March 2011 - Career Point

(v) (F) is sodium peroxide as only peroxides gives H 2 O 2
on reaction with dil. acids.
2 Na 2 O
(B)
⎯
Na 2O 2 +
(F)
670 ⎯⎯
K →
∆
(F)
Na 2O 2 +
2 Na
(A)
H 2 SO 4 → H 2O 2 + Na 2 SO 4
dil.
(F) gives the following oxidations :
Cr(OH) 3 + 5OH – → CrO 2– 4 + 4H 2 O + 3e –
Mn 2+ + 8OH – → MnO – 4 + 4H 2 O + 5e –
S 2– + 8OH – → SO 2– 4 + 4H 2 O + 8e –
The reduction equation of (F) is
O 2– 2 + 2H 2 O + 2e – → 4OH –
(vi) (G) is sodamide because it is used in the
dehydrohalogenation reactions.
Na + NH 3 (l) →
2O
(B)
NaNH +
(G)
CH 3 – CH – CH 2 + 2NaNH 2
Br
Br
2
∆
NaOH
(E)
CH 3 – C ≡ CH
Propyne
+ 2NaBr + 2NH 3
5. (i) A blue coloured compound (A) on heating gives
two products, (B) and (C).
(ii) A metal (D) is deposited on passing hydrogen
through heated (B).
(iii) The solution of (D) in HCl on treatment with
K 4 [Fe(CN) 6 ] gives a chocolate brown coloured
precipitate of compound (E)
(iv) (C) turns lime water milky which disappears on
continuous passage of (C) forming a compound
(F).
Identify (A) to (F) and give chemical equations
for the reactions at steps (i) to (iv).
Sol. [A]
Blue colour
Heated [B]
⎯ heat ⎯→ [B] + [C]
lime
H
⎯⎯→
2
[D]
Metal
(i)HCl
⎯ ⎯⎯⎯⎯⎯→
[E]
( ii)K4[Fe(CN)
6 ]
chocolate coloured
[C] ⎯⎯ →Milky ⎯ [ ⎯→
C]
Milkiness disappears
water
[F]
Conclusions from the above set of reactions
(a) Reaction of the solution of the metal [D] in HCl
with potassium ferrocyanide to give chocolate
coloured precipitate indicates that the metal [D] is
copper. Hence the blue coloured compound [A] must
be a copper salt, most probably copper sulphate.
(b) Reaction of [C] with lime water indicates that
[C] is SO 2 gas.
Hence all the given reactions in steps (i) to (iv) can
be written as below.
(i) CuSO 4 ⎯
750o ⎯⎯
C → CuO + SO 3 SO 2 + ½ O 2
[A] [B] [C]
(ii) CuO (hot) + H 2 ⎯→ Cu + H 2 O
[B]
[D]
(iii) Cu + 2HCl ⎯→ CuCl 2 + H 2
[D]
2CuCl 2 +K 4 [Fe(CN) 6 ] ⎯→ Cu [Fe(CN)
(iv) SO 2 + Ca(OH) 2 ⎯→
2 6]
chocolate ppt
CaSO3
↓
(F) ppt
↓ + 4KCl
An acid-base indicator is a weak acid or a weak
base. The undissociated form of the indicator is a
different color than the iogenic form of the indicator.
An Indicator does not change color from pure acid to
pure alkaline at specific hydrogen ion concentration,
but rather, color change occurs over a range of
hydrogen ion concentrations. This range is termed
the color change interval. It is expressed as a pH
range.
How is an indicator used?
Weak acids are titrated in the presence of indicators
which change under slightly alkaline conditions.
Weak bases should be titrated in the presence of
indicators which change under slightly acidic
conditions.
Some common acid-base indicators?
Indicator pH Range Acid Base
Thymol Blue 1.2-2.8 red yellow
Pentamethoxy red 1.2-2.3 red-violet colorless
2,4-Dinitrophenol 2.4-4.0 colorless yellow
Methyl yellow 2.9-4.0 red yellow
Methyl orange 3.1-4.4 red orange
-Naphthyl red 3.7-5.0 red yellow
Bromcresol green 4.0-5.6 yellow blue
Methyl red 4.4-6.2 red yellow
Bromcresol purple 5.2-6.8 yellow purple
Chlorphenol red 5.4-6.8 yellow red
Phenol red 6.4-8.0 yellow red
Thymol blue 8.0-9.6 yellow blue
Phenolphthalein 8.0-10.0 colorless red
-Naphtholbenzein 9.0-11.0 yellow blue
Thymolphthalein 9.4-10.6 colorless blue
Nile blue 10.1-11.1 blue red
Poirrier's blue 11.0-13.0 blue violet-pink
Trinitrobenzoic acid 12.0-13.4 colorless orange-red
XtraEdge for IIT-JEE 38 MARCH 2011