_7 + 2 KCl -> K_2Cr_2O_7 + 2 NaCl`

`(b)` `text(Physical Properties)` :

It is orange- red crystalline compound having melting point `670` `K`.

`(c)` `text(Chemical Properties of)` `K_2Cr_2O_7` :

(i) See fig.1

(ii) `text(Chromyl Chloride Test)` :

(A) This is the test of Chloride

`K_2Cr_2O_7 + 2H_2SO_4 -> 2 KHSO_4 + 2CrO_3 + H_2O`

`[NaCl + H_2SO_4 -> NaHSO_4 + HCl] xx 4`

`[CrO_3 + 2HCl -> CrO_2Cl_2 + H_2 O] xx 2`

`=> (text(Total Reaction))` `K_2Cr_2O_7 + 6H_2SO_4 + 4NaCl -> 2 KHSO_4 + 4 NaHSO_4 + 2CrO_2Cl_2 + 3H_2O`

(B) When Chromyl Chloride vapours are passed through `NaOH` solution, yellow coloured solution is obtained.

`4 NaOH +CrO_2 Cl_2 -> Na_2 Cr O_4 + 2NaCl +H_2O`

(iii) `text(Action with HCl)` :

`K_2Cr_2O_7 + 14 HCl -> 2 KCl + 2CrCl_3 + 7H_2O + undersettext(Chlorine)(3Cl_2) uparrow`

`(iv)` `text(Oxidising character)` :

(A) The dichromates act. as powerful oxidizing agent in acidic medium.

`K_2Cr_2O _7+ 4 H_2SO_4 -> K_2SO_4 + Cr_2 (SO_4)_3 + 4 H_2O + undersettext(Nascent oxygen) (3 [O])`

(B) In term of electronic concept, the `Cr_2O_7^(2-)` ion take up electron in Acidic medium and hence acts as an oxidizing agent.

`underset(+6)[Cr_2O_7^(2-)] + 14 H^(+) + 6 e^(-) -> undersettext(+3)[2Cr^(3+)] + 7H_2O`

`text(Note)` : Both `Na_2Cr_2O_7` & `K_2Cr_2O_7` are oxidizing agents but `K_2Cr_2O_7` is preferred since it is not hygroscopic and can be used as primary standard.

(C) Some oxidizing reactions of `K_2Cr_2O_7` are:

It liberate `I_2` from `Kl`

`K_2Cr_2O_7 + 4H_2SO_4 -> K_2SO_4 + Cr_2 ( SO_4)_3 + 4 H_2O + 3O`

`[2 KI + H_2SO_4+ O -> K_2SO_4 + H_2O + I_2 ] xx 3`

`K_2 Cr_2O_7+ 6 KI + 7H_2SO_4 -> 4K_2SO_4 + Cr_2(SO_4)_3 + 7 H_2O + 3I_2` (Iodine)

`text(Total Reaction)` `Cr_2O_7^(2-) + 14 H^+ + 6I^(-) + 6e^(-) -> 2 Cr^(3+) + 7H_2O + 3 I_2 + 6e^-`

or `Cr_2O_7^(2-) + 14H^+ + 6I^(-) -> 2Cr^(3+) + 3I_2 + 7H_2O`

It oxidises ferrous salts to ferric salts.

`K_2Cr_2O_7 + 4H_2SO_4 -> K_2SO_4 + Cr_2 (SO_4)_3 + 4H_2O + 3(O)`

`2 [FeSO_4 + H_2SO_4 + O -> Fe_2(SO_4)_3 + H_2O] xx 3`

`text(Total Reaction)` `K_2Cr_2O_7 + 7H_2SO_4 + 6FeSO_4 -> 3Fe_2(SO_4)_3 + K_2SO_4 + Cr_2(SO_4)_3 + 7H_2O`

or `6Fe^(2+) + Cr_2O_7^(2-) + 14 H^+ -> 6Fe^(3+) + 2Cr^(3+) + 7H_2O`

It oxidises hydrogen sulphide to sulphur

`K_2Cr_2O_7 + 4H_2SO_4 -> K_2SO_4 + Cr_2(SO_4)_3 + 4H_2O + 3 [O]`

`[H_2S + O -> H_2 O + S] xx 3`

`text(Total Reaction)` `=>K_2Cr_2O_7 + 4H_2SO_4 + H_2S -> K_2SO_4 + Cr_2(SO_4)_3, + 3S + 7 H_2O`

or `H_2S + Cr_2O_7^(-2) + 8H^+ -> 2Cr^(3+) + 3S + 7H_2O`

It oxidises sulphites to sulphates

`K_2Cr_2O_7 + 4H_2SO_4 -> K_2SO_4 + Cr_2 (SO_4)_3 + 4H_2O + 3(O)`

`[Na_2SO_3 + O -> Na_2SO_4] xx 3`

`text(Total Reaction)` `=> 3 Na_2SO_3 +K_2Cr_2O_7 + 4 H_2SO_4 -> K_2SO_4 +Cr_2(SO_4)_3 + 3Na_2 SO_4 + 4H_2O`

or `3SO_3^(2-) + Cr_2O_7^(2-) + 8 H^+ -> 3 SO_4^(2-) + 2 Cr^(3+) + 4H_2O`

`SO_2` is oxidised to `H_2SO_4`

`K_2Cr_2O_7 + 4 H_2SO_4 -> K_2SO_4 +Cr_2(SO_4)_3 + [O]`

`[SO_2 +(O) + H_2O -> H_2SO_4] xx 3`

`text(Total Reaction) ` `=> K_2Cr_2O_7 + H_2SO_4 + 3SO_2 -> Cr_2(SO_4)_3 H_2O`

or `Cr_2O_7^(2-) + 3 SO_2 + 2H^(+) -> 2 Cr ^(3+) + 3 (SO_4)^(2-) + H_2 O`

`text(NOTE)` :

`K_2Cr_2O_7 + H_2SO_4 + 3SO_4 -> K_2SO_4 +Cr_2(SO_4)_3 +H_2O`

`undersettext(Chrome Alum)(K_2SO_4Cr_2(SO_4)_3 . 24 H_2O)`

Similarly, it oxidises, chlorides to chlorine, nitrites to nitrates, arsenites to arsenates thiosulphate to sulphate and sulphur `(S_2O_3^(2-) + O -> SO_4^(2-)+ S)` , `HBr` to `Br_2`, `Hl` to `I_2` .

It oxidises ethyl alcohol to acetaldehyde and acetic acid

`K_2Cr_2O_7 + 4H_2SO_4 -> K_2SO_4 + Cr_2(SO_4)_3 + 4H_2O + 3 [O]`

`undersettext(Ethyl Alcohol)(CH_3CH_2OH) + [O] -> CH_3CHO + H_2O`

`undersettext(Acetaldehyde)(CH_3CHO) + O -> CH_3COOH`

`text(Structure of chromate and dichromate ions)` : See fig.2.

`text(Uses)` : Potassium dichromate is used :

(i) As a volumetric reagent in laboratory for the estimation of ferrous ions, iodide ions etc.

(ii) For the preparation of chrome yellow `(PbCrO_4)`, Chrome red `(PbCrO_4.PbO)` , Zinc yellow `(ZnCrO_4)`, Gugrets green `(Cr_2O_3. 2H_2O)`, chromic acid `(CrO_3^-` orange), `K_3 [CrO_8]` (Red brown).

(iii) In organic chemistry as oxidising agents.

(iv) In photography for hardening gelatine films.

Potassium Permanganate(`KMnO_4`) :

`text(Preparation)` : Potassium Penmanganate is prepared from mineral pyrolusite (`MnO_2`).
The preparation involves the following steps.

(A) `text(Conversion of Pyrolusite ore to Potassium Manganate)` :

(a) The pyrolusite (`MnO_2`) is fused with caustic potash (`KOH`) or potassium carbonate in presence of air to give a green mass due to the formation of potassium manganate.

`2 MnO_2 + 4 KOH + O_2 overset(Delta)-> undersettext[Pot. Manganate (Green mass)](2K_2MnO_4) + 2H_2O`

`2MnO_2 +2 K_2CO_3 +O_2 overset(Delta)-> undersettext[Pot. Manganate(Green mass)](2K_2MnO_4) +2CO_2`

`(B)` `text(Oxidation of Potassium Manganate to Potassium permanganate)` :

(a) The fused mass is extracted with water and the solution is green (`MnO_4^(2-)`). The solution is treated with `Cl_2` or ozone (`O_3`) or `CO_2` to oxidize `K_2MnO_4` to `KMnO_4`.

`2K_2MnO_4 + Cl_2 -> 2KCl + 2KMnO_4`

`K_2MnO_4 + 2CO_2 -> 2K_2CO_3 +2 MnO_2 downarrow + undersettext(Purple)(2KMnO_4)`

(b) `MnO_4^(2-)` can be oxidized to `MnO_4^-` electrochemically at anode.

`K_2MnO_4 ⇋ 2 K^+ + MnO_4^(2-)`

At anode : `undersettext(Green)(MnO_4^(2-)) -> undersettext(Purple)(MnO_4^(-)) +e^-`

At cathode : `2H^+ + e^(-) -> 2H -> H_2`

`text(Properties)` :

`(A)` It is a dark violet crystalline solid having a metallic lustre. It has M.P. `523` `K`.

`(B)` It is fairly soluble in water giving a purple solution.

`(C)` `text(Action of heat)` :

`2KMnO_4 overset(Delta)-> underset(Pot.Manganate)(K_2 MnO_4) + MnO_2 +O_2 uparrow `

`(D)` `text(Action of alkalies)` :

`4KMnO_4 + 4KOH -> underset(Pot.Manganate)(4K_2MnO_4) + 2H_2O + O_2 uparrow`

`(E)` `text(Oxidising character)` : Potassium permanganate act as an oxidizing agent in neutral, alkaline and acidic solutions.

(a) In Neutral Medium : `MnO_2` is formed

`2KMnO_4 + H_2O -> 2 KOH + 2MnO_2 +3O`

`underset(+7)(MnO_4^(-)) +2H_2O + 3e^(-) -> underset(+4)(MnO_2) + 4 OH^(-)`

`text(Important reactions of Neutral)` `KMnO_4`

`(i) 2 KMnO_4 + 3H_2S -> 2KOH + 2 MnO_2 + 3S + 2H_2O`

(ii) It oxidises manganese sulphate to manganese dioxide.

`2KMnO_4 + 3MnSO_4 + 2H_2O -> 5 MnO_2 + K_2SO_4 + 2H_2SO_4`

(iii) It oxidises sodium thiosulphate to sulphate and Sulphur

`2KMnO_4 + 3Na_2S_2O_3 + H_2O -> 2MnO_2 + 3Na_2SO_4 + 2 KOH + 3S`

(b) Alkaline medium :

(i) `2KMnO_4 + 2KOH -> K_2MnO_4 + H_2O + [O]`

`2K_2MnO_4 + 2H_2O -> 2MnO_2 + 4KOH + 2[O]`

`2KMnO_4^(-) + H_2O oversettext(alkalies)-> 2MnO_2 + 2KOH + 3[O]`

`text(Important reactions of alkaline)` `KMnO_4` :

(i) It oxidises iodides to iodates in alkaline medium :

`2KMnO_4 + H_2O oversettext(alkaline) -> 2MnO_2 + 2KOH + 3[O]`

`KI + 3[O] -> KIO_3`

`text(Total Reaction )` `=> 2K_2MnO_4 + KI + H_2O -> 2MnO_2 + 2KOH + KIO_3`

`2MnO_4^(-) + I^(-) + H_2O -> 2 MnO_2 +IO_3^(-) + 2OH^(-)`

(ii) Oxidises ethylene to ethylene glycol :

`CH_2 = CH_2 + H_2O + O -> undersettext(Ethylene Glycol)(CH_2(OH)-CH_2(OH))`

In alkaline medium it is called `text(Bayer's Reagent)`.

`text(Note)` :

(a) `undersettext[Conc.][KMnO_4] + H_2SO_4 -> K^(+) + undersettext[Green][MnO_3^(+)] + 3 HSO_4^(-) + H_3O^(+)`

(b) `undersettext[Excess][2KMnO_4]` `+undersettext[Conc.][H_2SO_4]` `overset(Delta)->` `undersettext[(Explosive oil)][Mn_2O_7] + K_2SO_4 +H_2O`

(c) `text(Acidic solution)` :

`2KMnO_4 + 3H_2SO_4 -> K_2SO_4 +2MnSO_4 + 3H_2O + 5O`

`MnO_4^(-) + 8 H^(+) + 5 e^(-) -> Mn^(2+) + 4H_2O`

`text(Some of the important reactions of acidified)` `KMnO_4` `text(are)`

(i) It oxidises acidified ferrous salt to ferric salts

`2KMnO_4 + 10 FeSO_4 + 8H_2SO_4 -> 2MnSO_4+ K_2SO_4 + 5Fe_2(SO_4)_3 + 8H_2O`

(ii) lt oxidises acidified `KI` to `I_2`

`2 KMnO_4 + 6K_2SO_4 +8H_2O +5I_2`

(iii) It oxidises `H_2S` to Sulphur

`2KMnO_4 + 3H_2SO_4 + 5H_2S -> 2MnSO_4 + K_2SO_4 + 8H_2O + 5 S`

(iv) It oxidises sulphur dioxide to sulphuric acid

`2KMnO_4 + 5SO_2 + 2H_2O -> K_2SO_4 + 2MnSO_4 + 2H_2SO_4`

(v) lt oxidises nirites to nitrates

`2KMnO_4 + 3H_2SO_4 + 5KNO_2 -> K_2SO_4 + 2MnSO_4 + 5KNO_3 + 3H_2O`

(vi) It oxidises oxalates or oxalic acid to carbon dioxide

`2KMnO_4 + 5C_2H_2O_4 + 3H_2SO_4 -> K_2SO_4 + 2MnSO_4 + 10 CO_2 + 8 H_2 O`