BZ Reaction
Teachers Notes (1 of 2)
This page summarises the answers to the questions asked of the students. The next page has all
the answers to the questions with the questions attached.
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2. i) 1s 2s 2p63s23p64s2 ii) Fe2+ —> Fe3+ + e
3. i) Ce3+ ii) BrO2 ii) BrO2
4. i) A reaction where a species is
simultaneously reduced and oxidised.
ii) Br in HOBr: oxidation number +1
Br in HBrO2: oxidation number +3
(oxidation)
Br in Br2: oxidation number 0 (reduction)
5. Chloride ions are more reactive than
bromide ions; therefore chloride ions will
displace bromide ions in the system.
6. A species which has an unpaired electron
available for bonding
7.
Colour
Iron ion
Cerium ion
Red
Fe2+
Ce3+
Blue
Fe3+
Ce3+
Green
Fe3+
Ce4+
Purple
Fe2+, Fe3+
Ce3+
We have
recorded the
duration of
each state of
the reaction for
each oscillation
and recorded
them here.
This graph
shows the
general trend
for the period to
elongate over
time.
References
A. N. Zakin, A. M. Zhabotinsky, Nature 225, 535 (1970)
R. J. Field, Chemie in unserer Zeit 7, 171 (1973)
R. J. Field, E. Körös, R. M. Noyes, J. Amer. Chem. Soc. 94, 8649 (1972)
www.syngenta.co.uk/learningzone
BZ Reaction
Teacher’s Notes: Further Questions (2 of 2)
1. Malonic acid is also
(oxidation)
known as
 Br in Br2: oxidation number 0
propanedioic acid.
(reduction)
Draw the structure of
5. The oscillations of the BZ reaction are very
this substance:
easily interrupted by the presence of chloride
2. Ferroin indicator
ions in the system. Suggest why chloride ions
contains Fe2+ ions in solution.
may have such an effect.
2+
 Write the full electronic structure of an Fe
Chloride ions are more reactive than
bromide ions; therefore chloride ions will
ion. 1s22s22p63s23p64s2
displace bromide ions in the system.
 When oxidised ferroin changes in colour,
from red to blue. Suggest the half-equation
The colour changes in the BZ reaction are
which represents this colour change.
caused by the redox reactions of cerium (III)
Fe2+ —> Fe3+ + eions (colourless), cerium (IV) ions (yellow),
as well as iron (II) and iron (III) ions.
The colour change from blue to green
involves the following redox reaction:
3+
BrO2· + Ce
+
4+
+ H HBrO2 + Ce
3. In this equation which species is:

+
Being oxidised? Ce3
 Being reduced? BrO2
 Acting as an oxidising agent? BrO2
Another reaction occurring in the BZ system
is as follows:
3HOBr HBrO2 + Br2 + H2O
The first colour change, from red to blue, is
caused by the following reactions:
BrO3- + HBrO2 + H+ 2BrO2· + H2O
BrO2· + Fe2+ + H+ HBrO2 + Fe3+
6. The first of these equations generates a free
radical species. Define a free radical.
A species which has an unpaired electron
available for bonding
7. For each colour in the reaction, fill in the table
to show the metal ions of iron and cerium in
the solution.
4. This is an example of a disproportionation
Colour
Iron ion
Cerium ion
reaction.
2+
Red
Fe
Ce3+
 Explain what is meant by the term
Blue
Fe3+
Ce3+
‘disproportionation reaction’.
A reaction where a species is
Green
Fe3+
Ce4+
simultaneously reduced and oxidised.
Purple
Fe2+, Fe3+
Ce3+
 Show, by use of oxidation numbers, which
species are reduced and oxidised.
References: R. J. Field, Chemie in unserer Zeit 7, 171 (1973)
N. Zakin, A. M. Zhabotinsky, Nature 225, 535 (1970)
 Br in HOBr: oxidation number +1
R. J. Field, E. Körös, R. M. Noyes, J. Amer. Chem. Soc. 94, 8649
 Br in HBrO2: oxidation number +3
(1972)
www.syngenta.co.uk/learningzone