Preparation and properties of Potassium dichromate (K2Cr2O7)

By HARI -

Preparation of Potassium dichromate (K2Cr2O7)

Potassium dichromate (K2Cr2O7) is prepared from chromite ore FeCr2O4. The chromite ore is fused with sodium or potassium carbonate in free access of air.

4FeCr2O4 + 8Na2CO3 + 7O2 -------> 8Na2CrO4 + 2FeO3 + 8CO2

The yellow solution of sodium chromate is filtered and acidified with sulfuric acid to give a solution from which orange sodium dichromate , Na2Cr2O7 2H2O can be crystallized.

2Na2CrO4 + 2H+ ---------> Na2Cr2O7 + 2Na+ + H2O

Sodium dichromate is more soluble than potassium dichromate. Hence sodium dichromate when fused with KCl forms orange crystals of potassium dichromate.

Na2Cr2O7 + 2KCl --------> K2Cr2O7 + 2NaCl

The chromates and dichromates can be inter convertible.

2CrO4 2- + 2H2+ ---------> Cr2O7 2- + H2O

Cr2O7 2- + 2OH- ---------> 2CrO4 2- + H2O

The dichromate ion and chromate ion exist in equilibrium with each other at a pH of 4. Yellow chromate changes into orange dichromate in acid medium and dichromate changes into chromate in basic medium.


2CrO4 2- + 2H+ <-_-_AcidAlkali-_-_> 2HCrO4- (Hydrogen chromate)


2HCrO4-<-_-_AcidAlkali-_-_> Cr2O7 2- + H2O Dichromate (orange)

The chromate ion is tetrahedral and the dichromate ion consists of two tetrahedral sharing at one corner, with Cr-O-Cr bond angle 126 degree.


Properties Potassium dichromate (K2Cr2O7)

Action of heat

Potassium dichromate decomposes on heating to from potassium chromate, chromic oxide and oxygen.

4K2Cr2O7 ------> 4K2CrO4 + 2CrO3 + 3O2

Oxidizing properties

Potassium dichromate is a powerful oxidizing-agent in acidic medium.

Cr2O7 2- + 14H+ + 6 electron -------> 2Cr3+ + 7H2O

A few examples for this oxidizing character in acidic medium are

1. It oxidize iodides to iodine.

Cr2O7 2- + 14H+ + 6I- ------> 2Cr 3+ + 7H2O + 3I2

2. It oxidizes ferrous salts to ferric salts.

Cr2O7 2- + 14H+ + 6 Fe2+ -------> 2Cr 3++ 7H2O + 6Fe3+

3. It oxidizes stannous salts to stannic salts.

Cr2O7 2- + 14H+ + 3Sn 2+ --------> 2Cr 3+ + 7H2O + 3Sn 4+

4. It oxidizes H2S to sulphur

Cr2O7 2- + 8H+ + 3H2S -------> 2Cr 3+ + 7H2O + 3S

Uses of potassium dichromate
(K2Cr2O7)

Sodium and potassium dichromate are strong oxidizing agents. Potassium dichromate is used as a primary standard in volumetric analysis. It is also used in chrome plating.


Related articles Copper sulphate penta hydrate

For articles about Extraction of metals visit Extraction

Comments

Post a Comment

Popular posts from this blog

Reaction of Ether with Hydrogen Iodide (HI)

By HARI -
Image
Chemical properties of Ethers (With HI) On heating with concentrated Hydrogen iodide (HI) the C-O bond in ethers breaks forming alcohol and alkyl iodide. For example, C 2 H 5 -O-C 2 H 5 + HI ------------> C 2 H 5 - I + C 2 H 5 OH On boiling with excess of concentrated Hydrogen iodide (HI), Alkyl iodide is formed. C 2 H 5 -O-C 2 H 5 + 2HI ------------> 2C 2 H 5 I + H 2 O In the case of mixed ethers with two different alkyl groups, the site of cleavage and hence the alcohol and alkyl iodide that form depend on the nature of the alkyl groups. When one group is methyl and the other is primary or secondary alkyl group, it is the lower alkyl group that forms alkyl iodide due to steric factors. For Example, When one group is methyl and the other alkyl group is a tertiary group, the halide formed is a tertiary group, the halide formed is a tertiary halide. For Example, It is because the attack by I- takes place at that carbon of alkyl group, which has a greater electron pus...
Read more

Reaction of Ether with Sulphuric Acid

By HARI -
Image
Chemical Properties of Ethers (with H 2 SO 4 ) On heating with dilute sulfuric acid under pressure, ethers are hydrolysed to alcohols. For example, C 2 H 5 OC 2 H 5 + H 2 O ----( dil.H2so4,high pressure )-----> 2C 2 H 5 OH Mixed ethers under similar conditions give a mixture of alcohols. CH 3 OC 2 H 5 + H 2 O ------( dil.H2so4,high pressure )------>C 2 H 5 OH + CH 3 OH But if concentrated sulfuric acid is used, then the products are alcohol and alkyl hydrogen sulphate. C 2 H 5 OC 2 H 5 (Diethyl ether) -----( heat, conc. H2SO4 )-----> C 2 H 5 OH (ethanol)+ C 2 H 5 OSO 2 OH (ethyl hydrogen sulphate) Ethers containing secondary and tertiary alkyl groups form alkenes with conc. sulphuric acid. For example For more about chemical reaction of ether visit http://entrancechemistry.blogspot.in/2012/07/chemical-properties-of-ether.html
Read more

Reaction of alcohol and phenol with carboxylic acid

By HARI -
Image
ESTERIFICATION Alcohols and phenols react with carboxylic acids, acid chlorides and acid anhydrides to form esters. R-O-H + R' - COOH -----H+-----> ROCOR' + H 2 O Ethanol Acetic acid Ethyl acetate(Ester) C 2 -OH + CH 3 -COOH -----H 2 SO 4 ----------> C 2 H 5 O COCH 3 + H 2 O Ar-OH + R'-COOH ------------------> ArOCOR' + H 2 O Phenol Ester Ar-OH/ R-OH + (R'CO) 2 ---------H + ----------> Ar/ ROCOR' + R'COOH Anhydride Ease of esterification of alcohol is in the order primary> secondary> tertiary . The reaction with carboxylic acid and acid anhydride is carried out in presence of small amount of concentrated sulphuric acid. The reaction with carboxylic acid is reversible, and therefore, water is removed as soon as it is formed. The Reaction With Acid Chloride Is Carried Out In Presence Of A Base (Pyridine) so as to remove HCl. It also shifts the equilibrium to right hand side. The introduction of acetyl (CH3CO-) ...
Read more