Question

K2cr2o7 when reacts with cold conc. H2so4 gives red crystal of

Answer 1

Potassium Dichromate, K2Cr207
Preparation
It is prepared from the ore called chromate or
ferrochrome or chrome iron, FeO.Cr203. The various
steps involved are
(a) Preparation of sodium chromate
4FeO.Cr203 + 02 Fe203 + 4Cr203
4Na2C03 + 2Cr203 + 302 -
4Na2Cr04 + 4C02
(b) Conversion of sodium chromate into sodium
dichromate.
2Na2Cr04 + H2504 Na2Cr207 + Na2504 + H2(
(c) Conversion of sodium dichromate into potassium
dichromate.
Na2Cr207 + 2KCI K2Cr207 + 2NaCI
Properties
It forms orange red crystals. It is moderately soluble in
cold water but freely soluble in hot water.
1 . Action of heat
When heated, it decomposed to its chromate
A
4K2Cr207 --l4K2Cr04 + 2Cr203 + 302
2. Action of alkalis
With alkalis it is converted into chromate which on
acidifying gives back dichromate.
K2Cr207 + 2KOH 2K2Cr04 + H20
2K2Cr207 + H2504 K2Cr207 + K2504 + H20

In dichromate solution the Cr2072- ions are in
equilibrium with Cr2072- ions at pH ; 4.
PH:4

Cr2072- + H20 ? 2Cr042- + 2HI
orange red yellow
3. Action of conc. H2504 solution
(a) in cold conditions

K2Cr207 + 2H2504 -
(b) in hot conditions
2K2Cr207 + 8H2504 2K2504 + 2Cr2(S04)3 +
8H20 + 302

2Cr03 + 2KHS04 + H20

4. Oxidising properties
It is a powerful oxidising agent. In the presence of dil.
H2504 it furnishes 3 atoms of available oxygen.
K2504 + Cr2(S04)3 + 4H20
K2Cr207 + 4H2504 -
+ 30
Some of the oxidizing properties of K2Cr207 are
(a) it liberates 12 from KI
K2Cr207 + 7H2504 + 6KI 4K2504 + Cr2(S04)3 +
312 + 7H20
(b) it oxidises ferrous salts to ferric salts
K2Cr207 + 7H2504 + 6FeS04 K2504 +
Cr2(S04)3 + 3Fe2(S04)3 + 2H20
(c) it oxidises S-2 to S
K2Cr207 + 4H2504 + 3H25 K2504 + Cr2(S04)3
+ 7H20 + 35
(d) it oxidises nitrites to nitrates
K2Cr207 + 4H2504 + 3NaN02 K2504 +
Cr2rS04)3 + 3NaN03 + 4H20