5) It oxidises
ethyl alcohol to acetaldehyde and acetic acid:
K2Cr2O7 + 4 H2SO4
→ K2SO4 + Cr2(SO4)3 + 4H2O
+3O
CH2CH2OH + O → CH3CHO + H2O
CH3CHO + O → CH3COOH
Acetaldehyde Acetic Acid
Test for a drunken driver. The above reaction helps to test
whether a driver has consumed alcohol or not. he is asked to breathe into the
acidified K2Cr2O7 solution taken in a test
tube. If the orange colour of the solution changes into green colour (due to Cr2(SO4)3
formed in the reaction), the driver is drunk, otherwise not.
o
Chromyl chloride test (Reaction with
a chloride and conc. sulphuric acid): When heated with
concentrated hydrochloric acid or with a chloride and strong sulphuric acid, reddish brown
vapours of Chromyl Chloride are obtained.
K2Cr2O7
+ 6 H2SO4 + 4 KCl → CrO2Cl2 + 6 KHSO4
+ 3H2O
Chromyl Chloride
Ø
Uses:
· In volumetric analysis, it is used as a primary standard for the
estimation of Fe2+ (ferrous ions) and I– (iodides) in
redox titrations.
· In industry, it is used
(a) In chrome tanning
in leather industry.
(b) In the preparation
of chrome alum K2SO4.Cr2(SO4)3.
24H2O and other industrially important compounds such as Cr2O3,
CrO3, CrO2Cl2, K2CrO4,
CrCl3 etc.
(c) In calico printing
and dyeing.
(d) In photography and
in hardening gelatine film.
ü Preparation
of Potassium Permanganate (KMnO4): Potassium permanganate is commercially prepared from Pyrolusite
(MnO2). The preparation involves two steps.
Step
1:
MnO2
is fused with KOH to form potassium manganate
(K2MnO4).
2 MnO2 + 4 KOH + O2 → 2 K2MnO4
+ 2 H2O
Step 2: K2MnO4 is
electrolytically oxidised to potassium permanganate.
3 MnO42- + 4 H+ → 2 MnO4-
+ MnO2 + 2 H2O
Ø Properties: Potassium permanganate forms dark purple crystals which are
iso-structural with those of KClO4. When
heated it decomposes and liberate O2.
2KMnO4 → K2MnO4
+ MnO2 + O2
The manganate and permanganate ions are tetrahedral
F The green manganate is
paramagnetic with one unpaired electron but the permanganate is diamagnetic.
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