1 of 1 Teacher/Lecturer
Practical 5.8 Stabilising an unusual oxidation number:
chromium(II) ethanoate, Cr2(CH3CO2)4(H2O)2
Purpose
Safety
To carry out experiments to:
(iii) prepare a sample of a complex,
e.g. chromium(II) ethanoate
[5.3.2g]
The chemicals and procedures used in this experiment are
extremely hazardous, so you must take even more care than
usual to reduce risks from them by using suitable control
measures. Solid sodium dichromate(VI) is very toxic and
oxidising and is classed as a category 2 carcinogen. It is
an irritant to all tissues and you must wear gloves when
handling this solid. Avoid inhaling any tiny crystals. Zinc
powder is highly flammable. Concentrated hydrochloric acid
is corrosive. The hydrogen evolved is extremely flammable.
As hydrogen is evolved, naked flames must be kept clear.
Throughout this experiment, you must wear eye protection.
Notes on the method
Some tap funnels have long stems
below the ground glass joints which
prevent the funnels being fitted
into the multiple adaptor as shown.
In this case the excess length has to
be cut off or some tuber tubing can
be sleeved over the stem and the
stem then carefully fitted in place
as shown here or two adaptors to
increase and decrease the size of
side arm can be added.
tap funnel
rubber tubing
multiple adaptor
Answers
1 The relevant electrode potentials are:
Zn21(aq) Z Zn(s)
E 5 20.76 V
Cr (aq), Cr (aq) Z Pt
E 5 20.41 V
[Cr2O722(aq) 1 14H1(aq)], [2Cr31(aq) 1 7H2O(l)] Z Pt
E 5 11.33 V
31
21
Application of the anticlockwise rule reveals that solid zinc should reduce Cr2O722(aq) to
Cr31(aq) and Cr31 to Cr21(aq).
2 The blue colour of chromium(II) changes to the green colour of chromium(III) on
exposure to air.
3 It takes much longer for any change to occur to the red colour of chromium(II)
ethanoate.
Edexcel practical materials adapted from Nuffield Advanced Chemistry materials
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