NZIC 2005
CHEMISTRY - 2.7
(Describe oxidation-reduction reactions)
ASSESSMENT SCHEDULE
While the writers of this assessment have worked to compile a resource that meets NCEA requirements, it has no
official status and teachers may wish to adjust questions and the assessment schedule as they see fit
Question
ONE
TWO
1&2
Three
Four
1&2
Evidence
O2
0
NO3–
+5
HCl
–1
Cr2O72–
+6
SO2
+4
(a) Yes Oxidation number of Pb changes from +2
to 0 OR oxidation number of C changes
from 0 to +2 OR equivalent answer using
transfer of oxygen.
(b) No Oxidation numbers do not change from
one side of the equation to the other.
(c) Yes Oxidation number of Mn changes from +4
to +2 OR Oxidation number of Cl changes
from –1 to 0.
1
2
3
Cu
Cu2+ + 2e–
NO3– + 4H+ + 3e–
NO + 2H2O
+
H2S
S + 2H + 2e
4
Cr2O72– + 14H+ +6e–
3
Merit
Correctly
identifies all
three reactions.
Correctly
identifies all
three reactions
and justifies
any two.
Three half
equations
correct.
Excellence
Full and
correct
justifications
for all the
reactions.
All four half
equations
correctly
balanced.
2Cr3+ + 7H2O
(SO32– + H2O
SO42– + 2H+ +2e–) 5
2IO3– + 12H+ + 10e–
I2 + 6H2O
2–
–
+
5SO3 + 2IO3 +2H
5SO42– + I2 +H2O
(NH3 + 2H2O
(O2 + 4H+ + 4e–
4NH3 + 7O2
Achievement
3 oxidation
number correct.
One equation
correct or
multiplies both
sets correctly
Both
equations
correctly
balanced
including
cancelling.
Correct half
equations
Correct
combined
equation
NO2 + 7H+ + 7e–) 4
2H2O) 7
4NO2 + 6 H2O
(a)
Fe2+
Fe3+ + e–
–
MnO4 + 8H+ + 5eMn2+ + 4H2O
(b)
5Fe2+ + MnO4– + 8H+
Mn2+ + 5Fe3+ + 4H2O
(c) The purple colour of the permanganate ion
changes to the very pale pink (colourless) of
Mn2+. The pale green of Fe2+ changes to the
pale yellow of Fe3+.
Correct
equations plus
two
observations
linked to
correct ions.
2
Question
Five
1&2
Evidence
Iron/chlorine
Oxidant is chlorine; reductant is iron
Sulfur dioxide is oxidant; hydrogen sulfide is
reductant
Achievement
Correct choice
of oxidants and
reductants for
one reaction.
Merit
Correct choice
for both OR
one
justification.
Excellence
Correct choice
for both.
Both
explanations
correct.
Recognises
that sulfur is
both oxidized
and reduced in
the second
reaction.
Correct
equations but
wrong
electrode.
Correct
equations at
correct
electrode.
Correct
equations at
correct
electrode plus
correct
justification.
Reasons: an oxidant is the reactant that is reduced:
in the first reaction, chlorine is reduced from Cl2 to
2Cl--; its oxidation number has decrease from 0 to 1, OR chlorine has gained two electrons to form
two chloride ions.
In the second reaction, the sulfur in H2S is oxidised
from –2 to 0 and is therefore the reductant, and the
sulfur in SO2 is reduced from +4 to 0 therefore
acting as the oxidant.
SIX
1
Cathode reaction: Al3+ +3e
2
Anode reaction: 2O2–
Al
O2 + 4e
3 O2– ions are attracted to the positive electrode
where they lose electrons
Al3+ ions are attracted to the negative electrode
where they gain electrons
Sufficiency Statement. These exams are not pre-tested. The statements below are a guide and schools should feel
free to adjust them to get a sensible distribution.
Achieved
A total of FOUR opportunities answered correctly at Achieved or higher.
Merit
A total of SIX opportunities answered correctly with 3 at Merit and 3 at Achieved
Excellence
A total of EIGHT opportunities answered correctly with 2 at Excellence, 3 at Merit and 3 at Achieved.
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